Aperiodic magnetic turbulence produced by relativistic ion beams
Niemiec, Jacek; Bret, Antoine; Stroman, Thomas
2009-01-01
Magnetic-field generation by a relativistic ion beam propagating through an electron-ion plasma along a homogeneous magnetic field is investigated with 2.5D high-resolution particle-in-cell (PIC) simulations. The studies test predictions of a strong amplification of short-wavelength modes of magnetic turbulence upstream of nonrelativistic and relativistic parallel shocks associated with supernova remnants, jets of active galactic nuclei, and gamma-ray bursts. We find good agreement in the properties of the turbulence observed in our simulations compared with the dispersion relation calculated for linear waves with arbitrary orientation of ${\\vec k}$. Depending on the parameters, the backreaction on the ion beam leads to filamentation of the ambient plasma and the beam, which in turn influences the properties of the magnetic turbulence. For mildly- and ultra-relativistic beams, the instability saturates at field amplitudes a few times larger than the homogeneous magnetic field strength. This result matches our...
Schlickeiser, R
2012-12-28
It is shown that an unmagnetized nonrelativistic thermal electron-proton plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of strength |δB|=3.5β(e)g(1/3)W(e)(1/2) G, where β(e) is the normalized thermal electron temperature, W(e) the thermal plasma energy density, and g the plasma parameter. For the unmagnetized intergalactic medium, immediately after the reionization onset, the field strengths from this mechanism are about 2×10(-16) G in cosmic voids and 2×10(-10) G in protogalaxies, both too weak to affect the dynamics of the plasma. Accounting for simultaneous viscous damping reduces these estimates to 2×10(-21) G in cosmic voids and 2×10(-12) G in protogalaxies. The shear and/or compression of the intergalactic and protogalactic medium exerted by the first supernova explosions locally amplify these seed fields and make them anisotropic, until the magnetic restoring forces affect the gas dynamics at ordered plasma betas near unity.
Explosive turbulent magnetic reconnection.
Higashimori, K; Yokoi, N; Hoshino, M
2013-06-21
We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This fast turbulent reconnection is achieved by the localization of turbulent diffusion. Additionally, localized structure forms through the interaction of the mean field and turbulence.
Explosive Turbulent Magnetic Reconnection
Higashimori, Katsuaki; Yokoi, Nobumitsu; Hoshino, Masahiro
2013-01-01
We report simulation results for turbulent magnetic reconnection obtained using a newly developed Reynolds-averaged magnetohydrodynamics model. We find that the initial Harris current sheet develops in three ways, depending on the strength of turbulence: laminar reconnection, turbulent reconnection, and turbulent diffusion. The turbulent reconnection explosively converts the magnetic field energy into both kinetic and thermal energy of plasmas, and generates open fast reconnection jets. This ...
Turbulent General Magnetic Reconnection
Eyink, Gregory L
2014-01-01
Plasma flows with an MHD-like turbulent inertial range, such as the solar wind, require a generalization of General Magnetic Reconnection (GMR) theory. We introduce the slip-velocity source vector, which gives the rate of development of slip velocity per unit arc length of field line. The slip source vector is the ratio of the curl of the non ideal electric field in the Generalized Ohm's Law and the magnetic field strength. It diverges at magnetic nulls, unifying GMR with magnetic null-point reconnection. Only under restrictive assumptions is the slip velocity related to the gradient of the quasi potential (integral of parallel electric field along field lines). In a turbulent inertial range the curl becomes extremely large while the parallel component is tiny, so that line slippage occurs even while ideal MHD becomes accurate. The resolution of this paradox is that ideal MHD is valid for a turbulent inertial-range only in a weak sense which does not imply magnetic line freezing. The notion of weak solution i...
Turbulent diffusion and galactic magnetism
Brandenburg, Axel
2009-01-01
Using the test-field method for nearly irrotational turbulence driven by spherical expansion waves it is shown that the turbulent magnetic diffusivity increases with magnetic Reynolds numbers. Its value levels off at several times the rms velocity of the turbulence multiplied by the typical radius of the expansion waves. This result is discussed in the context of the galactic mean-field dynamo.
Static magnetic fields enhance turbulence
Pothérat, Alban
2015-01-01
More often than not, turbulence occurs under the influence of external fields, mostly rotation and magnetic fields generated either by planets, stellar objects or by an industrial environment. Their effect on the anisotropy and the dissipative behaviour of turbulence is recognised but complex, and it is still difficult to even tell whether they enhance or dampen turbulence. For example, externally imposed magnetic fields suppress free turbulence in electrically conducting fluids (Moffatt 1967), and make it two-dimensional (2D) (Sommeria & Moreau 1982); but their effect on the intensity of forced turbulence, as in pipes, convective flows or otherwise, is not clear. We shall prove that since two-dimensionalisation preferentially affects larger scales, these undergo much less dissipation and sustain intense turbulent fluctuations. When higher magnetic fields are imposed, quasi-2D structures retain more kinetic energy, so that rather than suppressing forced turbulence, external magnetic fields indirectly enha...
Wave turbulence in magnetized plasmas
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S. Galtier
2009-02-01
Full Text Available The paper reviews the recent progress on wave turbulence for magnetized plasmas (MHD, Hall MHD and electron MHD in the incompressible and compressible cases. The emphasis is made on homogeneous and anisotropic turbulence which usually provides the best theoretical framework to investigate space and laboratory plasmas. The solar wind and the coronal heating problems are presented as two examples of application of anisotropic wave turbulence. The most important results of wave turbulence are reported and discussed in the context of natural and simulated magnetized plasmas. Important issues and possible spurious interpretations are also discussed.
Turbulent Dynamos and Magnetic Helicity
Energy Technology Data Exchange (ETDEWEB)
Ji, Hantao
1999-04-01
It is shown that the turbulent dynamo alpha-effect converts magnetic helicity from the turbulent field to the mean field when the turbulence is electromagnetic while the magnetic helicity of the mean-field is transported across space when the turbulence is elcetrostatic or due to the elcetron diamagnetic effect. In all cases, however, the dynamo effect strictly conserves the total helicity expect for a battery effect which vanishes in the limit of magnetohydrodynamics. Implications for astrophysical situations, especially for the solar dynamo, are discussed.
Turbulent transport in magnetized plasmas
Horton, Wendell
2012-01-01
This book explains how magnetized plasmas self-organize in states of electromagnetic turbulence that transports particles and energy out of the core plasma faster than anticipated by the fusion scientists designing magnetic confinement systems in the 20th century. It describes theory, experiments and simulations in a unified and up-to-date presentation of the issues of achieving nuclear fusion power.
Magnetized Turbulent Dynamo in Protogalaxies
Malyshkin, L M; Malyshkin, Leonid; Kulsrud, Russell
2002-01-01
The prevailing theory for the origin of cosmic magnetic fields is that they have been amplified to their present values by the turbulent dynamo inductive action in the protogalactic and galactic medium. Up to now, in calculation of the turbulent dynamo, it has been customary to assume that there is no back reaction of the magnetic field on the turbulence, as long as the magnetic energy is less than the turbulent kinetic energy. This assumption leads to the kinematic dynamo theory. However, the applicability of this theory to protogalaxies is rather limited. The reason is that in protogalaxies the temperature is very high, and the viscosity is dominated by magnetized ions. As the magnetic field strength grows in time, the ion cyclotron time becomes shorter than the ion collision time, and the plasma becomes strongly magnetized. As a result, the ion viscosity becomes the Braginskii viscosity. Thus, in protogalaxies the back reaction sets in much earlier, at field strengths much lower than those which correspond...
MHD Turbulence and Magnetic Dynamos
Shebalin, John V
2014-01-01
Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much
Inverse scattering problem in turbulent magnetic fluctuations
Treumann, R A; Narita, Y
2016-01-01
We apply a particular form of the inverse scattering theory to turbulent magnetic fluctuations in a plasma. In the present note we develop the theory, formulate the magnetic fluctuation problem in terms of its electrodynamic turbulent response function, and reduce it to the solution of a special form of the famous Gel$'$fand-Levitan-Marchenko equation of quantum mechanical scattering theory.
Cosmic Ray transport in turbulent magnetic field
Yan, Huirong
2013-01-01
Cosmic ray (CR) transport and acceleration is determined by the properties of magnetic turbulence. Recent advances in MHD turbulence call for revisions in the paradigm of cosmic ray transport. We use the models of magnetohydrodynamic turbulence that were tested in numerical simulation, in which turbulence is injected at large scale and cascades to to small scales. We shall address the issue of the transport of CRs, both parallel and perpendicular to the magnetic field. We shall demonstrate compressible fast modes are dominant cosmic ray scatterer from both quasilinear and nonlinear theories. We shall also show that the self-generated wave growth by CRs are constrained by preexisting turbulence and discuss the process in detail in the context of shock acceleration at supernova remnants and their implications. In addition, we shall dwell on the nonlinear growth of kinetic gyroresonance instability of cosmic rays induced by large scale compressible turbulence. This gyroresonance of cosmic rays on turbulence is d...
CHARACTERIZING MAGNETIZED TURBULENCE IN M51
Energy Technology Data Exchange (ETDEWEB)
Houde, Martin [Department of Physics and Astronomy, The University of Western Ontario, London, ON N6A 3K7 (Canada); Fletcher, Andrew [School of Mathematics and Statistics, Newcastle University, Newcastle-upon-Tyne NE1 7RU (United Kingdom); Beck, Rainer [Max-Planck-Institut fuer Radioastronomie, Auf dem Huegel 69, D-53121 Bonn (Germany); Hildebrand, Roger H. [Department of Astronomy and Astrophysics and Enrico Fermi Institute, The University of Chicago, Chicago, IL 60637 (United States); Vaillancourt, John E. [Stratospheric Observatory for Infrared Astronomy, Universities Space Research Association, NASA Ames Research Center, Moffet Field, CA 94035 (United States); Stil, Jeroen M. [Department of Physics and Astronomy, The University of Calgary, Calgary, AB T2N 1N4 (Canada)
2013-03-20
We use previously published high-resolution synchrotron polarization data to perform an angular dispersion analysis with the aim of characterizing magnetized turbulence in M51. We first analyze three distinct regions (the center of the galaxy, and the northwest and southwest spiral arms) and can clearly discern the turbulent correlation length scale from the width of the magnetized turbulent correlation function for two regions and detect the imprint of anisotropy in the turbulence for all three. Furthermore, analyzing the galaxy as a whole allows us to determine a two-dimensional Gaussian model for the magnetized turbulence in M51. We measure the turbulent correlation scales parallel and perpendicular to the local mean magnetic field to be, respectively, {delta}{sub ||} = 98 {+-} 5 pc and {delta} = 54 {+-} 3 pc, while the turbulent-to-ordered magnetic field strength ratio is found to be B{sub t}/B{sub 0} = 1.01 {+-} 0.04. These results are consistent with those of Fletcher et al., who performed a Faraday rotation dispersion analysis of the same data, and our detection of anisotropy is consistent with current magnetized turbulence theories.
Scaling laws in magnetized plasma turbulence
Energy Technology Data Exchange (ETDEWEB)
Boldyrev, Stanislav [Univ. of Wisconsin, Madison, WI (United States)
2015-06-28
Interactions of plasma motion with magnetic fields occur in nature and in the laboratory in an impressively broad range of scales, from megaparsecs in astrophysical systems to centimeters in fusion devices. The fact that such an enormous array of phenomena can be effectively studied lies in the existence of fundamental scaling laws in plasma turbulence, which allow one to scale the results of analytic and numerical modeling to the sized of galaxies, velocities of supernovae explosions, or magnetic fields in fusion devices. Magnetohydrodynamics (MHD) provides the simplest framework for describing magnetic plasma turbulence. Recently, a number of new features of MHD turbulence have been discovered and an impressive array of thought-provoking phenomenological theories have been put forward. However, these theories have conflicting predictions, and the currently available numerical simulations are not able to resolve the contradictions. MHD turbulence exhibits a variety of regimes unusual in regular hydrodynamic turbulence. Depending on the strength of the guide magnetic field it can be dominated by weakly interacting Alfv\\'en waves or strongly interacting wave packets. At small scales such turbulence is locally anisotropic and imbalanced (cross-helical). In a stark contrast with hydrodynamic turbulence, which tends to ``forget'' global constrains and become uniform and isotropic at small scales, MHD turbulence becomes progressively more anisotropic and unbalanced at small scales. Magnetic field plays a fundamental role in turbulent dynamics. Even when such a field is not imposed by external sources, it is self-consistently generated by the magnetic dynamo action. This project aims at a comprehensive study of universal regimes of magnetic plasma turbulence, combining the modern analytic approaches with the state of the art numerical simulations. The proposed study focuses on the three topics: weak MHD turbulence, which is relevant for laboratory devices
Characterizing Magnetized Turbulence in M51
Houde, Martin; Beck, Rainer; Hildebrand, Roger H; Vaillancourt, John E; Stil, Jeroen M
2013-01-01
We use previously published high-resolution synchrotron polarization data to perform an angular dispersion analysis with the aim of charactering magnetized turbulence in M51. We first analyze three distinct regions (the center of the galaxy, and the northwest and southwest spiral arms) and can clearly discern the turbulent correlation length scale from the width of the magnetized turbulent correlation function for two regions and detect the imprint of anisotropy in the turbulence for all three. Furthermore, analyzing the galaxy as a whole allows us to determine a two-dimensional Gaussian model for the magnetized turbulence in M51. We measure the turbulent correlation scales parallel and perpendicular to the local mean magnetic field to be, respectively, delta_{para} = 98 +/- 5 pc and delta_{perp} = 54 +/- 3 pc, while the turbulent to ordered magnetic field strength ratio is found to be Bt/B0 = 1.01 +/- 0.04. These results are consistent with those of Fletcher et al. (2011), who performed a Faraday rotation di...
Magnetic curvature effects on plasma interchange turbulence
Li, B.; Liao, X.; Sun, C. K.; Ou, W.; Liu, D.; Gui, G.; Wang, X. G.
2016-06-01
The magnetic curvature effects on plasma interchange turbulence and transport in the Z-pinch and dipole-like systems are explored with two-fluid global simulations. By comparing the transport levels in the systems with a different magnetic curvature, we show that the interchange-mode driven transport strongly depends on the magnetic geometry. For the system with large magnetic curvature, the pressure and density profiles are strongly peaked in a marginally stable state and the nonlinear evolution of interchange modes produces the global convective cells in the azimuthal direction, which lead to the low level of turbulent convective transport.
MAGNETIC TURBULENCE IN CLUSTERS OF GALAXIES
Directory of Open Access Journals (Sweden)
T. A. EnBlin
2009-01-01
Full Text Available Galaxy clusters are large laboratories for magnetic plasma turbulence which permit us to confront our theoretical concepts of magnetogenesis with detailed observations. Magnetic turbulence in clusters can be studied via the radio-synchrotron emission from the intra-cluster medium in the form of cluster radio relics and halos. The power spectrum of turbulent magnetic elds can be examined via Faraday rotation analysis of extended radio sources. In case of the Hydra A cool core, the observed magnetic spectrum can be understood in terms of a turbulence-mediated feedback loop between gas cooling and the jet activity of the central galaxy. Finally, methods to measure higher-order statistics of the magnetic eld using Stokes-parameter correlations are discussed, which permit us to determine the power spectrum of the magnetic tension force. This fourth-order statistical quantity o ers a way to discriminate between di erent magnetic turbulence scenarios and di erent eld structures using radio polarimetric observations.
Turbulent reconnection of magnetic bipoles in stratified turbulence
Jabbari, Sarah; Mitra, Dhrubaditya; Kleeorin, Nathan; Rogachevskii, Igor
2016-01-01
We consider strongly stratified forced turbulence in a plane-parallel layer with helicity and corresponding large-scale dynamo action in the lower part and nonhelical turbulence in the upper. The magnetic field is found to develop strongly concentrated bipolar structures near the surface. They form elongated bands with a sharp interface between opposite polarities. Unlike earlier experiments with imposed magnetic field, the inclusion of rotation does not strongly suppress the formation of these structures. We perform a systematic numerical study of this phenomenon by varying magnetic Reynolds number, scale separation ratio, and Coriolis number. We also focus on the formation of the current sheet between bipolar regions where reconnection of oppositely oriented field lines occurs. We determine the reconnection rate by measuring either the inflow velocity in the vicinity of the current sheet or by measuring the electric field in the reconnection region. We demonstrate that for small Lundquist number, S1000, the...
Turbulent dynamo with advective magnetic helicity flux
Del Sordo, Fabio; Brandenburg, Axel
2012-01-01
Many astrophysical bodies harbor magnetic fields that are thought to be sustained by dynamo processes. However, it has been argued that the production of large-scale magnetic fields by a mean-field dynamo is strongly suppressed at large magnetic Reynolds numbers owing to the conservation of magnetic helicity. This phenomenon is known as catastrophic quenching. Advection of magnetic field toward the outer boundaries and away from the dynamo is expected to alleviate such quenching. Examples are stellar and galactic winds. Such advection might be able to overcome the constraint imposed by the conservation of magnetic helicity, transporting a fraction of it outside the domain in which the dynamo operates. We study how the dynamo process is affected by advection. In particular, we study the relative roles played by advective and diffusive fluxes of magnetic helicity. We do this by performing direct numerical simulations of a turbulent dynamo of alpha^2 type driven by forced turbulence in a Cartesian domain in the ...
Polarimetric studies of magnetic turbulence with interferometer
Lee, Hyeseung; Cho, Jungyeon
2016-01-01
We study statistical properties of synchrotron polarization emitted from media with magnetohydrodynamic (MHD) turbulence. We use both synthetic and MHD turbulence simulation data for our studies. We obtain the spatial spectrum and its derivative with respect to wavelength of synchrotron polarization arising from both synchrotron radiation and Faraday rotation fluctuations. In particular, we investigate how the spectrum changes with frequency. We find that our simulations agree with the theoretical predication in Lazarian \\& Pogosyan (2016). We conclude that the spectrum of synchrotron polarization and it derivative can be very informative tools to get detailed information about the statistical properties of MHD turbulence from radio observations of diffuse synchrotron polarization. Especially, they are useful to recover the statistics of turbulent magnetic field as well as turbulent density of electrons. We also simulate interferometric observations that incorporate the effects of noise and finite telesco...
Inverse scattering problem in turbulent magnetic fluctuations
Treumann, Rudolf A.; Baumjohann, Wolfgang; Narita, Yasuhito
2016-08-01
We apply a particular form of the inverse scattering theory to turbulent magnetic fluctuations in a plasma. In the present note we develop the theory, formulate the magnetic fluctuation problem in terms of its electrodynamic turbulent response function, and reduce it to the solution of a special form of the famous Gelfand-Levitan-Marchenko equation of quantum mechanical scattering theory. The last of these applies to transmission and reflection in an active medium. The theory of turbulent magnetic fluctuations does not refer to such quantities. It requires a somewhat different formulation. We reduce the theory to the measurement of the low-frequency electromagnetic fluctuation spectrum, which is not the turbulent spectral energy density. The inverse theory in this form enables obtaining information about the turbulent response function of the medium. The dynamic causes of the electromagnetic fluctuations are implicit to it. Thus, it is of vital interest in low-frequency magnetic turbulence. The theory is developed until presentation of the equations in applicable form to observations of turbulent electromagnetic fluctuations as input from measurements. Solution of the final integral equation should be done by standard numerical methods based on iteration. We point to the possibility of treating power law fluctuation spectra as an example. Formulation of the problem to include observations of spectral power densities in turbulence is not attempted. This leads to severe mathematical problems and requires a reformulation of inverse scattering theory. One particular aspect of the present inverse theory of turbulent fluctuations is that its structure naturally leads to spatial information which is obtained from the temporal information that is inherent to the observation of time series. The Taylor assumption is not needed here. This is a consequence of Maxwell's equations, which couple space and time evolution. The inversion procedure takes advantage of a particular
Leprovost, Nicolas; Kim, Eun-Jin
2009-08-01
We investigate three-dimensional magnetohydrodynamics turbulence in the presence of velocity and magnetic shear (i.e., with both a large-scale shear flow and a nonuniform magnetic field). By assuming a turbulence driven by an external forcing with both helical and nonhelical spectra, we investigate the combined effect of these two shears on turbulence intensity and turbulent transport represented by turbulent diffusivities (turbulent viscosity, alpha and beta effect) in Reynolds-averaged equations. We show that turbulent transport (turbulent viscosity and diffusivity) is quenched by a strong flow shear and a strong magnetic field. For a weak flow shear, we further show that the magnetic shear increases the turbulence intensity while decreasing the turbulent transport. In the presence of a strong flow shear, the effect of the magnetic shear is found to oppose the effect of flow shear (which reduces turbulence due to shear stabilization) by enhancing turbulence and transport, thereby weakening the strong quenching by flow shear stabilization. In the case of a strong magnetic field (compared to flow shear), magnetic shear increases turbulence intensity and quenches turbulent transport.
Kinetic intermittency in magnetized plasma turbulence
Teaca, Bogdan; Told, Daniel; Jenko, Frank
2016-01-01
We employ magnetized plasma turbulence, described by a gyrokinetic formalism in an interval ranging from the end of the fluid scales to the electron gyroradius, to introduce the first study of kinetic intermittency, in which nonlinear structures formed directly in the distribution functions are analyzed by accounting for velocity space correlations generated by linear (Landau resonance) and nonlinear phase mixing. Electron structures are found to be strongly intermittent and dominated by linear phase mixing, while nonlinear phase mixing dominates the weakly intermittent ions. This is the first time spatial intermittency and linear phase mixing are shown to be self-consistently linked for the electrons and, as the magnetic field follows the intermittency of the electrons at small scales, explain why magnetic islands are places dominated by Landau damping in steady state turbulence.
Magnetic reconnection as an element of turbulence
Directory of Open Access Journals (Sweden)
S. Servidio
2011-10-01
Full Text Available In this work, recent advances on the study of reconnection in turbulence are reviewed. Using direct numerical simulations of decaying incompressible two-dimensional magnetohydrodynamics (MHD, it was found that in fully developed turbulence complex processes of reconnection locally occur (Servidio et al., 2009, 2010a. In this complex scenario, reconnection is spontaneous but locally driven by the fields, with the boundary conditions provided by the turbulence. Matching classical turbulence analysis with a generalized Sweet-Parker theory, the statistical features of these multiple-reconnection events have been identified. A discussion on the accuracy of our algorithms is provided, highlighting the necessity of adequate spatial resolution. Applications to the study of solar wind discontinuities are reviewed, comparing simulations to spacecraft observations. New results are shown, studying the time evolution of these local reconnection events. A preliminary study on the comparison between MHD and Hall MHD is reported. Our new approach to the study of reconnection as an element of turbulence has broad applications to space plasmas, shedding a new light on the study of magnetic reconnection in nature.
Jurčišinová, E; Jurčišin, M; Remecký, R
2011-10-01
The turbulent magnetic Prandtl number in the framework of the kinematic magnetohydrodynamic (MHD) turbulence, where the magnetic field behaves as a passive vector field advected by the stochastic Navier-Stokes equation, is calculated by the field theoretic renormalization group technique in the two-loop approximation. It is shown that the two-loop corrections to the turbulent magnetic Prandtl number in the kinematic MHD turbulence are less than 2% of its leading order value (the one-loop value) and, at the same time, the two-loop turbulent magnetic Prandtl number is the same as the two-loop turbulent Prandtl number obtained in the corresponding model of a passively advected scalar field. The dependence of the turbulent magnetic Prandtl number on the spatial dimension d is investigated and the source of the smallness of the two-loop corrections for spatial dimension d=3 is identified and analyzed.
Transport of magnetic turbulence in supernova remnants
Brose, R.; Telezhinsky, I.; Pohl, M.
2016-08-01
Context. Supernova remnants are known as sources of Galactic cosmic rays for their nonthermal emission of radio waves, X-rays, and gamma rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims: We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods: We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvénic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a comoving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acceleration site. The transport equation for cosmic rays is solved in a test-particle approach. Results: We demonstrate that the system is typically not in a steady state. In fact, even after several thousand years of evolution, no equilibrium situation is reached. The resulting time-dependent particle spectra strongly differ from those derived assuming a steady state and Bohm diffusion. Our results indicate that proper accounting for the evolution of the scattering turbulence and hence the particle diffusion coefficient is crucial for the formation of the observed soft spectra. In any case, the need to continuously develop magnetic turbulence upstream of the shock introduces nonlinearity in addition to that imposed by cosmic-ray feedback.
Magnetic field amplification in turbulent astrophysical plasmas
Federrath, Christoph
2016-01-01
Magnetic fields play an important role in astrophysical accretion discs, and in the interstellar and intergalactic medium. They drive jets, suppress fragmentation in star-forming clouds and can have a significant impact on the accretion rate of stars. However, the exact amplification mechanisms of cosmic magnetic fields remain relatively poorly understood. Here I start by reviewing recent advances in the numerical and theoretical modelling of the 'turbulent dynamo', which may explain the origin of galactic and inter-galactic magnetic fields. While dynamo action was previously investigated in great detail for incompressible plasmas, I here place particular emphasis on highly compressible astrophysical plasmas, which are characterised by strong density fluctuations and shocks, such as the interstellar medium. I find that dynamo action works not only in subsonic plasmas, but also in highly supersonic, compressible plasmas, as well as for low and high magnetic Prandtl numbers. I further present new numerical simu...
Solar wind magnetic turbulence: Inferences from spectral shape
Treumann, R A; Narita, Y
2016-01-01
Some differences between theoretical, numerical and observational determinations of spectral slopes of solar wind turbulence are interpreted in the thermodynamical sense. Confirmations of turbulent Kolmogorov slopes in solar wind magnetic turbulence and magnetohydrodynamic simulations exhibit tiny differences. These are used to infer about entropy generation in the turbulent cascade and to infer about the anomalous turbulent collision frequency in the dissipative range as well as the average energy input in solar wind turbulence. Anomalous turbulent collision frequencies are obtained of the order of v < 200 Hz. The corresponding stationary solar wind magnetic energy input into magnetic turbulence in the Kolmogorov inertial range is obtained to be of the order of 50 eV/s. Its thermal fate is discussed.
Magnetic flux concentrations from turbulent stratified convection
Käpylä, P J; Kleeorin, N; Käpylä, M J; Rogachevskii, I
2015-01-01
(abridged) Context: The mechanisms that cause the formation of sunspots are still unclear. Aims: We study the self-organisation of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains that are $8.5$-$24$ Mm deep and $34$-$96$ Mm wide. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface with domain depths of $12.5$ and $24$ Mm. The size of the concentrations increases as the box size increases and the largest structures ($20$ Mm horizontally) are obtained in the 24 Mm deep models. The field strength in the concentrations is in the range of $3$-$5$ kG. The concentrations grow approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its ...
Jurčišinová, E; Jurčišin, M; Remecký, R; Zalom, P
2013-04-01
Using the field theoretic renormalization group technique, the influence of helicity (spatial parity violation) on the turbulent magnetic Prandtl number in the kinematic magnetohydrodynamic turbulence is investigated in the two-loop approximation. It is shown that the presence of helicity decreases the value of the turbulent magnetic Prandtl number and, at the same time, the two-loop helical contribution to the turbulent magnetic Prandtl number is at most 4.2% (in the case with the maximal helicity) of its nonhelical value. These results demonstrate, on one hand, the potential importance of the presence of asymmetries in processes in turbulent environments and, on the other hand, the rather strong stability of the properties of diffusion processes of the magnetic field in the conductive turbulent environment with the spatial parity violation in comparison to the corresponding systems without the spatial parity violation. In addition, obtained results are compared to the corresponding results found for the two-loop turbulent Prandtl number in the model of passively advected scalar field. It is shown that the turbulent Prandtl number and the turbulent magnetic Prandtl number, which are the same in fully symmetric isotropic turbulent systems, are essentially different when one considers the spatial parity violation. It means that the properties of the diffusion processes in the turbulent systems with a given symmetry breaking can considerably depend on the internal tensor structure of advected quantities.
Magnetic moment nonconservation in magnetohydrodynamic turbulence models.
Dalena, S; Greco, A; Rappazzo, A F; Mace, R L; Matthaeus, W H
2012-07-01
The fundamental assumptions of the adiabatic theory do not apply in the presence of sharp field gradients or in the presence of well-developed magnetohydrodynamic turbulence. For this reason, in such conditions the magnetic moment μ is no longer expected to be constant. This can influence particle acceleration and have considerable implications in many astrophysical problems. Starting with the resonant interaction between ions and a single parallel propagating electromagnetic wave, we derive expressions for the magnetic moment trapping width Δμ (defined as the half peak-to-peak difference in the particle magnetic moments) and the bounce frequency ω(b). We perform test-particle simulations to investigate magnetic moment behavior when resonance overlapping occurs and during the interaction of a ring-beam particle distribution with a broadband slab spectrum. We find that the changes of magnetic moment and changes of pitch angle are related when the level of magnetic fluctuations is low, δB/B(0) = (10(-3),10(-2)), where B(0) is the constant and uniform background magnetic field. Stochasticity arises for intermediate fluctuation values and its effect on pitch angle is the isotropization of the distribution function f(α). This is a transient regime during which magnetic moment distribution f(μ) exhibits a characteristic one-sided long tail and starts to be influenced by the onset of spatial parallel diffusion, i.e., the variance grows linearly in time as in normal diffusion. With strong fluctuations f(α) becomes completely isotropic, spatial diffusion sets in, and the f(μ) behavior is closely related to the sampling of the varying magnetic field associated with that spatial diffusion.
Magnetic turbulence in the plasma sheet
Vörös, Z; Nakamura, R; Runov, A; Zhang, T L; Eichelberger, H U; Treumann, R A; Georgescu, E; Balogh, A; Klecker, B; R`eme, H
2004-01-01
Small-scale magnetic turbulence observed by the Cluster spacecraft in the plasma sheet is investigated by means of a wavelet estimator suitable for detecting distinct scaling characteristics even in noisy measurements. The spectral estimators used for this purpose are affected by a frequency dependent bias. The variances of the wavelet coefficients, however, match the power-law shaped spectra, which makes the wavelet estimator essentially unbiased. These scaling characteristics of the magnetic field data appear to be essentially non-steady and intermittent. The scaling properties of bursty bulk flow (BBF) and non-BBF associated magnetic fluctuations are analysed with the aim of understanding processes of energy transfer between scales. Small-scale ($\\sim 0.08-0.3$ s) magnetic fluctuations having the same scaling index $\\alpha \\sim 2.6$ as the large-scale ($\\sim 0.7-5$ s) magnetic fluctuations occur during BBF-associated periods. During non-BBF associated periods the energy transfer to small scales is absent, ...
Anchoring Magnetic Field in Turbulent Molecular Clouds
Li, Hua-bai; Goodman, Alyssa; Hildebrand, Roger; Novak, Giles
2009-01-01
One of the key problems in star formation research is to determine the role of magnetic fields. Starting from the atomic inter-cloud medium (ICM) which has density nH ~ 1 per cubic cm, gas must accumulate from a volume several hundred pc across in order to form a typical molecular cloud. Star formation usually occurs in cloud cores, which have linear sizes below 1 pc and densities nH2 > 10^5 per cubic cm. With current technologies, it is hard to probe magnetic fields at scales lying between the accumulation length and the size of cloud cores, a range corresponds to many levels of turbulent eddy cascade, and many orders of magnitude of density amplification. For field directions detected from the two extremes, however, we show here that a significant correlation is found. Comparing this result with molecular cloud simulations, only the sub-Alfvenic cases result in field orientations consistent with our observations.
Turbulent behaviour in magnetic hydrodynamics is not universal
Dmitriy, W
1996-01-01
A short distance expansion method (SDE) that is well known in the quantum field theory for analysis of turbulent behaviour of stochastic magnetic hydrodynamics of incompressible conductive fluid is applied. As a result is shown that in an inertial range the turbulent spectra of magnetic hydrodynamics depend on a scale of arising of curls.
Mathematics of aperiodic order
Lenz, Daniel; Savinien, Jean
2015-01-01
What is order that is not based on simple repetition, that is, periodicity? How must atoms be arranged in a material so that it diffracts like a quasicrystal? How can we describe aperiodically ordered systems mathematically? Originally triggered by the – later Nobel prize-winning – discovery of quasicrystals, the investigation of aperiodic order has since become a well-established and rapidly evolving field of mathematical research with close ties to a surprising variety of branches of mathematics and physics. This book offers an overview of the state of the art in the field of aperiodic order, presented in carefully selected authoritative surveys. It is intended for non-experts with a general background in mathematics, theoretical physics or computer science, and offers a highly accessible source of first-hand information for all those interested in this rich and exciting field. Topics covered include the mathematical theory of diffraction, the dynamical systems of tilings or Delone sets, their cohomolog...
Heat Transfer and Reconnection Diffusion in Turbulent Magnetized Plasmas
Lazarian, A
2011-01-01
It is well known that magnetic fields constrain motions of charged particles, impeding the diffusion of charged particles perpendicular to magnetic field direction. This modification of transport processes is of vital importance for a wide variety of astrophysical processes including cosmic ray transport, transfer of heavy elements in the interstellar medium, star formation etc. Dealing with these processes one should keep in mind that in realistic astrophysical conditions magnetized fluids are turbulent. In this review we single out a single transport process, namely, heat transfer and consider how it occurs in the presence of the magnetized turbulence. We show that the ability of magnetic field lines to constantly change topology and connectivity is at the heart of the correct description of the 3D magnetic field stochasticity in turbulent fluids. This ability is ensured by fast magnetic reconnection in turbulent fluids and puts forward the concept of reconnection diffusion at the core of the physical pictu...
Cyclotomic Aperiodic Substitution Tilings
Directory of Open Access Journals (Sweden)
Stefan Pautze
2017-01-01
Full Text Available The class of Cyclotomic Aperiodic Substitution Tilings (CASTs is introduced. Its vertices are supported on the 2 n -th cyclotomic field. It covers a wide range of known aperiodic substitution tilings of the plane with finite rotations. Substitution matrices and minimal inflation multipliers of CASTs are discussed as well as practical use cases to identify specimen with individual dihedral symmetry D n or D 2 n , i.e., the tiling contains an infinite number of patches of any size with dihedral symmetry D n or D 2 n only by iteration of substitution rules on a single tile.
Aperiodic Quantum Random Walks
Ribeiro, P; Mosseri, R; Ribeiro, Pedro; Milman, Perola; Mosseri, Remy
2004-01-01
We generalize the quantum random walk protocol for a particle in a one-dimensional chain, by using several types of biased quantum coins, arranged in aperiodic sequences, in a manner that leads to a rich variety of possible wave function evolutions. Quasiperiodic sequences, following the Fibonacci prescription, are of particular interest, leading to a sub-ballistic wavefunction spreading. In contrast, random sequences leads to diffusive spreading, similar to the classical random walk behaviour. We also describe how to experimentally implement these aperiodic sequences.
Assessing the Structure of Isotropic and Anisotropic Turbulent Magnetic Fields
Fatuzzo, Marco; Holden, Lisa; Grayson, Lindsay; Wallace, Kirk
2016-10-01
Turbulent magnetic fields permeate our universe, impacting a wide range of astronomical phenomena across all cosmic scales. A clear example is the magnetic field that threads the interstellar medium (ISM), which impacts the motion of cosmic rays through that medium. Understanding the structure of magnetic turbulence within the ISM and how it relates to the physical quantities that characterize it can thus inform our analysis of particle transport within these regions. Toward that end, we probe the structure of magentic turbulence through the use of Lyapunov exponents for a suite of isotropic and nonisotropic Alfvénic turbulence profiles. Our results provide a means of calculating a “turbulence lengthscale” that can then be connected to how cosmic rays propagate through magentically turbulent environments, and we perform such an analysis for molecular cloud environments.
Toward the Theory of Turbulence in Magnetized Plasmas
Energy Technology Data Exchange (ETDEWEB)
Boldyrev, Stanislav [University of Wisconsin - Madison
2013-07-26
The goal of the project was to develop a theory of turbulence in magnetized plasmas at large scales, that is, scales larger than the characteristic plasma microscales (ion gyroscale, ion inertial scale, etc.). Collisions of counter-propagating Alfven packets govern the turbulent cascade of energy toward small scales. It has been established that such an energy cascade is intrinsically anisotropic, in that it predominantly supplies energy to the modes with mostly field-perpendicular wave numbers. The resulting energy spectrum of MHD turbulence, and the structure of the fluctuations were studied both analytically and numerically. A new parallel numerical code was developed for simulating reduced MHD equations driven by an external force. The numerical setting was proposed, where the spectral properties of the force could be varied in order to simulate either strong or weak turbulent regimes. It has been found both analytically and numerically that weak MHD turbulence spontaneously generates a “condensate”, that is, concentration of magnetic and kinetic energy at small k{sub {parallel}}. A related topic that was addressed in the project is turbulent dynamo action, that is, generation of magnetic field in a turbulent flow. We were specifically concentrated on the generation of large-scale magnetic field compared to the scales of the turbulent velocity field. We investigate magnetic field amplification in a turbulent velocity field with nonzero helicity, in the framework of the kinematic Kazantsev-Kraichnan model.
Turbulent magnetic fluctuations in laboratory reconnection
Von Stechow, Adrian; Grulke, Olaf; Klinger, Thomas
2016-07-01
The role of fluctuations and turbulence is an important question in astrophysics. While direct observations in space are rare and difficult dedicated laboratory experiments provide a versatile environment for the investigation of magnetic reconnection due to their good diagnostic access and wide range of accessible plasma parameters. As such, they also provide an ideal chance for the validation of space plasma reconnection theories and numerical simulation results. In particular, we studied magnetic fluctuations within reconnecting current sheets for various reconnection parameters such as the reconnection rate, guide field, as well as plasma density and temperature. These fluctuations have been previously interpreted as signatures of current sheet plasma instabilities in space and laboratory systems. Especially in low collisionality plasmas these may provide a source of anomalous resistivity and thereby contribute a significant fraction of the reconnection rate. We present fluctuation measurements from two complementary reconnection experiments and compare them to numerical simulation results. VINETA.II (Greifswald, Germany) is a cylindrical, high guide field reconnection experiment with an open field line geometry. The reconnecting current sheet has a three-dimensional structure that is predominantly set by the magnetic pitch angle which results from the superposition of the guide field and the in-plane reconnecting field. Within this current sheet, high frequency magnetic fluctuations are observed that correlate well with the local current density and show a power law spectrum with a spectral break at the lower hybrid frequency. Their correlation lengths are found to be extremely short, but propagation is nonetheless observed with high phase velocities that match the Whistler dispersion. To date, the experiment has been run with an external driving field at frequencies higher than the ion cyclotron frequency f_{ci}, which implies that the EMHD framework applies
Radiative diagnostics for sub-Larmor scale magnetic turbulence
Reynolds, Sarah J
2011-01-01
Radiative diagnostics of high-energy density plasmas is addressed in this paper. We propose that the radiation produced by energetic particles in small-scale magnetic field turbulence, which can occur in laser-plasma experiments, collisionless shocks, and during magnetic reconnection, can be used to deduce some properties of the turbulent magnetic field. Particles propagating through such turbulence encounter locally strong magnetic fields, but over lengths much shorter than a particle gyroradius. Consequently, the particle is accelerated but not deviated substantially from a straight line path. We develop the general jitter radiation solutions for this case and show that the resulting radiation is directly dependent upon the spectral distribution of the magnetic field through which the particle propagates. We demonstrate the power of this approach in considering the radiation produced by particles moving through a region in which a (Weibel-like) filamentation instability grows magnetic fields randomly orient...
Magnetic reconnection in two-dimensional magnetohydrodynamic turbulence.
Servidio, S; Matthaeus, W H; Shay, M A; Cassak, P A; Dmitruk, P
2009-03-20
Systematic analysis of numerical simulations of two-dimensional magnetohydrodynamic turbulence reveals the presence of a large number of X-type neutral points where magnetic reconnection occurs. We examine the statistical properties of this ensemble of reconnection events that are spontaneously generated by turbulence. The associated reconnection rates are distributed over a wide range of values and scales with the geometry of the diffusion region. Locally, these events can be described through a variant of the Sweet-Parker model, in which the parameters are externally controlled by turbulence. This new perspective on reconnection is relevant in space and astrophysical contexts, where plasma is generally in a fully turbulent regime.
Turbulent Amplification and Structure of the Intracluster Magnetic Field
Beresnyak, Andrey
2015-01-01
We compare DNS calculations of homogeneous isotropic turbulence with the statistical properties of intra-cluster turbulence from the Matryoshka Run (Miniati 2014) and find remarkable similarities between their inertial ranges. This allowed us to use the time dependent statistical properties of intra-cluster turbulence to evaluate dynamo action in the intra-cluster medium, based on earlier results from numerically resolved nonlinear magneto-hydrodynamic turbulent dynamo (Beresnyak 2012). We argue that this approach is necessary (a) to properly normalize dynamo action to the available intra-cluster turbulent energy and (b) to overcome the limitations of low Re affecting current numerical models of the intra-cluster medium. We find that while the properties of intra-cluster magnetic field are largely insensitive to the value and origin of the seed field, the resulting values for the Alfven speed and the outer scale of the magnetic field are consistent with current observational estimates, basically confirming th...
Turbulent magnetic field amplification from the smallest to the largest magnetic Prandtl numbers
Bovino, Stefano; Schober, Jennifer
2012-01-01
The small-scale dynamo provides a highly efficient mechanism for the conversion of turbulent into magnetic energy. In astrophysical environments, such turbulence often occurs at high Mach numbers, implying steep slopes in the turbulent spectra. It is thus a central question whether the small-scale dynamo can amplify magnetic fields in the interstellar or intergalactic media, where such Mach numbers occur. To address this long-standing issue, we employ the Kazantsev model for turbulent magnetic field amplification, systematically exploring the effect of different turbulent slopes, as expected for Kolmogorov, Burgers, the Larson laws and results derived from numerical simulations. With the framework employed here, we give the first solution encompassing the complete range of magnetic Prandtl numbers, including Pm > 1. We derive scaling laws of the growth rate as a function of hydrodynamic and magnetic Reynolds number for Pm > 1 for all types of turbulence. A central result concerns the regime of Pm ~ 1, where t...
Plasmoid-induced turbulence in collisionless magnetic reconnection.
Fujimoto, Keizo; Sydora, Richard D
2012-12-28
The dissipation mechanism in collisionless magnetic reconnection in a quasisteady period is investigated for the antiparallel field configuration. A three-dimensional simulation in a fully kinetic system reveals that a current-aligned electromagnetic mode produces turbulent electron flow that facilitates the transport of the momentum responsible for the current density. It is found that the electromagnetic turbulence is drastically enhanced by plasmoid formations and has a significant impact on the dissipation at the magnetic x-line. The linear analyses confirm that the mode survives in the real ion-to-electron mass ratio, which assures the importance of the turbulence in collisionless reconnection.
Observation of a Turbulence-Generated Large Scale Magnetic Field
Spence, E J; Kendrick, R D; Nornberg, M D
2006-01-01
A uniform magnetic field is applied to a spherical, turbulent flow of liquid sodium. An induced magnetic dipole moment is measured which cannot be generated by the interaction of the axisymmetric mean flow with the applied field, indicating the presence of a turbulent electromotive force. It is shown that the induced dipole moment should vanish for any axisymmetric laminar flow. Also observed is the production of toroidal magnetic field from applied poloidal magnetic field (the omega-effect). Its potential role in the production of the induced dipole is discussed.
Magnetic flux concentration and zonal flows in magnetorotational instability turbulence
Energy Technology Data Exchange (ETDEWEB)
Bai, Xue-Ning [Institute for Theory and Computation, Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS-51, Cambridge, MA 02138 (United States); Stone, James M., E-mail: xbai@cfa.harvard.edu [Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, NJ 08544 (United States)
2014-11-20
Accretion disks are likely threaded by external vertical magnetic flux, which enhances the level of turbulence via the magnetorotational instability (MRI). Using shearing-box simulations, we find that such external magnetic flux also strongly enhances the amplitude of banded radial density variations known as zonal flows. Moreover, we report that vertical magnetic flux is strongly concentrated toward low-density regions of the zonal flow. Mean vertical magnetic field can be more than doubled in low-density regions, and reduced to nearly zero in high-density regions in some cases. In ideal MHD, the scale on which magnetic flux concentrates can reach a few disk scale heights. In the non-ideal MHD regime with strong ambipolar diffusion, magnetic flux is concentrated into thin axisymmetric shells at some enhanced level, whose size is typically less than half a scale height. We show that magnetic flux concentration is closely related to the fact that the turbulent diffusivity of the MRI turbulence is anisotropic. In addition to a conventional Ohmic-like turbulent resistivity, we find that there is a correlation between the vertical velocity and horizontal magnetic field fluctuations that produces a mean electric field that acts to anti-diffuse the vertical magnetic flux. The anisotropic turbulent diffusivity has analogies to the Hall effect, and may have important implications for magnetic flux transport in accretion disks. The physical origin of magnetic flux concentration may be related to the development of channel flows followed by magnetic reconnection, which acts to decrease the mass-to-flux ratio in localized regions. The association of enhanced zonal flows with magnetic flux concentration may lead to global pressure bumps in protoplanetary disks that helps trap dust particles and facilitates planet formation.
Magnetic Flux Transport by turbulent reconnection in astrophysical flows
Pino, Elisabete M de Gouveia Dal; Santos-Lima, Reinaldo; Guerrero, Gustavo; Kowal, Grzegorz; Lazarian, Alex
2011-01-01
The role of MHD turbulence in astrophysical environments is still highly debated. An important question that permeates this debate is the transport of magnetic flux. This is particularly important, for instance, in the context of star formation. When clouds collapse gravitationally to form stars, there must be some magnetic flux transport. otherwise the new born stars would have magnetic fields several orders of magnitude larger than the observed ones. Also, the magnetic flux that is dragged in the late stages of the formation of a star can remove all the rotational support from the accretion disk that grows around the protostar. The efficiency of the mechanism which is often invoked to allow the transport of magnetic fields in the different stages of star formation, namely, the ambipolar diffusion, has been lately put in check. We here discuss an alternative mechanism for magnetic flux transport which is based on turbulent fast magnetic reconnection. We review recent results obtained from 3D MHD numerical si...
Solar Polarimetry - from Turbulent Magnetic Fields to Sunspots
Kleint, Lucia
2016-07-01
Polarimetric measurements are essential to investigate the solar magnetic field. Scattering polarization and the Hanle effect allow us to probe the turbulent magnetic field and the still open questions of its strength and variability. Directed magnetic fields can be detected via the Zeeman effect. To derive their orientation and strength, so-called inversion codes are used, which iteratively modify a model atmosphere and calculate the resulting polarization profiles that are then compared to the observations. While photospheric polarimetry is well-established, chromospheric polarimetry is still in its infancy, especially because it requires a treatment in non-LTE, making it a complex non-linear problem. But some of the most important open questions concern the strength and geometry of the chromospheric magnetic field. In this talk, I will review different polarimetric analysis techniques and recent advances in magnetic field measurements going from the small scales of turbulent magnetic fields to changes of magnetic fields in an active region during flares.
Gerke, Tim D.
Presented in this thesis is an investigation into aperiodic volume optical devices. The three main topics of research and discussion are the aperiodic volume optical devices that we call computer-generated volume holograms (CGVH), defects within periodic 3D photonic crystals, and non-periodic, but ordered 3D quasicrystals. The first of these devices, CGVHs, are designed and investigated numerically and experimentally. We study the performance of multi-layered amplitude computer-generated volume holograms in terms of efficiency and angular/frequency selectivity. Simulation results show that such aperiodic devices can increase diffraction efficiency relative to periodic amplitude volume holograms while maintaining angular and wavelength selectivity. CGVHs are also designed as voxelated volumes using a new projection optimization algorithm. They are investigated using a volumetric diffraction simulation and a standard 3D beam propagation technique as well as experimentally. Both simulation and experiment verify that the structures function according to their design. These represent the first diffractive structures that have the capacity for generating arbitrary transmission and reflection wave fronts and that provide the ability for multiplexing arbitrary functionality given different illumination conditions. Also investigated and discussed in this thesis are 3D photonic crystals and quasicrystals. We demonstrate that these devices can be fabricated using a femtosecond laser direct writing system that is particularly appropriate for fabrication of such arbitrary 3D structures. We also show that these devices can provide 3D partial bandgaps which could become complete bandgaps if fabricated using high index materials or by coating lower index materials with high index metals. Our fabrication method is particularly suited to the fabrication of engineered defects within the periodic or quasi-periodic systems. We demonstrate the potential for fabricating defects within
Inverse cascade of magnetic helicity in magnetohydrodynamic turbulence.
Müller, Wolf-Christian; Malapaka, Shiva Kumar; Busse, Angela
2012-01-01
The nonlinear dynamics of magnetic helicity HM, which is responsible for large-scale magnetic structure formation in electrically conducting turbulent media, is investigated in forced and decaying three-dimensional magnetohydrodynamic turbulence. This is done with the help of high-resolution direct numerical simulations and statistical closure theory. The numerically observed spectral scaling of HM is at variance with earlier work using a statistical closure model [Pouquet et al., J. Fluid Mech. 77, 321 (1976)]. By revisiting this theory, a universal dynamical balance relation is found that includes the effects of kinetic helicity as well as kinetic and magnetic energies on the inverse cascade of HM and explains the above-mentioned discrepancy. Consideration of the result in the context of mean-field dynamo theory suggests a nonlinear modification of the α-dynamo effect, which is important in the context of magnetic-field excitation in turbulent plasmas.
The inverse cascade of magnetic helicity in magnetohydrodynamic turbulence
Müller, Wolf-Christian; Busse, Angela
2012-01-01
The nonlinear dynamics of magnetic helicity, $H^M$, which is responsible for large-scale magnetic structure formation in electrically conducting turbulent media is investigated in forced and decaying three-dimensional magnetohydrodynamic turbulence. This is done with the help of high resolution direct numerical simulations and statistical closure theory. The numerically observed spectral scaling of $H^M$ is at variance with earlier work using a statistical closure model [Pouquet et al., J. Fluid Mech. \\textbf{77} 321 (1976)]. By revisiting this theory a universal dynamical balance relation is found that includes effects of kinetic helicity, as well as kinetic and magnetic energy on the inverse cascade of $H^M$ and explains the above-mentioned discrepancy. Considering the result in the context of mean-field dynamo theory suggests a nonlinear modification of the $\\alpha$-dynamo effect important in the context of magnetic field excitation in turbulent plasmas.
Fixed Point and Aperiodic Tilings
Durand, Bruno; Shen, Alexander
2008-01-01
An aperiodic tile set was first constructed by R.Berger while proving the undecidability of the domino problem. It turned out that aperiodic tile sets appear in many topics ranging from logic (the Entscheidungsproblem) to physics (quasicrystals) We present a new construction of an aperiodic tile set that is based on Kleene's fixed-point construction instead of geometric arguments. This construction is similar to J. von Neumann self-reproducing automata; similar ideas were also used by P. Gacs in the context of error-correcting computations. The flexibility of this construction allows us to construct a ``robust'' aperiodic tile set that does not have periodic (or close to periodic) tilings even if we allow some (sparse enough) tiling errors. This property was not known for any of the existing aperiodic tile sets.
Simulations of Energetic Particles Interacting with Dynamical Magnetic Turbulence
Hussein, M.; Shalchi, A.
2016-02-01
We explore the transport of energetic particles in interplanetary space by using test-particle simulations. In previous work such simulations have been performed by using either magnetostatic turbulence or undamped propagating plasma waves. In the current paper we simulate for the first time particle transport in dynamical turbulence. To do so we employ two models, namely the damping model of dynamical turbulence and the random sweeping model. We compute parallel and perpendicular diffusion coefficients and compare our numerical findings with solar wind observations. We show that good agreement can be found between simulations and the Palmer consensus range for both dynamical turbulence models if the ratio of turbulent magnetic field and mean field is δB/B0 = 0.5.
INVERSE CASCADE OF NONHELICAL MAGNETIC TURBULENCE IN A RELATIVISTIC FLUID
Energy Technology Data Exchange (ETDEWEB)
Zrake, Jonathan [Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Menlo Park, CA 94025 (United States)
2014-10-20
The free decay of nonhelical relativistic magnetohydrodynamic turbulence is studied numerically, and found to exhibit cascading of magnetic energy toward large scales. Evolution of the magnetic energy spectrum P{sub M} (k, t) is self-similar in time and well modeled by a broken power law with subinertial and inertial range indices very close to 7/2 and –2, respectively. The magnetic coherence scale is found to grow in time as t {sup 2/5}, much too slow to account for optical polarization of gamma-ray burst afterglow emission if magnetic energy is to be supplied only at microphysical length scales. No bursty or explosive energy loss is observed in relativistic MHD turbulence having modest magnetization, which constrains magnetic reconnection models for rapid time variability of GRB prompt emission, blazars, and the Crab nebula.
From Weakly to Strongly Magnetized Isotropic MHD Turbulence
Alexakis, Alexandros
2012-01-01
High Reynolds number isotropic magneto-hydro-dynamic turbulence in the presence of large scale magnetic fields is investigated as a function of the magnetic field strength. For a variety of flow configurations the energy dissipation rate \\epsilon, follows the Kolmogorov scaling \\epsilon ~ U^3/L even when the large scale magnetic field energy is twenty times larger than the kinetic. Further increase of the magnetic energy showed a transition to the \\epsilon ~ U^2 B / L scaling implying that magnetic shear becomes more efficient at this point at cascading the energy than the velocity fluctuations. Strongly helical configurations form helicity condensates that deviate from these scalings. Weak turbulence scaling was absent from the investigation. Finally, the magnetic energy spectra showed support for the Kolmogorov spectrum k^{-5/3} while kinetic energy spectra are closer to the Iroshnikov-Kraichnan spectrum k^{-3/2}.
On Runaway Transport under Magnetic Turbulence in Tokamaks
Energy Technology Data Exchange (ETDEWEB)
Castejon, F.; Equilior, S.; Rodriguez-Rodrigo, L. [CIEMAT. Madrid (Spain)
2001-07-01
The influence of magnetic turbulence on runaway transport has been studied. The evolution of runaway distribution function has been calculated using Electra a 2D code in momentum space and 1D in radius coordinate. The code considers the effect of averaging the turbulence by runaway orbits. Then Hard X-Ray emission spectrum is estimated and compared with experimental results of TJ-1 tokamak, obtaining a remarkable agreement. (Author) 15 refs.
Turbulence-induced magnetic flux asymmetry at nanoscale junctions.
Bushong, Neil; Pershin, Yuriy; Di Ventra, Massimiliano
2007-11-30
It was recently predicted [J. Phys. Condens. Matter 18, 11059 (2006)] that turbulence of electron flow may develop at nonadiabatic nanoscale junctions under appropriate conditions. Here we show that such an effect leads to an asymmetric current-induced magnetic field on the two sides of an otherwise symmetric junction. We propose that measuring the fluxes ensuing from these fields across two surfaces placed at the two sides of the junction would provide direct and noninvasive evidence of the transition from laminar to turbulent electron flow. The flux asymmetry is predicted to first increase, reach a maximum, and then decrease with increasing current, i.e., with increasing amount of turbulence.
Magnetic discontinuities in magnetohydrodynamic turbulence and in the solar wind.
Zhdankin, Vladimir; Boldyrev, Stanislav; Mason, Joanne; Perez, Jean Carlos
2012-04-27
Recent measurements of solar wind turbulence report the presence of intermittent, exponentially distributed angular discontinuities in the magnetic field. In this Letter, we study whether such discontinuities can be produced by magnetohydrodynamic (MHD) turbulence. We detect the discontinuities by measuring the fluctuations of the magnetic field direction, Δθ, across fixed spatial increments Δx in direct numerical simulations of MHD turbulence with an imposed uniform guide field B(0). A large region of the probability density function (pdf) for Δθ is found to follow an exponential decay, proportional to exp(-Δθ/θ(*)), with characteristic angle θ(*)≈(14°)(b(rms)/B(0))(0.65) for a broad range of guide-field strengths. We find that discontinuities observed in the solar wind can be reproduced by MHD turbulence with reasonable ratios of b(rms)/B(0). We also observe an excess of small angular discontinuities when Δx becomes small, possibly indicating an increasing statistical significance of dissipation-scale structures. The structure of the pdf in this case closely resembles the two-population pdf seen in the solar wind. We thus propose that strong discontinuities are associated with inertial-range MHD turbulence, while weak discontinuities emerge from dissipation-range turbulence. In addition, we find that the structure functions of the magnetic field direction exhibit anomalous scaling exponents, which indicates the existence of intermittent structures.
TURBULENT AMPLIFICATION AND STRUCTURE OF THE INTRACLUSTER MAGNETIC FIELD
Energy Technology Data Exchange (ETDEWEB)
Beresnyak, Andrey [Nordita, KTH Royal Institute of Technology and Stockholm University, SE-10691 Stockholm (Sweden); Miniati, Francesco [Physics Dept., ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 (Switzerland)
2016-02-01
We compare DNS calculations of homogeneous isotropic turbulence with the statistical properties of intracluster turbulence from the Matryoshka Run and find remarkable similarities between their inertial ranges. This allowed us to use the time-dependent statistical properties of intracluster turbulence to evaluate dynamo action in the intracluster medium, based on earlier results from a numerically resolved nonlinear magneto-hydrodynamic turbulent dynamo. We argue that this approach is necessary (a) to properly normalize dynamo action to the available intracluster turbulent energy and (b) to overcome the limitations of low Re affecting current numerical models of the intracluster medium. We find that while the properties of intracluster magnetic field are largely insensitive to the value and origin of the seed field, the resulting values for the Alfvén speed and the outer scale of the magnetic field are consistent with current observational estimates, basically confirming the idea that the magnetic field in today’s galaxy clusters is a record of its past turbulent activity.
Hall MHD Stability and Turbulence in Magnetically Accelerated Plasmas
Energy Technology Data Exchange (ETDEWEB)
H. R. Strauss
2012-11-27
The object of the research was to develop theory and carry out simulations of the Z pinch and plasma opening switch (POS), and compare with experimental results. In the case of the Z pinch, there was experimental evidence of ion kinetic energy greatly in excess of the ion thermal energy. It was thought that this was perhaps due to fine scale turbulence. The simulations showed that the ion energy was predominantly laminar, not turbulent. Preliminary studies of a new Z pinch experiment with an axial magnetic field were carried out. The axial magnetic is relevant to magneto - inertial fusion. These studies indicate the axial magnetic field makes the Z pinch more turbulent. Results were also obtained on Hall magnetohydrodynamic instability of the POS.
Cosmic-ray diffusion in magnetized turbulence
Tautz, R C
2015-01-01
The problem of cosmic-ray scattering in the turbulent electromagnetic fields of the interstellar medium and the solar wind is of great importance due to the variety of applications of the resulting diffusion coefficients. Examples are diffusive shock acceleration, cosmic-ray observations, and, in the solar system, the propagation of coronal mass ejections. In recent years, it was found that the simple diffusive motion that had been assumed for decades is often in disagreement both with numerical and observational results. Here, an overview is given of the interaction processes of cosmic rays and turbulent electromagnetic fields. First, the formation of turbulent fields due to plasma instabilities is treated, where especially the non-linear behavior of the resulting unstable wave modes is discussed. Second, the analytical and the numerical side of high-energy particle propagation will be reviewed by presenting non-linear analytical theories and Monte-Carlo simulations. For the example of the solar wind, the im...
Polarization of radiation of electrons in highly turbulent magnetic fields
Prosekin, A. Yu.; Kelner, S. R.; Aharonian, F. A.
2016-09-01
We study the polarization properties of the jitter and synchrotron radiation produced by electrons in highly turbulent anisotropic magnetic fields. The net polarization is provided by the geometry of the magnetic field the directions of which are parallel to a certain plane. Such conditions may appear in the relativistic shocks during the amplification of the magnetic field through the so-called Weibel instability. While the polarization properties of the jitter radiation allows extraction of direct information on the turbulence spectrum as well as the geometry of magnetic field, the polarization of the synchrotron radiation reflects the distribution of the magnetic field over its strength. For the isotropic distribution of monoenergetic electrons, we found that the degree of polarization of the synchrotron radiation is larger than the polarization of the jitter radiation. For the power-law energy distribution of electrons the relation between the degree of polarization of synchrotron and jitter radiation depends on the spectral index of the distribution.
Polarization of radiation of electrons in highly turbulent magnetic fields
Prosekin, A Yu; Aharonian, F A
2016-01-01
We study the polarization properties of the jitter and synchrotron radiation produced by electrons in highly turbulent anisotropic magnetic fields. The net polarization is provided by the geometry of the magnetic field the directions of which are parallel to a certain plane. Such conditions may appear in the relativistic shocks during the amplification of the magnetic field through the so-called Weibel instability. While the polarization properties of the jitter radiation allows extraction of direct information on the turbulence spectrum as well as the geometry of magnetic field, the polarization of the synchrotron radiation reflects the distribution of the magnetic field over its strength. For the isotropic distribution of monoenergetic electrons, we found that the degree of polarization of the synchrotron radiation is larger than the polarization of the jitter radiation. For the power-law energy distribution of electrons the relation between the degree of polarization of synchrotron and jitter radiation dep...
Magnetic reversals in a modified shell model for magnetohydrodynamics turbulence.
Nigro, Giuseppina; Carbone, Vincenzo
2010-07-01
The aim of the paper is the study of dynamo action using a simple nonlinear model in the framework of magnetohydrodynamic turbulence. The nonlinear behavior of the system is described by using a shell model for velocity field and magnetic field fluctuations, modified for the magnetic field at the largest scale by a term describing a supercritical pitchfork bifurcation. Turbulent fluctuations generate a dynamical situation where the large-scale magnetic field jumps between two states which represent the opposite polarities of the magnetic field. Despite its simplicity, the model has the capability to describe a long time series of reversals from which we infer results about the statistics of persistence times and scaling laws of cancellations between opposite polarities for different magnetic diffusivity coefficients. These properties of the model are compared with real paleomagnetic data, thus revealing the origin of long-range correlations in the process.
Turbulence-induced magnetic fields in shock precursors
del Valle, Maria Victoria; Santos-Lima, Reinaldo
2016-01-01
Galactic cosmic rays are believed to be mostly accelerated at supernova shocks. However, the interstellar magnetic field is too weak to efficiently accelerate galactic cosmic rays up to the highest energies, i.e. $10^{15}$ eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic-ray nonresonant streaming instability has been claimed to be responsible for the amplification of precursor magnetic fields. However, an alternative mechanism has been proposed in which the cosmic-ray pressure gradient forms the shock precursor and drives turbulence, amplifying the magnetic field via the small-scale dynamo. A key ingredient for the mechanism to operate are the inhomogeneities present in the interstellar medium (ISM). These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different ISM conditions through 3D MHD numerical simulations.
Simulation of turbulent magnetic reconnection in the smallscale solar wind
Institute of Scientific and Technical Information of China (English)
魏奉思; 胡强; R.Schwen; 冯学尚
2000-01-01
Some observational examples for the possible occurrence of the turbulent magnetic reconnection in the solar wind are found by analysing Helios spacecraft’s high resolution data. The phenom-ena of turbulent magnetic reconnections in small scale solar wind are simulated by introducing a third order accuracy upwind compact difference scheme to the compressible two-dimensional MHD flow. Numerical results verify that the turbulent magnetic reconnection process could occur in small scale in-terplanetary solar wind, which is a basic feature characterizing the magnetic reconnection in high-mag-netie Peynolds number ( RM = 2 000-10 000) solar wind. The configurations of the magnetic reconnection could evolve from a single X-line to a multiple X-line reconnection, exhibiting a complex picture of the formation, merging and evolution of magnetic islands, and finally the magnetic reconnection would evolve into a low-energy state. Its life-span of evolution is about one hour order of magnitude. Various magnetic and f
On the Nature of Magnetic Turbulence in Rotating, Shearing Flows
Walker, Justin; Boldyrev, Stanislav
2015-01-01
The local properties of turbulence driven by the magnetorotational instability (MRI) in rotating, shearing flows are studied in the framework of a shearing-box model. Based on numerical simulations, we propose that the MRI-driven turbulence comprises two components: the large-scale shear-aligned strong magnetic field and the small-scale fluctuations resembling magnetohydrodynamic (MHD) turbulence. The energy spectrum of the large-scale component is close to $k^{-2}$, whereas the spectrum of the small-scale component agrees with the spectrum of strong MHD turbulence $k^{-3/2}$. While the spectrum of the fluctuations is universal, the outer-scale characteristics of the turbulence are not; they depend on the parameters of the system, such as the net magnetic flux. However, there is remarkable universality among the allowed turbulent states -- their intensity $v_0$ and their outer scale $\\lambda_0$ satisfy the balance condition $v_0/\\lambda_0\\sim \\mathrm d\\Omega/\\mathrm d\\ln r$, where $\\mathrm d\\Omega/\\mathrm d\\l...
Transport of magnetic turbulence in Supernova remnants
Brose, Robert; Pohl, Martin
2016-01-01
Context. Supernova remnants are known as sources of galactic cosmic rays for their non-thermal emission of radio waves, X-rays, and gamma-rays. However, the observed soft broken power-law spectra are hard to reproduce within standard acceleration theory based on the assumption of Bohm diffusion and steady-state calculations. Aims. We point out that a time-dependent treatment of the acceleration process together with a self-consistent treatment of the scattering turbulence amplification is necessary. Methods. We numerically solve the coupled system of transport equations for cosmic rays and isotropic Alfvenic turbulence. The equations are coupled through the growth rate of turbulence determined by the cosmic-ray gradient and the spatial diffusion coefficient of cosmic rays determined by the energy density of the turbulence. The system is solved on a co-moving expanding grid extending upstream for dozens of shock radii, allowing for the self-consistent study of cosmic-ray diffusion in the vicinity of their acce...
Magnetic moment conservation and particles acceleration in turbulence
Dalena, S.; Greco, A.; Matthaeus, W. H.
2010-12-01
The present work concerns the study of particle magnetic moment conservation in the presence of turbulent magnetic fields. As we know from the particle orbit theory, for slow temporal and spatial magnetic field variations(i.e. if their characteristic length and time are greater than the particle orbit diameter and the time spent by a particle to execute one orbit, respectively), the magnetic moment, defined as μ = (v^2⊥ /B) (averaged over the particle gyroperiod) is an adiabatic invariant and remains constant during particle motion. But in presence of a well developed magnetic turbulence μ can undergo rapid variations and might not be constant anymore. Of course, this fact could influence particle acceleration and could have a considerable implications in many astrophysical problems, such as coronal heating. In order to reproduce and extend some of the results obtained by Karimabadi et al. 1992, we study the interaction between ions and a single or a couple of electromagnetic waves. We varied both the wave frequency and the cosine of pitch angle at which particles are injected, in order to observe in this very simple case which is the limit for magnetic moment conservation. We also will reconsider the results of Dmitruk and Matthaeus (2006) regarding particle acceleration in turbulence, taking into account statistics of the magnetic moment (see also Lehe et al., 2010). Later we will add more waves to obtain a complete turbulent spectrum. The final aim of this research work is the understanding the behavior of particles magnetic moment during magnetic reconnection phenomena. H. Karimabadi, D. Krauss-Varban and T. Teresawa, JGR, 97, 13853, 1992. P. Dmitruk and W. H. Matthaeus, JGR, 11, A12110, 2006. R. Lehe, I. J. Parrish and E. Quataert, Astrophys. J. 707, 404, 2009.
Thermal and Magnetic Pressure in a Turbulent Bistable Medium
Gazol, A.; Kim, J.; Vazquez-Semadeni, E.; Luis, L.
2006-06-01
We present results from a systematic numerical study of the effect of turbulent velocity fluctuations on the thermal and magnetic pressure distributions in thermally bistable flows. The turbulent fluctuations are characterized by their rms Mach number M (with respect to the warm medium) and the energy injection wavenumber, kfor. The behavior of the thermal pressure is consistent with the picture that as either of these parameters is increased, the local ratio of turbulent crossing time to cooling time decreases, causing transient structures in which the effective behavior is intermediate between the thermal-equilibrium and adiabatic regimes. As a result, the effective polytropic exponent of the simulations ranges between ˜ 0.2 to ˜ 1.1, and the mean pressure of the diffuse gas is generally reduced below the thermal equilibrium pressure Peq, while that of the dense gas is increased with respect to Peq. For the magnetic and for the non-magnetic cases, a preliminary comparison of the fraction of high-density zones (n > 7.1cm-3) with P > 104 K cm-3 with the recent pressure measurements of Jenkins (2004) in CI favors our case with M=0.5 and kfor=2. The presence of the magnetic field has an important effect on the thermal pressure distribution, which does not longer show a power-law shape and develops a low pressure tail due to the presence of magnetically dominated regions.The magnetic pressure distribution shows a large dynamical range and a large spread for a given density, indicating that the magnetic field intensity is determined by turbulent motions.We do not observe any correlation between the density and the magnetic field for low densities (n< 80cm-3 ) but the denser gas in our simulations is however over-pressurized thermally and magnetically, indicating that the formation of dense regions by TI, which do not involve strong compressions, prevents this correlation as long as the density is low enough.
Magnetohydrodynamic turbulent cascade of coronal loop magnetic fields.
Rappazzo, A F; Velli, M
2011-06-01
The Parker model for coronal heating is investigated through a high resolution simulation. An inertial range is resolved where fluctuating magnetic energy EMk[Please see symbol]) [Please see symbol] k[Please see symbol](-2.7) exceeds kinetic energy EK(k[Please see symbol])[Please see symbol]k[Please see symbol](-0.6). Increments scale as δbℓ ~/= ℓ(-0.85) and δuℓ ~/= ℓ(+0.2) with velocity increasing at small scales, indicating that magnetic reconnection plays a prime role in this turbulent system. We show that spectral energy transport is akin to standard magnetohydrodynamic (MHD) turbulence even for a system of reconnecting current sheets sustained by the boundary. In this new MHD turbulent cascade, kinetic energy flows are negligible while cross-field flows are enhanced, and through a series of "reflections" between the two fields, cascade more than half of the total spectral energy flow.
Numerical insights into magnetic dynamo action in a turbulent regime
Kenjeres, S.; Hanjalic, K.
2007-01-01
We report on hybrid numerical simulations of a turbulent magnetic dynamo. The simulated set-up mimics the Riga dynamo experiment characterized by Re ≈ 3.5 × 106 and (Gailitis et al 2000 Phys. Rev. Lett. 84 4365–8). The simulations were performed by a simultaneous fully coupled solution of the trans
Characteristic Lengths of Magnetic Field in Magnetohydrodynamic Turbulence
Cho, Jungyeon
2009-01-01
In the framework of turbulence dynamo, flow motions amplify a weak seed magnetic field through the stretching of field lines. Although the amplification process has been a topic of active research, less attention has been paid to the length scales of magnetic field. In this paper, we described a numerical study on characteristic lengths of magnetic field in magnetohydrodynamic turbulence. We considered the case of very weak or zero mean magnetic field, which is applicable to the turbulence in the intergalactic space. Our findings are as follows. (1) At saturation, the peak of magnetic field spectrum occurs at $\\sim L_0/2$, where $L_0$ is the energy injection scale, while the most energy containing scale is $\\sim L_0/5$. The peak scale of spectrum of projected, two-dimensional field is $\\sim L_0$. (2) During the stage of magnetic field amplification, the energy equipartition scale shows a power-law increase of $\\sim t^{1.5}$, while the integral and curvature scales show a linear increase. The equipartition, in...
Unsteady wandering magnetic field lines, turbulence and laboratory flux ropes
Intrator, T.; Sears, J.; Weber, T.; Liu, D.; Pulliam, D.; Lazarian, A.
2011-12-01
We describe earth bound laboratory experiment investigations of patchy, unsteady, bursty, patchy magnetic field structures that are unifying features of magnetic reconnection and turbulence in helio, space and astro physics. Macroscopic field lines occupy cross sectional areas, fill up three dimensional (3D) volumes as flux tubes. They contain mass with Newtonian dynamics that follow magneto-hydro-dynamic (MHD) equations of motion. Flux rope geometry can be ubiquitous in laminar reconnection sheet geometries that are themselves unstable to formation of secondary "islands" that in 3D are really flux ropes. Flux ropes are ubiquitous structures on the sun and the rest of the heliosphere. Understanding the dynamics of flux ropes and their mutual interactions offers the key to many important astrophysical phenomena, including magnetic reconnection and turbulence. We describe laboratory investigations on RSX, where 3D interaction of flux ropes can be studied in great detail. We use experimental probes inside the the flux ropes to measure the magnetic and electric fields, current density, density, temperatures, pressure, and electrostatic and vector plasma potentials. Macroscopic magnetic field lines, unsteady wandering characteristics, and dynamic objects with structure down to the dissipation scale length can be traced from data sets in a 3D volume. Computational approaches are finally able to tackle simple 3D systems and we sketch some intriguing simulation results that are consistent with 3D extensions of typical 2D cartoons for magnetic reconnection and turbulence.
Electronic properties of aperiodic quantum dot chains
Korotaev, P. Yu.; Vekilov, Yu. Kh.; Kaputkina, N. E.
2012-04-01
The electronic spectral and transport properties of aperiodic quantum dot chains are investigated. The systems with singular continuous energy spectrum are considered: Thue-Morse chain, double-periodic chain, Rudin-Shapiro chain. The influence of electronic energy in quantum dot on the spectral properties, band structure, density of states and spectral resistivity, is discussed. Low resistivity regions correspond to delocalized states and these states could be current states. Also we discuss the magnetic field application as the way to tune electronic energy in quantum dot and to obtain metallic or insulating conducting states of the systems.
MAGNETIC ENERGY PRODUCTION BY TURBULENCE IN BINARY NEUTRON STAR MERGERS
Energy Technology Data Exchange (ETDEWEB)
Zrake, Jonathan; MacFadyen, Andrew I. [Center for Cosmology and Particle Physics, Physics Department, New York University, New York, NY 10003 (United States)
2013-06-01
The simultaneous detection of electromagnetic and gravitational wave emission from merging neutron star binaries would greatly aid in their discovery and interpretation. By studying turbulent amplification of magnetic fields in local high-resolution simulations of neutron star merger conditions, we demonstrate that magnetar-level ({approx}> 10{sup 16} G) fields are present throughout the merger duration. We find that the small-scale turbulent dynamo converts 60% of the randomized kinetic energy into magnetic fields on a merger timescale. Since turbulent magnetic energy dissipates through reconnection events that accelerate relativistic electrons, turbulence may facilitate the conversion of orbital kinetic energy into radiation. If 10{sup -4} of the {approx}10{sup 53} erg of orbital kinetic available gets processed through reconnection and creates radiation in the 15-150 keV band, then the fluence at 200 Mpc would be 10{sup -7} erg cm{sup -2}, potentially rendering most merging neutron stars in the advanced LIGO and Virgo detection volumes detectable by Swift BAT.
Magnetic energy production by turbulence in binary neutron star mergers
Zrake, Jonathan
2013-01-01
The simultaneous detection of electromagnetic and gravitational wave emission from merging neutron star binaries would aid greatly in their discovery and interpretation. By studying turbulent amplification of magnetic fields in local high-resolution simulations of neutron star merger conditions, we demonstrate that magnetar-level (~10^16) G fields are present throughout the merger duration. We find that the small-scale turbulent dynamo converts 60% of the randomized kinetic energy into magnetic fields on a merger time scale. Since turbulent magnetic energy dissipates through reconnection events which accelerate relativistic electrons, turbulence may facilitate the conversion of orbital kinetic energy into radiation. If 10^-4 of the ~ 10^53 erg of orbital kinetic available gets processed through reconnection, and creates radiation in the 15-150 keV band, then the fluence at 200 Mpc would be 10^-7 erg/cm^2, potentially rendering most merging neutron stars in the advanced LIGO and Virgo detection volumes detecta...
Turbulent Erosion of Magnetic Flux Tubes
Petrovay, K
1997-01-01
Results from a numerical and analytical investigation of the solution of a nonlinear axially symmetric diffusion equation for the magnetic field are presented for the case when the nonlinear dependence of the diffusivity $\
JOINT INVERSE CASCADE OF MAGNETIC ENERGY AND MAGNETIC HELICITY IN MHD TURBULENCE
Energy Technology Data Exchange (ETDEWEB)
Stepanov, R.; Frick, P.; Mizeva, I. [Institute of Continuous Media Mechanics, Korolyov str. 1, 614013 Perm (Russian Federation)
2015-01-10
We show that oppositely directed fluxes of energy and magnetic helicity coexist in the inertial range in fully developed magnetohydrodynamic (MHD) turbulence with small-scale sources of magnetic helicity. Using a helical shell model of MHD turbulence, we study the high Reynolds number MHD turbulence for helicity injection at a scale that is much smaller than the scale of energy injection. In a short range of scales larger than the forcing scale of magnetic helicity, a bottleneck-like effect appears, which results in a local reduction of the spectral slope. The slope changes in a domain with a high level of relative magnetic helicity, which determines that part of the magnetic energy is related to the helical modes at a given scale. If the relative helicity approaches unity, the spectral slope tends to –3/2. We show that this energy pileup is caused by an inverse cascade of magnetic energy associated with the magnetic helicity. This negative energy flux is the contribution of the pure magnetic-to-magnetic energy transfer, which vanishes in the non-helical limit. In the context of astrophysical dynamos, our results indicate that a large-scale dynamo can be affected by the magnetic helicity generated at small scales. The kinetic helicity, in particular, is not involved in the process at all. An interesting finding is that an inverse cascade of magnetic energy can be provided by a small-scale source of magnetic helicity fluctuations without a mean injection of magnetic helicity.
Energy transfer and dual cascade in kinetic magnetized plasma turbulence.
Plunk, G G; Tatsuno, T
2011-04-22
The question of how nonlinear interactions redistribute the energy of fluctuations across available degrees of freedom is of fundamental importance in the study of turbulence and transport in magnetized weakly collisional plasmas, ranging from space settings to fusion devices. In this Letter, we present a theory for the dual cascade found in such plasmas, which predicts a range of new behavior that distinguishes this cascade from that of neutral fluid turbulence. These phenomena are explained in terms of the constrained nature of spectral transfer in nonlinear gyrokinetics. Accompanying this theory are the first observations of these phenomena, obtained via direct numerical simulations using the gyrokinetic code AstroGK. The basic mechanisms that are found provide a framework for understanding the turbulent energy transfer that couples scales both locally and nonlocally.
Turbulent amplification of supernova magnetic fields in the laboratory
Gregori, Gianluca
2014-10-01
X-ray and radio observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. Field coincident with the outer shock probably arises through a non-linear feedback process involving cosmic rays. The origin of the large magnetic field in the interior of the remnant is less clear but it is probably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas. However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar cloud bank formed prior to the supernova explosion. We have conducted a series of laboratory experiments using high power laser facilities in order to reproduce the essential features of the supernova shock interacting with strong density perturbations. Our results indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. These experiments provide an example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.
Current filaments in turbulent magnetized plasmas
DEFF Research Database (Denmark)
Martines, E.; Vianello, N.; Sundkvist, D.;
2009-01-01
Direct measurements of current density perturbations associated with non-linear phenomena in magnetized plasmas can be carried out using in situ magnetic measurements. In this paper we report such measurements for three different kinds of phenomena. Current density fluctuations in the edge density...... gradient region of a fusion plasma confined in reversed field pinch configuration and in a density gradient region in the Earth magnetosphere are measured and compared, showing that in both environments they can be attributed to drift-Alfvén vortices. Current structures associated with reconnection events...
Growth of Magnetic Fields Induced by Turbulent Motions
Cho, J; Beresnyak, A; Lazarian, A; Ryu, D
2008-01-01
We present numerical simulations of driven magnetohydrodynamic (MHD) turbulence with weak/moderate imposed magnetic fields. The main goal is to clarify dynamics of magnetic field growth. We also investigate the effects of the imposed magnetic fields on the MHD turbulence, including, as a limit, the case of zero external field. Our findings are as follows. First, when we start off simulations with weak mean magnetic field only (or with small scale random field with zero imposed field), we observe that there is a stage at which magnetic energy density grows linearly with time. Runs with different numerical resolutions and/or different simulation parameters show consistent results for the growth rate at the linear stage. Second, we find that, when the strength of the external field increases, the equilibrium kinetic energy density drops by roughly the product of the rms velocity and the strength of the external field. The equilibrium magnetic energy density rises by roughly the same amount. Third, when the exter...
Large-scale magnetic fields in magnetohydrodynamic turbulence.
Alexakis, Alexandros
2013-02-22
High Reynolds number magnetohydrodynamic turbulence in the presence of zero-flux large-scale magnetic fields is investigated as a function of the magnetic field strength. For a variety of flow configurations, the energy dissipation rate [symbol: see text] follows the scaling [Symbol: see text] proportional U(rms)(3)/ℓ even when the large-scale magnetic field energy is twenty times larger than the kinetic energy. A further increase of the magnetic energy showed a transition to the [Symbol: see text] proportional U(rms)(2) B(rms)/ℓ scaling implying that magnetic shear becomes more efficient at this point at cascading the energy than the velocity fluctuations. Strongly helical configurations form nonturbulent helicity condensates that deviate from these scalings. Weak turbulence scaling was absent from the investigation. Finally, the magnetic energy spectra support the Kolmogorov spectrum k(-5/3) while kinetic energy spectra are closer to the Iroshnikov-Kraichnan spectrum k(-3/2) as observed in the solar wind.
Growth of a localized seed magnetic field in a turbulent medium
2012-01-01
Turbulence dynamo deals with amplification of a seed magnetic field in a turbulent medium and has been studied mostly for uniform or spatially homogeneous seed magnetic fields. However, some astrophysical processes (e.g. jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy clusters) can provide localized seed magnetic fields. In this paper, we numerically study amplification of localized seed magnetic fields in a turbulent medium. Throughout the paper, we assume that...
Transport of solar electrons in the turbulent interplanetary magnetic field
Energy Technology Data Exchange (ETDEWEB)
Ablaßmayer, J.; Tautz, R. C., E-mail: robert.c.tautz@gmail.com [Zentrum für Astronomie und Astrophysik, Technische Universität Berlin, Hardenbergstraße 36, D-10623 Berlin (Germany); Dresing, N., E-mail: dresing@physik.uni-kiel.de [Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Leibnizstraße 11, D-24118 Kiel (Germany)
2016-01-15
The turbulent transport of solar energetic electrons in the interplanetary magnetic field is investigated by means of a test-particle Monte-Carlo simulation. The magnetic fields are modeled as a combination of the Parker field and a turbulent component. In combination with the direct calculation of diffusion coefficients via the mean-square displacements, this approach allows one to analyze the effect of the initial ballistic transport phase. In that sense, the model complements the main other approach in which a transport equation is solved. The major advancement is that, by recording the flux of particles arriving at virtual detectors, intensity and anisotropy-time profiles can be obtained. Observational indications for a longitudinal asymmetry can thus be explained by tracing the diffusive spread of the particle distribution. The approach may be of future help for the systematic interpretation of observations for instance by the solar terrestrial relations observatory (STEREO) and advanced composition explorer (ACE) spacecrafts.
Sustained Turbulence in Differentially Rotating Magnetized Fluids at Low Magnetic Prandtl Number
DEFF Research Database (Denmark)
Nauman, Farrukh; Pessah, Martin E.
2016-01-01
We show for the first time that sustained turbulence is possible at low magnetic Prandtl number for Keplerian flows with no mean magnetic flux. Our results indicate that increasing the vertical domain size is equivalent to increasing the dynamical range between the energy injection scale and the ...
Turbulence-induced magnetic flux asymmetry at nanoscale junctions
2007-01-01
It was recently predicted [J. Phys.: Condens. Matter 18, 11059 (2006)] that turbulence of the electron flow may develop at nonadiabatic nanoscale junctions under appropriate conditions. Here we show that such an effect leads to an asymmetric current-induced magnetic field on the two sides of an otherwise symmetric junction. We propose that by measuring the fluxes ensuing from these fields across two surfaces placed at the two sides of the junction would provide direct and noninvasive evidence...
Turbulent amplification of magnetic fields in the laboratory
Gregori, Gianluca
2014-10-01
Magnetic fields exist ubiquitously in the Universe, as revealed by either diffuse radio-synchrotron emission, or Faraday rotation observations, with strengths from a few nG to tens of μG. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the Universe. At present, the origin and the distribution of the magnetic fields are far from being understood. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent processes to the level consistent with current observations. We have conducted a series of laboratory experiments using high power laser facilities to exploit the scale invariance of the magneto-hydrodynamics equations. While the scaling is not perfect (e.g., in what concerns dissipation coefficients such as resistivity or viscosity), the similarity is sufficiently close to make such experiments interesting - and the results have been showing up the fundamental physical process at play. Our results indicate the magnetic field is indeed amplified by turbulent mechanisms. We relate our findings with processes occurring in supernova remnants and in cluster of galaxies. These experiments provide an example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena. The research leading to these results has received funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 256973.
Magnetic reconnection and kinetic effects in Vlasov turbulence
Servidio, Sergio
2015-04-01
The process of magnetic reconnection is ubiquitous in nature, being typical of large scale magnetic configurations. Recently [1], reconnection has been observed to emerge locally and intermittently in plasmas, being a crucial element of turbulence itself. Systematic analysis of MHD simulations reveals the presence of a large number of X-type neutral points, where magnetic reconnection occurs. More recently, the same phenomenon has been inspected within plasma models [2]. The link between magnetic reconnection and kinetic effects in the turbulent solar-wind has been investigated by means of multi-dimensional simulations of the hybrid Vlasov-Maxwell (HVM) code [3], using 5D (2D in space and 3D in velocity space) and full 6D simulations of plasma turbulence. Kinetic effects manifest through the deformation of the proton distribution function, with patterns of non-Maxwellian features being concentrated near regions of strong magnetic gradients. Recent analyses [4] of solar-wind data from spacecraft aimed to quantify kinetic effects through the temperature anisotropy T⊥/T|| on the proton velocity distribution function. Values of the anisotropy range broadly, with most values between 10-1 and 101. Moreover, the distribution of temperature anisotropy depends systematically on the ambient proton parallel beta β|| (the ratio of parallel kinetic pressure to magnetic pressure), manifesting a characteristic rhomboidal shape. In order to make contact with solar-wind observations, temperature anisotropy has been evaluated from an ensemble of HVM simulations [5], obtained by varying the global plasma beta and fluctuation level, in such a way to cover distinct regions of the parameter space defined by T⊥/T|| and β||. The HVM simulations presented here demonstrate that, when the distribution function is free to explore the entire velocity subspace, new features appear as complex interactions between the particles and the turbulent background. Comparison of numerical results
Plasma Beta Dependence of Magnetic Compressibility in Solar Wind Turbulence
Chapman, S. C.; Hnat, B.; Kiyani, K. H.; Sahraoui, F.
2014-12-01
The turbulent signature of MHD scales in the near-Earth solar wind are known to be primarily incompressible which manifests itself in magnetic field fluctuation vector components to be aligned primarily perpendicular to the background magnetic field -- so-called "Variance Anisotropy". This, and other facts, have been seen as evidence for a majority Alfvenic turbulence cascade; with a small component (10%) of compressible fluctuations. When one approaches scales on the order of the ion-inertial length and the Larmor radius, this behaviour changes and it is now becoming increasingly evident that the spectral break at these scales is also accompanied by an increase in magnetic compressibility. This has been attributed to a phase change in the physics at these scales -- from fluid to kinetic -- and in particular to the dominant role of the Hall-effect at sub-ion scales. We will be presenting results from the Cluster mission to show how this increase in the compressibility is dependent on the ion plasma beta and what implications this has for the physics at sub-ion scales in the context of prominent theories and models for kinetic plasma turbulence.
Cosmic Ray Small Scale Anisotropies and Local Turbulent Magnetic Fields
López-Barquero, Vanessa; Xu, S; Desiati, P; Lazarian, A
2015-01-01
Cosmic ray anisotropy is observed in a wide energy range and at different angular scales by a variety of experiments. However, a comprehensive and satisfactory explanation has been elusive for over a decade now. The arrival distribution of cosmic rays on Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field. On the contrary, the medium and small angular scale structure could be an effect of non diffusive propagation of cosmic rays in perturbed magnetic fields. In particular, a possible explanation of the observed small scale anisotropy observed at TeV energy scale, may come from the effect of particle scattering in turbulent magnetized plasmas. We perform numerical integration of test particle trajectories in low-$\\beta$ compressible mag...
Role of helicities for the dynamics of turbulent magnetic fields
Mueller, Wollf-Christian
2013-01-01
Investigations of the inverse cascade of magnetic helicity are conducted with pseudospectral, three-dimensional direct numerical simulations of forced and decaying incompressible magnetohydrodynamic turbulence. The high-resolution simulations which allow for the necessary scale-separation show that the observed self-similar scaling behavior of magnetic helicity and related quantities can only be understood by taking the full nonlinear interplay of velocity and magnetic fluctuations into account. With the help of the eddy-damped quasi-normal Markovian approximation a probably universal relation between kinetic and magnetic helicities is derived that closely resembles the extended definition of the prominent dynamo pseudoscalar $\\alpha$. This unexpected similarity suggests an additional nonlinear quenching mechanism of the current-helicity contribution to $\\alpha$.
Sustained Turbulence in Differentially Rotating Magnetized Fluids at Low Magnetic Prandtl Number
Nauman, Farrukh
2016-01-01
We show for the first time that sustained turbulence is possible at low magnetic Prandtl number for Keplerian flows with no mean magnetic flux. Our results indicate that increasing the vertical domain size is equivalent to increasing the dynamical range between the energy injection scale and the dissipative scale. This has important implications for a large variety of differentially rotating systems with low magnetic Prandtl number such as protostellar disks and laboratory experiments.
Aperiodic order and spectral properties
Baake, Michael; Damanik, David; Grimm, Uwe
2015-01-01
This article presents a very gentle introduction to the field of aperiodic order, aimed at a general audience. It is intended to provide a "Snapshot of Modern Mathematics" relating to the Oberwolfach mini-workshop "Dynamical versus Diffraction Spectra in the Theory of Quasicrystals" in November/December 2014.
Magnetic turbulence in space plasmas: in and around the Earth's magnetosphere
Energy Technology Data Exchange (ETDEWEB)
Zimbardo, Gaetano [Universita della Calabria, Dipartimento di Fisica, Ponte P. Bucci, Cubo 31C, I-87036 Arcavacata di Rende (Italy)
2006-12-15
In collisionless space plasmas most phenomena are governed by wave particle interaction and by the interaction with the large scale fields. Low frequency magnetic turbulence in the solar wind is relatively well characterized and understood. The situation is more complicated for magnetic turbulence in and around the Earth's magnetosphere, where the turbulence feature can vary widely with the location. Recent spacecraft observations of magnetic turbulence in the magnetosheath, in the polar cusp regions and in the magnetotail are considered. Turbulence features like the fluctuation level, the spectral power law index, the turbulence drivers and the turbulence anisotropy and intermittency are addressed. The influence of such a turbulence on the plasma transport and dynamics is briefly described, also using the results of numerical simulations.
BIPOLAR MAGNETIC SPOTS FROM DYNAMOS IN STRATIFIED SPHERICAL SHELL TURBULENCE
Energy Technology Data Exchange (ETDEWEB)
Jabbari, Sarah; Brandenburg, Axel; Kleeorin, Nathan; Mitra, Dhrubaditya; Rogachevskii, Igor, E-mail: sarahjab@kth.se [Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, SE-10691 Stockholm (Sweden)
2015-06-01
Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75° latitude and an azimuthal extent of 180°. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker’s Ω-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10°. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.
Electron turbulence and transport in large magnetic islands
Morton, Lucas
2016-10-01
Magnetic islands, observed in both reversed-field pinches (RFPs) and tokamaks, often display unexpected turbulence and transport characteristics. For the first time in an RFP, the high repetition rate Thomson scattering diagnostic on MST has captured a 2D image of the rotating electron temperature structure of a magnetic island in a single discharge. MHD modeling using edge magnetic signals implies a 16 cm wide m,n =1,6 tearing mode island which completely overlaps a 5.5 cm n =7 island (12 cm between island centers). The 3D field is partially chaotic, but still reflective of the n =6 island structure. The measured temperature structure matches the shape and location of the n =6 partially chaotic (or `remnant') island. Contrary to the usual assumption that islands have flat internal temperature, the electron temperature is peaked inside the remnant magnetic island due to ohmic heating. The temperature peaking implies a local effective perpendicular conductivity 10-40 m2/s inside the remnant island. This agrees quantitatively with an effective perpendicular conductivity of 16 m2/s estimated using the magnetic diffusion coefficient (evaluated at the electron mean free path) calculated from the modeled chaotic field. Statistical analysis of measurement ensembles with lower time resolution implies that remnant island heating is common in MST discharges. To investigate the role of turbulence near a magnetic island, the 2D structure of long-wavelength density turbulence has been mapped around a large applied static m,n =2,1 L-mode island in the DIII-D tokamak. The turbulence exhibits intriguing spatial structure. Fluctuations are enhanced several-fold (compared to the no-island case) on the inboard side of the X-point, but not on the outboard side of the X-point and are also reduced near the O-point. This work is supported by the NSF and US DOE under DE-FC02-04ER54698, and DE-FG02-89ER53296.
PREFACE: 6th International Conference on Aperiodic Crystals (APERIODIC'09)
Grimm, Uwe; McGrath, Rónán; Degtyareva, Olga; Sharma, Hem Raj
2010-04-01
Aperiodic Logo Aperiodic'09, the sixth International Conference on Aperiodic Crystals, took place in Liverpool 13-18 September 2009. It was the first major conference in this interdisciplinary research field held in the UK. The conference, which was organised under the auspices of the Commission on Aperiodic Crystals of the International Union of Crystallography (IUCr), followed on from Aperiodic'94 (Les Diablerets, Switzerland), Aperiodic'97 (Alpe d'Huez, France), Aperiodic'2000 (Nijmegen, The Netherlands), Aperiodic'03 (Belo Horizonte, Brazil) and Aperiodic'06 (Zao, Japan). The next conference in the series will take place in Australia in 2012. The Aperiodic conference series is itself the successor to a series of Conferences on Modulated Structures, Polytypes and Quasicrystals (MOSPOQ), which were held in Marseilles (France) in 1984, Wroclaw (Poland) in 1986, Varanasi (India) in 1988 and Balatonszeplak (Hungary) in 1991. The remit of the conference covers two broad areas of research on aperiodic crystals, incommensurately modulated and composite crystals on the one hand, and quasicrystals on the other hand, sharing the property that they are aperiodically ordered solids. In addition, the conference also featured recent research on complex metal alloys, which are in fact periodically ordered solids. However, the term complex refers to their large unit cells, which may contain thousands of atoms, and as a consequence complex metal alloys share some of the properties of quasicrystalline solids. Aperiodic'09 attracted about 110 participants from across the world, including 20 UK-based scientists (the second largest group after Japan who sent 21 delegates). A particular feature of the conference series is its interdisciplinary character, and once again the range of disciplines of participants included mathematics, physics, crystallography and materials science. The programme started with three tutorial lectures on Sunday afternoon, presenting introductory overviews
GROWTH OF A LOCALIZED SEED MAGNETIC FIELD IN A TURBULENT MEDIUM
Energy Technology Data Exchange (ETDEWEB)
Cho, Jungyeon; Yoo, Hyunju, E-mail: jcho@cnu.ac.kr [Department of Astronomy and Space Science, Chungnam National University, Daejeon (Korea, Republic of)
2012-11-10
Turbulence dynamo deals with the amplification of a seed magnetic field in a turbulent medium and has been studied mostly for uniform or spatially homogeneous seed magnetic fields. However, some astrophysical processes (e.g., jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy clusters) can provide localized seed magnetic fields. In this paper, we numerically study amplification of localized seed magnetic fields in a turbulent medium. Throughout the paper, we assume that the driving scale of turbulence is comparable to the size of the system. Our findings are as follows. First, turbulence can amplify a localized seed magnetic field very efficiently. The growth rate of magnetic energy density is as high as that for a uniform seed magnetic field. This result implies that magnetic field ejected from an astrophysical object can be a viable source of a magnetic field in a cluster. Second, the localized seed magnetic field disperses and fills the whole system very fast. If turbulence in a system (e.g., a galaxy cluster or a filament) is driven at large scales, we expect that it takes a few large-eddy turnover times for the magnetic field to fill the whole system. Third, growth and turbulence diffusion of a localized seed magnetic field are also fast in high magnetic Prandtl number turbulence. Fourth, even in decaying turbulence, a localized seed magnetic field can ultimately fill the whole system. Although the dispersal rate of the magnetic field is not fast in purely decaying turbulence, it can be enhanced by an additional forcing.
Evolving magnetic equilibria in anomalous turbulent transport simulations
Lee, Jungpyo; Cerfon, Antoine; Highcock, Edmund; Barnes, Michael
2014-10-01
The evolution of poloidal and toroidal magnetic fluxes in a tokamak are determined by Faraday's law in which electric field needs to be consistent with 1-D radial transports of density, temperature, and toroidal angular momentum. Consistency is required because the transport of the thermodynamic variables depends on the 2-D magnetic equilibrium that changes depending on the radial pressure profile. For neoclassical transport, consistency is achieved through a proper treatment of the parallel electric field and Ohm's law [Hinton and Hazeltine (1976), Hirshman and Jardin (1979)]. Recently, consistency for the anomalous turbulent transport has been studied analytically using a Lagrangian formulation of gyrokinetics [Sugama et al. (2014)]. In this poster, we propose a simple numerical model to evolve both the magnetic equilibrium and the radial profile of density, temperature, and toroidal angular frequency due to turbulent transport with a fixed q (safety factor) profile. The constraint of fixed q profile makes the evolution self-consistent only if the transport time scale is much smaller than the resistive current diffusion time scale. In this model, we use the transport code TRINITY coupled with the local gyrokinetic code GS2 and the q-solver version of the Grad-Shafranov code ECOM.
Turbulence and intermittent transport at the boundary of magnetized plasmas
DEFF Research Database (Denmark)
Garcia, O.E.; Naulin, V.; Nielsen, A.H.
2005-01-01
a forcing region with spatially localized sources of particles and heat outside which losses due to the motion along open magnetic-field lines dominate, corresponding to the edge region and the scrape-off layer, respectively. Turbulent states reveal intermittent eruptions of hot plasma from the edge region......, propagating radially far into the scrape-off layer in the form of field-aligned filaments, or blobs. This results in positively skewed and flattened single-point probability distribution functions of particle density and temperature, reflecting the frequent appearance of large fluctuations. The conditional...
Influence of Spontaneously Generated Turbulence on Magnetic Reconnection
Daughton, William; Roytershteyn, Vadim; Karimabadi, Homa
2012-10-01
The 3D dynamics of reconnection is examined for electron-positron plasmas within Harris sheet geometry with a guide field. This configuration is unstable to tearing modes at resonant surfaces across the layer, corresponding to oblique angles relative to 2D models. Vlasov theory predicts a spectrum of oblique modes which can destroy the flux surfaces and produce interacting flux ropes. These structures coalesce to larger scales leading to the continual formation and break-up of new current sheets and the generation of turbulence. The fluctuation spectrum is highly anisotropic and is characterized by two power-laws with a break at k di˜1, where di is the inertial length. In the large 3D simulations, the dissipation rate is reduced by ˜40% relative to small 2D cases which are steady and laminar. In both limits, the reconnection remains fast (i.e. Alfv'enic), is insensitive to the system size and ultimately occurs within inertial-scale current sheets. These results imply that the physics responsible for setting the time scale is not radically altered by the turbulence. However, the results indicate that a larger fraction of the magnetic energy is accessible in 3D and that many more particles are accelerated into the high energy tails due to the turbulence.
Energy Technology Data Exchange (ETDEWEB)
Garbet, X
2001-06-01
The purpose of this work is to introduce the main processes that occur in a magnetized plasma. During the last 2 decades, the understanding of turbulence has made great progress but analytical formulas and simulations are far to produce reliable predictions. The values of transport coefficients in a tokamak plasma exceed by far those predicted by the theory of collisional transport. This phenomenon is called abnormal transport and might be due to plasma fluctuations. An estimation of turbulent fluxes derived from the levels of fluctuations, is proposed. A flow description of plasma allows the understanding of most micro-instabilities. The ballooning representation deals with instabilities in a toric geometry. 3 factors play an important role to stabilize plasmas: density pinch, magnetic shear and speed shear. The flow model of plasma gives an erroneous value for the stability threshold, this is due to a bad description of the resonant interaction between wave and particle. As for dynamics, flow models can be improved by adding dissipative terms so that the linear response nears the kinetic response. The kinetic approach is more accurate but is complex because of the great number of dimensions involved. (A.C.)
Analysis of turbulent pipe flow with transverse magnetic Field. Ph.D. Thesis
Energy Technology Data Exchange (ETDEWEB)
Ji, Hyum-chul
1994-01-01
This research focuses on the turbulent pipe flow of an electrically conducting fluid in a horizontal pipe with transverse magnetic field. Techniques are proposed for modeling the interaction of the magnetic field with turbulence, the damping of the turbulent flow by the magnetic field, and the field`s influence on the momentum and the heat transfer. The physics of the electromagnetic damping of turbulence is presented and an electromagnetic damping model is formulated for the k-epsilon turbulence model. The results of the turbulent pipe flow calculations show good agreement with available experimental data. The positive results of the computations demonstrate the utility of the k-epsilon damping model in describing the interaction of a transverse magnetic field with heat and momentum transfer.
Magnetic helicity and the evolution of decaying magnetohydrodynamic turbulence.
Berera, Arjun; Linkmann, Moritz
2014-10-01
Ensemble-averaged high resolution direct numerical simulations of reverse spectral transfer are presented, extending on the many single realization numerical studies done up to now. This identifies this type of spectral transfer as a statistical property of magnetohydrodynamic turbulence and thus permits reliable numerical exploration of its dynamics. The magnetic energy decay exponent from these ensemble runs has been determined to be nE=(0.47±0.03)+(13.9±0.8)/Rλ for initially helical magnetic fields. We show that even after removing the Lorentz force term in the momentum equation, thus decoupling it from the induction equation, reverse spectral transfer still persists. The induction equation is now linear with an externally imposed velocity field, thus amenable to numerous analysis techniques. A new door has opened for analyzing reverse spectral transfer, with various ideas discussed.
Role of nonlinear localized structures and turbulence in magnetized plasma
Pathak, Neha; Yadav, Nitin; Uma, R.; Sharma, R. P.
2016-09-01
In the present study, we have analyzed the field localization of kinetic Alfvén wave (KAW) due to the presence of background density perturbation, which are assumed to be originated by the three dimensionally propagating low frequency KAW. These localized structures play an important role for energy transportation at smaller scales in the dispersion range of magnetic power spectrum. For the present model, governing dynamic equations of high frequency pump KAW and low frequency KAW has been derived by considering ponderomotive nonlinearity. Further, these coupled equations have been numerically solved to analyze the resulting localized structures of pump KAW and magnetic power spectrum in the magnetopause regime. Numerically calculated spectrum exhibits inertial range having spectral index of -3/2 followed by steeper scaling; this steepening in the turbulent spectrum is a signature of energy transportation from larger to smaller scales. In this way, the proposed mechanism, which is based on nonlinear wave-wave interaction, may be useful for understanding the particle acceleration and turbulence in magnetopause.
Inefficient star formation through turbulence, magnetic fields and feedback
Federrath, Christoph
2015-01-01
Star formation is inefficient. Only a few percent of the available gas in molecular clouds forms stars, leading to the observed low star formation rate (SFR). The same holds when averaged over many molecular clouds, such that the SFR of whole galaxies is again surprisingly low. Indeed, considering the low temperatures, molecular clouds should be highly gravitationally unstable and collapse on their global mean freefall timescale. And yet, they are observed to live about 10-100 times longer, i.e., the SFR per freefall time (SFR_ff) is only a few percent. Thus, other physical mechanisms must provide support against quick global collapse. Magnetic fields, turbulence and stellar feedback have been proposed as stabilising agents, but it is still unclear which of these processes is the most important and what their relative contributions are. Here we run high-resolution simulations including gravity, turbulence, magnetic fields, and jet/outflow feedback. We confirm that clouds collapse on a mean freefall time, if o...
Memory Effects in Turbulent Dynamo Generation and Propagation of Large Scale Magnetic Field
Fedotov, S; Zubarev, A; Fedotov, Sergei; Ivanov, Alexey; Zubarev, Andrey
2001-01-01
We are concerned with large scale magnetic field dynamo generation and propagation of magnetic fronts in turbulent electrically conducting fluids. An effective equation for the large scale magnetic field is developed here that takes into account the finite correlation times of the turbulent flow. This equation involves the memory integrals corresponding to the dynamo source term describing the alpha-effect and turbulent transport of magnetic field. We find that the memory effects can drastically change the dynamo growth rate, in particular, non-local turbulent transport might increase the growth rate several times compared to the conventional gradient transport expression. Moreover, the integral turbulent transport term leads to a large decrease of the speed of magnetic front propagation.
Vertical structure of a supernova-driven turbulent magnetized ISM
Hill, Alex S; Mac Low, Mordecai-Mark; Benjamin, Robert A; Haffner, L Matthew; Klingenberg, Christian; Waagan, Knut
2012-01-01
Stellar feedback drives the circulation of matter from the disk to the halo of galaxies. We perform three-dimensional magnetohydrodynamic simulations of a vertical column of the interstellar medium with initial conditions typical of the solar circle in which supernovae drive turbulence and determine the vertical stratification of the medium. The simulations were run using a stable, positivity-preserving scheme for ideal MHD implemented in the FLASH code. We find that the majority (\\approx 90 %) of the mass is contained in thermally-stable temperature regimes of cold molecular and atomic gas at T 3 kpc. The magnetic field in our models has no significant impact on the scale heights of gas in each temperature regime; the magnetic tension force is approximately equal to and opposite the magnetic pressure, so the addition of the field does not significantly affect the vertical support of the gas. The addition of a magnetic field does reduce the fraction of gas in the cold (< 200 K) regime with a corresponding...
Fundamental Statistical Descriptions of Plasma Turbulence in Magnetic Fields
Energy Technology Data Exchange (ETDEWEB)
John A. Krommes
2001-02-16
A pedagogical review of the historical development and current status (as of early 2000) of systematic statistical theories of plasma turbulence is undertaken. Emphasis is on conceptual foundations and methodology, not practical applications. Particular attention is paid to equations and formalism appropriate to strongly magnetized, fully ionized plasmas. Extensive reference to the literature on neutral-fluid turbulence is made, but the unique properties and problems of plasmas are emphasized throughout. Discussions are given of quasilinear theory, weak-turbulence theory, resonance-broadening theory, and the clump algorithm. Those are developed independently, then shown to be special cases of the direct-interaction approximation (DIA), which provides a central focus for the article. Various methods of renormalized perturbation theory are described, then unified with the aid of the generating-functional formalism of Martin, Siggia, and Rose. A general expression for the renormalized dielectric function is deduced and discussed in detail. Modern approaches such as decimation and PDF methods are described. Derivations of DIA-based Markovian closures are discussed. The eddy-damped quasinormal Markovian closure is shown to be nonrealizable in the presence of waves, and a new realizable Markovian closure is presented. The test-field model and a realizable modification thereof are also summarized. Numerical solutions of various closures for some plasma-physics paradigms are reviewed. The variational approach to bounds on transport is developed. Miscellaneous topics include Onsager symmetries for turbulence, the interpretation of entropy balances for both kinetic and fluid descriptions, self-organized criticality, statistical interactions between disparate scales, and the roles of both mean and random shear. Appendices are provided on Fourier transform conventions, dimensional and scaling analysis, the derivations of nonlinear gyrokinetic and gyrofluid equations
Energy Technology Data Exchange (ETDEWEB)
Kobayashi, T., E-mail: kobayashi.tatsuya@LHD.nifs.ac.jp [National Institute for Fusion Science, Toki 509-5292 (Japan); Inagaki, S.; Sasaki, M.; Nagashima, Y.; Kasuya, N.; Fujisawa, A.; Itoh, S.-I. [Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580 (Japan); Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580 (Japan); Kosuga, Y. [Research Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580 (Japan); Institute for Advanced Study, Kyushu University, Fukuoka 812-8581 (Japan); Arakawa, H. [Teikyo University, 6-22 Misakimachi, Omuta 836-8505 (Japan); Yamada, T. [Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580 (Japan); Faculty of Arts and Science, Kyushu University, Nishi-ku, Fukuoka 819-0395 (Japan); Miwa, Y. [Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga 816-8580 (Japan); Itoh, K. [National Institute for Fusion Science, Toki 509-5292 (Japan); Research Center for Plasma Turbulence, Kyushu University, Kasuga 816-8580 (Japan)
2015-11-15
Fluctuation component in the turbulence regime is found to be azimuthally localized at a phase of the global coherent modes in a linear magnetized plasma PANTA. Spatial distribution of squared bicoherence is given in the azimuthal cross section as an indicator of nonlinear energy transfer function from the global coherent mode to the turbulence. Squared bicoherence is strong at a phase where the turbulence amplitude is large. As a result of the turbulence localization, time evolution of radial particle flux becomes bursty. Statistical features such as skewness and kurtosis are strongly modified by the localized turbulence component, although contribution to mean particle flux profile is small.
Growth of a localized seed magnetic field in a turbulent medium
Cho, Jungyeon
2012-01-01
Turbulence dynamo deals with amplification of a seed magnetic field in a turbulent medium and has been studied mostly for uniform or spatially homogeneous seed magnetic fields. However, some astrophysical processes (e.g. jets from active galaxies, galactic winds, or ram-pressure stripping in galaxy clusters) can provide localized seed magnetic fields. In this paper, we numerically study amplification of localized seed magnetic fields in a turbulent medium. Throughout the paper, we assume that driving scale of turbulence is comparable to the size of the system. Our findings are as follows. First, turbulence can amplify a localized seed magnetic field very efficiently. The growth rate of magnetic energy density is as high as that for a uniform seed magnetic field. This result implies that a magnetic field ejected from an astrophysical object can be a viable source of magnetic field in a cluster. Second, the localized seed magnetic field disperses and fills the whole system very fast. If turbulence in a system (...
Generation of Alfvenic Waves and Turbulence in Magnetic Reconnection Jets
Hoshino, M.
2014-12-01
The magneto-hydro-dynamic (MHD) linear stability for the plasma sheet with a localized bulk plasma flow parallel to the neutral sheet is investigated. We find three different unstable modes propagating parallel to the anti-parallel magnetic field line, and we call them as "streaming tearing'', "streaming sausage'', and "streaming kink'' mode. The streaming tearing and sausage modes have the tearing mode-like structure with symmetric density fluctuation to the neutral sheet, and the streaming kink mode has the asymmetric fluctuation. The growth rate of the streaming tearing mode decreases with increasing the magnetic Reynolds number, while those of the streaming sausage and kink modes do not strongly depend on the Reynolds number. The wavelengths of these unstable modes are of the order of the thickness of plasma sheet, which behavior is almost same as the standard tearing mode with no bulk flow. Roughly speaking the growth rates of three modes become faster than the standard tearing mode. The situation of the plasma sheet with the bulk flow can be realized in the reconnection exhaust with the Alfvenic reconnection jet, and the unstable modes may be regarded as one of the generation processes of Alfvenic turbulence in the plasma sheet during magnetic reconnection.
Savane, Y S; Faza-Barry, M; Lomonossov, V
2002-01-01
We study the acceleration of charged particles by the variable magnetic field. The study is based on the determination of spectrum of accelerated particles and the spectrum of hydro magnetic turbulence. We plan the self-consistent system of equation and we also find out the solution of the system for the spectrum of particles and hydro magnetic turbulence with the conditions of effective acceleration in the cosmic space of solar system.
Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium
Frick, Peter; Denisov, Sergey; Stepanov, Rodion
2010-01-01
The first direct measurements of effective magnetic diffusivity in turbulent flow of electro-conductive fluids (the so-called beta-effect) under magnetic Reynolds number Rm >> 1 are reported. The measurements are performed in a nonstationary turbulent flow of liquid sodium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re \\approx 3 10^6, which corresponds to the magnetic Reynolds number Rm \\approx 30. The magnetic diffusivity of the liquid metal was determined by measuring the phase shift between the induced and the applied magnetic fields. The maximal deviation of magnetic diffusivity from its basic (laminar) value reaches about 50% .
Direct measurement of effective magnetic diffusivity in turbulent flow of liquid sodium.
Frick, Peter; Noskov, Vitaliy; Denisov, Sergey; Stepanov, Rodion
2010-10-29
The first direct measurements of effective magnetic diffusivity in turbulent flow of electroconductive fluids (the so-called β effect) under the magnetic Reynolds number Rm≫1 are reported. The measurements are performed in a nonstationary turbulent flow of liquid sodium, generated in a closed toroidal channel. The peak level of the Reynolds number reached Re≈3×10(6), which corresponds to the magnetic Reynolds number Rm≈30. The magnetic diffusivity of the liquid metal was determined by measuring the phase shift between the induced and the applied magnetic fields. The maximal deviation of magnetic diffusivity from its laminar value reaches about 50%.
Molecular processes and turbulent magnetic fields in the solar atmosphere
Shapiro, A. I.
2008-08-01
Coherent scattering in the solar atmosphere leads to the formation of the linearly polarized solar spectrum, just like Rayleigh scattering leads to the polarization of the blue sky. One of the most prominent features of the linearly polarized solar spectrum is the CN violet system as it is also in the unpolarized spectrum. This thesis is devoted to the modeling and interpretation of this system in both spectra and developing it into a very sensitive tool for studying the magnetic fields and the temperature structure of the solar atmosphere. The understanding of the solar magnetic field structure is very important as it is connected with and even controls most of the solar activity phenomena. Zeeman effect diagnostics allows to measure strong directed magnetic fields which only cover about 1% of the solar atmosphere. The remaining part is occupied by weak entangled magnetic fields with mixed polarity, which might significantly contribute to the overall solar magnetic energy. These fields are invisible to the Zeeman effect due to signal cancellation. Therefore the discovery of the linearly polarized solar spectrum opened a new epoch in solar physics. The polarization due to the scattering processes is modified by weak entangled magnetic fields via the Hanle effect and thus, provides us with a unique possibility to access and study such "hidden" magnetic fields. Molecular lines are very useful for probing magnetic fields as, due to their strong temperature sensitivity, different molecules sample different, narrow layers of the solar atmosphere. Therefore the extension of the atomic Hanle effect to molecular lines can provide the 3D structure of the solar turbulent magnetic field. Moreover, due to the broad range of magnetic sensitivities within narrow spectral regions molecular lines can be used for employing the differential Hanle effect technique, which allows dramatically reduced model dependence of the obtained magnetic field. This thesis consists of two main
Marschalkó, G
2014-01-01
A simple analytical relation of form {\\alpha} = 2 {\\kappa} -1 between the magnetic energy spectral exponent {\\alpha} of the turbulent magnetic field in the solar photosphere and its magnetic flux cancellation exponent {\\kappa}, valid under certain restrictive assumptions, is tested and extended outside its range of validity in a series of Monte Carlo simulations. In these numerical tests artificial "magnetograms" are constructed in 1D and 2D by superposing a discrete set of Fourier modes of the magnetic field distribution with amplitudes following a power law spectrum and measuring the cancellation function on these simulated magnetograms. Our results confirm the validity of the analytical relation and extend it to the domain {\\alpha} 0 as {\\alpha} ---> - infinity. The observationally derived upper limit of 0.38 on {\\kappa} implies {\\alpha} < -0.24 in the granular size range, apparently at odds with a small scale dynamo driven in the inertial range.
Influence of magnetic field on the turbulence of plane high temperature gas flow
Energy Technology Data Exchange (ETDEWEB)
Levitan, Yu.S.
1977-01-01
Expressions are obtained within the framework of the Prandtl hypothesis and proposition on local-isotropic turbulence for turbulent friction and thermal flow in a channel flow of an electrical-conducting medium with a current along the channel's axis in the internal magnetic field, and which account for temperature fluctuations of the medium. 6 references.
MMS observations of ion-scale magnetic island in the magnetosheath turbulent plasma
Huang, S. Y.; Sahraoui, F.; Retino, A.; Le Contel, O.; Yuan, Z. G.; Chasapis, A.; Aunai, N.; Breuillard, H.; Deng, X. H.; Zhou, M.; Fu, H. S.; Pang, Y.; Wang, D. D.; Torbert, R. B.; Goodrich, K. A.; Ergun, R. E.; Khotyaintsev, Y. V.; Lindqvist, P.-A.; Russell, C. T.; Strangeway, R. J.; Magnes, W.; Bromund, K.; Leinweber, H.; Plaschke, F.; Anderson, B. J.; Pollock, C. J.; Giles, B. L.; Moore, T. E.; Burch, J. L.
2016-08-01
In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized by bipolar variation of magnetic fields with magnetic field compression, strong core field, density depletion, and strong currents dominated by the parallel component to the local magnetic field. The estimated size of magnetic island is about 8 di, where di is the ion inertial length. Distinct particle behaviors and wave activities inside and at the edges of the magnetic island are observed: parallel electron beam accompanied with electrostatic solitary waves and strong electromagnetic lower hybrid drift waves inside the magnetic island and bidirectional electron beams, whistler waves, weak electromagnetic lower hybrid drift waves, and strong broadband electrostatic noise at the edges of the magnetic island. Our observations demonstrate that highly dynamical, strong wave activities and electron-scale physics occur within ion-scale magnetic islands in the magnetosheath turbulent plasma.
ENHANCED DISSIPATION RATE OF MAGNETIC FIELD IN STRIPED PULSAR WINDS BY THE EFFECT OF TURBULENCE
Energy Technology Data Exchange (ETDEWEB)
Takamoto, Makoto [Department of Physics, Kyoto University, Oiwake-cho, Kitashirakawa, Sakyo-ku, Kyoto 606-8502 (Japan); Inoue, Tsuyoshi [Department of Physics and Mathematics, Aoyama Gakuin University, Fuchinobe, Chuou-ku, Sagamihara 252-5258 (Japan); Inutsuka, Shu-ichiro, E-mail: takamoto@tap.scphys.kyoto-u.ac.jp, E-mail: inouety@phys.aoyama.ac.jp, E-mail: inutsuka@nagoya-u.jp [Department of Physics, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602 (Japan)
2012-08-10
In this paper, we report on turbulent acceleration of the dissipation of the magnetic field in the post-shock region of a Poynting flux-dominated flow, such as the Crab pulsar wind nebula. We have performed two-dimensional resistive relativistic magnetohydrodynamics simulations of subsonic turbulence driven by the Richtmyer-Meshkov instability at the shock fronts of the Poynting flux-dominated flows in pulsar winds. We find that turbulence stretches current sheets which substantially enhances the dissipation of the magnetic field, and that most of the initial magnetic field energy is dissipated within a few eddy-turnover times. We also develop a simple analytical model for turbulent dissipation of the magnetic field that agrees well with our simulations. The analytical model indicates that the dissipation rate does not depend on resistivity even in the small resistivity limit. Our findings can possibly alleviate the {sigma}-problem in the Crab pulsar wind nebulae.
Generation of a magnetic island by edge turbulence in tokamak plasmas
Poyé, A.; Agullo, O.; Muraglia, M.; Garbet, X.; Benkadda, S.; Sen, A.; Dubuit, N.
2015-03-01
We investigate, through extensive 3D magneto-hydro-dynamics numerical simulations, the nonlinear excitation of a large scale magnetic island and its dynamical properties due to the presence of small-scale turbulence. Turbulence is induced by a steep pressure gradient in the edge region [B. D. Scott, Plasma Phys. Controlled Fusion 49, S25 (2007)], close to the separatrix in tokamaks where there is an X-point magnetic configuration. We find that quasi-resonant localized interchange modes at the plasma edge can beat together and produce extended modes that transfer energy to the lowest order resonant surface in an inner stable zone and induce a seed magnetic island. The island width displays high frequency fluctuations that are associated with the fluctuating nature of the energy transfer process from the turbulence, while its mean size is controlled by the magnetic energy content of the turbulence.
Generation of a magnetic island by edge turbulence in tokamak plasmas
Energy Technology Data Exchange (ETDEWEB)
Poyé, A. [Aix-Marseille Université, CNRS, PIIM, UMR 7345, Marseille (France); Université de Bordeaux, CELIA Laboratory, Talence 33405 (France); Agullo, O.; Muraglia, M.; Benkadda, S.; Dubuit, N. [Aix-Marseille Université, CNRS, PIIM, UMR 7345, Marseille (France); France-Japan Magnetic Fusion Laboratory, LIA 336 CNRS, Marseille (France); Garbet, X. [IRFM, CEA, St-Paul-Lez-Durance 13108 (France); Sen, A. [Institute for Plasma Research, Bhat, Gandhinagar 382428 (India)
2015-03-15
We investigate, through extensive 3D magneto-hydro-dynamics numerical simulations, the nonlinear excitation of a large scale magnetic island and its dynamical properties due to the presence of small-scale turbulence. Turbulence is induced by a steep pressure gradient in the edge region [B. D. Scott, Plasma Phys. Controlled Fusion 49, S25 (2007)], close to the separatrix in tokamaks where there is an X-point magnetic configuration. We find that quasi-resonant localized interchange modes at the plasma edge can beat together and produce extended modes that transfer energy to the lowest order resonant surface in an inner stable zone and induce a seed magnetic island. The island width displays high frequency fluctuations that are associated with the fluctuating nature of the energy transfer process from the turbulence, while its mean size is controlled by the magnetic energy content of the turbulence.
Bailly, Christophe
2015-01-01
This book covers the major problems of turbulence and turbulent processes, including physical phenomena, their modeling and their simulation. After a general introduction in Chapter 1 illustrating many aspects dealing with turbulent flows, averaged equations and kinetic energy budgets are provided in Chapter 2. The concept of turbulent viscosity as a closure of the Reynolds stress is also introduced. Wall-bounded flows are presented in Chapter 3, and aspects specific to boundary layers and channel or pipe flows are also pointed out. Free shear flows, namely free jets and wakes, are considered in Chapter 4. Chapter 5 deals with vortex dynamics. Homogeneous turbulence, isotropy, and dynamics of isotropic turbulence are presented in Chapters 6 and 7. Turbulence is then described both in the physical space and in the wave number space. Time dependent numerical simulations are presented in Chapter 8, where an introduction to large eddy simulation is offered. The last three chapters of the book summarize remarka...
Brandenburg, Axel; Kleeorin, Nathan
2016-01-01
In the presence of strong density stratification, hydromagnetic turbulence attains qualitatively new properties: the formation of magnetic flux concentrations. We review here the theoretical foundations of this mechanism in terms of what is now called the negative effective magnetic pressure instability. We also present direct numerical simulations of forced turbulence in strongly stratified layers and discuss the qualitative and quantitative similarities with corresponding mean-field simulations. Finally, the relevance to sunspot formation is discussed.
Vorticity, Shocks and Magnetic Fields in Subsonic, ICM-like Turbulence
Porter, David H; Ryu, Dongsu
2015-01-01
We analyze high resolution simulations of compressible, MHD turbulence with properties resembling conditions in galaxy clusters. The flow is driven to turbulence Mach number $\\mathcal{M}_t \\sim 1/2$ in an isothermal medium with an initially very weak, uniform seed magnetic field ($\\beta = P_g/P_B = 10^6$). Since cluster turbulence is likely to result from a mix of sheared (solenoidal) and compressive forcing processes, we examine the distinct turbulence properties for both cases. In one set of simulations velocity forcing is entirely solenoidal ($\
Garbage Collection Scheduling of Aperiodic Tasks
Institute of Scientific and Technical Information of China (English)
Ning Zhang; Guang-Ze Xiong
2009-01-01
In the previous work of garbage collection (GC) models, scheduling analysis was given based on an assumption that there were no aperiodic mutator tasks. However, it is not true in practical real-time systems. The GC algorithm which can schedule aperiodic tasks is proposed, and the variance of live memory is analyzed. In this algorithm, active tasks are deferred to be processed by GC until the states of tasks become inactive, and the saved sporadic server time can be used to schedule aperiodic tasks. Scheduling the sample task sets demonstrates that this algorithm in this paper can schedule aperiodic tasks and decrease GC work. Thus, the GC algorithm proposed is more flexible and portable.
First Measurement of the Magnetic Turbulence Induced Reynolds Stress in a Tokamak
Institute of Scientific and Technical Information of China (English)
徐国盛; 万宝年; 宋梅
2003-01-01
Reynolds stress component due to magnetic turbulence was first measured in the plasma edge region of the HT- 7 superconducting tokamak using an insertable magnetic probe. A radial gradient of magnetic Reynolds stress was observed to be dose to the velocity shear layer location; however, in this experiment its contribution to driving the poloidal flows is small compared to the electrostatic component. The electron heat transport driven by magnetic turbulence is quite small and cannot account for the total energy transport at the plasma edge.
Plunian, Franck; Stepanov, Rodion
2010-10-01
A phenomenology of isotropic magnetohydrodynamic (MHD) turbulence subject to both rotation and applied magnetic field is presented. It is assumed that the triple correlation decay time is the shortest between the eddy turn-over time and the ones associated to the rotating frequency and the Alfvén wave period. For Pm=1 it leads to four kinds of piecewise spectra, depending on four parameters: injection rate of energy, magnetic diffusivity, rotation rate, and applied field. With a shell model of MHD turbulence (including rotation and applied magnetic field), spectra for Pm ≤ 1 are presented, together with the ratio between magnetic and viscous dissipations.
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
Nornber, M D; Kendrick, R D; Jacobson, C M; Forest, C B
2006-01-01
The magnetic field measured in the Madison Dynamo Experiment shows intermittent periods of growth when an axial magnetic field is applied. The geometry of the intermittent field is consistent with the fastest growing magnetic eigenmode predicted by kinematic dynamo theory using a laminar model of the mean flow. Though the eigenmodes of the mean flow are decaying, it is postulated that turbulent fluctuations of the velocity field change the flow geometry such that the eigenmode growth rate is temporarily positive. Therefore, it is expected that a characteristic of the onset of a turbulent dynamo is magnetic intermittency.
Strong plasma turbulence and anomalous diffusion in a magnetic field
Energy Technology Data Exchange (ETDEWEB)
Okuda, H.
1979-04-01
Plasma diffusion in the presence of strong turbulence has been studied by means of analytic theory and numerical simulations. First, diffusion and turbulent fluctuation spectrum in the presence of convective cells are studied using a two-dimensional guiding center model and a two-fluid model keeping the ion inertia. Second, particle diffusion associated with drift wave turbulence using full dynamic ions and Debye shielding electrons is considered.
Direct numerical simulation of turbulent liquid metal flow entering a magnetic field
Energy Technology Data Exchange (ETDEWEB)
Albets-Chico, X., E-mail: xalbets@ucy.ac.cy; Grigoriadis, D.G.E.; Votyakov, E.V.; Kassinos, S.
2013-12-15
Highlights: • Analysis of turbulence persistence of fully developed MHD pipe flow at Re{sub b} = 4000. • Turbulence decay of fully developed turbulence flow entering low, moderate and strong magnetic fields. • Analysis of the wall conductivity on the aforementioned phenomena. • Discovering and further analysis of flow instabilities of the flow entering a strong magnetic field. -- Abstract: This paper presents direct numerical simulations (DNS) of fully developed turbulent liquid-metal flow in a circular duct entering a magnetic field. The case of a magnetohydrodynamic flow leaving a strong magnetic field has been extensively studied experimentally and numerically owing to its similarity to typical flow configurations appearing in liquid metal blankets of nuclear fusion reactors. Although also relevant to the design of fusion reactor blankets, the flow entering the fringing field of a magnet remains unexplored because its high intricacy precludes any simplification of the governing equations. Indeed, the complexity of the magnetohydrodynamic–turbulence interaction can only be analysed by direct numerical simulations or experiments. With that purpose, this paper addresses the case of a fully developed turbulent flow (Re{sub τ} ≈ 520) entering low, intermediate and strong magnetic fields under electrically insulating and poorly conducting walls by means of three-dimensional direct numerical simulations. Purely hydrodynamic computations (without the effect of the magnetic field) reveal an excellent agreement against previous experimental and numerical results. Current MHD results provide a very detailed information of the turbulence decay and reveal new three-dimensional features related to liquid-metal flow entering strong increasing magnetic fields, such as flow instabilities due to the effect of the Lorentz forces within the fringing region at high Ha numbers.
Simulation of turbulent magnetic reconnection in the small-scale solar wind
Institute of Scientific and Technical Information of China (English)
无
2000-01-01
Some observational examples for the possible occurrence of the turbulent magnetic reconnection in the solar wind are found by analysing Helios spacecraft's high resolution data. The phenomena of turbulent magnetic reconnections in small scale solar wind are simulated by introducing a third order accuracy upwind compact difference scheme to the compressible two_dimensional MHD flow. Numerical results verify that the turbulent magnetic reconnection process could occur in small scale interplanetary solar wind, which is a basic feature characterizing the magnetic reconnection in high_magnetic Reynolds number (RM=2 000-10 000) solar wind. The configurations of the magnetic reconnection could evolve from a single X_line to a multiple X-line reconnection, exhibiting a complex picture of the formation, merging and evolution of magnetic islands, and finally the magnetic reconnection would evolve into a low_energy state. Its life_span of evolution is about one hour order of magnitude. Various magnetic and flow signatures are recorded in the numerical test for different evolution stages and along different crossing paths, which could in principle explain and confirm the observational samples from the Helios spacecraft. These results are helpful for revealing the basic physical processes in the solar wind turbulence.
Li, Jiquan; Kishimoto, Y.; Miyato, N.; Matsumoto, T.
2004-11-01
We investigate how the magnetic shear governs the dynamics of large-scale structures, such as zonal flows and streamers, in electron temperature gradient (ETG) driven turbulence. Based on the well-known 2D Hasegawa-Mima turbulence modeling, which is the inviscid version of fluid (or gyrofluid) ETG turbulence [1], we derive a general dispersion relation of secondary fluctuations through modulation instability analysis. The results show that the formation of different large-scale structures including zonal flow, streamer and so-called generalized Kelvin-Helmholtz (GKH) mode in ETG turbulence depends on the spectral anisotropy of turbulent fluctuation. In a slab geometry, the magnetic shear closely relates to the ETG mode structures so that it may determine the pattern selection in the quasi-steady ETG turbulence. 3D gyrofluid slab ETG simulations show that turbulent ETG fluctuation energy condenses to the zonal flows in the weak shear plasmas and to the streamer component for the high shears. 2D ETG simulations with rather high resolution not only exhibits the global spectral distribution of zonal flows, but also further confirm a mechanism: enhanced zonal flow in weak shear ETG turbulence is limited by exciting a KH mode [1]. Furthermore, in toroidal ETG simulations, streamer structures are observed at around good curvature region along the flux tube in the quasisteady state in some medium shear regime. Related streamer dynamics are also investigated. [1] Jiquan Li and Y. Kishimoto, Phys. Plasmas 11, 1493(2004)
Energy Technology Data Exchange (ETDEWEB)
Dubuit, N
2006-10-15
This work deals with the transport of impurities in magnetically confined thermonuclear plasmas. The accumulation of impurities in the core of the plasma would imply dramatic losses of energy that may lead to the extinction of the plasma. On the opposite, the injection of impurities in the plasma edge is considered as an efficient means to extract heat without damaging the first wall. The balance between these 2 contradictory constraints requires an accurate knowledge of the impurity transport inside the plasma. The effect of turbulence, the main transport mechanism for impurities is therefore a major issue. In this work, the complete formula of a turbulent flow of impurities for a given fluctuation spectrum has been inferred. The origin and features of the main accumulation processes have been identified. The main effect comes from the compressibility of the electrical shift speed in a plane perpendicular to the magnetic field. This compressibility appears to be linked to the curvature of the magnetic field. A less important effect is a thermal-diffusion process that is inversely proportional to the number of charges and then disappears for most type of impurities except the lightest. This effect implies an impurity flux proportional to the temperature gradient and its direction can change according to the average speed of fluctuations. A new version of the turbulence code TRB has been developed. This new version allows the constraints of the turbulence not by the gradients but by the flux which is more realistic. The importance of the processes described above has been confirmed by a comparison between calculation and experimental data from Tore-supra and the Jet tokamak. The prevailing role of the curvature of the magnetic field in the transport impurity is highlighted. (A.C.)
Institute of Scientific and Technical Information of China (English)
Z. Lin; R.E. Waltz
2007-01-01
@@ Turbulent transport driven by plasma pressure gradients [Tangl978] is one of the most important scientific challenges in burning plasma experiments since the balance between turbulent transport and the self-heating by the fusion products (a-particles) determines the performance of a fusion reactor like ITER.
Alignment of Velocity and Magnetic Fluctuations in Simulations of Anisotropic MHD Turbulence
Ng, C. S.; Bhattacharjee, A.
2007-11-01
There has been recent theoretical interest in the effect of the alignment of velocity and magnetic fluctuations in three-dimensional (3D) MHD turbulence with a large-scale magnetic field [Boldyrev 2005, 2006]. This theory predicts that the angle θ between the velocity and magnetic fluctuation vectors has a scaling of θ&1/4circ;, where λ is the spatial scale of the fluctuations. There have also been simulations on 3D forced MHD turbulence that supports this prediction [Mason et al. 2006, 2007]. The scaling has also been tested against observations of solar wind turbulence [Podesta et al. 2007]. We report here simulation results based on decaying 2D turbulence. The scaling of θ&1/4circ; and Iroshnikov-Kraichnan scaling has also been observed within a range of time interval and spatial scales, despite the fact that Boldyrev's theory was developed for fully 3D turbulence in the presence of a strong external field. As the external field is reduced in magnitude and becomes comparable to the magnitude of magnetic fluctuations or lower, the scale-dependent alignment is weakened. Implications for observations of solar wind turbulence will be discussed.
Howes, Gregory G
2009-01-01
Measurements of small-scale turbulent fluctuations in the solar wind find a non-zero right-handed magnetic helicity. This has been interpreted as evidence for ion cyclotron damping. However, theoretical and empirical evidence suggests that the majority of the energy in solar wind turbulence resides in low frequency anisotropic kinetic Alfven wave fluctuations that are not subject to ion cyclotron damping. We demonstrate that a dissipation range comprised of kinetic Alfven waves also produces a net right-handed fluctuating magnetic helicity signature consistent with observations. Thus, the observed magnetic helicity signature does not necessarily imply that ion cyclotron damping is energetically important in the solar wind.
Evolution of clustered magnetic nulls in a turbulent-like reconnection region in the magnetotail
Guo, Ruilong; Pu, Zuyin; Fu, Suiyan; Xie, Lun; Dunlop, Malcolm; Bogdanova, Yulia V.; He, Jiansen; Wang, Xin; Yao, Zhonghua
2016-07-01
Magnetic null points and flux ropes play important roles in the three-dimensional process of magnetic reconnection. In this study, a cluster of null points are reconstructed in the reconnection region in the magnetotail by applying a fitting-reconstruction method to measurements from the Cluster mission. The number of reconstructed null points varies rapidly, presenting a turbulent-like evolution of the magnetic structure. The electron density and the flux of the accelerated electrons were enhanced in this turbulent-like region. During this unstable reconnection process, a B-As-B null structure was formed, showing flux rope features and resembling a secondary island in the observation.
Nonlinear Dynamics of Magnetic Islands Imbedded in Small-Scale Turbulence
Muraglia, Magali; Benkadda, Sadruddin; Garbet, Xavier; Beyer, P; Sen, Abhijit; 10.1103/PhysRevLett.103.145001
2011-01-01
The nonlinear dynamics of magnetic tearing islands imbedded in a pressure gradient driven turbulence is investigated numerically in a reduced magnetohydrodynamic model. The study reveals regimes where the linear and nonlinear phases of the tearing instability are controlled by the properties of the pressure gradient. In these regimes, the interplay between the pressure and the magnetic flux determines the dynamics of the saturated state. A secondary instability can occur and strongly modify the magnetic island dynamics by triggering a poloidal rotation. It is shown that the complex nonlinear interaction between the islands and turbulence is nonlocal and involves small scales.
Falceta-Goncalves, D
2015-01-01
In this work we report a numerical study of the cosmic magnetic field amplification due to collisionless plasma instabilities. The collisionless magnetohydrodynamic equations derived account for the pressure anisotropy that leads, in specific conditions, to the firehose and mirror instabilities. We study the time evolution of seed fields in turbulence under the influence of such instabilities. An approximate analytical time evolution of magnetic field is provided. The numerical simulations and the analytical predictions are compared. We found that i) amplification of magnetic field was efficient in firehose unstable turbulent regimes, but not in the mirror unstable models, ii) the growth rate of the magnetic energy density is much faster than the turbulent dynamo, iii) the efficient amplification occurs at small scales. The analytical prediction for the correlation between the growth timescales with pressure anisotropy ratio is confirmed by the numerical simulations. These results reinforce the idea that pres...
Influence of an external magnetic field on forced turbulence in a swirling flow of liquid metal
Gallet, Basile; Mordant, Nicolas
2009-01-01
We report an experimental investigation on the influence of an external magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We observe an exponential damping of the turbulent velocity fluctuations as a function of the interaction parameter N (ratio of Lorentz force over inertial terms of the Navier-Stokes equation). The flow structures develop some anisotropy but do not become bidimensional. From a dynamical viewpoint, the damping first occurs homogeneously over the whole spectrum of frequencies. For larger values of N, a very strong additional damping occurs at the highest frequencies. However, the injected mechanical power remains independent of the applied magnetic field. The simultaneous measurement of induced magnetic field and electrical potential differences shows a very weak correlation between magnetic field and velocity fluctuations. The observed reduction of the fluctuations is in agreement with a previously proposed mechanism for the saturation of turbulent dynamos and wit...
The Effects of Turbulence on Three-Dimensional Magnetic Reconnection at the Magnetopause
Price, L; Drake, J F; Cassak, P A; Dahlin, J; Ergun, R E
2016-01-01
Two- and three-dimensional particle-in-cell simulations of a recent encounter of the Magnetospheric Multiscale Mission (MMS) with an electron diffusion region at the magnetopause are presented. While the two-dimensional simulations are laminar, in the three-dimensional simulation turbulence develops at both the x-line and along the magnetic separatrices. The turbulence is strong enough to make the magnetic field around the reconnection island chaotic and produces both anomalous resistivity and anomalous viscosity. Each contribute significantly to breaking the frozen-in condition in the electron diffusion region. A surprise is that the crescent-shaped features seen in velocity distributions during the recent MMS observations and in two-dimensional simulations survive even in the turbulent environment of the three-dimensional system. This suggests that MMS's measurements of crescent distributions do not exclude the possibility that turbulence plays an important role in magnetopause reconnection.
Large-scale magnetic structure formation in three-dimensional magnetohydrodynamic turbulence
Energy Technology Data Exchange (ETDEWEB)
Malapaka, Shiva Kumar; Müller, Wolf-Christian [Max-Planck Institute for Plasmaphysics, Boltzmannstrasse 2, D-85748, Garching bei Muenchen (Germany)
2013-11-20
The inverse cascade of magnetic helicity in three-dimensional magnetohydrodynamic (3D-MHD) turbulence is believed to be one of the processes responsible for large-scale magnetic structure formation in astrophysical systems. In this work, we present an exhaustive set of high-resolution direct numerical simulations of both forced and decaying 3D-MHD turbulence, to understand this structure formation process. It is first shown that an inverse cascade of magnetic helicity in small-scale driven turbulence does not necessarily generate coherent large-scale magnetic structures. The observed large-scale magnetic field, in this case, is severely perturbed by magnetic fluctuations generated by the small-scale forcing. In the decaying case, coherent large-scale structures form similarly to those observed astronomically. Based on the numerical results, the formation of large-scale magnetic structures in some astrophysical systems is suggested to be the consequence of an initial forcing that imparts the necessary turbulent energy into the system, which, after the forcing shuts off, decays to form the large-scale structures. This idea is supported by representative examples, e.g., clusters of galaxies.
Large-scale Magnetic Structure Formation in 3D-MHD Turbulence
Malapaka, Shiva Kumar
2013-01-01
The inverse cascade of magnetic helicity in 3D-MHD turbulence is believed to be one of the processes responsible for large scale magnetic structure formation in astrophysical systems. In this work we present an exhaustive set of high resolution direct numerical simulations (DNS) of both forced and decaying 3D-MHD turbulence, to understand this structure formation process. It is first shown that an inverse cascade of magnetic helicity in small-scale driven turbulence does not necessarily generate coherent large-scale magnetic structures. The observed large-scale magnetic field, in this case, is severely perturbed by magnetic fluctuations generated by the small-scale forcing. In the decaying case, coherent large-scale structure form similar to those observed astronomically. Based on the numerical results the formation of large-scale magnetic structures in some astrophysical systems, is suggested to be the consequence of an initial forcing which imparts the necessary turbulent energy into the system, which, afte...
Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks
DEFF Research Database (Denmark)
G. Blackman, Eric; Pessah, Martin Elias
2009-01-01
Non-thermal X-ray emission in compact accretion engines can be interpreted to result from magnetic dissipation in an optically thin magnetized corona above an optically thick accretion disk. If coronal magnetic field originates in the disk and the disk is turbulent, then only magnetic structures...... large enough for their turbulent shredding time to exceed their buoyant rise time survive the journey to the corona. We use this concept and a physical model to constrain the minimum fraction of magnetic energy above the critical scale for buoyancy as a function of the observed coronal to bolometric...... AGN, for which of order 30 per cent of the bolometric flux is in the X-ray band, we find that more than 20 per cent of the magnetic energy must be of large enough scale to rise and dissipate in the corona....
Timofeev, I V
2012-01-01
The power of second harmonic electromagnetic emission is calculated for the case when strong plasma turbulence is excited by a powerful electron beam in a magnetized plasma. It is shown that the simple analytical model of strong plasma turbulence with the assumption of a constant pump power is able to explain experimentally observed bursts of electromagnetic radiation as a consequence of separate collapse events. It is also found that the electromagnetic emission power calculated for three-wave interaction processes occurring in the long-wavelength part of turbulent spectrum is in order-of-magnitude agreement with experimental results.
Turbulence in magnetized plasmas and financial markets: comparative study of multifractal statistics
Budaev, V. P.
2004-12-01
The turbulence in magnetized plasma and financial data of Russian market have been studied in terms of the multifractal formalism revisited with wavelets. The multifractal formalism based on wavelet calculations allows one to study the scaling properties of turbulent fluctuations. It is observed that both plasma edge turbulence in fusion devices and Russian financial markets demonstrate multifractal statistics, i.e., the scaling behaviour of absolute moments is described by a convex function. Multifractality parameter defined in multiplicative cacade model, seems to be of the same magnitude for the plasma and financial time series considered in this paper.
Federrath, Christoph; Schober, Jennifer; Banerjee, Robi; Klessen, Ralf S; Schleicher, Dominik R G; 10.1103/PhysRevLett.107.114504
2011-01-01
We study the growth rate and saturation level of the turbulent dynamo in magnetohydrodynamical simulations of turbulence, driven with solenoidal (divergence-free) or compressive (curl-free) forcing. For models with Mach numbers ranging from 0.02 to 20, we find significantly different magnetic field geometries, amplification rates, and saturation levels, decreasing strongly at the transition from subsonic to supersonic flows, due to the development of shocks. Both extreme types of turbulent forcing drive the dynamo, but solenoidal forcing is more efficient, because it produces more vorticity.
Pucci, F.; Malara, F.; Perri, S.; Zimbardo, G.; Sorriso-Valvo, L.; Valentini, F.
2016-07-01
The transport of energetic particles in the presence of magnetic turbulence is an important but unsolved problem of space physics and astrophysics. Here, we aim at advancing the understanding of energetic particle transport by means of a new numerical model of synthetic magnetic turbulence. The model builds up a turbulent magnetic field as a superposition of space-localized fluctuations at different spatial scales. The resulting spectrum is isotropic with an adjustable spectral index. The model allows us to reproduce a spectrum broader than four decades, and to regulate the level of intermittency through a technique based on the p-model. Adjusting the simulation parameters close to solar wind conditions at 1 au, we inject ˜1 MeV protons in the turbulence realization and compute the parallel and perpendicular diffusion coefficients as a function of spectral extension, turbulence level, and intermittency. While a number of previous results are recovered in the appropriate limits, including anomalous transport regimes for low turbulence levels, we find that long spectral extensions tend to reduce the diffusion coefficients. Furthermore, we find for the first time that intermittency has an influence on parallel transport but not on perpendicular transport, with the parallel diffusion coefficient increasing with the level of intermittency. We also obtain the distribution of particle inversion times for parallel velocity, a power law for more than one decade, and compare it with the pitch angle scattering times observed in the solar wind. This parametric study can be useful to interpret particle propagation properties in astrophysical systems.
Joos, Marc; Ciardi, Andrea; Fromang, Sebastien
2013-01-01
[Abridged] Theoretical and numerical studies of star formation have shown that magnetic field (B) has a strong influence on both disk formation and fragmentation; even a relatively low B can prevent these processes. However, very few studies investigated the combined effects of B and turbulence. We study the effects of turbulence in magnetized core collapse, focusing on the magnetic diffusion, the orientation of the angular momentum (J) of the protostellar core, and on its consequences on disk formation, fragmentation and outflows. We perform 3D, AMR, MHD simulations of magnetically supercritical collapsing dense cores of 5 Msun using the MHD code RAMSES. A turbulent velocity field is imposed as initial conditions, characterised by a Kolmogorov power spectrum. Different levels of turbulence and magnetization are investigated, as well as 3 realisations for the turbulent velocity field. Magnetic diffusion, orientation of the rotation axis with respect to B, transport of J, disk formation, fragmentation and outf...
Turbulent magnetic field amplification driven by cosmic-ray pressure gradients
Drury, Luke O'C
2012-01-01
Observations of non-thermal emission from several supernova remnants suggest that magnetic fields close to the blastwave are much stronger than would be naively expected from simple shock compression of the field permeating the interstellar medium (ISM). We present a simple model which is capable of achieving sufficient magnetic field amplification to explain the observations. We propose that the cosmic-ray pressure gradient acting on the inhomogeneous ISM upstream of the supernova blastwave induces strong turbulence upstream of the supernova blastwave. The turbulence is generated through the differential acceleration of the upstream ISM which occurs as a result of density inhomogeneities in the ISM. This turbulence then amplifies the pre-existing magnetic field. Numerical simulations are presented which demonstrate that amplification factors of 20 or more are easily achievable by this mechanism when reasonable parameters for the ISM and supernova blastwave are assumed. The length scale over which this amplif...
On the angular distribution of cosmic rays from an individual source in a turbulent magnetic field
Harari, Diego; Roulet, Esteban
2015-01-01
We obtain the angular distribution of the cosmic rays reaching an observer from an individual source and after propagation through a turbulent magnetic field, for different ratios between the source distance and the diffusion length. We study both the high-energy quasi-rectilinear regime as well as the transition towards the diffusive regime at lower energies where the deflections become large. We consider the impact of energy losses, showing that they tend to enhance the anisotropy of the source at a given energy. We also discuss lensing effects, in particular those that could result from the regular galactic magnetic field component, and show that the effect of the turbulent extragalactic magnetic fields can smooth out the divergent magnification peaks that would result for point-like sources in the limit of no turbulent deflections.
Kunz, M W; Cowley, S C; Binney, J J; Sanders, J S
2010-01-01
We consider the problem of self-regulated heating and cooling in galaxy clusters and the implications for cluster magnetic fields and turbulence. Viscous heating of a weakly collisional magnetised plasma is regulated by the pressure anisotropy with respect to the local direction of the magnetic field. The intracluster medium is a high-beta plasma, where pressure anisotropies caused by the turbulent stresses and the consequent local changes in the magnetic field will trigger very fast microscale instabilities. We argue that the net effect of these instabilities will be to pin the pressure anisotropies at a marginal level, controlled by the plasma beta parameter. This gives rise to local heating rates that turn out to be comparable to the radiative cooling rates. Furthermore, we show that a balance between this heating and Bremsstrahlung cooling is thermally stable, unlike the often conjectured balance between cooling and thermal conduction. Given a sufficient (and probably self-regulating) supply of turbulent ...
Giannattasio, F.; Berrilli, F.; Del Moro, D.; Bellot Rubio, L.; Orozco Suarez, D.; Gosic, M.
2012-12-01
Solar atmosphere is a unique laboratory for the study of turbulent flows under extreme conditions (e.g. very high Reynolds numbers). The turbulent nature of the flow may be approached by determining how magnetic flux elements are transported on the solar surface, and measuring the spatio-temporal scales on which these small magnetic structures are organized. The process involved is diffusion. Several works explored this topic, both by simulations and observations, and the results are often contradictory, ranging from fully-developed turbulent scenarios to normal-diffusive motions. We analyze 24-hour continuous Hinode SOT observations of a supergranular region (for the first time these long scales are explored), studying the evolution of the mutual distance between magnetic element pairs and its scaling laws, in order to investigate the diffusion process. We find a super-diffusive behavior, with a gamma index depending on the spatial scale selected.
Magnetic turbulence and particle dynamics in the Earth’s magnetotail
Directory of Open Access Journals (Sweden)
G. Zimbardo
Full Text Available The influence of magnetic turbulence in the near-Earth magnetotail on ion motion is investigated by numerical simulation. The magnetotail current sheet is modelled as a magnetic field reversal with a normal magnetic field com-ponent Bn , plus a three-dimensional spectrum of magnetic fluctuations dB which represents the observed magnetic turbulence. The dawn-dusk electric field Ey is also considered. A test particle simulation is performed using different values of Bn and of the fluctuation level dB/B_{0}. We show that when the magnetic fluctuations are taken into account, the particle dynamics is deeply affected, giving rise to an increase in the cross tail transport, ion heating, and current sheet thickness. For strong enough turbulence, the current splits in two layers, in agreement with recent Cluster observations.
Key words. Magnetospheric physics (magnetospheric configuration and dynamics – Interplanetary physics (MHD waves and turbulence – Electromagnetics (numerical methods
Cosmic-Ray Small-scale Anisotropies and Local Turbulent Magnetic Fields
López-Barquero, V.; Farber, R.; Xu, S.; Desiati, P.; Lazarian, A.
2016-10-01
Cosmic-ray anisotropy has been observed in a wide energy range and at different angular scales by a variety of experiments over the past decade. However, no comprehensive or satisfactory explanation has been put forth to date. The arrival distribution of cosmic rays at Earth is the convolution of the distribution of their sources and of the effects of geometry and properties of the magnetic field through which particles propagate. It is generally believed that the anisotropy topology at the largest angular scale is adiabatically shaped by diffusion in the structured interstellar magnetic field. On the contrary, the medium- and small-scale angular structure could be an effect of nondiffusive propagation of cosmic rays in perturbed magnetic fields. In particular, a possible explanation for the observed small-scale anisotropy observed at the TeV energy scale may be the effect of particle propagation in turbulent magnetized plasmas. We perform numerical integration of test particle trajectories in low-β compressible magnetohydrodynamic turbulence to study how the cosmic rays’ arrival direction distribution is perturbed when they stream along the local turbulent magnetic field. We utilize Liouville’s theorem for obtaining the anisotropy at Earth and provide the theoretical framework for the application of the theorem in the specific case of cosmic-ray arrival distribution. In this work, we discuss the effects on the anisotropy arising from propagation in this inhomogeneous and turbulent interstellar magnetic field.
Magnetic and Kinetic Power Spectra as a Tool to Probe the Turbulent Dynamo
Abramenko, V I; Goode, P R
2011-01-01
Generation and diffusion of the magnetic field on the Sun is a key mechanism responsible for solar activity on all spatial and temporal scales - from the solar cycle down to the evolution of small-scale magnetic elements in the quiet Sun. The solar dynamo operates as a non-linear dynamical process and is thought to be manifest in two types: as a global dynamo responsible for the solar cycle periodicity, and as a small-scale turbulent dynamo responsible for the formation of magnetic carpet in the quiet Sun. Numerous MHD simulations of the solar turbulence did not yet reach a consensus as to the existence of a turbulent dynamo on the Sun. At the same time, high-resolution observations of the quiet Sun from Hinode instruments suggest possibilities for the turbulent dynamo. Analysis of characteristics of turbulence derived from observations would be beneficial in tackling the problem. We analyse magnetic and velocity energy spectra as derived from Hinode/SOT, SOHO/MDI, SDO/HMI and the New Solar Telescope (NST) of...
Fractal signatures in the aperiodic Fibonacci grating.
Verma, Rupesh; Banerjee, Varsha; Senthilkumaran, Paramasivam
2014-05-01
The Fibonacci grating (FbG) is an archetypal example of aperiodicity and self-similarity. While aperiodicity distinguishes it from a fractal, self-similarity identifies it with a fractal. Our paper investigates the outcome of these complementary features on the FbG diffraction profile (FbGDP). We find that the FbGDP has unique characteristics (e.g., no reduction in intensity with increasing generations), in addition to fractal signatures (e.g., a non-integer fractal dimension). These make the Fibonacci architecture potentially useful in image forming devices and other emerging technologies.
The efficiency of magnetic field amplification at shocks by turbulence
Ji (), Suoqing; Oh, S. Peng; Ruszkowski, M.; Markevitch, M.
2016-12-01
Turbulent dynamo field amplification has often been invoked to explain the strong field strengths in thin rims in supernova shocks ( ˜ 100 μG) and in radio relics in galaxy clusters ( ˜ μG). We present high-resolution magnetohydrodynamic simulations of the interaction between pre-shock turbulence, clumping and shocks, to quantify the conditions under which turbulent dynamo amplification can be significant. We demonstrate numerically converged field amplification which scales with Alfvén Mach number, B/B_0 ∝ M_A, up to M_A ˜ 150. This implies that the post-shock field strength is relatively independent of the seed field. Amplification is dominated by compression at low M_A, and stretching (turbulent amplification) at high M_A. For high M_A, the B-field grows exponentially and saturates at equipartition with turbulence, while the vorticity jumps sharply at the shock and subsequently decays; the resulting field is orientated predominately along the shock normal (an effect only apparent in 3D and not 2D). This agrees with the radial field bias seen in supernova remnants. By contrast, for low M_A, field amplification is mostly compressional, relatively modest, and results in a predominantly perpendicular field. The latter is consistent with the polarization seen in radio relics. Our results are relatively robust to the assumed level of gas clumping. Our results imply that the turbulent dynamo may be important for supernovae, but is only consistent with the field strength, and not geometry, for cluster radio relics. For the latter, this implies strong pre-existing B-fields in the ambient cluster outskirts.
Diffusion and radiation in magnetized collisionless plasmas with small-scale Whistler turbulence
Keenan, Brett D.; Medvedev, Mikhail V.
2016-04-01
> Magnetized high-energy-density plasmas can often have strong electromagnetic fluctuations whose correlation scale is smaller than the electron Larmor radius. Radiation from the electrons in such plasmas - which markedly differs from both synchrotron and cyclotron radiation - is tightly related to their energy and pitch-angle diffusion. In this paper, we present a comprehensive theoretical and numerical study of particle transport in cold, `small-scale' Whistler-mode turbulence and its relation to the spectra of radiation simultaneously produced by these particles. We emphasize that this relation is a superb diagnostic tool of laboratory, astrophysical, interplanetary and solar plasmas with a mean magnetic field and strong small-scale turbulence.
Vikhrev, V. V.; Baronova, E. O.
2006-01-01
Pinch dynamics is described, which takes into account generation of turbulent magnetic fields. Turbulent/chaotic magnetic fields (TMF) appear due to MHD and kinetic instabilities. It is shown, that TMF arises near the moment of maximal compression and essentially affects plasma dynamics at the expansion stage.
Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks
Niemiec, Jacek; Stroman, Thomas; Nishikawa, and Ken-Ichi
2008-01-01
We present results of 2D and 3D PIC simulations of magnetic turbulence production by isotropic cosmic-ray ions drifting upstream of SNR shocks. The studies aim at testing recent predictions of a strong amplification of short wavelength non-resonant wave modes and at studying the evolution of the magnetic turbulence and its backreaction on cosmic rays. We confirm the generation of the turbulent magnetic field due to the drift of cosmic rays in the upstream plasma, but show that an oblique filamentary mode grows more rapidly than the non resonant parallel modes found in analytical theory. The growth rate of the field perturbations is much slower than is estimated using a quasi-linear approach, and the amplitude of the turbulence saturates at about dB/B~1. The backreaction of the turbulence on the particles leads to an alignment of the bulk-flow velocities of the cosmic rays and the background medium, which is an essential characteristic of cosmic-ray modified shocks. It accounts for the saturation of the instab...
Energy Technology Data Exchange (ETDEWEB)
Santos-Lima, R.; De Gouveia Dal Pino, E. M. [Instituto de Astronomia, Geofisica e Ciencias Atmosfericas, Universidade de Sao Paulo, R. do Matao, 1226, Sao Paulo, SP 05508-090 (Brazil); Lazarian, A. [Department of Astronomy, University of Wisconsin, Madison, WI 53706 (United States)
2012-03-01
The formation of protostellar disks out of molecular cloud cores is still not fully understood. Under ideal MHD conditions, the removal of angular momentum from the disk progenitor by the typically embedded magnetic field may prevent the formation of a rotationally supported disk during the main protostellar accretion phase of low-mass stars. This has been known as the magnetic braking problem and the most investigated mechanism to alleviate this problem and help remove the excess of magnetic flux during the star formation process, the so-called ambipolar diffusion (AD), has been shown to be not sufficient to weaken the magnetic braking at least at this stage of the disk formation. In this work, motivated by recent progress in the understanding of magnetic reconnection in turbulent environments, we appeal to the diffusion of magnetic field mediated by magnetic reconnection as an alternative mechanism for removing magnetic flux. We investigate numerically this mechanism during the later phases of the protostellar disk formation and show its high efficiency. By means of fully three-dimensional MHD simulations, we show that the diffusivity arising from turbulent magnetic reconnection is able to transport magnetic flux to the outskirts of the disk progenitor at timescales compatible with the collapse, allowing the formation of a rotationally supported disk around the protostar of dimensions {approx}100 AU, with a nearly Keplerian profile in the early accretion phase. Since MHD turbulence is expected to be present in protostellar disks, this is a natural mechanism for removing magnetic flux excess and allowing the formation of these disks. This mechanism dismisses the necessity of postulating a hypothetical increase of the ohmic resistivity as discussed in the literature. Together with our earlier work which showed that magnetic flux removal from molecular cloud cores is very efficient, this work calls for reconsidering the relative role of AD in the processes of star
Magnetic reconnection in turbulent space plasmas: null-points or pinches?
Olshevsky, Vyacheslav; Lapenta, Giovanni; Markidis, Stefano; Divin, Andrey
2014-05-01
We report particle-in-cell simulations of magnetic reconnection in the configuration containing both null-points and pinches. All indicators suggest that secondary magnetic reconnection driven by kinking of the pinches plays a dominant role in the energetics of the system. While there is no substantial energy dissipation in the vicinity of X-type null-points. Such reconnection results in tremendous release of magnetic energy, generation of suprathermal particles and waves. Similar scenario may take place in turbulent space plasmas, where current channels and twisted magnetic fields are detected.
Tzeferacos, P.; Rigby, A.; Bott, A.; Bell, A. R.; Bingham, R.; Casner, A.; Cattaneo, F.; Churazov, E. M.; Emig, J.; Flocke, N.; Fiuza, F.; Forest, C. B.; Foster, J.; Graziani, C.; Katz, J.; Koenig, M.; Li, C.-K.; Meinecke, J.; Petrasso, R.; Park, H.-S.; Remington, B. A.; Ross, J. S.; Ryu, D.; Ryutov, D.; Weide, K.; White, T. G.; Reville, B.; Miniati, F.; Schekochihin, A. A.; Froula, D. H.; Gregori, G.; Lamb, D. Q.
2017-04-01
The universe is permeated by magnetic fields, with strengths ranging from a femtogauss in the voids between the filaments of galaxy clusters to several teragauss in black holes and neutron stars. The standard model behind cosmological magnetic fields is the nonlinear amplification of seed fields via turbulent dynamo to the values observed. We have conceived experiments that aim to demonstrate and study the turbulent dynamo mechanism in the laboratory. Here, we describe the design of these experiments through simulation campaigns using FLASH, a highly capable radiation magnetohydrodynamics code that we have developed, and large-scale three-dimensional simulations on the Mira supercomputer at the Argonne National Laboratory. The simulation results indicate that the experimental platform may be capable of reaching a turbulent plasma state and determining the dynamo amplification. We validate and compare our numerical results with a small subset of experimental data using synthetic diagnostics.
On the effect of rotation on magnetohydrodynamic turbulence at high magnetic Reynolds number
Favier, Benjamin F N; Cambon, Claude; 10.1080/03091929.2010.544655
2011-01-01
This article is focused on the dynamics of a rotating electrically conducting fluid in a turbulent state. As inside the Earth's core or in various industrial processes, a flow is altered by the presence of both background rotation and a large scale magnetic field. In this context, we present a set of 3D direct numerical simulations of incompressible decaying turbulence. We focus on parameters similar to the ones encountered in geophysical and astrophysical flows, so that the Rossby number is small, the interaction parameter is large, but the Elsasser number, defining the ratio between Coriolis and Lorentz forces, is about unity. These simulations allow to quantify the effect of rotation and thus inertial waves on the growth of magnetic fluctuations due to Alfv\\'en waves. Rotation prevents the occurrence of equipartition between kinetic and magnetic energies, with a reduction of magnetic energy at decreasing Elsasser number {\\Lambda}. It also causes a decrease of energy transfer mediated by cubic correlations....
Energy Technology Data Exchange (ETDEWEB)
Falceta-Gonçalves, D. [SUPA, School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, Fife KY16 9SS (United Kingdom); Kowal, G. [Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, Rua Arlindo Bettio, 1000, São Paulo, SP 03828-000 (Brazil)
2015-07-20
In this work we report on a numerical study of the cosmic magnetic field amplification due to collisionless plasma instabilities. The collisionless magnetohydrodynamic equations derived account for the pressure anisotropy that leads, in specific conditions, to the firehose and mirror instabilities. We study the time evolution of seed fields in turbulence under the influence of such instabilities. An approximate analytical time evolution of the magnetic field is provided. The numerical simulations and the analytical predictions are compared. We found that (i) amplification of the magnetic field was efficient in firehose-unstable turbulent regimes, but not in the mirror-unstable models; (ii) the growth rate of the magnetic energy density is much faster than the turbulent dynamo; and (iii) the efficient amplification occurs at small scales. The analytical prediction for the correlation between the growth timescales and pressure anisotropy is confirmed by the numerical simulations. These results reinforce the idea that pressure anisotropies—driven naturally in a turbulent collisionless medium, e.g., the intergalactic medium, could efficiently amplify the magnetic field in the early universe (post-recombination era), previous to the collapse of the first large-scale gravitational structures. This mechanism, though fast for the small-scale fields (∼kpc scales), is unable to provide relatively strong magnetic fields at large scales. Other mechanisms that were not accounted for here (e.g., collisional turbulence once instabilities are quenched, velocity shear, or gravitationally induced inflows of gas into galaxies and clusters) could operate afterward to build up large-scale coherent field structures in the long time evolution.
Turbulent Dynamo Amplification of Magnetic Fields in Laser-Produced Plasmas
Tzeferacos, Petros
2016-10-01
Magnetic fields are ubiquitous in the Universe, as revealed by diffuse radio-synchrotron emission and Faraday rotation observations, with strengths from a few nG to tens of μG. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter in the Universe. The standard model for the origin of these intergalactic magnetic fields is through the amplification of seed fields via turbulent dynamo to the level consistent with current observations. We have conceived and conducted a series of experiments using high-power laser facilities to study the amplification of magnetic fields via turbulence. In these experiments, we characterize the properties of the fluid and the magnetic field turbulence using a comprehensive suite of plasma and magnetic field diagnostics. We describe the large-scale 3D simulations we performed with the radiation-MHD code FLASH on ANL's Mira to help design and interpret the experiments. We then discuss the results of the experiments, which indicate magnetic Reynolds numbers above the expected dynamo threshold are achieved and seed magnetic fields produced by the Biermann battery mechanism are amplified by turbulent dynamo. We relate our findings to processes occurring in galaxy clusters. We acknowledge funding and resources from the ERC (FP7/2007-2013, no. 256973 and 247039), and the U.S. DOE, Contract No. B591485 to LLNL, FWP 57789 to ANL, Grant No. DE-NA0002724 to the University of Chicago, and contract DE-AC02-06CH11357 to ALCF at ANL.
Comparing Numerical Methods for Isothermal Magnetized Supersonic Turbulence
Kritsuk, Alexei G; Collins, David; Padoan, Paolo; Norman, Michael L; Abel, Tom; Banerjee, Robi; Federrath, Christoph; Flock, Mario; Lee, Dongwook; Li, Pak Shing; Mueller, Wolf-Christian; Teyssier, Romain; Ustyugov, Sergey D; Vogel, Christian; Xu, Hao
2011-01-01
We employ simulations of supersonic super-Alfv\\'enic turbulence decay as a benchmark test problem to assess and compare the performance of nine astrophysical MHD methods actively used to model star formation. The set of nine codes includes: ENZO, FLASH, KT-MHD, LL-MHD, PLUTO, PPML, RAMSES, STAGGER, and ZEUS. We present a comprehensive set of statistical measures designed to quantify the effects of numerical dissipation in these MHD solvers. We compare power spectra for basic fields to determine the effective spectral bandwidth of the methods and rank them based on their relative effective Reynolds numbers. We also compare numerical dissipation for solenoidal and dilatational velocity components to check for possible impacts of the numerics on small-scale density statistics. Finally, we discuss convergence of various characteristics for the turbulence decay test and impacts of various components of numerical schemes on the accuracy of solutions. We show that the best performing codes employ a consistently high...
Turbulence simulations of blob formation and radial propagation in toroidally magnetized plasmas
DEFF Research Database (Denmark)
Garcia, O.E.; Naulin, V.; Nielsen, A.H.
2006-01-01
of particles and heat, which is coupled to a scrape-off layer with linear damping terms for all dependent variables corresponding to transport along open magnetic field lines. The formation of blob structures is related to profile variations caused by bursting in the global turbulence level, which is due...
The effects of a magnetic field on planetary migration in laminar and turbulent discs
Comins, Megan L.; Romanova, Marina M.; Koldoba, Alexander V.; Ustyugova, Galina V.; Blinova, Alisa A.; Lovelace, Richard V. E.
2016-07-01
We investigate the migration of low-mass planets (1, 5 and 20 M⊕) in accretion discs threaded with a magnetic field using 2D magnetohydrodynamic code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is β ˜ 2-4, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e. the plasma parameter β is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magnetorotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been investigated.
The effects of a magnetic field on planetary migration in laminar and turbulent discs
Comins, M L; Koldoba, A V; Ustyugova, G V; Lovelace, R V E
2015-01-01
We investigate the migration of low-mass planets ($5 M_{\\oplus}$ and $20 M_{\\oplus}$) in accretion discs threaded with a magnetic field using 2D MHD code in polar coordinates. We observed that, in the case of a strong azimuthal magnetic field where the plasma parameter is $\\beta\\sim 1-2$, density waves at the magnetic resonances exert a positive torque on the planet and may slow down or reverse its migration. However, when the magnetic field is weaker (i.e., the plasma parameter $\\beta$ is relatively large), then non-axisymmetric density waves excited by the planet lead to growth of the radial component of the field and, subsequently, to development of the magneto-rotational instability, such that the disc becomes turbulent. Migration in a turbulent disc is stochastic, and the migration direction may change as such. To understand migration in a turbulent disc, both the interaction between a planet and individual turbulent cells, as well as the interaction between a planet and ordered density waves, have been ...
Dynamics of ion temperature gradient turbulence and transport with a static magnetic island
Izacard, Olivier; Holland, Christopher; James, Spencer D.; Brennan, Dylan P.
2016-02-01
Understanding the interaction mechanisms between large-scale magnetohydrodynamic instabilities and small-scale drift-wave microturbulence is essential for predicting and optimizing the performance of magnetic confinement based fusion energy experiments. We report progress on understanding these interactions using both analytic theory and numerical simulations performed with the BOUT++ [Dudson et al., Comput. Phys. Commun. 180, 1467 (2009)] framework. This work focuses upon the dynamics of the ion temperature gradient instability in the presence of a background static magnetic island, using a weakly electromagnetic two-dimensional five-field fluid model. It is found that the island width must exceed a threshold size (comparable with the turbulent correlation length in the no-island limit) to significantly impact the turbulence dynamics, with the primary impact being an increase in turbulent fluctuation and heat flux amplitudes. The turbulent radial ion energy flux is shown to localize near the X-point, but does so asymmetrically in the poloidal dimension. An effective turbulent resistivity which acts upon the island outer layer is also calculated and shown to always be significantly (10×-100×) greater than the collisional resistivity used in the simulations.
The Magnetic Field as a Turbulence Suppressor in Molecular Cloud Formation
Manuel, Zamora-Avilés; Bastian, Körtgen; Robi, Banerjee; Lee, Hartmann
2016-01-01
We present magnetohydrodynamic simulations aimed at studying the effect of the magnetic field on the production of turbulence through various instabilities during the formation of molecular clouds (MCs) by converging flows. We particularly focus on the subsequent star formation (SF) activity. We study four magnetically supercritical models with magnetic field strengths $B= 0$, 1, 2, and 3 $\\mu$G (corresponding to mass-to-flux ratios of $\\infty$, 4.76, 2.38, and 1.59 times the critical value), with the magnetic field initially aligned with the flows. We find that, for increasing magnetic field strength, the clouds formed tend to be more massive, denser, less turbulent, and with higher SF activity. This causes the onset of star formation activity in the non-magnetic or more weakly magnetized cases to be delayed by a few Myr in comparison to the more strongly magnetized cases. We attribute this behavior to a suppression of the nonlinear thin shell instability (NTSI), which is the main mechanism responsible for t...
Zhai, X. M.; Yeung, P. K.
2016-11-01
Turbulence in an electrically conducting fluid in the limit of low magnetic Reynolds number is, because of the Lorentz force due to an external magnetic field, very different from classical turbulence at both the large scales and the small scales. The importance of minimizing finite domain-size effects on the large scale development has often tended to limit the Reynolds number reached in the past. In this work we use periodic domains stretched along the magnetic field with aspect ratio up to 8 and beyond. The initial state is obtained from decaying isotropic turbulence with large-eddy length scales of order 1% of the length of the domain. After a transient period the kinetic energy returns to a power law decay while the integral length scales in the direction parallel to the magnetic field show preferential growth. At early times the parallel velocity component becomes stronger than the other two but this anisotropy is subsequently reversed under the combined effects of anisotropic Joule dissipation and viscous dissipation. The small scales show characteristics of quasi two-dimensional behavior in the transverse plane. Results over a range of magnetic interaction parameters and Reynolds numbers are compared with known theoretical predictions. Supported by NSF Grant CBET-1510749 and supercomputer resources at TACC/XSEDE and ALCF.
Supernova-driven Turbulence and Magnetic Field Amplification in Disk Galaxies
Gressel, Oliver
2010-01-01
Supernovae are known to be the dominant energy source for driving turbulence in the interstellar medium. Yet, their effect on magnetic field amplification in spiral galaxies is still poorly understood. Analytical models based on the uncorrelated-ensemble approach predicted that any created field will be expelled from the disk before a significant amplification can occur. By means of direct simulations of supernova-driven turbulence, we demonstrate that this is not the case. Accounting for vertical stratification and galactic differential rotation, we find an exponential amplification of the mean field on timescales of 100Myr. The self-consistent numerical verification of such a "fast dynamo" is highly beneficial in explaining the observed strong magnetic fields in young galaxies. We, furthermore, highlight the importance of rotation in the generation of helicity by showing that a similar mechanism based on Cartesian shear does not lead to a sustained amplification of the mean magnetic field. This finding impr...
Magnetically-regulated fragmentation of a massive, dense and turbulent clump
Fontani, F; Giannetti, A; Beltrán, M T; Sánchez-Monge, Á; Testi, L; Brand, J; Caselli, P; Cesaroni, R; Dodson, R; Longmore, S; Rioja, M; Tan, J C; Walmsley, C M
2016-01-01
Massive stars, multiple stellar systems and clusters are born from the gravitational collapse of massive dense gaseous clumps, and the way these systems form strongly depends on how the parent clump fragments into cores during collapse. Numerical simulations show that magnetic fields may be the key ingredient in regulating fragmentation. Here we present ALMA observations at ~0.25'' resolution of the thermal dust continuum emission at ~278 GHz towards a turbulent, dense, and massive clump, IRAS 16061-5048c1, in a very early evolutionary stage. The ALMA image shows that the clump has fragmented into many cores along a filamentary structure. We find that the number, the total mass and the spatial distribution of the fragments are consistent with fragmentation dominated by a strong magnetic field. Our observations support the theoretical prediction that the magnetic field plays a dominant role in the fragmentation process of massive turbulent clump.
``Pheudo-cyclotron'' radiation of non-relativistic particles in small-scale magnetic turbulence
Keenan, Brett; Ford, Alex; Medvedev, Mikhail V.
2014-03-01
Plasma turbulence in some astrophysical objects (e.g., weakly magnetized collisionless shocks in GRBs and SN) has small-scale magnetic field fluctuations. We study spectral characteristics of radiation produced by particles moving in such turbulence. It was shown earlier that relativistic particles produce jitter radiation, which spectral characteristics are markedly different from synchrotron radiation. Here we study radiation produced by non-relativistic particles. In the case of a homogeneous fields, such radiation is cyclotron and its spectrum consists of just a single harmonic at the cyclotron frequency. However, in the sub-Larmor-scale turbulence, the radiation spectrum is much reacher and reflects statistical properties of the underlying magnetic field. We present both analytical estimates and results of ab initio numerical simulations. We also show that particle propagation in such turbulence is diffusive and evaluate the diffusion coefficient. We demonstrate that the diffusion coefficient correlates with some spectral parameters. These results can be very valuable for remote diagnostics of laboratory and astrophysical plasmas. Supported by grant DOE grant DE-FG02-07ER54940 and NSF grant AST-1209665.
Decay rates of magnetic modes below the threshold of a turbulent dynamo.
Herault, J; Pétrélis, F; Fauve, S
2014-04-01
We measure the decay rates of magnetic field modes in a turbulent flow of liquid sodium below the dynamo threshold. We observe that turbulent fluctuations induce energy transfers between modes with different symmetries (dipolar and quadrupolar). Using symmetry properties, we show how to measure the decay rate of each mode without being restricted to the one with the smallest damping rate. We observe that the respective values of the decay rates of these modes depend on the shape of the propellers driving the flow. Dynamical regimes, including field reversals, are observed only when the modes are both nearly marginal. This is in line with a recently proposed model.
DEFF Research Database (Denmark)
W. Davis, S.; M. Stone, J.; Pessah, Martin Elias
2010-01-01
leads to convergence in the turbulent energy densities and stresses as the resolution increases, contrary to results for zero net flux, unstratified boxes. The ratio of total stress to midplane pressure has a mean of ~0.01, although there can be significant fluctuations on long (>~50 orbit) timescales....... We find that the time averaged stresses are largely insensitive to both the radial or vertical aspect ratio of our simulation domain. For simulations with explicit dissipation, we find that stratification extends the range of Reynolds and magnetic Prandtl numbers for which turbulence is sustained...
Kubo number and magnetic field line diffusion coefficient for anisotropic magnetic turbulence
Energy Technology Data Exchange (ETDEWEB)
Pommois, P.; Veltri, P.; Zimbardo, G.
2001-06-01
The magnetic field line diffusion coefficients D{sub x} and D{sub y} are obtained by numerical simulations in the case that all the magnetic turbulence correlation lengths l{sub x}, l{sub y}, and l{sub z} are different. We find that the variety of numerical results can be organized in terms of the Kubo number, the definition of which is extended from R=({delta}B/B{sub 0})(l{sub {parallel}}/l{sub {perpendicular}}) to R=({delta}B/B{sub 0})(l{sub z}/l{sub x}), for l{sub x}{ge}l{sub y}. Here, l{sub {parallel}} (l{sub {perpendicular}}) is the correlation length along (perpendicular to) the average field B{sub 0}=B{sub 0}{cflx e}{sub z}. We have anomalous, non-Gaussian transport for R{approx_lt}0.1, in which case the mean square deviation scales nonlinearly with time. For R{approx_gt}1 we have several Gaussian regimes: an almost quasilinear regime for 0.1{approx_lt}R{approx_lt}1, an intermediate, transition regime for 1{approx_lt}R{approx_lt}10, and a percolative regime for R{approx_gt}10. An analytical form of the diffusion coefficient is proposed, D{sub i}=D({delta}Bl{sub z}/B{sub 0}l{sub x}){sup {mu}}(l{sub i}/l{sub x}){sup {nu}}l{sub x}{sup 2}/l{sub z}, which well describes the numerical simulation results in the quasilinear, intermediate, and percolative regimes.
Santos-Lima, R; Lazarian, A
2012-01-01
Recent numerical analysis of Keplerian disk formation in turbulent, magnetized cloud cores by Santos-Lima, de Gouveia Dal Pino, & Lazarian (2012) demonstrated that reconnection diffusion is an efficient process to remove the magnetic flux excess during the build up of a rotationally supported disk. This process is induced by fast reconnection of the magnetic fields in a turbulent flow. In a similar numerical study, Seifried et al. (2012) concluded that reconnection diffusion or any other non-ideal MHD effects would not be necessary and turbulence shear alone would provide a natural way to build up a rotating disk without requiring magnetic flux loss. Their conclusion was based on the fact that the mean mass-to-flux ratio ({\\mu}) evaluated over a spherical region with a radius much larger than the disk is nearly constant in their models. In this letter we compare the two sets of simulations and show that this averaging over large scales can mask significant real increases of {\\mu} in the inner regions wher...
Turbulent magnetic pumping in a Babcock-Leighton solar dynamo model
Guerrero, G.; de Gouveia Dal Pino, E. M.
2008-07-01
Context: The turbulent pumping effect corresponds to the transport of magnetic flux due to the presence of density and turbulence gradients in convectively unstable layers. In the induction equation it appears as an advective term and for this reason it is expected to be important in the solar and stellar dynamo processes. Aims: We explore the effects of turbulent pumping in a flux-dominated Babcock-Leighton solar dynamo model with a solar-like rotation law. Methods: As a first step, only vertical pumping has been considered through the inclusion of a radial diamagnetic term in the induction equation. In the second step, a latitudinal pumping term was included and then, a near-surface shear was included. Results: The results reveal the importance of the pumping mechanism in solving current limitations in mean field dynamo modeling, such as the storage of the magnetic flux and the latitudinal distribution of the sunspots. If a meridional flow is assumed to be present only in the upper part of the convective zone, it is the full turbulent pumping that regulates both the period of the solar cycle and the latitudinal distribution of the sunspot activity. In models that consider shear near the surface, a second shell of toroidal field is generated above r=0.95~R⊙ at all latitudes. If the full pumping is also included, the polar toroidal fields are efficiently advected inwards, and the toroidal magnetic activity survives only at the observed latitudes near the equator. With regard to the parity of the magnetic field, only models that combine turbulent pumping with near-surface shear always converge to the dipolar parity. Conclusions: This result suggests that, under the Babcock-Leighton approach, the equartorward motion of the observed magnetic activity is governed by the latitudinal pumping of the toroidal magnetic field rather than by a large scale coherent meridional flow. Our results support the idea that the parity problem is related to the quadrupolar imprint of
Interstellar Turbulent Magnetic Field Generation by Plasma Instabilities
Tautz, R C
2013-01-01
The maximum magnetic field strength generated by Weibel-type plasma instabilities is estimated for typical conditions in the interstellar medium. The relevant kinetic dispersion relations are evaluated by conducting a parameter study both for Maxwellian and for suprathermal particle distributions showing that micro Gauss magnetic fields can be generated. It is shown that, depending on the streaming velocity and the plasma temperatures, either the longitudinal or a transverse instability will be dominant. In the presence of an ambient magnetic field, the filamentation instability is typically suppressed while the two-stream and the classic Weibel instability are retained.
Energy partition, scale by scale, in magnetic Archimedes Coriolis weak wave turbulence.
Salhi, A; Baklouti, F S; Godeferd, F; Lehner, T; Cambon, C
2017-02-01
Magnetic Archimedes Coriolis (MAC) waves are omnipresent in several geophysical and astrophysical flows such as the solar tachocline. In the present study, we use linear spectral theory (LST) and investigate the energy partition, scale by scale, in MAC weak wave turbulence for a Boussinesq fluid. At the scale k^{-1}, the maximal frequencies of magnetic (Alfvén) waves, gravity (Archimedes) waves, and inertial (Coriolis) waves are, respectively, V_{A}k,N, and f. By using the induction potential scalar, which is a Lagrangian invariant for a diffusionless Boussinesq fluid [Salhi et al., Phys. Rev. E 85, 026301 (2012)PLEEE81539-375510.1103/PhysRevE.85.026301], we derive a dispersion relation for the three-dimensional MAC waves, generalizing previous ones including that of f-plane MHD "shallow water" waves [Schecter et al., Astrophys. J. 551, L185 (2001)AJLEEY0004-637X10.1086/320027]. A solution for the Fourier amplitude of perturbation fields (velocity, magnetic field, and density) is derived analytically considering a diffusive fluid for which both the magnetic and thermal Prandtl numbers are one. The radial spectrum of kinetic, S_{κ}(k,t), magnetic, S_{m}(k,t), and potential, S_{p}(k,t), energies is determined considering initial isotropic conditions. For magnetic Coriolis (MC) weak wave turbulence, it is shown that, at large scales such that V_{A}k/f≪1, the Alfvén ratio S_{κ}(k,t)/S_{m}(k,t) behaves like k^{-2} if the rotation axis is aligned with the magnetic field, in agreement with previous direct numerical simulations [Favier et al., Geophys. Astrophys. Fluid Dyn. (2012)] and like k^{-1} if the rotation axis is perpendicular to the magnetic field. At small scales, such that V_{A}k/f≫1, there is an equipartition of energy between magnetic and kinetic components. For magnetic Archimedes weak wave turbulence, it is demonstrated that, at large scales, such that (V_{A}k/N≪1), there is an equipartition of energy between magnetic and potential components
Rouson, D. W. I.; Kassinos, S. C.; Moulitsas, I.; Sarris, I. E.; Xu, X.
2008-02-01
A new tensor statistic, the dispersed-phase structure dimensionality Dp, is defined to describe the preferred orientation of clusters of discrete bodies. The evolution of Dp is calculated via direct numerical simulations of passive, Stokesian particles driven by initially isotropic, decaying magnetohydrodynamic turbulence. Results are presented for five magnetic field strengths as characterized by magnetic interaction parameters, N, in the range 0-50. Four field strengths are studied at a grid resolution of 1283. The strongest field strength is also studied at 2563 resolution. In each case, the externally applied magnetic field was spatially uniform and followed a step function in time. Particles with initially uniform distributions were tracked through hydrodynamic turbulence for up to 2800 particle response times before the step change in the magnetic field. In the lower resolution simulation, the particle response time, τp, matched the Kolmogorov time scale at the magnetic field application time t0. The higher-resolution simulation tracked ten sets of particles with τp spanning four decades bracketing the Kolmogorov time scale and the Joule time. The results demonstrate that Dp distinguishes between uniformly distributed particles, those organized into randomly oriented clusters, and those organized into two-dimensional sheets everywhere tangent to the magnetic field lines. Lumley triangles are used to demonstrate that the degree of structural anisotropy depends on τp, N, and the time span over which the magnetic field is applied.
Davis, S W; Pessah, M E
2009-01-01
We examine the effects of density stratification on magnetohydrodynamic turbulence driven by the magnetorotational instability in local simulations that adopt the shearing box approximation. Our primary result is that, even in the absence of explicit dissipation, the addition of vertical gravity leads to convergence in the turbulent energy densities and stresses as the resolution increases, contrary to results for zero net flux, unstratified boxes. The ratio of total stress to midplane pressure has a mean of ~0.01, although there can be significant fluctuations on long (>~50 orbit) timescales. We find that the time averaged stresses are largely insensitive to both the radial or vertical aspect ratio of our simulation domain. For simulations with explicit dissipation, we find that stratification extends the range of Reynolds and magnetic Prandtl numbers for which turbulence is sustained. Confirming the results of previous studies, we find oscillations in the large scale toroidal field with periods of ~10 orbit...
Kinetic-scale magnetic turbulence and finite Larmor radius effects at Mercury
Uritsky, V M; Khazanov, G V; Donovan, E F; Boardsen, S A; Anderson, B J; Korth, H
2011-01-01
We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near Mercury's space environment, with the emphasis on key boundary regions participating in the solar wind -- magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable time scale ~20 s imposed by the signal nonstationarity, suggesting that turbulence at this planet is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered...
Takamoto, Makoto
2016-01-01
In this Letter, we report compressible mode effects on relativistic magnetohydrodynamic (RMHD) turbulence in Poynting-dominated plasmas using 3-dimensional numerical simulations. We decomposed fluctuations in the turbulence into 3 MHD modes (fast, slow, and Alfv\\'en) following the procedure mode decomposition in (Cho & Lazarian 2002), and analyzed their energy spectra and structure functions separately. We also analyzed the ratio of compressible mode to Alfv\\'en mode energy with respect to its Mach number. We found the ratio of compressible mode increases not only with the Alfv\\'en Mach number but with the background magnetization, which indicates a strong coupling between the fast and Alfv\\'en modes and appearance of a new regime of RMHD turbulence in Poynting-dominated plasmas where the fast and Alfv\\'en modes strongly couples and cannot be distinguished, different from the non-relativistic MHD case. This finding will affect particle acceleration efficiency obtained by assuming Alfv\\'enic critical balan...
Effect of Aperiodicity on the Charge Transfer Through DNA Molecules
Ghosh, Angsula; Chaudhuri, Puspitapallab
The effect of aperiodicity on the charge transfer process through DNA molecules is investigated using a tight-binding model. Single-stranded aperiodic Fibonacci polyGC and polyAT sequences along with aperiodic Rudin-Shapiro poly(GCAT) sequences are used in the study. Based on the tight-binding model, molecular orbital calculations of the DNA chains are performed and ionization potentials compared, as this might be relevant to understanding the charge transfer process. Charges migrate through the sequences in a multistep hopping process. Results for current conduction through aperiodic sequences are compared with those for the corresponding periodic sequences. We find that dinucleotide aperiodic Fibonacci sequences decrease the current while tetranucleotide aperiodic Rudin-Shapiro sequences increase the current when compared with the corresponding periodic sequences. The conductance in all cases decays exponentially as the sequence length increases.
Renormalization of aperiodic model lattices: spectral properties
Kroon, L
2003-01-01
Many of the published results for one-dimensional deterministic aperiodic systems treat rather simplified electron models with either a constant site energy or a constant hopping integral. Here we present some rigorous results for more realistic mixed tight-binding systems with both the site energies and the hopping integrals having an aperiodic spatial variation. It is shown that the mixed Thue-Morse, period-doubling and Rudin-Shapiro lattices can be transformed to on-site models on renormalized lattices maintaining the individual order between the site energies. The character of the energy spectra for these mixed models is therefore the same as for the corresponding on-site models. Furthermore, since the study of electrons on a lattice governed by the Schroedinger tight-binding equation maps onto the study of elastic vibrations on a harmonic chain, we have proved that the vibrational spectra of aperiodic harmonic chains with distributions of masses determined by the Thue-Morse sequence and the period-doubli...
Anomalous diffusion and Levy random walk of magnetic field lines in three dimensional turbulence
Energy Technology Data Exchange (ETDEWEB)
Zimbardo, G.; Veltri, P.; Basile, G.; Principato, S. [Dipartimento di Fisica, Universita della Calabria, I-87030 Arcavacata di Rende (Italy)
1995-07-01
The transport of magnetic field lines is studied numerically where three dimensional (3-D) magnetic fluctuations, with a power law spectrum, and periodic over the simulation box are superimposed on an average uniform magnetic field. The weak and the strong turbulence regime, {delta}{ital B}{similar_to}{ital B}{sub 0}, are investigated. In the weak turbulence case, magnetic flux tubes are separated from each other by percolating layers in which field lines undergo a chaotic motion. In this regime the field lines may exhibit Levy, rather than Gaussian, random walk, changing from Levy flights to trapped motion. The anomalous diffusion laws {l_angle}{Delta}{ital x}{sup 2}{sub {ital i}}{r_angle}{proportional_to}{ital s}{sup {alpha}} with {alpha}{gt}1 and {alpha}{lt}1, are obtained for a number of cases, and the non-Gaussian character of the field line random walk is pointed out by computing the kurtosis. Increasing the fluctuation level, and, therefore stochasticity, normal diffusion ({alpha}{congruent}1) is recovered and the kurtoses reach their Gaussian value. However, the numerical results show that neither the quasi-linear theory nor the two dimensional percolation theory can be safely extrapolated to the considered 3-D strong turbulence regime. {copyright} {ital 1995} {ital American} {ital Institute} {ital of} {ital Physics}.
ICPP: Scale size of of magnetic turbulence as probed with 30 MeV runaway electrons
Jaspers, Roger
2000-10-01
This paper reviews results concerning generation, confinement and transport of runaway electrons in the energy range 20-30 MeV in the TEXTOR tokamak. For future fusion reactors, it is of major importance to know the processes of runaway generation and runaway loss after disruptions, because of the potential damage to first wall components. Second, since the runaway electrons are effectively collisionless, their confinement is determined by the magnetic field turbulence. In this way the runaway transport provides a unique opportunity to probe turbulence in the core of a thermonuclear plasma. Runaway electrons above 20 MeV emit synchrotron radiation in the (near) infrared, which can easily be detected by thermographic cameras. This technique is developed and exploited at the TEXTOR-94 tokamak and has resulted in some spectacular results. These include: the experimental evidence of the secondary (`knock-on') runaway generation; the discovery of the runaway snake; the observation of disruption generated runaways; the probing of magnetic turbulence in the core of the plasma in Ohmic and additionally heated plasmas. The paper reviews these results with special emphasis on the subject of probing magnetic turbulence in the core of the plasma. Measurements in the TEXTOR-94 tokamak show that after switching on Neutral Beam Injection, the runaway population decays. The decay only starts with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaway electrons. These observations allow to estimate the mode width δ of the magnetic perturbations:δ smaller than 0.5 cm in Ohmic discharges, increasing to δ=3D 4.4 cm for 0.6 MW NBI power.
Schober, Jennifer; Schleicher, Dominik; Federrath, Christoph; Klessen, Ralf; Banerjee, Robi
2012-02-01
The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus it is expected to depend crucially on the nature of the turbulence. In this paper, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(ℓ)proportional ℓ([symbol see text])V}, where ℓ and v(ℓ) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_{crit}, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence ([symbol see text] = 1/3), we find that the critical magnetic Reynolds number is Rm(crit) (K) ≈ 110 and for Burgers turbulence ([symbol see text] = 1/2) Rm(crit)(B) ≈ 2700. Furthermore, we derive that the growth rate of the small-scale magnetic field for a general type of turbulence is Γ proportional Re((1-[symbol see text])/(1+[symbol see text])) in the limit of infinite magnetic Prandtl number. For decreasing magnetic Prandtl number (down to Pm >/~ 10), the growth rate of the small-scale dynamo decreases. The details of this drop depend on the WKB approximation, which becomes invalid for a magnetic Prandtl number of about unity.
Impact of resonant magnetic perturbations on nonlinearly driven modes in drift-wave turbulence
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Leconte, M. [WCI Center for Fusion Theory, NFRI (Korea, Republic of); Diamond, P. H. [WCI Center for Fusion Theory, NFRI (Korea, Republic of); CMTFO and CASS, UCSD, California 92093 (United States)
2012-05-15
In this work, we study the effects of resonant magnetic perturbations (RMPs) on turbulence, flows, and confinement in the framework of resistive drift wave turbulence. We extend the Hasegawa-Wakatani model to include RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large ({delta}B{sub r}/B{sub 0}). Both the vorticity flux (Reynolds stress) and particle flux are modulated. We derive an extended predator prey model which couples zonal potential and density dynamics to the evolution of turbulence intensity. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. We find states that are similar to the ZF-dominated state of the standard predator-prey model, but for which the power threshold is now a function of the RMP strength. For small RMP amplitude, the energy of zonal flows decreases and the turbulence energy increases with ({delta}B{sub r}/B{sub 0}), corresponding to a damping of zonal flows.
Bañón Navarro, A.; Bardóczi, L.; Carter, T. A.; Jenko, F.; Rhodes, T. L.
2017-03-01
Neoclassical tearing modes have deleterious effects on plasma confinement and, if they grow large enough, they can lead to discharge termination. Therefore, they impose a major barrier in the development of operating scenarios of present-day tokamaks. Gyrokinetics offers a path toward studying multi-scale interactions with turbulence and the effect on plasma confinement. As a first step toward this goal, we have implemented static magnetic islands in nonlinear gyrokinetic simulations with the GENE code. We investigate the effect of the islands on profiles, flows, turbulence and transport and the scaling of these effects with respect to island size. We find a clear threshold island width, below which the islands have little or no effect while beyond this point the islands significantly perturb flows, increase turbulence and transport. Additionally, we study the effect of radially asymmetric islands on shear flows for the first time. We find that island induced shear flows can regulate turbulent fluctuation levels in the vicinity of the island separatrices. Throughout this work, we focus on experimentally relevant quantities, such as rms levels of density and electron temperature fluctuations, as well as amplitude and phasing of turbulence modulation. These simulations aim to provide guidelines for interpreting experimental results by comparing qualitative trends in the simulations with those obtained in tokamak experiments.
Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales
Lapenta, Giovanni
2008-01-01
Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well-known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: i) it is much faster, developing on scales of the order of the Alfv\\'en time, and ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed circulation patterns where the jets going out of the reconnection regions turn around and forces their way back in, carrying along copious amounts of magnetic flux.
Dispersion blue-shift in an aperiodic Bragg reflection waveguide
Fesenko, Volodymyr I
2016-01-01
A particular feature of an aperiodic design of cladding of Bragg reflection waveguides to demonstrate a dispersion blue-shift is elucidated. It is made on the basis of a comparative study of dispersion characteristics of both periodic and aperiodic configurations of Bragg mirrors in the waveguide system, wherein for the aperiodic configuration three procedures for layers alternating, namely Fibonacci, Thue-Morse and Kolakoski substitutional rules are considered. It was found out that, in a Bragg reflection waveguide with any considered aperiodic cladding, dispersion curves of guided modes appear to be shifted to shorter wavelengths compared to the periodic configuration regardless of the modes polarization.
Aperiodic structures in condensed matter fundamentals and applications
Macia Barber, Enrique
2008-01-01
One of the Top Selling Physics Books according to YBP Library ServicesOrder can be found in all the structures unfolding around us at different scales, including in the arrangements of matter and in energy flow patterns. Aperiodic Structures in Condensed Matter: Fundamentals and Applications focuses on a special kind of order referred to as aperiodic order.The book covers several topics dealing with the role of aperiodic order in numerous domains of the physical sciences and technology. It first presents the most characteristic features of various aperiodic systems. The author then describes t
Keenan, Brett D
2015-01-01
Magnetized high-energy-density plasmas can often have strong electromagnetic fluctuations whose correlation scale is smaller than the electron Larmor radius. Radiation from the electrons in such plasmas, which markedly differs from both synchrotron and cyclotron radiation, and their energy and pitch-angle diffusion are tightly related. In this paper, we present a comprehensive theoretical and numerical study of the particles' transport in both cold, "small-scale" Langmuir and Whistler-mode turbulence and its relation to the spectra of radiation simultaneously produced by these particles. We emphasize that this relation is a superb diagnostic tool of laboratory, astrophysical, interplanetary, and solar plasmas with a mean magnetic field and strong small-scale turbulence.
The role of turbulence, magnetic fields and feedback for star formation
Federrath, Christoph
2016-01-01
Star formation is inefficient. Only a few percent of the available gas in molecular clouds forms stars, leading to the observed low star formation rate (SFR). The same holds when averaged over many molecular clouds, such that the SFR of whole galaxies is again surprisingly low. Indeed, considering the low temperatures, molecular clouds should be highly gravitationally unstable and collapse on their global mean freefall timescale. And yet, they are observed to live about 10-100 times longer, i.e., the SFR per freefall time is only a few percent. Thus, other physical mechanisms must provide support against quick global collapse. Magnetic fields, turbulence and stellar feedback have been proposed as stabilising agents controlling star formation, but it is still unclear which of these processes is the most important and what their relative contributions are. Here I present high-resolution, adaptive-mesh-refinement simulations of star cluster formation that include turbulence, magnetic fields, and protostellar jet...
Magnetorotational turbulence in Taylor--Couette flow with imposed azimuthal magnetic field
Guseva, A; Hollerbach, R; Avila, M
2015-01-01
The magnetorotational instability (MRI) is thought to be a powerful source of turbulence and momentum transport in astrophysical accretion discs, but obtaining observational evidence of its operation is challenging. Recently, laboratory experiments of Taylor--Couette flow with externally imposed axial and azimuthal magnetic fields have revealed the kinematic and dynamic properties of the MRI close to the instability onset. While good agreement was found with linear stability analyses, little is known about the transition to turbulence and transport properties of the MRI. We here report on a numerical investigation of the MRI with an imposed azimuthal magnetic field. We show that the laminar Couette flow becomes unstable to a wave rotating in the azimuthal direction and standing in the axial direction via a supercritical Hopf bifurcation. Subsequently, the flow features a catastrophic transition to spatio-temporal defects which is mediated by a subcritical subharmonic Hopf bifurcation. Our results are in agree...
Current Structure and Non-Ideal Behavior at Magnetic Null Points in the Turbulent Magnetosheath
Wendel, D. E.; Adrian, M. L.
2012-12-01
The Poincaré index indicates the Cluster spacecraft tetrahedron entraps a number of magnetic nulls during an encounter with the turbulent magnetosheath. Previous researchers have found evidence for reconnection at one of the many filamentary current layers observed by Cluster in this region. We find that many of the entrained nulls are also associated with strong currents. We dissect the current structure of a pair of spiral nulls that may be topologically connected. At both nulls, we find a strong current along the spine, accompanied by a modest current perpendicular to the spine that tilts the axis of the spine toward the fan plane. At least one of the nulls manifests a non-ideal rotational flow pattern in the fan plane that is consistent with torsional spine reconnection as predicted by theory. These results emphasize the importance of examining the magnetic topology in interpreting the nature of currents and reconnection in three-dimensional turbulence.
Low, B.-C.
1972-01-01
The generation of a magnetic field by statistically homogeneous, stationary velocity turbulence is considered. The generation of rms magnetic fluctuation is explicitly demonstrated in the limit of short turbulence correlation time. It is shown that the fluctuation associated with a growing or stationary mean field grows with time such that the ratio of the fluctuation and the square of the mean field tends to a steady value, which is a monotonically decreasing function of the growth rate of the mean field.
Photoionization feedback in a self-gravitating, magnetized, turbulent cloud
Geen, Sam; Hennebelle, Patrick; Tremblin, Pascal; Rosdahl, Joakim
2015-12-01
We present a new set of analytic models for the expansion of H II regions powered by ultraviolet (UV) photoionization from massive stars and compare them to a new suite of radiative magnetohydrodynamic simulations of turbulent, self-gravitating molecular clouds. To perform these simulations, we use RAMSES-RT, a Eulerian adaptive mesh magnetohydrodynamics code with radiative transfer of UV photons. Our analytic models successfully predict the global behaviour of the H II region provided the density and velocity structure of the cloud are known. We give estimates for the H II region behaviour based on a power-law fit to the density field assuming that the system is virialized. We give a radius at which the ionization front should stop expanding (`stall'). If this radius is smaller than the distance to the edge of the cloud, the H II region will be trapped by the cloud. This effect is more severe in collapsing clouds than in virialized clouds, since the density in the former increases dramatically over time, with much larger photon emission rates needed for the H II region to escape a collapsing cloud. We also measure the response of Jeans unstable gas to the H II regions to predict the impact of UV radiation on star formation in the cloud. We find that the mass in unstable gas can be explained by a model in which the clouds are evaporated by UV photons, suggesting that the net feedback on star formation should be negative.
Self-feeding turbulent magnetic reconnection on macroscopic scales.
Lapenta, Giovanni
2008-06-13
Within a MHD approach we find magnetic reconnection to progress in two entirely different ways. The first is well known: the laminar Sweet-Parker process. But a second, completely different and chaotic reconnection process is possible. This regime has properties of immediate practical relevance: (i) it is much faster, developing on scales of the order of the Alfvén time, and (ii) the areas of reconnection become distributed chaotically over a macroscopic region. The onset of the faster process is the formation of closed-circulation patterns where the jets going out of the reconnection regions turn around and force their way back in, carrying along copious amounts of magnetic flux.
Leonardis, E; Chapman, S C; Daughton, W; Roytershteyn, V; Karimabadi, H
2013-05-17
Recent fully nonlinear, kinetic three-dimensional simulations of magnetic reconnection [W. Daughton et al., Nat. Phys. 7, 539 (2011)] evolve structures and exhibit dynamics on multiple scales, in a manner reminiscent of turbulence. These simulations of reconnection are among the first to be performed at sufficient spatiotemporal resolution to allow formal quantitative analysis of statistical scaling, which we present here. We find that the magnetic field fluctuations generated by reconnection are anisotropic, have nontrivial spatial correlation, and exhibit the hallmarks of finite range fluid turbulence: they have non-Gaussian distributions, exhibit extended self-similarity in their scaling, and are spatially multifractal. Furthermore, we find that the rate at which the fields do work on the particles, J · E, is also multifractal, so that magnetic energy is converted to plasma kinetic energy in a manner that is spatially intermittent. This suggests that dissipation in this sense in collisionless reconnection on kinetic scales has an analogue in fluidlike turbulent phenomenology, in that it proceeds via multifractal structures generated by an intermittent cascade.
Polaritonic excitations in aperiodic nanolayers
Vasconcelos, M. S.; Anselmo, Dory Hélio A. L.; Mello, Vamberto D.
2014-03-01
The magnetic polariton propagation in magnonic layered structures is theoretically studied by using a a transfer matrix approach. The layered structures considered here are made up by the periodic and quasiperiodic stacking of two different layers (also known as building blocks, named A and B), where one of them is a ferromagnetic nanofilm (A), while the other is a metamaterial nanolayer (B). For the periodic arrangement, the bulk modes are characterized by two large symmetric bands, with non-reciprocal surface modes between them. The quasiperiodic metamagnetic structure is then built up by following the Fibonacci sequence, whose long range order effect is then investigated, giving rise to an interesting self-similar spectra.
Energy Technology Data Exchange (ETDEWEB)
Votsish, A.D.; Kolesnikov, Yu.B.
1977-01-01
Results are given for an experimental study of two-dimensional turbulent flow with shifts in a plane duct in an azimuthal magnetic field. The turbulent flow was shown to become practically equal to zero in a sufficiently strong field whereas the intensity of the pulsation rate has a finite value. This is explained by the fact that the magnetic field transforms the structure of turbulence into a two-dimensional structure whose maintenance merely requires an insignificant portion of medium flow energy. 4 illustrations, 8 references.
Evidence of low-dimensional chaos in magnetized plasma turbulence
Zivkovic, Tatjana
2008-01-01
We analyze probe data obtained from a toroidal magnetized plasma configuration suitable for studies of low-frequency gradient-driven instabilities. These instabilities give rise to field-aligned convection rolls analogous to Rayleigh-Benard cells in neutral fluids, and may theoretically develop similar routes to chaos. When using mean-field dimension analysis, we observe low dimensionality, but this could originate from either low-dimensional chaos, periodicity or quasi-periodicity. Therefore, we apply recurrence plot analysis as well as estimation of the largest Lyapunov exponent. These analyses provide evidence of low-dimensional chaos, in agreement with theoretical predictions.
On the spatio-temporal behavior of magnetohydrodynamic turbulence in a magnetized plasma
Lugones, R; Mininni, P D; Wan, M; Matthaeus, W H
2016-01-01
Using direct numerical simulations of three-dimensional magnetohydrodynamic (MHD) turbulence the spatio-temporal behavior of magnetic field fluctuations is analyzed. Cases with relatively small, medium and large values of a mean background magnetic field are considered. The (wavenumber) scale dependent time correlation function is directly computed for different simulations, varying the mean magnetic field value. From this correlation function the time decorrelation is computed and compared with different theoretical times, namely, the local non-linear time, the random sweeping time, and the Alfv\\'enic time, the latter being a wave effect. It is observed that time decorrelations are dominated by sweeping effects, and only at large values of the mean magnetic field and for wave vectors mainly aligned with this field time decorrelations are controlled by Alfv\\'enic effects.
Okamoto, Naoya; Yoshimatsu, Katsunori; Schneider, Kai; Farge, Marie
2014-03-01
Small-scale anisotropic intermittency is examined in three-dimensional incompressible magnetohydrodynamic turbulence subjected to a uniformly imposed magnetic field. Orthonormal wavelet analyses are applied to direct numerical simulation data at moderate Reynolds number and for different interaction parameters. The magnetic Reynolds number is sufficiently low such that the quasistatic approximation can be applied. Scale-dependent statistical measures are introduced to quantify anisotropy in terms of the flow components, either parallel or perpendicular to the imposed magnetic field, and in terms of the different directions. Moreover, the flow intermittency is shown to increase with increasing values of the interaction parameter, which is reflected in strongly growing flatness values when the scale decreases. The scale-dependent anisotropy of energy is found to be independent of scale for all considered values of the interaction parameter. The strength of the imposed magnetic field does amplify the anisotropy of the flow.
Dynamics of Ion Temperature Gradient Turbulence and Transport with a Static Magnetic Island
Izacard, Olivier; James, Spencer D; Brennan, Dylan P
2015-01-01
The quantification of the interaction mechanism between large-scale magnetohydrodynamics instabilities and small-scale drift-wave microturbulence is essential for predicting and optimizing the performance of magnetic confinement based fusion energy experiments. We report progress on understanding these interactions using both analytic theory and numerical simulation, with BOUT++ [B. Dudson et al., Comput. Phys. Comm. 180, 1467 (2009)] used to evolve simple five-field fluid models in a sheared slab geometry. This work focuses upon understanding the dynamics of the ion temperature gradient instability in the presence of a background static magnetic island in a weakly electromagnetic two-dimensional five-field model as key parameters such as ion temperature gradient, magnetic gradients and static magnetic island size are varied. The simulation results are then used to calculate the effective turbulent transport coefficient (i.e. resistivity) that is compared against classical coefficient. As part of this work, t...
Magnetically dominated structures as an important component of the solar wind turbulence
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R. Bruno
2007-08-01
Full Text Available This study focuses on the role that magnetically dominated fluctuations have within the solar wind MHD turbulence. It is well known that, as the wind expands, magnetic energy starts to dominate over kinetic energy but we lack of a statistical study apt to estimate the relevance of these fluctuations depending on wind speed, radial distance from the sun and heliographic latitude. Our results suggest that this kind of fluctuations can be interpreted as non-propagating structures, advected by the wind during its expansion. In particular, observations performed in the ecliptic revealed a clear radial dependence of these magnetic structures within fast wind, but not within slow wind. At short heliocentric distances (~0.3 AU the turbulent population is largely dominated by Alfvénic fluctuations characterized by high values of normalized cross-helicity and a remarkable level of energy equipartition. However, as the wind expands, a new-born population, characterized by lower values of Alfvénicity and a clear imbalance in favor of magnetic energy becomes visible and clearly distinguishable from the Alfvénic population largely characterized by an outward sense of propagation. We estimate that more than 20% of all the analyzed intervals of hourly scale within fast wind are characterized by normalized cross-helicity close to zero and magnetic energy largely dominating over kinetic energy. Most of these advected magnetic structures result to be non-compressive and might represent the crossing of the border between adjacent flux tubes forming, as suggested in literature, the advected background structure of the interplanetary magnetic field. On the other hand, their features are also well fitted by the Magnetic Field Directional Turnings paradigm as proposed in literature.
Energy Technology Data Exchange (ETDEWEB)
Burrell, K.H.
1996-11-01
One of the scientific success stories of fusion research over the past decade is the development of the ExB shear stabilization model to explain the formation of transport barriers in magnetic confinement devices. This model was originally developed to explain the transport barrier formed at the plasma edge in tokamaks after the L (low) to H (high) transition. This concept has the universality needed to explain the edge transport barriers seen in limiter and divertor tokamaks, stellarators, and mirror machines. More recently, this model has been applied to explain the further confinement improvement from H (high)-mode to VH (very high)-mode seen in some tokamaks, where the edge transport barrier becomes wider. Most recently, this paradigm has been applied to the core transport barriers formed in plasmas with negative or low magnetic shear in the plasma core. These examples of confinement improvement are of considerable physical interest; it is not often that a system self-organizes to a higher energy state with reduced turbulence and transport when an additional source of free energy is applied to it. The transport decrease that is associated with ExB velocity shear effects also has significant practical consequences for fusion research. The fundamental physics involved in transport reduction is the effect of ExB shear on the growth, radial extent and phase correlation of turbulent eddies in the plasma. The same fundamental transport reduction process can be operational in various portions of the plasma because there are a number ways to change the radial electric field Er. An important theme in this area is the synergistic effect of ExB velocity shear and magnetic shear. Although the ExB velocity shear appears to have an effect on broader classes of microturbulence, magnetic shear can mitigate some potentially harmful effects of ExB velocity shear and facilitate turbulence stabilization.
Schober, Jennifer; Federrath, Christoph; Klessen, Ralf; Banerjee, Robi
2011-01-01
The small-scale dynamo is a process by which turbulent kinetic energy is converted into magnetic energy, and thus is expected to depend crucially on the nature of turbulence. In this work, we present a model for the small-scale dynamo that takes into account the slope of the turbulent velocity spectrum v(l) ~ l^theta, where l and v(l) are the size of a turbulent fluctuation and the typical velocity on that scale. The time evolution of the fluctuation component of the magnetic field, i.e., the small-scale field, is described by the Kazantsev equation. We solve this linear differential equation for its eigenvalues with the quantum-mechanical WKB-approximation. The validity of this method is estimated as a function of the magnetic Prandtl number Pm. We calculate the minimal magnetic Reynolds number for dynamo action, Rm_crit, using our model of the turbulent velocity correlation function. For Kolmogorov turbulence (theta=1/3), we find that the critical magnetic Reynolds number is approximately 110 and for Burger...
Pseudochaos and low-frequency percolation scaling for turbulent diffusion in magnetized plasma.
Milovanov, Alexander V
2009-04-01
The basic physics properties and simplified model descriptions of the paradigmatic "percolation" transport in low-frequency electrostatic (anisotropic magnetic) turbulence are theoretically analyzed. The key problem being addressed is the scaling of the turbulent diffusion coefficient with the fluctuation strength in the limit of slow fluctuation frequencies (large Kubo numbers). In this limit, the transport is found to exhibit pseudochaotic, rather than simply chaotic, properties associated with the vanishing Kolmogorov-Sinai entropy and anomalously slow mixing of phase-space trajectories. Based on a simple random-walk model, we find the low-frequency percolation scaling of the turbulent diffusion coefficient to be given by D/omega proportional, variantQ;{2/3} (here Q1 is the Kubo number and omega is the characteristic fluctuation frequency). When the pseudochaotic property is relaxed, the percolation scaling is shown to cross over to Bohm scaling. The features of turbulent transport in the pseudochaotic regime are described statistically in terms of a time fractional diffusion equation with the fractional derivative in the Caputo sense. Additional physics effects associated with finite particle inertia are considered.
Radiation from relativistic shocks with turbulent magnetic fields
Nishikawa, K -I; Medvedev, M; Zhang, B; Hardee, P; Nordlund, A; Frederiksen, J; Mizuno, Y; Sol, H; Pohl, M; Hartmann, D H; Oka, M; Fishman, G J
2009-01-01
Using our new 3-D relativistic electromagnetic particle (REMP) code parallelized with MPI, we investigated long-term particle acceleration associated with a relativistic electron-positron jet propagating in an unmagnetized ambient electron-positron plasma. The simulations were performed using a much longer simulation system than our previous simulations in order to investigate the full nonlinear stage of the Weibel instability and its particle acceleration mechanism. Cold jet electrons are thermalized and ambient electrons are accelerated in the resulting shocks. Acceleration of ambient electrons leads to a maximum ambient electron density three times larger than the original value. Behind the bow shock in the jet shock strong electromagnetic fields are generated. These fields may lead to time dependent afterglow emission. We calculated radiation from electrons propagating in a uniform parallel magnetic field to verify the technique. We also used the new technique to calculate emission from electrons based on...
The Interplay of Turbulence & Magnetic Fields in Star-Forming Regions: Simulations and Observations
Kirk, H; Basu, Shantanu
2009-01-01
We analyze a suite of thin sheet magnetohydrodynamical simulations based on the formulation of Basu, Ciolek, Dapp & Wurster. These simulations allow us to examine the observational consequences to a star-forming region of varying the input level of turbulence (between thermal and a Mach number of 4) and the initial magnetic field strength corresponding to a range of mass to flux ratios between subcritical (mu_0=0.5) and supercritical (mu_0=10). The input turbulence is allowed to decay over the duration of the simulation. We compare the measured observable quantities with those found from surveying the Perseus molecular cloud. We find that only the most turbulent of simulations (high Mach number and weak magnetic field) have sufficient large-scale velocity dispersion (at ~1 pc) to match that observed across extinction regions in Perseus. Generally, the simulated core (~0.02 pc) and line of sight velocity dispersions provide a decent match to observations. The motion between the simulated core and its local...
McIntosh, Scott W; Threlfall, James; De Moortel, Ineke; Leamon, Robert J; Tian, Hui
2013-01-01
The presence of turbulent phenomena in the outer solar atmosphere is a given. However, because we are reduced to remotely sensing the atmosphere of a star with instruments of limited spatial and/or spectral resolution, we can only infer the physical progression from macroscopic to microscopic phenomena. Even so, we know that many, if not all, of the turbulent phenomena that pervade interplanetary space have physical origins at the Sun and so in this brief article we consider some recent measurements which point to sustained potential source(s) of heliospheric turbulence in the magnetic and thermal domains. In particular, we look at the scales of magnetism that are imprinted on the outer solar atmosphere by the relentless magneto-convection of the solar interior and combine state-of-the-art observations from the Solar Dynamics Observatory (SDO) and the Coronal Multi-channel Polarimeter (CoMP) which are beginning to hint at the origins of the wave/plasma interplay prevalent closer to the Earth. While linking th...
Kinetic-Scale Magnetic Turbulence and Finite Larmor Radius Effects at Mercury
Uritsky, V. M.; Slavin, J. A.; Khazanov, G. V.; Donovan, E. F.; Boardsen, S. A.; Anderson, B. J.; Korth, H.
2011-01-01
We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near-Mercury space environment, with the emphasis on key boundary regions participating in the solar wind - magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable timescale (approx.20 s) imposed by the signal nonstationariry, suggesting that turbulence at this plane I is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.
Tzeferacos, Petros; Fatenejad, Milad; Flocke, Norbert; Graziani, Carlo; Gregori, Gianluca; Lamb, Donald; Lee, Dongwook; Meinecke, Jena; Scopatz, Anthony; Weide, Klaus
2014-10-01
In this study we present high-resolution numerical simulations of laboratory experiments that study the turbulent amplification of magnetic fields generated by laser-driven colliding jets. The radiative magneto-hydrodynamic (MHD) simulations discussed here were performed with the FLASH code and have assisted in the analysis of the experimental results obtained from the Vulcan laser facility. In these experiments, a pair of thin Carbon foils is placed in an Argon-filled chamber and is illuminated to create counter-propagating jets. The jets carry magnetic fields generated by the Biermann battery mechanism and collide to form a highly turbulent region. The interaction is probed using a wealth of diagnostics, including induction coils that are capable of providing the field strength and directionality at a specific point in space. The latter have revealed a significant increase in the field's strength due to turbulent amplification. Our FLASH simulations have allowed us to reproduce the experimental findings and to disentangle the complex processes and dynamics involved in the colliding flows. This work was supported in part at the University of Chicago by DOE NNSA ASC.
Constraining regular and turbulent magnetic field strengths in M51 via Faraday depolarization
Shneider, Carl; Fletcher, Andrew; Shukurov, Anvar
2014-01-01
We employ an analytical model that incorporates both wavelength-dependent and wavelength-independent depolarization to describe radio polarimetric observations of polarization at $\\lambda \\lambda \\lambda \\, 3.5, 6.2, 20.5$ cm in M51 (NGC 5194). The aim is to constrain both the regular and turbulent magnetic field strengths in the disk and halo, modeled as a two- or three-layer magneto-ionic medium, via differential Faraday rotation and internal Faraday dispersion, along with wavelength-independent depolarization arising from turbulent magnetic fields. A reduced chi-squared analysis is used for the statistical comparison of predicted to observed polarization maps to determine the best-fit magnetic field configuration at each of four radial rings spanning $2.4 - 7.2$ kpc in $1.2$ kpc increments. We find that a two-layer modeling approach provides a better fit to the observations than a three-layer model, where the near and far sides of the halo are taken to be identical, although the resulting best-fit magnetic...
Kolmogorov-Sinai entropy in field line diffusion by anisotropic magnetic turbulence
Energy Technology Data Exchange (ETDEWEB)
Milovanov, Alexander V [Associazione Euratom-ENEA sulla Fusione, Centro Ricerche Frascati, Via E. Fermi 45, C.P. 65, I-00044 Frascati, Rome (Italy); Bitane, Rehab [Laboratoire Cassiopee, UNSA, CNRS, Observatoire de la Cote d' Azur, BP 4229, 06304 Nice Cedex 4 (France); Zimbardo, Gaetano [Dipartimento di Fisica, Universita degli Studi della Calabria, Ponte P. Bucci, Cubo 31C, I-87036 Arcavacata di Rende (Italy)
2009-07-15
The Kolmogorov-Sinai (KS) entropy in turbulent diffusion of magnetic field lines is analyzed on the basis of a numerical simulation model and theoretical investigations. In the parameter range of strongly anisotropic magnetic turbulence the KS entropy is shown to deviate considerably from the earlier predicted scaling relations (1992 Rev. Mod. Phys. 64 961). In particular, a slowing down logarithmic behavior versus the so-called Kubo number R >> 1 (R = ({delta}B/B{sub 0}) ({xi}{sub ||}/{xi}{sub perpendicular}), where {delta}B/B{sub 0} is the ratio of the rms magnetic fluctuation field to the magnetic field strength, and {xi}{sub perpendicular} and {xi}{sub ||} are the correlation lengths in respective dimensions) is found instead of a power-law dependence. These discrepancies are explained from general principles of Hamiltonian dynamics. We discuss the implication of Hamiltonian properties in governing the paradigmatic 'percolation' transport, characterized by R {yields} {infinity}, associating it with the concept of pseudochaos (random non-chaotic dynamics with zero Lyapunov exponents). Applications of this study pertain to both fusion and astrophysical plasma and by mathematical analogy to problems outside the plasma physics.
Hull, Charles L. H.; Mocz, Philip; Burkhart, Blakesley K.; Miquel Girart, Josep; Goodman, Alyssa A.; Cortes, Paulo; Li, Zhi-Yun; Lai, Shih-Ping; Hernquist, Lars; Springel, Volker
2017-01-01
The first polarization data from ALMA have been delivered, and are both expanding and confounding our understanding of the role of magnetic fields in low-mass star formation. Here I will show the highest resolution and highest sensitivity polarization images ever made of a Class 0 protostellar source. These new ALMA observations of the source, known as Ser-emb 8, achieve 140 AU resolution, allowing us to probe polarization -- and thus magnetic field orientation -- in the innermost regions surrounding the protostar. The collapse of strongly magnetized dense gas is predicted to pinch the magnetic field into an hourglass shape that persists down to scales <100 AU. However, in contrast with more than 50 years of theory, the ALMA data definitively rule out an hourglass morphology and instead reveal a chaotic magnetic field that has not been inherited from the field in the interstellar medium surrounding the source. We have simulated the star formation process with cutting-edge, moving-mesh AREPO simulations on scales from a million AU (5 pc) down to 60 AU. We find that only in the case of a very strong magnetic field (~100 microgauss on 5 pc scales) is the field direction preserved from cloud to disk scales. When the field is weak, turbulence in the interstellar gas shapes the field on large scales, and the forming star system re-shapes the field again on small scales, divorcing the field from its history on larger scales. We conclude that this is what has happened in Ser-emb 8. The main distinction from the strong-field star formation model is that in the weak-field case it is turbulence -- not the magnetic field -- that shapes the material that forms the protostar.
Magnetic turbulent spectra in the magnetosheath: new insights
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F. Sahraoui
2004-06-01
Full Text Available The spectrum of the magnetic fluctuations measured by the Cluster satellites in the inner magnetosheath is investigated using the k-filtering technique. On a case study, it is shown first that the wave vectors calculated from the Flux Gate Magnetometer (FGM data fit well with those determined from the Spatio-Temporal Analysis of Field Fluctuations (STAFF data for their common range of frequency, which allows one to confirm that the high pass filter applied to STAFF data does not alter the spatial characteristics of its spectra. Both analyses confirm the dominance of the mirror mode for frequencies up to 1.4Hz. Furthermore, by comparing the experimental charateristics of the identified mirror mode to the prediction of the linear theory, it is shown that the predicted maximum growth rate is observed in the frequency range 0-0.15Hz, i.e. the FGM range. All the rest of the mirror mode, identified for higher frequencies is more likely to be a non linear extension of the most instable one. This cascade on the spatial scales is, in turn, observed in the satellite frame as a temporal spread due to Doppler shift. Further implications on the real nature of the observed spectrum are discussed.
FORMING CHONDRITES IN A SOLAR NEBULA WITH MAGNETICALLY INDUCED TURBULENCE
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Hasegawa, Yasuhiro; Turner, Neal J.; Masiero, Joseph [Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 (United States); Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi, E-mail: yasuhiro@caltech.edu [Center for Computational Astrophysics, National Astronomical Observatory of Japan, Osawa, Mitaka, Tokyo 181-8588 (Japan)
2016-03-20
Chondritic meteorites provide valuable opportunities to investigate the origins of the solar system. We explore impact jetting as a mechanism of chondrule formation and subsequent pebble accretion as a mechanism of accreting chondrules onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the solar nebula is ≤5 times more massive than the minimum-mass solar nebula at a ≃ 2–3 au and parent bodies of chondrites are ≤10{sup 24} g (≤500 km in radius) in the solar nebula, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, the formation timescale of chondrules, and the magnetic field strength of the nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies’ masses leads to the following implications: primordial asteroids that were originally ≥10{sup 24} g in mass were unlikely to contain chondrules, while less massive primordial asteroids likely had a chondrule-rich surface layer. The scenario developed from impact jetting and pebble accretion can therefore provide new insights into the origins of the solar system.
Cameron, R. H.; Schüssler, M.
2016-06-01
Context. In order to match observed properties of the solar cycle, flux-transport dynamo models require the toroidal magnetic flux to be stored in a region of low magnetic diffusivity, typically located at or below the bottom of the convection zone. Aims: We infer the turbulent magnetic diffusivity affecting the toroidal field on the basis of empirical data. Methods: We considered the time evolution of mean latitude and width of the activity belts of solar cycles 12-23 and their dependence on cycle strength. We interpreted the decline phase of the cycles as a diffusion process. Results: The activity level of a given cycle begins to decline when the centers of its equatorward propagating activity belts come within their (full) width (at half maximum) from the equator. This happens earlier for stronger cycles because their activity belts are wider. From that moment on, the activity and the belt width decrease in the same manner for all cycles, independent of their maximum activity level. In terms of diffusive cancellation of opposite-polarity toroidal flux across the equator, we infer the turbulent diffusivity experienced by the toroidal field, wherever it is located, to be in the range 150-450 km2 s-1. Strong diffusive latitudinal spreading of the toroidal flux underneath the activity belts can be inhibited by an inflow toward the toroidal field bands in the convection zone with a magnitude of several meters per second. Conclusions: The inferred value of the turbulent magnetic diffusivity affecting the toroidal field agrees, to order of magnitude, with estimates based on mixing-length models for the solar convection zone. This is at variance with the requirement of flux-transport dynamo models. The inflows required to keep the toroidal field bands together before they approach the equator are similar to the inflows toward the activity belts observed with local helioseismology.
Experimental Investigation of Active Feedback Control of Turbulent Transport in a Magnetized Plasma
Energy Technology Data Exchange (ETDEWEB)
Gilmore, Mark Allen [University of New Mexico
2013-07-07
A new and unique basic plasma science laboratory device - the HelCat device (HELicon-CAThode) - has been constructed and is operating at the University of New Mexico. HelCat is a 4 m long, 0.5 m diameter device, with magnetic field up to 2.2 kG, that has two independent plasmas sources - an RF helicon source, and a thermionic cathode. These two sources, which can operate independently or simultaneously, are capable of producing plasmas with a wide range of parameters and turbulence characteristics, well suited to a variety of basic plasma physics experiments. An extensive set of plasma diagnostics is also operating. Experiments investigating the active feedback control of turbulent transport of particles and heat via electrode biasing to affect plasma ExB flows are underway, and ongoing.
Radio Synchrotron Fluctuation Statistics as a Probe of Magnetized Interstellar Turbulence
Herron, C A; Lazarian, A; Gaensler, B M; McClure-Griffiths, N M
2016-01-01
We investigate how observations of synchrotron intensity fluctuations can be used to probe the sonic and Alfv\\'enic Mach numbers of interstellar turbulence, based on mock observations performed on simulations of magnetohydrodynamic turbulence. We find that the structure function slope, and a diagnostic of anisotropy that we call the integrated quadrupole ratio modulus, both depend on the Alfv\\'enic Mach number. However, these statistics also depend on the orientation of the mean magnetic field in the synchrotron emitting region relative to our line of sight, and this creates a degeneracy that cannot be broken by observations of synchrotron intensity alone. We conclude that the polarization of synchrotron emission could be analyzed to break this degeneracy, and suggest that this will be possible with the Square Kilometre Array.
Periodic and aperiodic synchronization in skilled action
Directory of Open Access Journals (Sweden)
Fred eCummins
2011-12-01
Full Text Available Synchronized action is considered as a manifestation of shared skill. Most synchronized behaviors in humans and other animals are based on periodic repetition. Aperiodic synchronization of complex action is found in the experimental task of synchronous speaking, in which naive subjects read a common text in lock step. The demonstration of synchronized behavior without a periodic basis is presented as a challenge for theoretical understanding. A unified treatment of periodic and aperiodic synchronization is suggested by replacing the sequential processing model of cognitivist approaches with the more local notion of a task-specific sensorimotor coordination. On this view, skilled action is the imposition of constraints on the co-variation of movement and sensory flux such that the boundary conditions that define the skill are met. This non-cognitivist approach originates in the work of John Dewey. It allows a unification of the treatment of sensorimotor synchronization in simple rhythmic behavior and in complex skilled behavior and it suggests that skill sharing is a uniquely human trait of considerable import.
Particle Diffusion and Acceleration by Shock Wave in Magnetized Filamentary Turbulence
Honda, M; Honda, Mitsuru; Honda, Yasuko S.
2005-01-01
We expand the off-resonant scattering theory for particle diffusion in magnetized current filaments that can be typically compared to astrophysical jets, including active galactic nucleus jets. In a high plasma beta region where the directional bulk flow is a free-energy source for establishing turbulent magnetic fields via current filamentation instabilities, a novel version of quasi-linear theory to describe the diffusion of test particles is proposed. The theory relies on the proviso that the injected energetic particles are not trapped in the small-scale structure of magnetic fields wrapping around and permeating a filament but deflected by the filaments, to open a new regime of the energy hierarchy mediated by a transition compared to the particle injection. The diffusion coefficient derived from a quasi-linear type equation is applied to estimating the timescale for the stochastic acceleration of particles by the shock wave propagating through the jet. The generic scalings of the achievable highest ener...
Development of a long pulse plasma gun discharge for magnetic turbulence studies
Schaffner, David
2016-10-01
A long pulse ( 300 μs) plasma gun discharge is in development at the Bryn Mawr College Plasma Laboratory for the production of sustained magnetized plasma injection for magnetohydrodynamic (MHD) turbulence studies. An array of eight 0.5mF parallel capacitors are used to create a pulse-forming-network (PFN) with a plateaued current output of 50kA for at least 200 of the 300 μs pulse. A 24cm inner diameter plasma gun provides stuffing flux fields at the stuffing threshold in order to allow for the continuous injection of magnetic helicity. Plasma is injected into a 24cm diameter flux-conserving aluminum chamber with a high density port array for fine spatial resolution diagnostic access. Fluctuations of magnetic field and saturation current are measured using pickup probes and Langmuir probes respectively.
Muñoz-Jaramillo, Andrés; Martens, Petrus C H
2010-01-01
The turbulent magnetic diffusivity in the solar convection zone is one of the most poorly constrained ingredients of mean-field dynamo models. This lack of constraint has previously led to controversy regarding the most appropriate set of parameters, as different assumptions on the value of turbulent diffusivity lead to radically different solar cycle predictions. Typically, the dynamo community uses double step diffusivity profiles characterized by low values of diffusivity in the bulk of the convection zone. However, these low diffusivity values are not consistent with theoretical estimates based on mixing-length theory -- which suggest much higher values for turbulent diffusivity. To make matters worse, kinematic dynamo simulations cannot yield sustainable magnetic cycles using these theoretical estimates. In this work we show that magnetic cycles become viable if we combine the theoretically estimated diffusivity profile with magnetic quenching of the diffusivity. Furthermore, we find that the main featur...
Directory of Open Access Journals (Sweden)
A. Greco
2000-01-01
Full Text Available The ion dynamics in the distant Earth's magnetotail is studied in the case that a cross tail electric field and reconnection parity magnetic turbulence are present in the neutral sheet. A test particle simulation is performed for the ions, and moments of the ion distribution function are obtained as a function of the magnetic fluctuation level, δB/B0, and of the value of the cross tail electric field, Ey. It is found that magnetic turbulence can split the current carrying region into a double current sheet, in agreement with inferences from observations in the distant magnetotail. The problem of ion conductivity is addressed by varying the value of the cross tail electric field from zero to the observed one: we find that Ohm's law is not enforced, and that a non local, system dependent conductivity is necessary to describe the ion response to the electric field. Also, it appears that the relation between current and electric field may be nonlinear.
HF wave propagation and induced ionospheric turbulence in the magnetic equatorial region
Eliasson, B.; Papadopoulos, K.
2016-03-01
The propagation and excitation of artificial ionospheric turbulence in the magnetic equatorial region by high-frequency electromagnetic (EM) waves injected into the overhead ionospheric layer is examined. EM waves with ordinary (O) mode polarization reach the critical layer only if their incidence angle is within the Spitze cone. Near the critical layer the wave electric field is linearly polarized and directed parallel to the magnetic field lines. For large enough amplitudes, the O mode becomes unstable to the four-wave oscillating two-stream instability and the three-wave parametric decay instability driving large-amplitude Langmuir and ion acoustic waves. The interaction between the induced Langmuir turbulence and electrons located within the 50-100 km wide transmitter heating cone at an altitude of 230 km can potentially accelerate the electrons along the magnetic field to several tens to a few hundreds of eV, far beyond the thresholds for optical emissions and ionization of the neutral gas. It could furthermore result in generation of shear Alfvén waves such as those recently observed in laboratory experiments at the University of California, Los Angeles Large Plasma Device.
The Link Between Shocks, Turbulence, and Magnetic Reconnection in Collisionless Plasmas
Karimabadi, H.; Roytershteyn, V.; Vu, H. X.; Omelchenko, Y. A.; Scudder, J.; Daughton, W.; Dimmock, A.; Nykyri, K.; Wan, M.; Sibeck, D.; Tatineni, M.; Majumdar, A.; Loring, B.; Geveci, B.
2014-01-01
Global hybrid (electron fluid, kinetic ions) and fully kinetic simulations of the magnetosphere have been used to show surprising interconnection between shocks, turbulence and magnetic reconnection. In particular collisionless shocks with their reflected ions that can get upstream before retransmission can generate previously unforeseen phenomena in the post shocked flows: (i) formation of reconnecting current sheets and magnetic islands with sizes up to tens of ion inertial length. (ii) Generation of large scale low frequency electromagnetic waves that are compressed and amplified as they cross the shock. These 'wavefronts' maintain their integrity for tens of ion cyclotron times but eventually disrupt and dissipate their energy. (iii) Rippling of the shock front, which can in turn lead to formation of fast collimated jets extending to hundreds of ion inertial lengths downstream of the shock. The jets, which have high dynamical pressure, 'stir' the downstream region, creating large scale disturbances such as vortices, sunward flows, and can trigger flux ropes along the magnetopause. This phenomenology closes the loop between shocks, turbulence and magnetic reconnection in ways previously unrealized. These interconnections appear generic for the collisionless plasmas typical of space, and are expected even at planar shocks, although they will also occur at curved shocks as occur at planets or around ejecta.
Stochastic acceleration by multi-island contraction during turbulent magnetic reconnection.
Bian, Nicolas H; Kontar, Eduard P
2013-04-12
The acceleration of charged particles in magnetized plasmas is considered during turbulent multi-island magnetic reconnection. The particle acceleration model is constructed for an ensemble of islands which produce adiabatic compression of the particles. The model takes into account the statistical fluctuations in the compression rate experienced by the particles during their transport in the acceleration region. The evolution of the particle distribution function is described as a simultaneous first- and second-order Fermi acceleration process. While the efficiency of the first-order process is controlled by the average rate of compression, the second-order process involves the variance in the compression rate. Moreover, the acceleration efficiency associated with the second-order process involves both the Eulerian properties of the compression field and the Lagrangian properties of the particles. The stochastic contribution to the acceleration is nonresonant and can dominate the systematic part in the case of a large variance in the compression rate. The model addresses the role of the second-order process, how the latter can be related to the large-scale turbulent transport of particles, and explains some features of the numerical simulations of particle acceleration by multi-island contraction during magnetic reconnection.
Energy Technology Data Exchange (ETDEWEB)
Zimbardo, G.
2005-07-01
Plasma transport in the presence of turbulence depends on a variety of parameters like the fluctuation level ? B/B0, the ratio between the particle Larmor radius and the turbulence correlation lengths, and the turbulence anisotropy. In this presentation, we review the results of numerical simulations of plasma and magnetic field line transport in the case of anisotropic magnetic turbulence, for parameter values close to those of the solar wind. We assume a uniform background magnetic field B0 = B0ez and a Fourier representation for magnetic fluctuations, with wavectors forming any angle with respect to B0. The energy density spectrum is a power law, and in k space the constant amplitude surfaces are ellipsoids, described by the correlation lengths lx, ly, lz, which quantify the anisotropy of turbulence. For magnetic field lines, we find that transport perpendicular to the background field depends on the Kubo number R = ? B B0 lz lx . For small Kubo numbers, R ? 1, we find anomalous, non Gaussian transport regimes (both sub and superdiffusive) which can be described as a Levy random walk. Increasing the Kubo number, i.e., the fluctuation level ? B/B0 and/or the ratio lz/lx, we find first a quasilinear and then a percolative regime, both corresponding to Gaussian diffusion. For particles, we find that transport parallel and perpendicular to the background magnetic field heavily depends on the turbulence anisotropy and on the particle Larmor radius. For turbulence levels typical of the solar wind, ? B/B0 ? 0.5 ?1, when the ratio between the particle Larmor radius and the turbulence correlation lengths is small, anomalous regimes are found in the case lz/lx ? 1, with Levy random walk (superdiffusion) along the magnetic field and subdiffusion in the perpendicular directions. Conversely, for lz/lx > 1 normal, Gaussian diffusion is found. Increasing the ratio between the particle Larmor radius and the turbulence correlation lengths, the parallel superdiffusion is
A study of periodic and aperiodic ferromagnetic antidot lattices
Bhat, Vinayak S.
This thesis reports our study of the effect of domain wall pinning by ferromagnetic (FM) metamaterials [1] in the form of periodic antidot lattices (ADL) on spin wave spectra in the reversible regime. This study was then extended to artificial quasicrystals in the form of Penrose P2 tilings (P2T). Our DC magnetization study of these metamaterials showed reproducible and temperature dependent knee anomalies in the hysteretic regime that are due to the isolated switching of the FM segments. Our dumbbell model analysis [2] of simulated magnetization maps indicates that FM switching in P2T is nonstochastic . We have also acquired the first direct, two-dimensional images of the magnetization of Permalloy films patterned into P2T using scanning electron microscopy with polarization analysis (SEMPA). Our SEMPA images demonstrate P2T behave as geometrically frustrated networks of narrow ferromagnetic film segments having near-uniform, bipolar (Ising-like) magnetization, similar to artificial spin ices (ASI). We find the unique aperiodic translational symmetry and diverse vertex coordination of multiply-connected P2T induce a more complex spin-ice behavior driven by exchange interactions in vertex domain walls, which differs markedly from the behavior of disconnected ASI governed only by dipolar interactions. Keywords: Ferromagnetic Antidot Lattices, Metamaterials, Ferromagnetic Resonance, Artificial Quasicrystal, Artificial Spin Ice. [1] VV Kruglyak et al. "Magnonic metamaterials". In: Metamaterial, edited by X.-Y. Jiang (InTech, 2012) (2012). [2] Claudio Castelnovo, Roderich Moessner, and Shivaji L Sondhi. "Magnetic monopoles in spin ice". In: Nature 451.7174 (2008), pp. 42--45.
Dynamo action in turbulent accretion discs around black holes. 2. The mean magnetic field
Energy Technology Data Exchange (ETDEWEB)
Pudritz, R.E. (British Columbia Univ., Vancouver (Canada). Dept. of Physics)
1981-06-01
The presence of strong differential rotation and vertical density gradients in thin accretion discs allows the generation of a large-scale, mean magnetic field by ..cap alpha.. ..omega.. dynamo action. Analytic expressions for the preferred steady-state solutions are found, which when matched on to an exterior vacuum field show that the class of quadrupole modes (Bsub(phi) and Bsub(r) even, and Bsub(z) odd functions of z) are most easily excited. The critical dynamo number of this mode is Qsub(e,c)sup(1/3) = 2.4, corresponding to a turbulent Mach number of Msub(t)sup(e) = 0.19.
Scaling and anisotropy in magnetohydrodynamic turbulence in a strong mean magnetic field.
Grappin, Roland; Müller, Wolf-Christian
2010-08-01
We present an analysis of the anisotropic spectral energy distribution in incompressible magnetohydrodynamic turbulence permeated by a strong mean magnetic field. The turbulent flow is generated by high-resolution pseudospectral direct numerical simulations with large-scale isotropic forcing. Examining the radial energy distribution for various angles θ with respect to B0 reveals a specific structure which remains hidden when not taking axial symmetry with respect to B0 into account. For each direction, starting at the forced large scales, the spectrum first exhibits an amplitude drop around a wave number k0 which marks the start of a scaling range and goes on up to a dissipative wave number k(d)(θ). The three-dimensional spectrum for k≥k0 is described by a single θ-independent functional form F(k/k(d)), with the scaling law being the same in every direction. The previous properties still hold when increasing the mean field from B0=5 up to B0=10b(rms), as well as when passing from resistive to ideal flows. We conjecture that at fixed B0 the direction-independent scaling regime is reached when increasing the Reynolds number above a threshold which raises with increasing B0. Below that threshold critically balanced turbulence is expected.
Effect of a Magnetic Field on Turbulent Flow in Continuous Casting Mold
Singh, Ramnik; Vanka, Pratap; Thomas, Brian G.
2012-11-01
Electromagnetic Braking (EMBr) fields are applied to control the turbulent mold flow for defect reduction in continuous steel casting. The effect of EMBr depends on the path of induced electric current which is modified by presence of the highly conducting solidifying shell. The mold geometry is complex involving flow in a high-aspect ratio closed channel with bifurcated jet impinging obliquely on the side walls. The extremely transient nature and the anisotropic behavior of turbulence under the EMBr field make numerical studies challenging. We use large eddy simulations to study effects of EMBr with electrically insulating and conducting boundary conditions. Magnetohydrodynamic equations are solved using a fractional step method with second order spatial and temporal accuracy. The electric potential method is used as magnetic Reynolds number is low for liquid metal flows. The solver was first validated with measurements from scaled GaInSn model and simulations were then performed to study real casters at industrial conditions. Time averaged and transient behaviors of the flow were studied by collecting distributions of mean velocities, turbulent fluctuations and vorticity. The simulations reveal that the electrical boundary conditions have a major effect on the flow structure. National Science Foundation Grant CMMI 11-30882.
Feedback Regulated Turbulence, Magnetic Fields, and Star Formation Rates in Galactic Disks
Kim, Chang-Goo
2015-01-01
We use three-dimensional magnetohydrodynamic (MHD) simulations to investigate the quasi-equilibrium states of galactic disks regulated by star formation feedback. We incorporate effects from massive-star feedback via time-varying heating rates and supernova (SN) explosions. We find that the disks in our simulations rapidly approach a quasi-steady state that satisfies vertical dynamical equilibrium. The star formation rate (SFR) surface density self-adjusts to provide the total momentum flux (pressure) in the vertical direction that matches the weight of the gas. We quantify feedback efficiency by measuring feedback yields, \\eta_c\\equiv P_c/\\Sigma_SFR (in suitable units), for each pressure component. The turbulent and thermal feedback yields are the same for HD and MHD simulations, \\eta_th~1 and \\eta_ turb~4, consistent with the theoretical expectations. In MHD simulations, turbulent magnetic fields are rapidly generated by turbulence, and saturate at a level corresponding to \\eta_mag,t~1. The presence of magn...
Lantz, Jonas; Ebbers, Tino; Karlsson, Matts
2012-11-01
In this study, turbulent kinetic energy (TKE) in an aortic coarctation was studied using both a numerical technique (large eddy simulation, LES) and in vivo measurements using magnetic resonance imaging (MRI). High levels of TKE are undesirable, as kinetic energy is extracted from the mean flow to feed the turbulent fluctuations. The patient underwent surgery to widen the coarctation, and the flow before and after surgery was computed and compared to MRI measurements. The resolution of the MRI was about 7 × 7 voxels in axial cross-section while 50x50 mesh cells with increased resolution near the walls was used in the LES simulation. In general, the numerical simulations and MRI measurements showed that the aortic arch had no or very low levels of TKE, while elevated values were found downstream the coarctation. It was also found that TKE levels after surgery were lowered, indicating that the diameter of the constriction was increased enough to decrease turbulence effects. In conclusion, both the numerical simulation and MRI measurements gave very similar results, thereby validating the simulations and suggesting that MRI measured TKE can be used as an initial estimation in clinical practice, while LES results can be used for detailed quantification and further research of aortic flows.
Break of universality for an Ising model with aperiodic Rudin-Shapiro interactions
Andrade, R. F. S.; Pinho, S. T. R.
2003-08-01
We analyze the ferromagnetic Ising model on non-Euclidean scale invariant lattices with aperiodic interactions ( J A , J B , J C , J D ) defined by Rudin-Shapiro substitution rules with Migdal-Kadanoff renormalization (MKR) and transfer matrix (TM) techniques. The analysis of the invariant sets of the zero-field MKR transformation indicates that the critical behavior, completely distinct from the one of the uniform model, is described by a new off-diagonal fixed point. This contrasts with other aperiodic models where the new critical behavior is described by a period-two cycle. With the new fixed point, values for the thermal critical exponents, α and ν, as well as the period of log-periodic oscillations, are obtained. Exact recursive maps for all thermodynamical functions are derived within the TM approach. The explicit dependence of the thermodynamical functions with respect to temperature is evaluated by the numerical iteration of the set of maps until a previously chosen convergence is achieved. They also indicate that, depending on the actual choice for the aperiodic coupling constants, the magnetic exponents (β and γ) assume different values. However the Rushbrook relation is always satisfied.
Hidden degrees of freedom in aperiodic materials.
Toudic, Bertrand; Garcia, Pilar; Odin, Christophe; Rabiller, Philippe; Ecolivet, Claude; Collet, Eric; Bourges, Philippe; McIntyre, Garry J; Hollingsworth, Mark D; Breczewski, Tomasz
2008-01-04
Numerous crystalline materials, including those of bioorganic origin, comprise incommensurate sublattices whose mutual arrangement is described in a superspace framework exceeding three dimensions. We report direct observation by neutron diffraction of superspace symmetry breaking in a solid-solid phase transition of an incommensurate host-guest system: the channel inclusion compound of nonadecane/urea. Strikingly, this phase transition generates a unit cell doubling that concerns only the modulation of one substructure by the other-an internal variable available only in superspace. This unanticipated pathway for degrees of freedom to rearrange leads to a second phase transition, which again is controlled by the higher dimensionality of superspace. These results reveal nature's capacity to explore the increased number of phases allowed in aperiodic crystals.
Phase-coded pulse aperiodic transmitter coding
Directory of Open Access Journals (Sweden)
I. I. Virtanen
2009-07-01
Full Text Available Both ionospheric and weather radar communities have already adopted the method of transmitting radar pulses in an aperiodic manner when measuring moderately overspread targets. Among the users of the ionospheric radars, this method is called Aperiodic Transmitter Coding (ATC, whereas the weather radar users have adopted the term Simultaneous Multiple Pulse-Repetition Frequency (SMPRF. When probing the ionosphere at the carrier frequencies of the EISCAT Incoherent Scatter Radar facilities, the range extent of the detectable target is typically of the order of one thousand kilometers – about seven milliseconds – whereas the characteristic correlation time of the scattered signal varies from a few milliseconds in the D-region to only tens of microseconds in the F-region. If one is interested in estimating the scattering autocorrelation function (ACF at time lags shorter than the F-region correlation time, the D-region must be considered as a moderately overspread target, whereas the F-region is a severely overspread one. Given the technical restrictions of the radar hardware, a combination of ATC and phase-coded long pulses is advantageous for this kind of target. We evaluate such an experiment under infinitely low signal-to-noise ratio (SNR conditions using lag profile inversion. In addition, a qualitative evaluation under high-SNR conditions is performed by analysing simulated data. The results show that an acceptable estimation accuracy and a very good lag resolution in the D-region can be achieved with a pulse length long enough for simultaneous E- and F-region measurements with a reasonable lag extent. The new experiment design is tested with the EISCAT Tromsø VHF (224 MHz radar. An example of a full D/E/F-region ACF from the test run is shown at the end of the paper.
Energy Technology Data Exchange (ETDEWEB)
Santos-Lima, R.; De Gouveia Dal Pino, E. M.; Kowal, G. [Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, R. do Matão, 1226, São Paulo, SP 05508-090 (Brazil); Falceta-Gonçalves, D. [Escola de Artes, Ciências e Humanidades, Universidade de São Paulo, Rua Arlindo Bettio, 1000, São Paulo, SP 03828-000 (Brazil); Lazarian, A. [Department of Astronomy, University of Wisconsin, Madison, WI 53706 (United States); Nakwacki, M. S. [Instituto de Astronomía y Física del Espacio (IAFE), CONICET (Argentina)
2014-02-01
The amplification of magnetic fields (MFs) in the intracluster medium (ICM) is attributed to turbulent dynamo (TD) action, which is generally derived in the collisional-MHD framework. However, this assumption is poorly justified a priori, since in the ICM the ion mean free path between collisions is of the order of the dynamical scales, thus requiring a collisionless MHD description. The present study uses an anisotropic plasma pressure that brings the plasma within a parametric space where collisionless instabilities take place. In this model, a relaxation term of the pressure anisotropy simulates the feedback of the mirror and firehose instabilities, in consistency with empirical studies. Our three-dimensional numerical simulations of forced transonic turbulence, aiming the modeling of the turbulent ICM, were performed for different initial values of the MF intensity and different relaxation rates of the pressure anisotropy. We found that in the high-β plasma regime corresponding to the ICM conditions, a fast anisotropy relaxation rate gives results that are similar to the collisional-MHD model, as far as the statistical properties of the turbulence are concerned. Also, the TD amplification of seed MFs was found to be similar to the collisional-MHD model. The simulations that do not employ the anisotropy relaxation deviate significantly from the collisional-MHD results and show more power at the small-scale fluctuations of both density and velocity as a result of the action of the instabilities. For these simulations, the large-scale fluctuations in the MF are mostly suppressed and the TD fails in amplifying seed MFs.
Federrath, Christoph; Schleicher, Dominik R G; Banerjee, Robi; Klessen, Ralf S
2011-01-01
Cosmic structure formation is characterized by the complex interplay between gravity, turbulence, and magnetic fields. The processes by which gravitational energy is converted into turbulent and magnetic energies, however, remain poorly understood. Here, we show with high-resolution, adaptive-mesh simulations that MHD turbulence is efficiently driven by extracting energy from the gravitational potential during the collapse of a dense gas cloud. Compressible motions generated during the contraction are converted into solenoidal, turbulent motions, leading to a natural energy ratio of E_sol/E_tot of approximately 2/3. We find that the energy injection scale of gravity-driven turbulence is close to the local Jeans scale. If small seeds of the magnetic field are present, they are amplified exponentially fast via the small-scale dynamo process. The magnetic field grows most efficiently on the smallest scales, for which the stretching, twisting, and folding of field lines, and the turbulent vortices are sufficientl...
Debye scale turbulence within the electron diffusion layer during magnetic reconnection
Energy Technology Data Exchange (ETDEWEB)
Jara-Almonte, J.; Ji, H. [Center for Magnetic Self-Organization, Max-Planck/Princeton Center for Plasma Physics, Princeton Plasma Physics Laboratory, Princeton, New Jersey 08543 (United States); Daughton, W. [Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (United States)
2014-03-15
During collisionless, anti-parallel magnetic reconnection, the electron diffusion layer is the region of both fieldline breaking and plasma mixing. Due to the in-plane electrostatic fields associated with collisionless reconnection, the inflowing plasmas are accelerated towards the X-line and form counter-streaming beams within the unmagnetized diffusion layer. This configuration is inherently unstable to in-plane electrostatic streaming instabilities provided that there is sufficient scale separation between the Debye length λ{sub D} and the electron skin depth c/ω{sub pe}. This scale separation has hitherto not been well resolved in kinetic simulations. Using both 2D fully kinetic simulations and a simple linear model, we demonstrate that these in-plane streaming instabilities generate Debye scale turbulence within the electron diffusion layer at electron temperatures relevant to magnetic reconnection both in the magnetosphere and in laboratory experiments.
Wave-vector dependence of magnetic-turbulence spectra in the solar wind.
Narita, Y; Glassmeier, K-H; Sahraoui, F; Goldstein, M L
2010-04-30
Using four-point measurements of the Cluster spacecraft, the energy distribution was determined for magnetic field fluctuations in the solar wind directly in the three-dimensional wave-vector domain in the range |k|
Hydraulic concentration of magnetic fields in the solar photosphere. I - Turbulent pumping
Parker, E. N.
1974-01-01
Observations suggest that most of the magnetic flux through the solar photosphere is concentrated in vertical filaments in the supergranule boundaries. Each filament appears to contain about 3 times 10 to the 18-th power maxwells, in the form of a field of 500 gauss or more, over a diameter of 700 km or less. The magnetic energy density in the filaments is 100 times the observed kinetic energy density of the observed supergranule motions, but comparable to the kinetic energy density of the granules. Force-free field configurations cannot duplicate the observational numbers, nor can such cooling effects as are believed responsible for the intense fields in sunspot umbrae. We point out a simple hydraulic mechanism (turbulent pumping) that appears to account for the observed concentration of fields.
Keohane, J W; Petre, R
1998-01-01
We present a deep X-ray observation of the young Galactic supernova remnant Cas A, acquired with the ROSAT High Resolution Imager. This high dynamic range (232 ks) image reveals low-surface-brightness X-ray structure, which appears qualitatively similar to corresponding radio features. We consider the correlation between the X-ray and radio morphologies and its physical implications. After correcting for the inhomogeneous absorption across the remnant, we performed a point by point (4" resolution) surface brightness comparison between the X-ray and radio images. We find a strong (r = 0.75) log-log correlation, implying an overall relationship of $\\log(\\Sigma_{_{\\rm X-ray}}) \\propto (2.21\\pm0.05) \\times \\log(\\Sigma_{_{\\rm radio}})$. This is consistent with proportionate partition (and possibly equipartition) between the local magnetic field and the hot gas --- implying that Cas A's plasma is fully turbulent and continuously amplifying the magnetic field.
Fromang, S; Terquem, C; De Villiers, J P; Fromang, Sebastien; Balbus, Steven A.; Terquem, Caroline; Villiers, Jean-Pierre De
2004-01-01
We present 3D magnetohydrodynamic (MHD) numerical simulations of the evolution of self--gravitating and weakly magnetized disks with an adiabatic equation of state. Such disks are subject to the development of both the magnetorotational and gravitational instabilities, which transport angular momentum outward. As in previous studies, our hydrodynamical simulations show the growth of strong m=2 spiral structure. This spiral disturbance drives matter toward the central object and disappears when the Toomre parameter Q has increased well above unity. When a weak magnetic field is present as well, the magnetorotational instability grows and leads to turbulence. In that case, the strength of the gravitational stress tensor is lowered by a factor of about~2 compared to the hydrodynamical run and oscillates periodically, reaching very small values at its minimum. We attribute this behavior to the presence of a second spiral mode with higher pattern speed than the one which dominates in the hydrodynamical simulations...
Electrostatic and magnetic measurements of turbulence and transport in Extrap T2
Möller, Anders; Sallander, Eva
1999-10-01
Langmuir probe and magnetic pick-up coil measurements are used to study edge turbulence in the Extrap T2 reversed field pinch. Magnetic fluctuations resonant outside the toroidal field reversal surface are observed where previously only fluctuations in the spectra of potential and electron density and temperature have been measured. Results are presented which imply that these fluctuations are coupled to and also correlated to the internally resonant tearing mode fluctuations. Evidence of coupling between low-frequency (<100 kHz) and high-frequency fluctuations is also presented. The normalized floating potential fluctuations are seen to increase with the edge electron temperature. This causes an increase of the potential and density fluctuation driven transport with the temperature which is faster than linear. These results, in combination, are consistent with a picture where internally resonant fluctuations couple to edge fluctuations through radial heat conduction from the stochastic core to the edge.
GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations
Wu, Benjamin; Nakamura, Fumitaka; Van Loo, Sven; Christie, Duncan; Collins, David
2016-01-01
We investigate giant molecular cloud (GMCs) collisions and their ability to induce gravitational instability and thus star formation. This mechanism may be a major driver of star formation activity in galactic disks. We carry out a series of three dimensional, magnetohydrodynamics (MHD), adaptive mesh refinement (AMR) simulations to study how cloud collisions trigger formation of dense filaments and clumps. Heating and cooling functions are implemented based on photo-dissociation region (PDR) models that span the atomic to molecular transition and can return detailed diagnostic information. The clouds are initialized with supersonic turbulence and a range of magnetic field strengths and orientations. Collisions at various velocities and impact parameters are investigated. Comparing and contrasting colliding and non-colliding cases, we characterize morphologies of dense gas, magnetic field structure, cloud kinematic signatures, and cloud dynamics. We present key observational diagnostics of cloud collisions, e...
The magnetic field and turbulence of the cosmic web measured using a brilliant fast radio burst
Ravi, V; Bailes, M; Bannister, K; Bhandari, S; Bhat, N D R; Burke-Spolaor, S; Caleb, M; Flynn, C; Jameson, A; Johnston, S; Keane, E F; Kerr, M; Tiburzi, C; Tuntsov, A V; Vedantham, H K
2016-01-01
Fast radio bursts (FRBs) are millisecond-duration events thought to originate beyond the Milky Way galaxy. Uncertainty surrounding the burst sources, and their propagation through intervening plasma, has limited their use as cosmological probes. We report on a mildly dispersed (dispersion measure 266.5+-0.1 pc cm^-3), exceptionally intense (120+-30 Jy), linearly polarized, scintillating burst (FRB 150807) that we directly localize to 9 arcmin^2. Based on a low Faraday rotation (12.0+-0.7 rad m^-2), we infer negligible magnetization in the circum-burst plasma and constrain the net magnetization of the cosmic web along this sightline to <21 nG, parallel to the line-of-sight. The burst scintillation suggests weak turbulence in the ionized intergalactic medium.
Cloud core collapse and the role of turbulent magnetic reconnection diffusion
Leão, M R M; Santos-Lima, R; Lazarian, A
2012-01-01
For a molecular cloud clump to form stars some transport of magnetic flux is required from the denser, inner regions to the outer regions of the cloud, otherwise this can prevent the gravitational collapse. Fast magnetic reconnection which takes place in the presence of turbulence can induce a process of reconnection diffusion. This paper continues our numerical study of this process and its implications. In particular, extending our earlier studies of reconnection diffusion in cylindrical clouds we consider more realistic clouds with spherical gravitational potentials (from embedded stars) and also account for the effects of the gas self-gravity. We demonstrate that within our setup reconnection diffusion takes place. We have also derived the conditions under which reconnection diffusion becomes efficient enough to make an initially subcritical cloud clump to become supercritical and collapse. Our results indicate that the formation of a supercritical core is regulated by a complex interplay between gravity,...
Detection of magnetic discontinuities in the dissipation regime of solar wind turbulence
Perri, S.; Goldstein, M. L.; Dorelli, J.; Sahraoui, F.
2012-12-01
Recent spacecraft observations of solar wind magnetic field fluctuations have shown the existence of a cascade of magnetic energy from the scale of the proton Larmor radius ρ_cp, where kinetic properties of ions invalidate fluid approximations, down to the electron Larmor radius ρ_ce, where electrons become demagnetized. This energy cascade has been conjectured to consist of highly oblique kinetic Alfvénic fluctuations (KAW) that are dissipated by proton and electron Landau damping. Analyzing the 450 vec/s resolution data from the STAFF search-coil magnetometer on Cluster, we report, for the first time, evidence for the existence in the solar wind of thin current sheets and discontinuities that exhibit spatial scales that range from the proton Larmor scale down to the electron Larmor scale. In the cases studied, the current sheets are very localized and have an extent between 20-200 km, size that is often close to both the proton Larmor radius and the proton inertial length. These isolated structures appear to be a manifestation of intermittency and may localize sites turbulent dissipation. Furthermore, we compare in-situ observations of thin current sheets and discontinuities in the solar wind at proton scales with results that come from two-dimensional Hall MHD turbulence simulations in the presence of a strong guide field. The initial condition in the simulations is a large scale flux rope structure which breaks down into smaller and smaller current sheets due to the turbulent energy transfer. The comparison shows good qualitative agreement between the properties of the structures observed in Cluster data and the properties of current sheets that arise in the simulations. Our results highlight two competing processes that contribute to the dissipation of solar wind turbulence when the plasma beta is of order unity; viz., kinetic (Landau) damping by protons and electrons and the general tendency of the cascade to form thin current sheets where reconnection and
Fluid-like dissipation of magnetic turbulence at electron scales in the solar wind
Alexandrova, O; Mangeney, A; Grappin, R
2011-01-01
The turbulent spectrum of magnetic fluctuations in the solar wind displays a spectral break at ion characteristic scales. At electron scales the spectral shape is not yet completely established. Here, we perform a statistical study of 102 spectra at plasma kinetic scales, measured by the Cluster/STAFF instrument in the free solar wind. We show that the magnetic spectrum in the high frequency range, [1,400] Hz, has a form similar to what is found in hydrodynamics in the dissipation range ~Ak^(-\\alpha)exp(-kl_d). The dissipation scale l_d is found to be correlated with the electron Larmor radius \\rho_e. The spectral index \\alpha varies in the range [2.2,2.9] and is anti-correlated with l_d, as expected in the case of the balance between the energy injection and the energy dissipation. The coefficient A is found to be proportional to the ion temperature anisotropy, suggesting that local ion instabilities may play some role for the solar wind turbulence at plasma kinetic scales. The exponential spectral shape fou...
Current Structure and Nonideal Behavior at Magnetic Null Points in the Turbulent Magnetosheath
Wendel, D. E.; Adrian, M. L.
2013-01-01
The Poincaré index indicates that the Cluster spacecraft tetrahedron entraps a number of 3-D magnetic nulls during an encounter with the turbulent magnetosheath. Previous researchers have found evidence for reconnection at one of the many filamentary current layers observed by Cluster in this region. We find that many of the entrained nulls are also associated with strong currents. We dissect the current structure of a pair of spiral nulls that may be topologically connected. At both nulls, we find a strong current along the spine, accompanied by a somewhat more modest current perpendicular to the spine that tilts the fan toward the axis of the spine. The current along the fan is comparable to the that along the spine. At least one of the nulls manifests a rotational flow pattern in the fan plane that is consistent with torsional spine reconnection as predicted by theory. These results emphasize the importance of examining the magnetic topology in interpreting the nature of currents and reconnection in 3-D turbulence.
Current structure and nonideal behavior at magnetic null points in the turbulent magnetosheath
Wendel, D. E.; Adrian, M. L.
2013-04-01
The Poincaré index indicates that the Cluster spacecraft tetrahedron entraps a number of 3-D magnetic nulls during an encounter with the turbulent magnetosheath. Previous researchers have found evidence for reconnection at one of the many filamentary current layers observed by Cluster in this region. We find that many of the entrained nulls are also associated with strong currents. We dissect the current structure of a pair of spiral nulls that may be topologically connected. At both nulls, we find a strong current along the spine, accompanied by a somewhat more modest current perpendicular to the spine that tilts the fan toward the axis of the spine. The current along the fan is comparable to the that along the spine. At least one of the nulls manifests a rotational flow pattern in the fan plane that is consistent with torsional spine reconnection as predicted by theory. These results emphasize the importance of examining the magnetic topology in interpreting the nature of currents and reconnection in 3-D turbulence.
Smith-Purcell radiation emission in aperiodic arrays
Saavedra, J. R. M.; Castells-Graells, D.; García de Abajo, F. Javier
2016-07-01
We study the Smith-Purcell light emission produced by electrons moving parallel to linear aperiodic particle arrays. This constitutes a generalization of this type of phenomenon from periodic to aperiodic structures. As in the periodic case, the emission is found to exhibit intense features in its angular and frequency distributions, associated with the condition of constructive interference between the contributions arising from different particles in the array. This condition can also be expressed in terms of momentum conservation involving reciprocal wave-vector transfers from the array. We consider two examples of quasiperiodic and hyperuniform aperiodic arrays that allow us to illustrate this idea. Our study provides insight into the interaction of fast electrons with aperiodic arrays characterized by strong features in reciprocal space, which dominate the electron-array coupling.
Aperiodic quantum XXZ chains: Renormalization-group results
Vieira, André P.
2005-04-01
We report a comprehensive investigation of the low-energy properties of antiferromagnetic quantum XXZ spin chains with aperiodic couplings. We use an adaptation of the Ma-Dasgupta-Hu renormalization-group method to obtain analytical and numerical results for the low-temperature thermodynamics and the ground-state correlations of chains with couplings following several two-letter aperiodic sequences, including the quasiperiodic Fibonacci and other precious-mean sequences, as well as sequences inducing strong geometrical fluctuations. For a given aperiodic sequence, we argue that in the easy-plane anisotropy regime, intermediate between the XX and Heisenberg limits, the general scaling form of the thermodynamic properties is essentially given by the exactly known XX behavior, providing a classification of the effects of aperiodicity on XXZ chains. We also discuss the nature of the ground-state structures and their comparison with the random-singlet phase characteristic of random-bond chains.
New construction of hybrid and aperiodic ordered PBG cavity
Institute of Scientific and Technical Information of China (English)
无
2011-01-01
The band gap of a photonic crystal (PhC) cavity intrinsically avoids HOM problems. In this paper, we present a new PBG structure based on the possible advantage of using hybrid structures and aperiodic lattices. This novel hybrid and aperiodically ordered cavity was designed for apparently higher Q-factor (more than 10300) and achieving large accelerating field gradient. The HOMs in the cavity are able to be absorbed efficiently.
Institute of Scientific and Technical Information of China (English)
LEE; ChunHian
2010-01-01
Direct numerical simulation (DNS) of incompressible magnetohydrodynamic (MHD) turbulent channel flow has been performed under the low magnetic Reynolds number assumption.The velocity-electric field and electric-electric field correlations were studied in the present work for different magnetic field orientations.The Kenjeres-Hanjalic (K-H) model was validated with the DNS data in a term by term manner.The numerical results showed that the K-H model makes good predictions for most components of the velocity-electric field correlations.The mechanisms of turbulence suppression were also analyzed for different magnetic field orientations utilizing the DNS data and the K-H model.The results revealed that the dissipative MHD source term is responsible for the turbulence suppression for the case of streamwise and spanwise magnetic orientation,while the Lorentz force which speeds up the near-wall fluid and decreases the production term is responsible for the turbulence suppression for the case of the wall normal magnetic orientation.
Energy Technology Data Exchange (ETDEWEB)
Zimbardo, Gaetano [Dipartimento di Fisica, Universita della Calabria, Ponte P. Bucci, Cubo 31C, I-87036 Arcavacata di Rende (Italy)
2005-12-15
Plasma transport in the presence of turbulence depends on a variety of parameters such as the fluctuation level, {delta}B/B{sub 0}, the ratio between the particle Larmor radius and the turbulence correlation length, and the turbulence anisotropy. In this paper, we present the results of numerical simulations of plasma and magnetic field line transport in the case of anisotropic magnetic turbulence, for parameter values close to those of the solar wind. We assume a uniform background magnetic field B{sub 0} = B{sub 0} e{sub z} and a Fourier representation for magnetic fluctuations, which includes wavectors oblique with respect to B{sub 0}. The energy density spectrum is a power law, and in k space it is described by the correlation lengths l{sub x}, l{sub y}, l{sub z}, which quantify the anisotropy of turbulence. For magnetic field lines, transport perpendicular to the background field depends on the Kubo number R ({delta}B/B{sub 0}) (l{sub z}/l{sub x}). For small Kubo numbers, R << 1, anomalous, non-Gaussian transport regimes (both sub- and superdiffusive) are found, which can be described as a Levy random walk. Increasing the Kubo number, i.e. the fluctuation level, {delta}B/B{sub 0}, or the ratio l{sub z}/l{sub x}, we find first a quasilinear regime and then a percolative regime, both corresponding to Gaussian diffusion. For particles, we find that transport parallel and perpendicular to the background magnetic field depends heavily on the turbulence anisotropy and on the particle Larmor radius. For turbulence levels typical of the solar wind, {delta}B/B{sub 0}{approx_equal} 0.5-1, when the ratio between the particle Larmor radius and the turbulence correlation lengths is small, anomalous regimes are found in the case l{sub z}/l{sub x} {<=} 1, with a Levy random walk (superdiffusion) along the magnetic field and subdiffusion in the perpendicular directions. Conversely, for l{sub z}/l{sub x} > 1 normal Gaussian diffusion is found. A possible expression for
Magnetic Draping as a Possible Solution to Turbulent Heating of the ICM in Kinetic Mode AGN Feedback
Bambic, Christopher John; Reynolds, Christopher S.; Morsony, Brian
2017-01-01
Recent x-ray measurements of the Perseus Cluster intracluster medium (ICM) by the Hitomi Mission found a velocity dispersion measure of σ ˜ 150 km/s, indicating a large-scale turbulent energy of approximately 4% of the thermal energy. If this energy is transferred to small scales via a turbulent cascade and dissipated as heat, radiative cooling can be offset and the cluster can remain in its observed thermal equilibrium. We investigate the role of AGN feedback in turbulent heating of galaxy clusters. Specifically, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work has shown that this process is inefficient, with less that 1% of the injected energy ending up in turbulence. This inefficiency is primarily due to the fact that the bubbles are shredded apart by hydrodynamic instabilies before they can excite sufficiently strong g-modes. Using a plane-parallel model of the ICM and 3D ideal MHD simulations, we examine the role of a large-scale magnetic field which is able to drape around these rising bubbles, preserving them from hydrodynamic instabilities. We present results for a magnetic field perpendicular to our gravitational field as well as for a helical field geometry. We find that, while magnetic draping is able to better preserve AGN-driven bubbles and excite stronger g-modes, the production of turbulence is still inefficient. This fact is likely due to the magnetic tension force preventing the production of vortices in the ICM plasma. Our work shows that ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects.
Effect of resonant magnetic perturbations on secondary structures in drift-wave turbulence
Energy Technology Data Exchange (ETDEWEB)
Leconte, M. [WCI Center for Fusion Theory, NFRI, Daejeon (Korea, Republic of); Diamond, P. H. [WCI Center for Fusion Theory, NFRI, Daejeon (Korea, Republic of); CMTFO and CASS, UCSD, California 92093 (United States)
2011-08-15
Recent experiments showed a decrease of long range correlations during the application of resonant magnetic perturbations (RMPs) [Y. Xu et al., Nucl. Fusion 51, 063020 (2011)]. This finding suggests that RMPs damp zonal flows. To elucidate the effect of the RMPs on zonal structures in drift wave turbulence, we construct a generalized Hasegawa-Wakatani model including RMP fields. The effect of the RMPs is to induce a linear coupling between the zonal electric field and the zonal density gradient, which drives the system to a state of electron radial force balance for large RMP amplitude. A predator-prey model coupling the primary drift wave dynamics to the zonal modes evolution is derived. This model has both turbulence drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. The novel regime has a power threshold which increases with RMP amplitude as {gamma}{sub c}{approx}[({delta}B{sub r}/B)]{sup 2}.
On-Off intermittency detected at the onset of turbulence in a magnetized plasma column
Pierre, Thiery
2016-10-01
The transition to turbulence is investigated in a rotating linear magnetized plasma column (MISTRAL device) and the role of the noise is emphasized. The destabilization is induced by injection of electrons on the axis of the device biasing the anode of the source plasma. Starting from a rotating plasma, that can be compared to a laminar regime in fluid dynamics, the slight increase of the potential of the source plasma leads to the onset of intermittent bursts in the edge corresponding to the expulsion of plasma blobs and to the transient destruction of the stable rotating plasma column. The statistical analysis of the time series of the density at the onset of the intermittency is performed. The distribution of the recurrence time of the turbulent bursts and the distribution of the duration of the laminar phases are analyzed. At the threshold, a power law is found for the distribution of the laminar duration with critical exponent -3/2. This dynamical behavior is similar to On-off intermittency (Platt, Spiegel, Tresser, PRL 70, 279,1993) induced by Gaussian noise superimposed on the control parameter. When the control parameter is increased, the distribution evolves towards an exponential decay law.
Applications the Lagrangian description in aperiodic flows
Mendoza, Carolina; Mancho, Ana Maria
2012-11-01
We use several recently developed Lagrangian tools for describing transport in general aperiodic flows. In our approach the first step is based in a Lagrangian descriptor (the so called function M). It measures the length of particle trajectories on the ocean surface over a given interval of time. We describe its output over satellite altimetry data on the Kuroshio current. The technique is combined with the direct computation of manifolds of Distinguished Hyperbolic trajectories and a very detailed description of transport is achieved across an eddy and a jet on the Kuroshio current,. A second velocity data set is examined with the M function tool. These are obtained from the HYCOM project on the Gulf of Mexico during the time of the oil-spill. We have identified underlying Lagrangian structures and dynamics. We acknowledge to the hospitality of the university of Delaware and the assistance of Bruce Lipphardt and Helga Huntley in accessing the model data sets. We acknowledge to the grants: UPM-AL12-PAC-09, Becas de Movilidad de Caja Madrid 2011, MTM2011-26696 and ILINK-0145.
Triangular dissections, aperiodic tilings and Jones algebras
Coquereaux, Robert
1994-01-01
The Brattelli diagram associated with a given bicolored Dynkin-Coxeter graph of type A_n determines planar fractal sets obtained by infinite dissections of a given triangle. All triangles appearing in the dissection process have angles that are multiples of \\pi/ (n+1). There are usually several possible infinite dissections compatible with a given n but a given one makes use of n/2 triangle types if n is even. Jones algebra with index [ 4 \\ \\cos^2{\\pi \\over n+1}]^{-1} (values of the discrete range) act naturally on vector spaces associated with those fractal sets. Triangles of a given type are always congruent at each step of the dissection process. In the particular case n=4, there are isometric and the whole structure lead, after proper inflation, to aperiodic Penrose tilings. The other "tilings" associated with other values of the index are discussed and shown to be encoded by equivalence classes of infinite sequences (with appropriate constraints) using only n/2 digits (if n is even) and generalizing the ...
Cameron, R H
2016-01-01
In order to match observed properties of the solar cycle, flux-transport dynamo models require the toroidal magnetic flux to be stored in a region of low magnetic diffusivity, typically located at or below the bottom of the convection zone. We infer the turbulent magnetic diffusivity affecting the toroidal field on the basis of empirical data. We consider the time evolution of mean latitude and width of the activity belts of solar cycles 12--23 and their dependence on cycle strength. We interpret the decline phase of the cycles as a diffusion process. The activity level of a given cycle begins to decline when the centers of its equatorward propagating activity belts come within their width (at half maximum) from the equator. This happens earlier for stronger cycles because their activity belts are wider. From that moment on, the activity and the belt width decrease in the same manner for all cycles, independent of their maximum activity level. In terms of diffusive cancellation of opposite-polarity toroidal f...
GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations
Wu, Benjamin; Tan, Jonathan C.; Nakamura, Fumitaka; Van Loo, Sven; Christie, Duncan; Collins, David
2017-02-01
We investigate giant molecular cloud collisions and their ability to induce gravitational instability and thus star formation. This mechanism may be a major driver of star formation activity in galactic disks. We carry out a series of 3D, magnetohydrodynamics (MHD), adaptive mesh refinement simulations to study how cloud collisions trigger formation of dense filaments and clumps. Heating and cooling functions are implemented based on photo-dissociation region models that span the atomic-to-molecular transition and can return detailed diagnostic information. The clouds are initialized with supersonic turbulence and a range of magnetic field strengths and orientations. Collisions at various velocities and impact parameters are investigated. Comparing and contrasting colliding and non-colliding cases, we characterize morphologies of dense gas, magnetic field structure, cloud kinematic signatures, and cloud dynamics. We present key observational diagnostics of cloud collisions, especially: relative orientations between magnetic fields and density structures, like filaments; 13CO(J = 2-1), 13CO(J = 3-2), and 12CO(J = 8-7) integrated intensity maps and spectra; and cloud virial parameters. We compare these results to observed Galactic clouds.
Turbulence and magnetic spots at the surface of hot massive stars
Cantiello, Matteo; Brandenburg, Axel; Del Sordo, Fabio; Käpylä, Petri; Langer, Norbert
2010-01-01
Hot luminous stars show a variety of phenomena in their photospheres and in their winds which still lack clear physical explanations at this time. Among these phenomena are non-thermal line broadening, line profile variability (LPVs), discrete absorption components (DACs), wind clumping and stochastically excited pulsations. Cantiello et al. (2009) argued that a convection zone close to the surface of hot, massive stars, could be responsible for some of these phenomena. This convective zone is caused by a peak in the opacity due to iron recombination and for this reason is referred as the "iron convection zone" (FeCZ). 3D MHD simulations are used to explore the possible effects of such subsurface convection on the surface properties of hot, massive stars. We argue that turbulence and localized magnetic spots at the surface are the likely consequence of subsurface convection in early type stars.
Magnetic Field Line Random Walk in Isotropic Turbulence with Zero Mean Field
Sonsrettee, W.; Subedi, P.; Ruffolo, D.; Matthaeus, W. H.; Snodin, A. P.; Wongpan, P.; Chuychai, P.
2015-01-01
In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b/B 0)(l∥/l) for rms magnetic fluctuation b, large-scale mean field B 0, and parallel and perpendicular coherence scales l∥ and l, respectively. Here we examine the FLRW when R → ∞ by taking B 0 → 0 for finite bz (fluctuation component along B 0), which differs from the well-studied route with bz = 0 or bz Lt B 0 as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B 0 = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k -1 or k -2 moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B 0 → 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.
MAGNETIC FIELD LINE RANDOM WALK IN ISOTROPIC TURBULENCE WITH ZERO MEAN FIELD
Energy Technology Data Exchange (ETDEWEB)
Sonsrettee, W.; Ruffolo, D.; Snodin, A. P.; Wongpan, P. [Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400 (Thailand); Subedi, P.; Matthaeus, W. H. [Bartol Research Institute, University of Delaware, Newark, DE 19716 (United States); Chuychai, P., E-mail: bturbulence@gmail.com, E-mail: david.ruf@mahidol.ac.th, E-mail: andrew.snodin@gmail.com, E-mail: pat.wongpan@postgrad.otago.ac.nz, E-mail: piyanate@gmail.com, E-mail: prasub@udel.edu, E-mail: whm@udel.edu [Thailand Center of Excellence in Physics, CHE, Ministry of Education, Bangkok 10400 (Thailand)
2015-01-01
In astrophysical plasmas, magnetic field lines often guide the motions of thermal and non-thermal particles. The field line random walk (FLRW) is typically considered to depend on the Kubo number R = (b/B {sub 0})(ℓ{sub ∥}/ℓ ) for rms magnetic fluctuation b, large-scale mean field B {sub 0}, and parallel and perpendicular coherence scales ℓ{sub ∥} and ℓ , respectively. Here we examine the FLRW when R → ∞ by taking B {sub 0} → 0 for finite b{sub z} (fluctuation component along B {sub 0}), which differs from the well-studied route with b{sub z} = 0 or b{sub z} << B {sub 0} as the turbulence becomes quasi-two-dimensional (quasi-2D). Fluctuations with B {sub 0} = 0 are typically isotropic, which serves as a reasonable model of interstellar turbulence. We use a non-perturbative analytic framework based on Corrsin's hypothesis to determine closed-form solutions for the asymptotic field line diffusion coefficient for three versions of the theory, which are directly related to the k {sup –1} or k {sup –2} moment of the power spectrum. We test these theories by performing computer simulations of the FLRW, obtaining the ratio of diffusion coefficients for two different parameterizations of a field line. Comparing this with theoretical ratios, the random ballistic decorrelation version of the theory agrees well with the simulations. All results exhibit an analog to Bohm diffusion. In the quasi-2D limit, previous works have shown that Corrsin-based theories deviate substantially from simulation results, but here we find that as B {sub 0} → 0, they remain in reasonable agreement. We conclude that their applicability is limited not by large R, but rather by quasi-two-dimensionality.
Energy Technology Data Exchange (ETDEWEB)
Matteini, L.; Horbury, T. S.; Schwartz, S. J. [The Blackett Laboratory, Imperial College London, SW7 2AZ (United Kingdom); Pantellini, F. [LESIA, Observatoire de Paris, CNRS, UPMC, Universit Paris-Diderot, 5 Place Jules Janssen, F-92195 Meudon (France); Velli, M. [Department of Earth, Planetary, and Space Sciences, UCLA, California (United States)
2015-03-20
We investigate the properties of plasma fluid motion in the large-amplitude, low-frequency fluctuations of highly Alfvénic fast solar wind. We show that protons locally conserve total kinetic energy when observed from an effective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame can be reasonably identified by alpha particles which, due to their drift with respect to protons at about the Alfvén speed along the magnetic field, do not partake in the fluid low-frequency fluctuations. Using their velocity to transform the proton velocity into the frame of Alfvénic turbulence, we demonstrate that the resulting plasma motion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity and magnetic field vectors as expected for unidirectional Alfvénic fluctuations in equilibrium. We propose that this constraint, via the correlation between velocity and magnetic field in Alfvénic turbulence, is the origin of the observed constancy of the magnetic field; while the constant velocity corresponding to constant energy can only be observed in the frame of the fluctuations, the corresponding constant total magnetic field, invariant for Galilean transformations, remains the observational signature in the spacecraft frame of the constant total energy in the Alfvén turbulence frame.
Directory of Open Access Journals (Sweden)
A. Taktakishvili
2007-08-01
Full Text Available Recent Cluster observations of the vicinity of the high latitude magnetopause indicate the presence of beams of singly charged oxygen ions, which are of ionospheric origin. In this paper we examine the role of magnetic turbulence combined with a dc electric field across the magnetopause in causing the cross field transport of protons and of singly charged oxygen ions, by means of a kinetic test particle simulation. We find that the observed values of magnetosheath turbulence and electric fields can produce a substantial escape of the oxygen ions relative to protons. By varying the magnetic turbulence level in the simulation, we find that the number of O^{+} crossing the magnetopause grows with δB/B_{0}, and that very few ions can cross the magnetopause for δB/B_{0}=0. The ion temperature also grows with δB/B_{0}, showing that magnetic turbulence is effective in thermalizing the kinetic energy gain due to the cross-magnetopause potential drop. We suggest that this mechanism can help to explain Cluster observations of energetic oxygen ions during a high-latitude magnetopause crossing.
Magnetic Draping as a Possible Solution to Turbulent Heating of the ICM in Kinetic Mode AGN Feedback
Bambic, Christopher; Reynolds, Christopher; Morsony, Brian
2017-01-01
Recent x-ray measurements of the Perseus Cluster intracluster medium (ICM) by the Hitomi Mission found a velocity dispersion measure of σ 150 km/s, indicating a large-scale turbulent energy of approximately 4 % of the thermal energy. If this energy is transferred to small scales via a turbulent cascade and dissipated as heat, radiative cooling can be offset and the cluster can remain in its observed thermal equilibrium. We investigate the role of AGN feedback, specifically the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles, in a plane-parallel model of the ICM using 3D ideal MHD simulations. We present results for a magnetic field perpendicular to the gravitational field as well as a helical field. We find that, while magnetic draping is able to better preserve AGN-driven bubbles and excite stronger g-modes, the production of turbulence is still inefficient. This fact is likely due to the magnetic tension force preventing the production of vortices in the ICM plasma. Our work shows that ideal MHD is an insufficient description for the cluster feedback process and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects. NSF grant AST1333514
Fasoli, A.; Avino, F.; Bovet, A.; Furno, I.; Gustafson, K.; Jolliet, S.; Loizu, J.; Malinverni, D.; Ricci, P.; Riva, F.; Theiler, C.; Spolaore, M.; Vianello, N.
2013-06-01
Progress in basic understanding of turbulence and its influence on the transport both of the plasma bulk and of suprathermal components is achieved in the TORPEX simple magnetized torus. This configuration combines a microwave plasma production scheme with a quasi-equilibrium generated by a toroidal magnetic field, onto which a small vertical component is superimposed, simulating a simplified form of tokamak scrape-off layers. After having clarified the formation of blobs in ideal interchange turbulence, TORPEX experiments elucidated the mechanisms behind the blob motion, with a general scaling law relating their size and speed. The parallel currents associated with the blobs, responsible for the damping of the charge separation that develops inside them, hence determining their cross-field velocity, have been measured. The blob dynamics is influenced by creating convective cells with biased electrodes, arranged in an array on a metal limiter. Depending on the biasing scheme, radial and vertical blob velocities can be varied. Suprathermal ion transport in small-scale turbulence is also investigated on TORPEX. Suprathermal ions are generated by a miniaturized lithium source, and are detected using a movable double-gridded energy analyser. We characterize vertical and radial spreading of the ion beam, associated with the ideal interchange-dominated plasma turbulence, as a function of the suprathermal ion energy and the plasma temperature. Experimental results are in good agreement with global fluid simulations, including in cases of non-diffusive behaviour. To investigate the interaction of plasma and suprathermal particles with instabilities and turbulence in magnetic configurations of increasing complexity, a closed field line configuration has recently been implemented on TORPEX, based on a current-carrying wire suspended in the vacuum chamber. First measurements indicate the creation of circular symmetric profiles centred on the magnetic axis, and instabilities
Corotation torques experienced by planets embedded in weakly magnetized turbulent discs
Baruteau, C; Nelson, R P; Masset, F
2011-01-01
The migration of low-mass planets is driven by the differential Lindblad torque and the corotation torque in non-magnetic viscous models of protoplanetary discs. The corotation torque has recently received detailed attention as it may slow down, stall, or reverse migration. In laminar viscous disc models, the long-term evolution of the corotation torque is intimately related to viscous and thermal diffusion processes in the planet's horseshoe region. This paper examines the properties of the corotation torque in discs where MHD turbulence develops as a result of the magnetorotational instability, considering a weak initial toroidal magnetic field. We present results of 3D MHD simulations carried out with two different codes. Non-ideal MHD effects and the disc's vertical stratification are neglected, and locally isothermal disc models are considered. The running time-averaged torque exerted by the disc on a fixed planet is evaluated in three disc models. We first present results with an inner disc cavity (plan...
Scale dependent partitioning of one-dimensional aperiodic set diffraction
Elkharrat, A.
2004-06-01
We give a multiresolution partition of pure point parts of diffraction patterns of one-dimensional aperiodic sets. When an aperiodic set is related to the Golden Ratio, denoted by tau, it is well known that the pure point part of its diffractive measure is supported by the extension ring of tau, denoted by mathbb{Z}[tau]. The partition we give is based on the formalism of the so called tau-integers, denoted by mathbb{Z}_tau. The set of tau-integers is a selfsimilar set obeying mathbb{Z}_tau/tau^{j-1}subsetmathbb{Z}_tau/tau^j subset mathbb{Z}_tau/tau^{j + 1} subsetmathbb{Z}[tau], jinmathbb{Z}. The pure point spectrum is then partitioned with respect to this “Russian doll” like sequence of subsets mathbb{Z}_tau/tau^j. Thus we deduce the partition of the pure point part of the diffractive measure of aperiodic sets.
Simulated Performance of Timescale Metrics for Aperiodic Light Curves
Findeisen, Krzysztof; Hillenbrand, Lynne
2014-01-01
Aperiodic variability is a characteristic feature of young stars, massive stars, and active galactic nuclei. With the recent proliferation of time domain surveys, it is increasingly essential to develop methods to quantify and analyze aperiodic variability. We develop three timescale metrics that have been little used in astronomy -- {\\Delta}m-{\\Delta}t plots, peak-finding, and Gaussian process regression -- and present simulations comparing their effectiveness across a range of aperiodic light curve shapes, characteristic timescales, observing cadences, and signal to noise ratios. We find that Gaussian process regression is easily confused by noise and by irregular sampling, even when the model being fit reflects the process underlying the light curve, but that {\\Delta}m-{\\Delta}t plots and peak-finding can coarsely characterize timescales across a broad region of parameter space. We make public the software we used for our simulations, both in the spirit of open research and to allow others to carry out ana...
Wawrzynczak, A.; Alania, M. V.
2015-08-01
We analyze the temporal changes in the rigidity spectrum of Forbush decrease (Fd) of the galactic cosmic ray (GCR) intensity observed in November 2004. We compute the rigidity spectrum in two energy ranges based on the daily data from the worldwide network of neutron monitors and Nagoya ground muon telescope. We demonstrate that the changes in the rigidity spectrum of Fd are linked to the evolution/decay of the interplanetary magnetic field (IMF) turbulence during various phases of the Fd. We analyze the time-evolution of the state of the turbulence of the IMF in various frequency ranges during the Fd. Performed analysis show that the decrease of the exponent ν of the Power Spectral Density (PSD ∝ f-ν, where f is frequency) of the IMF turbulence with decreasing frequency lead to the soft rigidity spectrum of Fd for GCR particles with relatively higher energies.
Graham, Jonathan Pietarila; Schuessler, Manfred
2008-01-01
Using turbulent MHD simulations (magnetic Reynolds numbers up to 8000) and Hinode observations, we study effects of turbulence on measuring the solar magnetic field outside active regions. Firstly, from synthetic Stokes V profiles for the FeI lines at 630.1 and 630.2 nm, we show that a peaked probability distribution function (PDF) for observationally-derived field estimates is consistent with a monotonic PDF for actual vertical field strengths. Hence, the prevalence of weak fields is greater than would be naively inferred from observations. Secondly, we employ the fractal self-similar geometry of the turbulent solar magnetic field to derive two estimates (numerical and observational) of the true mean vertical unsigned flux density. We also find observational evidence that the scales of magnetic structuring in the photosphere extend at least down to an order of magnitude smaller than 200 km: the self-similar power-law scaling in the signed measure from a Hinode magnetogram ranges (over two decades in length s...
Bertram, Erik; Banerjee, Robi; Klessen, Ralf S
2011-01-01
We study the mass-to-flux ratio (M/\\Phi) of clumps and cores in simulations of supersonic, magnetohydrodynamical turbulence for different initial magnetic field strengths. We investigate whether the (M/\\Phi)-ratio of core and envelope, R = (M/\\Phi)_{core}/(M/\\Phi)_{envelope} can be used to distinguish between theories of ambipolar diffusion and turbulence-regulated star formation. We analyse R for different Lines-of-Sight (LoS) in various sub-cubes of our simulation box. We find that, 1) the average and median values of |R| for different times and initial magnetic field strengths are typically greater, but close to unity, 2) the average and median values of |R| saturate at average values of |R| ~ 1 for smaller magnetic fields, 3) values of |R| < 1 for small magnetic fields in the envelope are caused by field reversals when turbulence twists the field lines such that field components in different directions average out. Finally, we propose two mechanisms for generating values |R| ~< 1 for the weak and st...
Dust coagulation and magnetic field strength in a planet-induced gap subject to MRI turbulence
Carballido, Augusto; Matthews, Lorin; Hyde, Truell
2017-01-01
We investigate the coagulation of dust particles in and around a gap opened by a Jupiter-mass planet. To this end, we carry out a high-resolution magnetohydrodynamic (MHD) simulation of the gap environment, which is turbulent due to the magneto rotational instability. From the MHD simulation, we obtain values of the gas velocities, densities and turbulent stresses close to the gap edge, in one of the two gas streams that accrete onto the planet, and inside the low-density gap. The MHD values are then supplied to a Monte Carlo dust coagulation algorithm, which models grain sticking, compaction and bouncing. We consider two dust populations for each region: one whose initial size distribution is monodisperse, with monomer radius equal to 1 micron, and another one whose initial size distribution follows the Mathis-Rumpl-Nordsieck distribution for interstellar dust grains, with an initial range of monomer radii between 0.5 and 10 microns. Without bouncing, our Monte Carlo calculations show steady growth of dust aggregates in all regions, and the mass-weighted (MW) average porosity of the initially mono disperse population reaches extremely high final values of 98%. The final MW porosities in all other cases without bouncing range from 30% to 82%. The efficiency of compaction is due to high turbulent relative speeds between dust particles. When bouncing is introduced, growth is slowed down in the planetary wake and inside the gap.We also analyze the strength of the magnetic field threading the gaps opened by planets of different sub-Jovian masses. Preliminary results show that, in a gap opened by a large-mass planet (~ 1 MJ), the time-averaged radial profile of the vertical component of the field (Bz) increases sharply inside the gap, and less sharply in the case of less massive planets. In gaps opened by intermediate-mass planets (~ 0.5 — 0.75 MJ), the radial profile of Bz exhibits local maxima in the vicinity of the planet, but not at the gap center.
Band structures and localization properties of aperiodic layered phononic crystals
Energy Technology Data Exchange (ETDEWEB)
Yan Zhizhong, E-mail: zzyan@bit.edu.cn [Department of Applied Mathematics, Beijing Institute of Technology, Beijing 100081 (China); Zhang Chuanzeng [Department of Civil Engineering, University of Siegen, D-57078 Siegen (Germany)
2012-03-15
The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.
Band structures and localization properties of aperiodic layered phononic crystals
Yan, Zhi-Zhong; Zhang, Chuanzeng
2012-03-01
The band structures and localization properties of in-plane elastic waves with coupling of longitudinal and transverse modes oblique propagating in aperiodic phononic crystals based on Thue-Morse and Rudin-Shapiro sequences are studied. Using transfer matrix method, the concept of the localization factor is introduced and the correctness is testified through the Rytov dispersion relation. For comparison, the perfect periodic structure and the quasi-periodic Fibonacci system are also considered. In addition, the influences of the random disorder, local resonance, translational and/or mirror symmetries on the band structures of the aperiodic phononic crystals are analyzed in this paper.
Lasing in an optimized deterministic aperiodic nanobeam cavity
Moon, Seul-Ki; Jeong, Kwang-Yong; Noh, Heeso; Yang, Jin-Kyu
2016-12-01
We have demonstrated lasing action from partially extended modes in deterministic aperiodic nanobeam cavities inflated by Rudin-Shapiro sequence with two different air holes at room temperature. By varying the size ratio of the holes and hence the structural aperiodicity, different optical lasing modes were obtained with maximized quality factors. The lasing characteristics of the partially extended modes were confirmed by numerical simulations based on scanning microscope images of the fabricated samples. We believe that this partially extended nanobeam modes will be useful for label-free optical biosensors.
Optical induction scheme for assembling nondiffracting aperiodic Vogel spirals
Energy Technology Data Exchange (ETDEWEB)
Diebel, Falko, E-mail: falko.diebel@uni-muenster.de; Rose, Patrick; Boguslawski, Martin; Denz, Cornelia [Institut für Angewandte Physik and Center for Nonlinear Science (CeNoS), Westfälische Wilhelms-Universität Münster, 48149 Münster (Germany)
2014-05-12
We introduce an experimental approach to realize aperiodic photonic lattices based on multiplexing of nondiffracting Bessel beams. This holographic optical induction scheme takes advantage of the well localized Bessel beam as a basis to assemble two-dimensional photonic lattices. We present the realization of an optically induced two-dimensional golden-angle Vogel spiral lattice, which belongs to the family of deterministic aperiodic structures. With our technique, a very broad class of photonic refractive index landscapes now becomes accessible to optical induction, which could not be realized with established distributed holographic techniques.
Structural and Spectral Properties of Deterministic Aperiodic Optical Structures
Directory of Open Access Journals (Sweden)
Luca Dal Negro
2016-12-01
Full Text Available In this comprehensive paper we have addressed structure-property relationships in a number of representative systems with periodic, random, quasi-periodic and deterministic aperiodic geometry using the interdisciplinary methods of spatial point pattern analysis and spectral graph theory as well as the rigorous Green’s matrix method, which provides access to the electromagnetic scattering behavior and spectral fluctuations (distributions of complex eigenvalues as well as of their level spacing of deterministic aperiodic optical media for the first time.
Magnetic Field Line Random Walk in Isotropic Turbulence with Varying Mean Field
Sonsrettee, W.; Subedi, P.; Ruffolo, D.; Matthaeus, W. H.; Snodin, A. P.; Wongpan, P.; Chuychai, P.; Rowlands, G.; Vyas, S.
2016-08-01
In astrophysical plasmas, the magnetic field line random walk (FLRW) plays an important role in guiding particle transport. The FLRW behavior is scaled by the Kubo number R=(b/{B}0)({{\\ell }}\\parallel /{{\\ell }}\\perp ) for rms magnetic fluctuation b, large-scale mean field {{\\boldsymbol{B}}}0, and coherence scales parallel ({{\\ell }}\\parallel ) and perpendicular ({{\\ell }}\\perp ) to {{\\boldsymbol{B}}}0. Here we use a nonperturbative analytic framework based on Corrsin’s hypothesis, together with direct computer simulations, to examine the R-scaling of the FLRW for varying B 0 with finite b and isotropic fluctuations with {{\\ell }}\\parallel /{{\\ell }}\\perp =1, instead of the well-studied route of varying {{\\ell }}\\parallel /{{\\ell }}\\perp for b \\ll {B}0. The FLRW for isotropic magnetic fluctuations is also of astrophysical interest regarding transport processes in the interstellar medium. With a mean field, fluctuations may have variance anisotropy, so we consider limiting cases of isotropic variance and transverse variance (with b z = 0). We obtain analytic theories, and closed-form solutions for extreme cases. Padé approximants are provided to interpolate all versions of theory and simulations to any B 0. We demonstrate that, for isotropic turbulence, Corrsin-based theories generally work well, and with increasing R there is a transition from quasilinear to Bohm diffusion. This holds even with b z = 0, when different routes to R\\to ∞ are mathematically equivalent; in contrast with previous studies, we find that a Corrsin-based theory with random ballistic decorrelation works well even up to R = 400, where the effects of trapping are barely perceptible in simulation results.
The role of MHD turbulence in magnetic self-excitation: A study of the Madison Dynamo Experiment
Nornberg, Mark D.
2006-07-01
Determining the onset conditions for magnetic field growth in magnetohydrodynamics is fundamental to understanding how astrophysical dynamos such as the Earth, the Sun, and the galaxy self-generate magnetic fields. The Madison Dynamo Experiment was constructed to explore the role of turbulence in changing these onset conditions for an impeller-driven flow of liquid sodium. The flow generates intermittent magnetic bursts with the spatial structure predicted from kinematic dynamo theory. A model of the mean flow was constructed from laser Doppler velocimetry measurements of the flow in an identical-scale water experiment. A kinematic eigenvalue code predicted that the flow would generate a predominantly dipolar magnetic field perpendicular to the symmetry axis for sufficiently high impeller speeds. The flow amplifies the magnetic field by stretching field lines. The field lines are then twisted back onto themselves creating a feedback loop for dynamo growth. The same flow was generated in the sodium experiment and was found to amplify an applied magnetic field oriented perpendicular to the drive shaft axis of the experiment. The amplification increased with motor rotation rate as the induced field became more closely aligned with the applied field, though a reduction in the amplitude is attributed to an enhanced resistivity due to turbulent diffusion. The turbulence was characterized by measurements of the velocity and magnetic power spectra. The velocity spectra have a Kolmogorov scaling. The wavenumber at which resistive dissipation range becomes dominant was observed to increase with flow speed indicating that smaller scale magnetic structures were generated. No amplification due to a small-scale dynamo was observed. The intermittent bursts were analyzed using conditional averaging. The growth rate was found to increase linearly with impeller rotation rate resulting in stronger bursts. The average duration decreased so that the bursts continued to satisfy Poisson
Zhao, K. J.; Shi, Yuejiang; Liu, H.; Diamond, P. H.; Li, F. M.; Cheng, J.; Chen, Z. P.; Nie, L.; Ding, Y. H.; Wu, Y. F.; Chen, Z. Y.; Rao, B.; Cheng, Z. F.; Gao, L.; Zhang, X. Q.; Yang, Z. J.; Wang, N. C.; Wang, L.; Jin, W.; Xu, J. Q.; Yan, L. W.; Dong, J. Q.; Zhuang, G.; J-TEXT Team
2016-07-01
The acceleration of the co-current toroidal rotations around resonant surfaces by resonant magnetic perturbations (RMPs) through turbulence is presented. These experiments were performed using a Langmuir probe array in the edge plasmas of the J-TEXT tokamak. This study aims at understanding the RMP effects on edge toroidal rotations and exploring its control method. With RMPs, the flat electron temperature T e profile, due to magnetic islands, appears around resonant surfaces (Zhao et al 2015 Nucl. Fusion 55 073022). When the resonant surface is closer to the last closed flux surface, the flat T e profile vanishes with RMPs. In both cases, the toroidal rotations significantly increase in the direction of the plasma current around the resonant surfaces with RMPs. The characteristics of turbulence are significantly affected by RMPs around the resonant surfaces. The turbulence intensity profile changes and the poloidal wave vector k θ increases with RMPs. The power fraction of the turbulence components in the ion diamagnetic drift direction increases with RMPs. The measurements of turbulent Reynolds stresses are consistent with the toroidal flows that can be driven by turbulence. The estimations of the energy transfer between the turbulence and toroidal flows suggest that turbulence energy transfers into toroidal flows. The result has the implication of the intrinsic rotation being driven by RMPs via turbulence.
Bloch oscillations in an aperiodic one-dimensional potential
de Moura, FABF; Lyra, ML; Dominguez-Adame, F; Malyshev, V.A.
2005-01-01
We study the dynamics of an electron subjected to a static uniform electric field within a one-dimensional tight-binding model with a slowly varying aperiodic potential. The unbiased model is known to support phases of localized and extended one-electron states separated by two mobility edges. We sh
Landry, Russell; Turner, Neal J; Abram, Greg
2013-01-01
Magnetorotational instability (MRI) is the most promising mechanism behind accretion in low-mass protostellar disks. Here we present the first analysis of the global structure and evolution of non-ideal MRI-driven T-Tauri disks on million-year timescales. We accomplish this in a 1+1D simulation by calculating magnetic diffusivities and utilizing turbulence activity criteria to determine thermal structure and accretion rate without resorting to a 3-D magnetohydrodynamical (MHD) simulation. Our major findings are as follows. First, even for modest surface densities of just a few times the minimum-mass solar nebula, the dead zone encompasses the giant planet-forming region, preserving any compositional gradients. Second, the surface density of the active layer is nearly constant in time at roughly 10 g/cm2, which we use to derive a simple prescription for viscous heating in MRI-active disks for those who wish to avoid detailed MHD computations. Furthermore, unlike a standard disk with constant-alpha viscosity, t...
Dual cascade and its possible variations in magnetized kinetic plasma turbulence
Zhu, Jian-Zhou
2010-01-01
An electrostatic gyrokinetic model for the deviation from Maxwellian distribution is used to study the dual cascade feature in the magnetized plasma (kinetic) turbulence in a 2D slab geometry. Only a finite range of spacial Fourier modes are kept and the Gibbs statistics are calculated with one ($E$) plus a continuum ($G(v)$) of constants of motion. The covariance density with continuous velocity is obtained by doing functional inversion and it is found that kinetic effects greatly enrich the physics of the absolute equilibria; but, the qualitative feature in physical space is similar to other 2D fluid models and that the conventional dual cascade arguments may be carried over \\textit{mutatis mutandis}. A finite extra contribution to the Fourier spectrum of $g^2$ emerges once a cutoff scale $\\Delta v$ of velocity arrises from the numerical discretization/coarse graining or other physical mechanisms. This contribution may seriously deteriorate the equipartition of $G(v)$ over the wave vectors and may drastical...
Meyer, Manuel; Montanino, Daniele; Conrad, Jan
2014-09-01
Background radiation fields pervade the Universe, and above a certain energy any γ-ray flux emitted by an extragalactic source should be attenuated due to e+e- pair production. The opacity could be alleviated if photons oscillated into hypothetical axion-like particles (ALPs) in ambient magnetic fields, leading to a γ-ray excess especially at high optical depths that could be detected with imaging air Cherenkov telescopes (IACTs). Here, we introduce a method to search for such a signal in γ-ray data and to estimate sensitivities for future observations. Different magnetic fields close to the γ-ray source are taken into account in which photons can convert into ALPs that then propagate unimpeded over cosmological distances until they re-convert in the magnetic field of the Milky Way. Specifically, we consider the coherent field at parsec scales in a blazar jet as well as the turbulent field inside a galaxy cluster. For the latter, we explicitly derive the transversal components of a magnetic field with gaussian turbulence which are responsible for the photon-ALP mixing. To illustrate the method, we apply it to a mock IACT array with characteristics similar to the Cherekov Telescope Array and investigate the dependence of the sensitivity to detect a γ-ray excess on the magnetic-field parameters.
Energy Technology Data Exchange (ETDEWEB)
Vafin, S.; Schlickeiser, R. [Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- and Astrophysik, Ruhr-Universität, Bochum (Germany); Lazar, M. [Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- and Astrophysik, Ruhr-Universität, Bochum (Germany); Center for Plasma Astrophysics, Celestijnenlaan 200 B, 3001 Leuven (Belgium)
2015-02-15
The aperiodic ordinary (O-) mode instability in homogeneous and collisionless plasmas with kinetic anisotropies has recently received renewed attention due to its potential application in the solar wind, as well as for equal-mass plasmas. The present paper revisits the marginal instability condition of the O-mode derived from the electromagnetic linear dispersion equation for waves propagating perpendicular to the background magnetic field. For a counterstreaming bi-Maxwellian plasma system, this condition is found to be significantly affected by the streaming parameters. New functional dependencies (not studied before) of the counterstreaming parameters on the magnetic field and the other plasma parameters lead to new conditions of this instability for the both equal mass and electron-proton plasmas.
Directory of Open Access Journals (Sweden)
D. Falceta-Gonçalves
2011-01-01
Full Text Available The Interstellar Medium (ISM is a complex, multi-phase system, where the history of the stars occurs. The processes of birth and death of stars are strongly coupled to the dynamics of the ISM. The observed chaotic and diffusive motions of the gas characterize its turbulent nature. Understanding turbulence is crucial for understanding the star-formation process and the energy-mass feedback from evolved stars. Magnetic fields, threading the ISM, are also observed, making this effort even more difficult. In this work, I briefly review the main observations and the characterization of turbulence from these observable quantities. Following on, I provide a review of the physics of magnetized turbulence. Finally, I will show the main results from theoretical and numerical simulations, which can be used to reconstruct observable quantities, and compare these predictions to the observations.
Linkmann, Moritz; Sahoo, Ganapati; McKay, Mairi; Berera, Arjun; Biferale, Luca
2016-11-01
We perform an analytical and numerical study of incompressible homogeneous conducting fluids by Fourier-helical decomposition of the equations of magnetohydrodynamics (MHD) and a subsequent reduction of the number of degrees of freedom. From the stability properties of the most general subset of interacting velocity and magnetic fields on a closed Fourier triad, we make predictions on the large-scale magnetic-field growth depending on the distribution of magnetic and kinetic helicities among the three wavenumbers. In the kinematic dynamo regime we predict the formation of a large-scale magnetic component with a magnetic helicity of opposite sign with respect to the kinetic helicity, a sort of triadic-by-triad α-effect in Fourier space, while in presence of strong small-scale magnetic helicity we predict an inverse cascade of magnetic helicity. We confirm these predictions through a series of Direct Numerical Simulations, either seeding different magnetic helical components in a strongly helical flow (turbulent/laminar) or directly injecting helical magnetic fluctuations at small scales. Our results show that important dynamical features of MHD flows can be predicted from an analytically tractable dynamical system derived directly from the MHD equations. ERC ADG NewTURB 2013.
Institute of Scientific and Technical Information of China (English)
Banibrata Mukhopadhyay; Kanak Saha
2011-01-01
The origin of hydrodynamic turbulence in rotating shear flow is a long standing puzzle.Resolving it is especially important in astrophysics when the flow's angular momentum profile is Keplerian which forms an accretion disk having negligible molecular viscosity.Hence, any viscosity in such systems must be due to turbulence, arguably governed by magnetorotational instability, especially when temperature T (≥)105.However, such disks around quiescent cataclysmic variables, protoplanetary and star-forming disks, and the outer regions of disks in active galactic nuclei are practically neutral in charge because of their low temperature, and thus are not expected to be coupled with magnetic fields enough to generate any transport due to the magnetorotational instability.This flow is similar to plane Couette flow including the Coriolis force, at least locally.What drives their turbulence and then transport,when such flows do not exhibit any unstable mode under linear hydrodynamic perturbation? We demonstrate that the three-dimensional secondary disturbance to the primarily perturbed flow that triggers elliptical instability may generate significant turbulent viscosity in the range 0.0001 (≤) vt (≤) 0.1, which can explain transport in accretion flows.
Energy Technology Data Exchange (ETDEWEB)
Leconte, M.
2008-11-15
The H confinement regime is set when the heating power reaches a threshold value P{sub c} and is linked to the formation of a transport barrier in the edge region of the plasma. Such a barrier is characterized by a high pressure gradient and is submitted to ELM (edge localized mode) instabilities. ELM instabilities trigger violent quasi-periodical ejections of matter and heat that induce quasi-periodical relaxations of the transport barrier called relaxation oscillations. In this work we studied the interaction between sheared flows and turbulence in fusion plasmas. In particular, we studied the complex dynamics of a transport barrier and we show through a simulation that resonant magnetic perturbations could control relaxation oscillations without a significant loss of confinement
Sahraoui, F
2008-08-01
Intermittency is usually identified in turbulent flows as non-Gaussian tails of the probability density functions (PDFs) of the turbulent field derivatives. Here we investigate the role of phase coherence among the Fourier modes in creating intermittency in magnetized space plasmas using the technique of surrogate data. We apply the technique to two examples: (i) synthetic data and (ii) magnetic field fluctuations recorded in the terrestrial magnetosheath by the Cluster spacecraft. We use a set of four series of data, one observed and three surrogate, and their PDFs and moments (q < or = 4) as discriminating statistics. We show that the technique allows for detecting coherent structures and estimating their scales. We show furthermore that the phases, but not the amplitudes, create the non-Gaussian tails of the PDFs. We show also that the surrogate data used cannot account for asymmetries of the PDFs of the observed data. This enables us to confirm a scenario of turbulent cascade of mirror structures proposed in previous publications, by showing the existence of an approximately constant energy flux in the inertial range.
Energy Technology Data Exchange (ETDEWEB)
Kiyani, K. H.; Fauvarque, O. [Department of Electrical and Electronic Engineering, Imperial College London, London SW7 2AZ (United Kingdom); Chapman, S. C.; Hnat, B. [Centre for Fusion, Space and Astrophysics, University of Warwick, Coventry CV4 7AL (United Kingdom); Sahraoui, F. [Laboratoire de Physique des Plasmas, Observatoire de Saint-Maur, F-94107 Saint-Maur-Des-Fosses (France); Khotyaintsev, Yu. V., E-mail: k.kiyani@imperial.ac.uk [Swedish Institute of Space Physics, SE-75121 Uppsala (Sweden)
2013-01-20
The anisotropic nature of solar wind magnetic turbulence fluctuations is investigated scale by scale using high cadence in situ magnetic field measurements from the Cluster and ACE spacecraft missions. The data span five decades in scales from the inertial range to the electron Larmor radius. In contrast to the inertial range, there is a successive increase toward isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. In the context of wave-mediated theories of turbulence, we show that this enhancement in magnetic fluctuations parallel to the local mean background field is qualitatively consistent with the magnetic compressibility signature of kinetic Alfven wave solutions of the linearized Vlasov equation. More generally, we discuss how these results may arise naturally due to the prominent role of the Hall term at sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show that the full statistical signature of the fluctuations at scales below the ion Larmor radius is that of a single isotropic globally scale-invariant process distinct from the anisotropic statistics of the inertial range.
Kiyani, K. H.; Chapman, S. C.; Sahraoui, F.; Hnat, B.; Fauvarque, O.; Khotyaintsev, Yu. V.
2013-01-01
The anisotropic nature of solar wind magnetic turbulence fluctuations is investigated scale by scale using high cadence in situ magnetic field measurements from the Cluster and ACE spacecraft missions. The data span five decades in scales from the inertial range to the electron Larmor radius. In contrast to the inertial range, there is a successive increase toward isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. In the context of wave-mediated theories of turbulence, we show that this enhancement in magnetic fluctuations parallel to the local mean background field is qualitatively consistent with the magnetic compressibility signature of kinetic Alfvén wave solutions of the linearized Vlasov equation. More generally, we discuss how these results may arise naturally due to the prominent role of the Hall term at sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show that the full statistical signature of the fluctuations at scales below the ion Larmor radius is that of a single isotropic globally scale-invariant process distinct from the anisotropic statistics of the inertial range.
Sahraoui, Fouad; Goldstein, Melvyn
2008-01-01
Several observations in space plasmas have reported the presence of coherent structures at different plasma scales. Structure formation is believed to be a direct consequence of nonlinear interactions between the plasma modes, which depend strongly on phase synchronization of those modes. Despite this important role of the phases in turbulence, very limited work has been however devoted to study the phases as a potential tracers of nonlinearities in comparison with the wealth of literature on power spectra of turbulence where phases are totally missed. We present a method based on surrogate data to systematically detect coherent structures in turbulent signals. The new method has been applied successfully to magnetosheath turbulence (Sahraoui, Phys. Rev. E, 2008, in press), where the relationship between the identified phase coherence and intermittency (classically identified as non Gaussian tails of the PDFs) as well as the energy cascade has been studied. Here we review the main results obtained in that study and show further applications to small scale solar wind turbulence. Implications of the results on theoretical modelling of space turbulence (applicability of weak/wave turbulence, its validity limits and its connection to intermittency) will be discussed.
Energy Technology Data Exchange (ETDEWEB)
Peterson, J. L. [Lawrence Livermore National Laboratory, Livermore, California 94550 (United States); Bell, R.; Guttenfelder, W.; Hammett, G. W.; Kaye, S. M.; LeBlanc, B.; Mikkelsen, D. R. [Princeton Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States); Candy, J. [General Atomics, San Diego, California 92186 (United States); Smith, D. R. [Department of Engineering Physics, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States); Yuh, H. Y. [Nova Photonics Inc., Princeton, New Jersey 08540 (United States)
2012-05-15
The National Spherical Torus Experiment (NSTX) [M. Ono et al., Nucl. Fusion 40, 557 (2000)] can achieve high electron plasma confinement regimes that are super-critically unstable to the electron temperature gradient driven (ETG) instability. These plasmas, dubbed electron internal transport barriers (e-ITBs), occur when the magnetic shear becomes strongly negative. Using the gyrokinetic code GYRO [J. Candy and R. E. Waltz, J. Comput. Phys. 186, 545 (2003)], the first nonlinear ETG simulations of NSTX e-ITB plasmas reinforce this observation. Local simulations identify a strongly upshifted nonlinear critical gradient for thermal transport that depends on magnetic shear. Global simulations show e-ITB formation can occur when the magnetic shear becomes strongly negative. While the ETG-driven thermal flux at the outer edge of the barrier is large enough to be experimentally relevant, the turbulence cannot propagate past the barrier into the plasma interior.
El-Alaoui, M.; Richard, R. L.; Ashour-Abdalla, M.; Walker, R. J.; Goldstein, M. L.
2012-01-01
We report the results of MHD simulations of Earth's magnetosphere for idealized steady solar wind plasma and interplanetary magnetic field (IMF) conditions. The simulations feature purely northward and southward magnetic fields and were designed to study turbulence in the magnetotail plasma sheet. We found that the power spectral densities (PSDs) for both northward and southward IMF had the characteristics of turbulent flow. In both cases, the PSDs showed the three scale ranges expected from theory: the energy-containing scale, the inertial range, and the dissipative range. The results were generally consistent with in-situ observations and theoretical predictions. While the two cases studied, northward and southward IMF, had some similar characteristics, there were significant differences as well. For southward IMF, localized reconnection was the main energy source for the turbulence. For northward IMF, remnant reconnection contributed to driving the turbulence. Boundary waves may also have contributed. In both cases, the PSD slopes had spatial distributions in the dissipative range that reflected the pattern of resistive dissipation. For southward IMF there was a trend toward steeper slopes in the dissipative range with distance down the tail. For northward IMF there was a marked dusk-dawn asymmetry with steeper slopes on the dusk side of the tail. The inertial scale PSDs had a dusk-dawn symmetry during the northward IMF interval with steeper slopes on the dawn side. This asymmetry was not found in the distribution of inertial range slopes for southward IMF. The inertial range PSD slopes were clustered around values close to the theoretical expectation for both northward and southward IMF. In the dissipative range, however, the slopes were broadly distributed and the median values were significantly different, consistent with a different distribution of resistivity.
Critical behavior of marginal aperiodic sequences: a Monte Carlo study
Branco, Nilton
2007-03-01
We study layered marginal aperiodic sequences for the Ising and q=8 Potts model on a square lattice. The phase transition is continuous for the former and first-order for the latter. Using the Wolff algorithm we calculate critical exponents and the critical temperature for the Potts model when the interaction constant can assume two values, according to an aperiodic sequence: the period-doubling one for the Ising model and the Fibonacci one for the Potts model. These sequences and models chracterize marginal critical behavior, according to the Luck criterion and, therefore, interaction-dependent exponents are expected for a continuous transition. For first-order phase transitions, no study so far has been done on the influence of marginal sequences.
Keohane, J W
1998-01-01
This thesis contains observational studies of shell-type supernova remnants (SNRs), primarily using the ROSAT and ASCA X-ray observatories. These results include: (1) evidence for turbulent magnetic field amplification in the young supernova remnant Cas A, (2) upper limits on the maximum energy which SNRs accelerate cosmic rays. (1) When inhomogeneous absorption is taken into account, the soft X- ray and radio morphologies of the young SNR, Cas A, are strikingly similar. We conclude from the slope of the X-ray/radio surface brightness correlation, that the density of gas (traced by the X-ray) is proportional to the square of the magnetic field (traced by the radio). This implies that Cas A's magnetic field is continuously being amplified on small scales, as would be expected in a turbulent plasma. (2) SNRs are known to accelerate cosmic rays to GeV range energies, and they are generally assumed to produce the majority of Galactic cosmic rays with energies below 1,000 TeV (the "knee"). We investigate X-ray syn...
Matteini, L; Pantellini, F; Velli, M; Schwartz, S J
2015-01-01
We investigate properties of the plasma fluid motion in the large amplitude low frequency fluctuations of highly Alfv\\'enic fast solar wind. We show that protons locally conserve total kinetic energy when observed from an effective frame of reference comoving with the fluctuations. For typical properties of the fast wind, this frame can be reasonably identified by alpha particles, which, owing to their drift with respect to protons at about the Alfv\\'en speed along the magnetic field, do not partake in the fluid low frequency fluctuations. Using their velocity to transform proton velocity into the frame of Alfv\\'enic turbulence, we demonstrate that the resulting plasma motion is characterized by a constant absolute value of the velocity, zero electric fields, and aligned velocity and magnetic field vectors as expected for unidirectional Alfv\\'enic fluctuations in equilibrium. We propose that this constraint, via the correlation between velocity and magnetic field in Alfv\\'enic turbulence, is at the origin of ...
On the scaling features of magnetic field fluctuations at non-MHD scales in turbulent space plasmas
Consolini, G.; Giannattasio, F.; Yordanova, E.; Vörös, Z.; Marcucci, M. F.; Echim, M.; Chang, T.
2016-11-01
In several different contexts space plasmas display intermittent turbulence at magneto-hydro-dynamic (MHD) scales, which manifests in anomalous scaling features of the structure functions of the magnetic field increments. Moving to smaller scales, i.e. below the ion-cyclotron and/or ion inertial length, these scaling features are still observed, even though its is not clear if these scaling features are still anomalous or not. Here, we investigate the nature of scaling properties of magnetic field increments at non-MHD scales for a period of fast solar wind to investigate the occurrence or not of multifractal features and collapsing of probability distribution functions (PDFs) using the novel Rank-Ordered Multifractal Analysis (ROMA) method, which is more sensitive than the traditional structure function approach. We find a strong evidence for the occurrence of a near mono-scaling behavior, which suggests that the observed turbulent regime at non-MHD scales mainly displays a mono-fractal nature of magnetic field increments. The results are discussed in terms of a non-compact fractal structure of the dissipation field.
Intrinsic periodic and aperiodic stochastic resonance in an electrochemical cell
Tiwari, Ishant; Phogat, Richa; Parmananda, P.; Ocampo-Espindola, J. L.; Rivera, M.
2016-08-01
In this paper we show the interaction of a composite of a periodic or aperiodic signal and intrinsic electrochemical noise with the nonlinear dynamics of an electrochemical cell configured to study the corrosion of iron in an acidic media. The anodic voltage setpoint (V0) in the cell is chosen such that the anodic current (I ) exhibits excitable fixed point behavior in the absence of noise. The subthreshold periodic (aperiodic) signal consists of a train of rectangular pulses with a fixed amplitude and width, separated by regular (irregular) time intervals. The irregular time intervals chosen are of deterministic and stochastic origins. The amplitude of the intrinsic internal noise, regulated by the concentration of chloride ions, is then monotonically increased, and the provoked dynamics are analyzed. The signal to noise ratio and the cross-correlation coefficient versus the chloride ions' concentration curves have a unimodal shape indicating the emergence of an intrinsic periodic or aperiodic stochastic resonance. The abscissa for the maxima of these unimodal curves correspond to the optimum value of intrinsic noise where maximum regularity of the invoked dynamics is observed. In the particular case of the intrinsic periodic stochastic resonance, the scanning electron microscope images for the electrode metal surfaces are shown for certain values of chloride ions' concentrations. These images, qualitatively, corroborate the emergence of order as a result of the interaction between the nonlinear dynamics and the composite signal.
Lyapunov exponents for one-dimensional aperiodic photonic bandgap structures
Kissel, Glen J.
2011-10-01
Existing in the "gray area" between perfectly periodic and purely randomized photonic bandgap structures are the socalled aperoidic structures whose layers are chosen according to some deterministic rule. We consider here a onedimensional photonic bandgap structure, a quarter-wave stack, with the layer thickness of one of the bilayers subject to being either thin or thick according to five deterministic sequence rules and binary random selection. To produce these aperiodic structures we examine the following sequences: Fibonacci, Thue-Morse, Period doubling, Rudin-Shapiro, as well as the triadic Cantor sequence. We model these structures numerically with a long chain (approximately 5,000,000) of transfer matrices, and then use the reliable algorithm of Wolf to calculate the (upper) Lyapunov exponent for the long product of matrices. The Lyapunov exponent is the statistically well-behaved variable used to characterize the Anderson localization effect (exponential confinement) when the layers are randomized, so its calculation allows us to more precisely compare the purely randomized structure with its aperiodic counterparts. It is found that the aperiodic photonic systems show much fine structure in their Lyapunov exponents as a function of frequency, and, in a number of cases, the exponents are quite obviously fractal.
High-pressure crystallography of periodic and aperiodic crystals
Directory of Open Access Journals (Sweden)
Clivia Hejny
2015-03-01
Full Text Available More than five decades have passed since the first single-crystal X-ray diffraction experiments at high pressure were performed. These studies were applied historically to geochemical processes occurring in the Earth and other planets, but high-pressure crystallography has spread across different fields of science including chemistry, physics, biology, materials science and pharmacy. With each passing year, high-pressure studies have become more precise and comprehensive because of the development of instrumentation and software, and the systems investigated have also become more complicated. Starting with crystals of simple minerals and inorganic compounds, the interests of researchers have shifted to complicated metal–organic frameworks, aperiodic crystals and quasicrystals, molecular crystals, and even proteins and viruses. Inspired by contributions to the microsymposium `High-Pressure Crystallography of Periodic and Aperiodic Crystals' presented at the 23rd IUCr Congress and General Assembly, the authors have tried to summarize certain recent results of single-crystal studies of molecular and aperiodic structures under high pressure. While the selected contributions do not cover the whole spectrum of high-pressure research, they demonstrate the broad diversity of novel and fascinating results and may awaken the reader's interest in this topic.
High-pressure crystallography of periodic and aperiodic crystals.
Hejny, Clivia; Minkov, Vasily S
2015-03-01
More than five decades have passed since the first single-crystal X-ray diffraction experiments at high pressure were performed. These studies were applied historically to geochemical processes occurring in the Earth and other planets, but high-pressure crystallography has spread across different fields of science including chemistry, physics, biology, materials science and pharmacy. With each passing year, high-pressure studies have become more precise and comprehensive because of the development of instrumentation and software, and the systems investigated have also become more complicated. Starting with crystals of simple minerals and inorganic compounds, the interests of researchers have shifted to complicated metal-organic frameworks, aperiodic crystals and quasicrystals, molecular crystals, and even proteins and viruses. Inspired by contributions to the microsymposium 'High-Pressure Crystallography of Periodic and Aperiodic Crystals' presented at the 23rd IUCr Congress and General Assembly, the authors have tried to summarize certain recent results of single-crystal studies of molecular and aperiodic structures under high pressure. While the selected contributions do not cover the whole spectrum of high-pressure research, they demonstrate the broad diversity of novel and fascinating results and may awaken the reader's interest in this topic.
Instability, Turbulence, and 3D Magnetic Reconnection in a Line-Tied, Zero Net Current Screw Pinch
Brookhart, Matthew I.; Stemo, Aaron; Zuberbier, Amanda; Zweibel, Ellen; Forest, Cary B.
2015-04-01
This Letter reports the first experimental investigation into a line-tied plasma with a reversed current profile. Discrete current sources create a cylindrical plasma equilibrium with an axial field and zero net current. Detailed magnetic measurements show that an internal m =1 mode with no external character grows exponentially. The nonlinear evolution of the mode drives 3D reconnection events that reorganize the plasma equilibrium. The plasma is turbulent and exhibits reconnection events on a range of scales. These data are consistent with recent simulations of coronal loops and the nanoflare coronal heating mechanism.
Instability, turbulence, and 3D magnetic reconnection in a line-tied, zero net current screw pinch.
Brookhart, Matthew I; Stemo, Aaron; Zuberbier, Amanda; Zweibel, Ellen; Forest, Cary B
2015-04-10
This Letter reports the first experimental investigation into a line-tied plasma with a reversed current profile. Discrete current sources create a cylindrical plasma equilibrium with an axial field and zero net current. Detailed magnetic measurements show that an internal m=1 mode with no external character grows exponentially. The nonlinear evolution of the mode drives 3D reconnection events that reorganize the plasma equilibrium. The plasma is turbulent and exhibits reconnection events on a range of scales. These data are consistent with recent simulations of coronal loops and the nanoflare coronal heating mechanism.
Karak, Bidya Binay
2012-01-01
Prediction of the Sun's magnetic activity is important because of its effect on space environmental conditions and climate. However, recent efforts to predict the amplitude of the solar cycle have resulted in diverging forecasts with no consensus. It is understood that the dynamical memory of the solar dynamo mechanism governs predictability and this memory is different for advection- and diffusion-dominated solar convection zones. By utilizing stochastically forced, kinematic dynamo simulations, we demonstrate that the inclusion of downward turbulent pumping of magnetic flux reduces the memory of both advection- and diffusion-dominated solar dynamos to only one cycle; stronger pumping degrades this memory further. We conclude that reliable predictions for the maximum of solar activity can be made only at the preceding minimum and for more accurate predictions, sequential data assimilation would be necessary in forecasting models to account for the Sun's short memory.
Globus, N; Parizot, E
2007-01-01
We extend previous studies of mixed-composition extragalactic cosmic-ray source models, by investigating the influence of a non-negligible extragalactic magnetic field on the propagated cosmic-ray spectrum and composition. We study the transport of charged particles in turbulent fields and the transition from a ballistic to a diffusive propagation regime. We introduce a method allowing a fast integration of the particle trajectories, which allows us to calculate extragalactic cosmic-ray spectra in the general case, without using either the diffusive or the rectilinear approximation. We find that the main features of the mixed-composition models -- regarding the interpretation of the ankle and the non-monotonous evolution of the average cosmic-ray mass -- remain essentially unchanged as long as the magnetic field intensity does not exceed a few nG.
Smolyakov, A. I.; Chapurin, O.; Frias, W.; Koshkarov, O.; Romadanov, I.; Tang, T.; Umansky, M.; Raitses, Y.; Kaganovich, I. D.; Lakhin, V. P.
2017-01-01
Partially-magnetized plasmas with magnetized electrons and non-magnetized ions are common in Hall thrusters for electric propulsion and magnetron material processing devices. These plasmas are usually in strongly non-equilibrium state due to presence of crossed electric and magnetic fields, inhomogeneities of plasma density, temperature, magnetic field and beams of accelerated ions. Free energy from these sources make such plasmas prone to various instabilities resulting in turbulence, anomalous transport, and appearance of coherent structures as found in experiments. This paper provides an overview of instabilities that exist in such plasmas. A nonlinear fluid model has been developed for description of the Simon-Hoh, lower-hybrid and ion-sound instabilities. The model also incorporates electron gyroviscosity describing the effects of finite electron temperature. The nonlinear fluid model has been implemented in the BOUT++ framework. The results of nonlinear simulations are presented demonstrating turbulence, anomalous current and tendency toward the formation of coherent structures.
Lionello, Roberto; Downs, Cooper; Linker, Jon A; Mikić, Zoran
2014-01-01
Although it is widely accepted that photospheric motions provide the energy source and that the magnetic field must play a key role in the process, the detailed mechanisms responsible for heating the Sun's corona and accelerating the solar wind are still not fully understood. Cranmer et al. (2007) developed a sophisticated, 1D, time-steady model of the solar wind with turbulence dissipation. By varying the coronal magnetic field, they obtain, for a single choice of wave properties, a realistic range of slow and fast wind conditions with a sharp latitudinal transition between the two streams. Using a 1D, time-dependent model of the solar wind of Lionello et al. (2014), which incorporates turbulent dissipation of Alfv\\'en waves to provide heating and acceleration of the plasma, we have explored a similar configuration, obtaining qualitatively equivalent results. However, our calculations suggest that the rapid transition between slow and fast wind suggested by this 1D model may be disrupted in multidimensional ...
Self-generated magnetic turbulence and the propagation of galactic cosmic rays
Aloisio, R
2013-01-01
Cosmic rays propagating in the Galaxy may excite a streaming instability when their motion is super-alfvenic, thereby producing the conditions for their own diffusion. We present the results of a self-consistent solution of the transport equation where diffusion occurs because of the self-generated turbulence together with a preexisting turbulence injected, for instance, by supernova explosions and cascading to smaller scales. All chemicals are included in our calculations, so that we are able to show the secondary to primary ratios in addition to the spectra of the individual elements. All predictions appear to be in good agreement with observations.
Park, Kiwan
2015-01-01
The inverse cascade of magnetic energy occurs when helicity or rotational instability exists in the magnetohydrodynamic (MHD) system. This well known phenomenon provides a basis for the large scale magnetic field in space. However even the decaying nonhelical magnetic energy can evolve to expand its scale. This phenomenon, inverse transfer of decaying nonhelical magnetic field may hold some vital clues to the origin of large scale magnetic field in the astrophysical system without helicity nor any significant driving source. Zeldovich's rope model has been considered as the basic principle with regard to the amplification of magnetic field. However, since the rope model assuming a driving force is not appropriate to the decaying system, we suggest a supplementary dynamo model based on the magnetic induction equation. The model explicitly shows the basic principle of migration and amplification of magnetic field. The expansion of scale and intensity of magnetic field is basically the consequent result of the r...
Turbulence and dynamo interlinks
de Gouveia Dal Pino, E. M.; Santos-Lima, R.; Kowal, G.; Falceta-Gonçalves, D.
2013-07-01
The role of turbulence in astrophysical environments and its interplay with magnetic fields is still highly debated. In this lecture, we will discuss this issue in the framework of dynamo processes. We will first present a very brief summary of turbulent dynamo theories, then will focus on small scale turbulent dynamos and their particular relevance on the origin and maintenance of magnetic fields in the intra-cluster media (ICM) of galaxies. In these environments, the very low density of the flow requires a collisionless-MHD treatment. We will show the implications of this approach in the turbulent amplification of the magnetic fields in these environments. To finalize, we will also briefly address the connection between MHD turbulence and fast magnetic reconnection and its possible implications in the diffusion of magnetic flux in the dynamo process.
Turbulence and Dynamo Interlinks
Pino, E M de Gouveia Dal
2013-01-01
The role of turbulence in astrophysical environments and its interplay with magnetic fields is still highly debated. In this lecture, we will discuss this issue in the framework of dynamo processes. We will first present a very brief summary of turbulent dynamo theories, then will focus on small scale turbulent dynamos and their particular relevance on the origin and maintenance of magnetic fields in the intra-cluster media (ICM) of galaxies. In these environments, the very low density of the flow requires a collisionless-MHD treatment. We will show the implications of this approach in the turbulent amplification of the magnetic fields in these environments. To finalize, we will also briefly address the connection between MHD turbulence and fast magnetic reconnection and its possible implications in the diffusion of magnetic flux in the dynamo process.
Federrath, C; Longmore, S N; Kruijssen, J M D; Bally, J; Contreras, Y; Crocker, R M; Garay, G; Jackson, J M; Testi, L; Walsh, A J
2016-01-01
Star formation is primarily controlled by the interplay between gravity, turbulence, and magnetic fields. However, the turbulence and magnetic fields in molecular clouds near the Galactic Center may differ substantially from spiral-arm clouds. Here we determine the physical parameters of the central molecular zone (CMZ) cloud G0.253+0.016, its turbulence, magnetic field and filamentary structure. Using column-density maps based on dust-continuum emission observations with ALMA+Herschel, we identify filaments and show that at least one dense core is located along them. We measure the filament width W_fil=0.17$\\pm$0.08pc and the sonic scale {\\lambda}_sonic=0.15$\\pm$0.11pc of the turbulence, and find W_fil~{\\lambda}_sonic. A strong velocity gradient is seen in the HNCO intensity-weighted velocity maps obtained with ALMA+Mopra, which is likely caused by large-scale shearing of G0.253+0.016, producing a wide double-peaked velocity PDF. After subtracting the gradient to isolate the turbulent motions, we find a near...
Lin, Chenxi; Povinelli, Michelle L
2013-01-01
We report the design, fabrication, and optical absorption measurement of silicon membranes patterned with partially aperiodic nanohole structures. We demonstrate excellent agreement between measurement and simulations. We optimize a partially aperiodic structure using a random walk algorithm and demonstrate an experimental broadband absorption of 4.9 times that of a periodic array.
Topological aperiodicity for product systems over semigroups of Ore type
DEFF Research Database (Denmark)
Kwasniewski, Bartosz; Szymanski, Wojciech
2016-01-01
We prove a version of uniqueness theorem for Cuntz-Pimsner algebras of discrete product systems over semigroups of Ore type. To this end, we introduce Doplicher-Roberts picture of Cuntz-Pimsner algebras, and the semigroup dual to a product system of 'regular' C*-correspondences. Under a certain...... aperiodicity condition on the latter, we obtain the uniqueness theorem and a simplicity criterion for the algebras in question. These results generalize the corresponding ones for crossed products by discrete groups, due to Archbold and Spielberg, and for Exel's crossed products, due to Exel and Vershik...
del Valle, M V; Kowal, G
2016-01-01
Fast magnetic reconnection may occur in different astrophysical sources, producing flare-like emission and particle acceleration. Currently, this process is being studied as an efficient mechanism to accelerate particles via a first-order Fermi process. In this work we analyse the acceleration rate and the energy distribution of test particles injected in three-dimensional magnetohydrodynamical (MHD) domains with large-scale current sheets where reconnection is made fast by the presence of turbulence. We study the dependence of the particle acceleration time with the relevant parameters of the embedded turbulence, i.e., the Alfv\\'en speed $V_{\\rm A}$, the injection power $P_{\\rm inj}$ and scale $k_{\\rm inj}$ ($k_{\\rm inj} = 1/l_{\\rm inj}$). We find that the acceleration time follows a power-law dependence with the particle kinetic energy: $t_{acc} \\propto E^{\\alpha}$, with $0.2 < \\alpha < 0.6$ for a vast range of values of $c / V_{\\rm A} \\sim 20 - 1000$. The acceleration time decreases with the Alfv\\'en...
Leconte, M.; Diamond, P. H.; Xu, Y.
2014-01-01
We study the effects of resonant magnetic perturbations (RMPs) on turbulence, flows and confinement in the framework of resistive drift-wave turbulence. This work was motivated, in parts, by experiments reported at the IAEA 2010 conference (Xu et al 2011 Nucl. Fusion 51 062030) which showed a decrease of long-range correlations during the application of RMPs. We derive and apply a zero-dimensional predator-prey model coupling the drift-wave-zonal-mode system (Leconte and Diamond 2012 Phys. Plasmas 19 055903) to the evolution of mean quantities. This model has both density-gradient drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. This model allows a description of the full L-H transition evolution with RMPs, including the mean sheared flow evolution. The key results are the following: (i) the L-I and I-H power thresholds both increase with RMP amplitude |\\tilde b_x| , the relative increase of the L-I threshold scales as \\Delta P_LI \\propto |\\tilde b_x|^2 \
Leconte, M; Xu, Y
2013-01-01
We study the effects of Resonant Magnetic Perturbations (RMPs) on turbulence, flows and confinement in the framework of resistive drift-wave turbulence. This work was motivated, in parts, by experiments reported at the IAEA 2010 conference [Y. Xu {\\it et al}, Nucl. Fusion \\textbf{51}, 062030] which showed a decrease of long-range correlations during the application of RMPs. We derive and apply a zero-dimensional predator-prey model coupling the Drift-Wave Zonal Mode system [M. Leconte and P.H. Diamond, Phys. Plasmas \\textbf{19}, 055903] to the evolution of mean quantities. This model has both density gradient drive and RMP amplitude as control parameters and predicts a novel type of transport bifurcation in the presence of RMPs. This model allows a description of the full L-H transition evolution with RMPs, including the mean sheared flow evolution. The key results are: i) The L-I and I-H power thresholds \\emph{both} increase with RMP amplitude $|\\bx|$, the relative increase of the L-I threshold scales as $\\D...
Extended State to Localization in Random Aperiodic Chains
Institute of Scientific and Technical Information of China (English)
GAO Hui-Fen; TAO Rui-Bao
2006-01-01
The electronic states in Thus-Morse chain (TMC) and generalized Fibonacci chain (GFC) are studied by solving eigenequation and using transfer matrix method. Two model Hamiltonians are studied. One contains the nearest neighbor (n.n.) hopping terms only and the other has additionally next nearest neighbor (n.n.n.) hopping terms. Based on the transfer matrix method, a criterion of transition from the extended to the localized states is suggested for GFC and TMC. The numerical calculation shows the existence of both extended and localized states in pure aperiodic system. A random potential is introduced to the diagonal term of the Hamiltonian and then the extended states are always changed to be localized. The exponents related to the localization length as a function of randomness are calculated. For different kinds of aperiodic chain, the critical value of randomness for the transition from extended to the localized states are found to be zero, consistent with the case of ordinary one-dimensional systems.
Meyer, Manuel; Conrad, Jan
2014-01-01
Background radiation fields pervade the Universe, and above a certain energy any $\\gamma$-ray flux emitted by an extragalactic source should be attenuated due to $e^+e^-$ pair production. The opacity could be alleviated if photons oscillated into hypothetical axion-like particles (ALPs) in ambient magnetic fields, leading to a $\\gamma$-ray excess especially at high optical depths that could be detected with imaging air Cherenkov telescopes (IACTs). Here, we introduce a method to search for such a signal in $\\gamma$-ray data and to estimate sensitivities for future observations. Different magnetic fields close to the $\\gamma$-ray source are taken into account in which photons can convert into ALPs that then propagate unimpeded over cosmological distances until they re-convert in the magnetic field of the Milky Way. Specifically, we consider the coherent field at parsec scales in a blazar jet as well as the turbulent field inside a galaxy cluster. For the latter, we explicitly derive the transversal components ...
Kobayashi, T.; Inagaki, S.; Kosuga, Y.; Sasaki, M.; Nagashima, Y.; Yamada, T.; Arakawa, H.; Kasuya, N.; Fujisawa, A.; Itoh, S.-I.; Itoh, K.
2016-10-01
In this paper, we show the direct observation of the parallel flow structure and the parallel Reynolds stress in a linear magnetized plasma, in which a cross-ferroic turbulence system is formed [Inagaki et al., Sci. Rep. 6, 22189 (2016)]. It is shown that the parallel Reynolds stress induced by the density gradient driven drift wave is the source of the parallel flow structure. Moreover, the generated parallel flow shear by the parallel Reynolds stress is found to drive the parallel flow shear driven instability D'Angelo mode, which coexists with the original drift wave. The excited D'Angelo mode induces the inward particle flux, which seems to help in maintaining the peaked density profile.
Dynamo action at low magnetic Prandtl numbers: mean flow versus fully turbulent motions
Energy Technology Data Exchange (ETDEWEB)
Ponty, Y [CNRS UMR6202, Laboratoire Cassiopee, Observatoire de la Cote d' Azur, BP 4229, Nice Cedex 04 (France); Mininni, P D [NCAR, P O Box 3000, Boulder Colorado 80307-3000 (United States); Pinton, J-F [CNRS UMR5672, Laboratoire de Physique, Ecole Normale Superieure de Lyon, 46 Allee d' Italie, 69007 Lyon (France); Politano, H [CNRS UMR6202, Laboratoire Cassiopee, Observatoire de la Cote d' Azur, BP 4229, Nice Cedex 04 (France); Pouquet, A [NCAR, P O Box 3000, Boulder Colorado 80307-3000 (United States)
2007-08-15
We compute numerically the threshold for dynamo action in Taylor-Green (TG) swirling flows. Kinematic dynamo calculations, for which the flow field is fixed to its time average, are compared to dynamical runs, with the Navier-Stokes and induction equations jointly solved. The dynamo instability for the kinematic calculations is found to have two branches. The dynamical dynamo threshold at low Reynolds numbers lies within the low branch, while at high Reynolds numbers it gets closer to the high branch. Based on these results, the effect of the mean flow and of the turbulent fluctuations in TG dynamos are discussed.
Klein, Kristopher G; TenBarge, Jason M; Podesta, John J
2014-01-01
Motivated by recent observations of distinct parallel and perpendicular signatures in magnetic helicity measurements segregated by wave period and angle between the local magnetic field and the solar wind velocity, this paper undertakes a comparison of three intervals of \\emph{Ulysses} data with synthetic time series generated from a physically motivated turbulence model. From these comparisons, it is hypothesized that the observed signatures result from a perpendicular cascade of \\Alfvenic fluctuations and a local, non-turbulent population of ion cyclotron or whistler waves generated by temperature anisotropy instabilities. By constraining the model's free parameters through comparison to \\emph{in situ} data, it is found that, on average, $ \\sim 95\\%$ of the power near dissipative scales is contained in a perpendicular \\Alfvenic cascade and that the parallel fluctuations are propagating nearly unidirectionally. The effects of aliasing on magnetic helicity measurements are considered and shown to be significa...
Energy Technology Data Exchange (ETDEWEB)
Klein, Kristopher G.; Howes, Gregory G. [Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242 (United States); TenBarge, Jason M. [IREAP, University of Maryland, College Park, MD 20742 (United States); Podesta, John J., E-mail: kristopher-klein@uiowa.edu [Center for Space Plasma Physics, Space Science Institute, Boulder, CO 80301 (United States)
2014-04-20
Motivated by recent observations of distinct parallel and perpendicular signatures in magnetic helicity measurements segregated by wave period and angle between the local magnetic field and the solar wind velocity, this paper undertakes a comparison of three intervals of Ulysses data with synthetic time series generated from a physically motivated turbulence model. From these comparisons, it is hypothesized that the observed signatures result from a perpendicular cascade of Alfvénic fluctuations and a local, non-turbulent population of ion-cyclotron or whistler waves generated by temperature anisotropy instabilities. By constraining the model's free parameters through comparison to in situ data, it is found that, on average, ∼95% of the power near dissipative scales is contained in a perpendicular Alfvénic cascade and that the parallel fluctuations are propagating nearly unidirectionally. The effects of aliasing on magnetic helicity measurements are considered and shown to be significant near the Nyquist frequency.
Guo, Fan
2012-01-01
After introduction we focus on: the transport of charged particles, the acceleration of ions at shocks, and the acceleration of electrons at shocks. Chapter 2 studies the propagation of solar energetic particles(SEPs) in turbulent magnetic fields. Particle trajectories in turbulent magnetic fields are numerically integrated. The turbulence includes a Kolmogorov-like power spectrum containing a broad range of scales. Small-scale variations in particle intensities(dropouts) and velocity dispersions can be reproduced. The result gives a constraint on the error of onset analysis for inferring SEP informations. We find that dropouts are rarely produced using the two-component model(Matthaeus et al., 1990). The result questions the turbulence model. Chapter 3 studies the acceleration of ions. We use 3-D hybrid simulations to study the acceleration of low-energy particles at parallel shocks. We find that particles gain energy by reflection at the shock. The protons can move off field lines in 3-D electric and magnet...
Energy Technology Data Exchange (ETDEWEB)
Bortsaikin, S.M.; Levitan, Yu.S.
1977-07-01
An examination is made of steady-state turbulent flow of a conducting liquid in a cylindrical channel in a longitudinal magnetic field. The system of motion equations and energy can be recorded in a convenient integral form by using the Prandtl hypothesis for computing turbulent coefficients with rather simple assumptions about the nature of the velocity gradient. A small parameter in the equations can be easily identified in direct proximity to the channel's wall that makes it possible to find an approximate analytical solution to this problem in this region. 6 references, 1 figure.
Dispersion properties of a nanophotonic Bragg waveguide with finite aperiodic cladding
Fesenko, Volodymyr I; Shulika, Oleksiy V; Sukhoivanov, Igor A
2015-01-01
A comprehensive analysis of guided modes of a novel type of a planar Bragg reflection waveguide which consists of a low refractive index guiding layer sandwiched between two finite aperiodic mirrors is presented. The layers in the mirrors are aperiodically arranged according to the Kolakoski substitution rule. In such a waveguide light is confined inside the core by Bragg reflection from the mirrors, while dispersion characteristics of guided modes strongly depend on aperiodicity of the cladding. Using the transfer matrix formalism bandgap conditions, dispersion characteristics and mode profiles of the guided modes of such Bragg reflection waveguide are studied.
Optical gaps, mode patterns and dipole radiation in two-dimensional aperiodic photonic structures
Boriskina, Svetlana V.; Gopinath, Ashwin; Negro, Luca Dal
2009-05-01
Based on the rigorous generalized Mie theory solution of Maxwell's equations for dielectric cylinders we theoretically investigate the optical properties of two-dimensional deterministic structures based on the Fibonacci, Thue-Morse and Rudin-Shapiro aperiodic sequences. In particular, we investigate bandgap formation and mode localization properties in aperiodic photonic structures based on the accurate calculation of their local density of states (LDOS). In addition, we explore the potential of photonic structures based on aperiodic order for the engineering of radiative rates and emission patterns in erbium-doped silicon-rich nitride photonic structures.
Chinks in Solar Dynamo Theory: Turbulent Diffusion, Dynamo Waves and Magnetic Helicity
DeLuca, E. E.; Wagner, William J. (Technical Monitor)
2001-01-01
We have investigated the generation of magnetic fields in the Sun using two-dimensional and three-dimensional numerical simulations. The results of our investigations have been presented at scientific meetings and published.
Guo, Fan
2010-01-01
We study the physics of electron acceleration at collisionless shocks that move through a plasma containing large-scale magnetic fluctuations. We numerically integrate the trajectories of a large number of electrons, which are treated as test particles moving in the time dependent electric and magnetic fields determined from 2-D hybrid simulations (kinetic ions, fluid electron). The large-scale magnetic fluctuations effect the electrons in a number of ways and lead to efficient and rapid energization at the shock front. Since the electrons mainly follow along magnetic lines of force, the large-scale braiding of field lines in space allows the fast-moving electrons to cross the shock front several times, leading to efficient acceleration. Ripples in the shock front occuring at various scales will also contribute to the acceleration by mirroring the electrons. Our calculation shows that this process favors electron acceleration at perpendicular shocks. The current study is also helpful in understanding the inje...
Turbulent ${\\alpha}$-effect in twisted magnetic flux tubes dynamos in Riemannian space
de Andrade, Garcia
2007-01-01
Analytical solution of first order torsion ${\\alpha}$-effect in twisted magnetic flux tubes representing a flux tube dynamo in Riemannian space is presented. Toroidal and poloidal component of the magnetic field decays as $r^{-1}$, while grow exponentially in time. The rate of speed of the helical dynamo depends upon the value of Frenet curvature of the tube. The $\\alpha$ factor possesses a fundamental contribution from constant torsion tube approximation. It is also assumed that the curvature of the magnetic axis of the tube is constant. Though ${\\alpha}$-effect dynamo equations are rather more complex in Riemann flux tube coordinates, a simple solution assuming force-free magnetic fields is shown to be possible. Dynamo solutions are possible if the dynamo action is able to change the signs of torsion and curvature of the dynamo flux tube simultaneously.
Simon, Jacob B; Armitage, Philip J; Stone, James M; Beckwith, Kris
2013-01-01
We carry out a series of local, vertically stratified shearing box simulations of protoplanetary disks that include ambipolar diffusion and a net vertical magnetic field. The ambipolar diffusion profiles we employ correspond to 30AU and 100AU in a minimum mass solar nebula (MMSN) disk model, which consists of a far-UV-ionized surface layer and low-ionization disk interior. These simulations serve as a follow up to Simon et al. (2013), in which we found that without a net vertical field, the turbulent stresses that result from the magnetorotational instability (MRI) are too weak to account for observed accretion rates. The simulations in this work show a very strong dependence of the accretion stresses on the strength of the background vertical field; as the field strength increases, the stress amplitude increases. For gas to magnetic pressure ratios of 1e4 and 1e5, we find accretion rates between 1e-8 and 1e-7 solar masses per year. These accretion rates agree with observational constraints, suggesting a vert...
Gilmore, M.; Desjardins, T. R.; Fisher, D. M.
2016-10-01
Ongoing experiments and numerical modeling on the effects of flow shear on electrostatic turbulence in the presence of electrode biasing are being conducted in helicon plasmas in the linear HelCat (Helicon-Cathode) device. It is found that changes in flow shear, affected by electrode biasing through Er x Bz rotation, can strongly affect fluctuation dynamics, including fully suppressing the fluctuations or inducing chaos. The fundamental underlying instability, at least in the case of low magnetic field, is identified as a hybrid resistive drift-Kelvin-Helmholtz mode. At higher magnetic fields, multiple modes (resistive drift, rotation-driven interchange and/or Kelvin-Helmholtz) are present, and interact nonlinearly. At high positive electrode bias (V >10Te), a large amplitude, global instability, identified as the potential relaxation instability is observed. Numerical modeling is also being conducted, using a 3 fluid global Braginskii solver for no or moderate bias cases, and a 1D PIC code for high bias cases. Recent experimental and numerical results will be presented. Supported by U.S. National Science Foundation Award 1500423.
Gain modulation by graphene plasmons in aperiodic lattice lasers
Chakraborty, S.; Marshall, O. P.; Folland, T. G.; Kim, Y.-J.; Grigorenko, A. N.; Novoselov, K. S.
2016-01-01
Two-dimensional graphene plasmon-based technologies will enable the development of fast, compact, and inexpensive active photonic elements because, unlike plasmons in other materials, graphene plasmons can be tuned via the doping level. Such tuning is harnessed within terahertz quantum cascade lasers to reversibly alter their emission. This is achieved in two key steps: first, by exciting graphene plasmons within an aperiodic lattice laser and, second, by engineering photon lifetimes, linking graphene’s Fermi energy with the round-trip gain. Modal gain and hence laser spectra are highly sensitive to the doping of an integrated, electrically controllable, graphene layer. Demonstration of the integrated graphene plasmon laser principle lays the foundation for a new generation of active, programmable plasmonic metamaterials with major implications across photonics, material sciences, and nanotechnology.
MHD Turbulence, Turbulent Dynamo and Applications
Beresnyak, Andrey
2014-01-01
MHD Turbulence is common in many space physics and astrophysics environments. We first discuss the properties of incompressible MHD turbulence. A well-conductive fluid amplifies initial magnetic fields in a process called small-scale dynamo. Below equipartition scale for kinetic and magnetic energies the spectrum is steep (Kolmogorov -5/3) and is represented by critically balanced strong MHD turbulence. In this paper we report the basic reasoning behind universal nonlinear small-scale dynamo and the inertial range of MHD turbulence. We measured the efficiency of the small-scale dynamo $C_E=0.05$, Kolmogorov constant $C_K=4.2$ and anisotropy constant $C_A=0.63$ for MHD turbulence in high-resolution direct numerical simulations. We also discuss so-called imbalanced or cross-helical MHD turbulence which is relevant for in many objects, most prominently in the solar wind. We show that properties of incompressible MHD turbulence are similar to the properties of Alfv\\'enic part of MHD cascade in compressible turbul...
Experimental validation of a filament transport model in turbulent magnetized plasmas
Carralero, D; Aho-Mantila, L; Birkenmeier, G; Brix, M; Groth, M; Müller, H W; Stroth, U; Vianello, N; Wolfrum, E; Contributors, JET
2015-01-01
In a wide variety of natural and laboratory magnetized plasmas, filaments appear as a result of interchange instability. These convective structures substantially enhance transport in the direction perpendicular to the magnetic field. According to filament models, their propagation may follow different regimes depending on the parallel closure of charge conservation. This is of paramount importance in magnetic fusion plasmas, as high collisionality in the scrape-off layer may trigger a regime transition leading to strongly enhanced perpendicular particle fluxes. This work reports for the first time on an experimental verification of this process, linking enhanced transport with a regime transition as predicted by models. Based on these results, a novel scaling for global perpendicular particle transport in reactor relevant tokamaks such as ASDEX-Upgrade and JET is found, leading to important implications for next generation fusion devices.
Length Scales and Turbulent Properties of Magnetic Fields in Simulated Galaxy Clusters
Egan, Hilary; Hallman, Eric; Burns, Jack; Xu, Hao; Collins, David; Li, Hui; Norman, Michael L
2016-01-01
Additional physics beyond standard hydrodynamics is needed to fully model the intracluster medium (ICM); however, as we move to more sophisticated models, it is important to consider the role of magnetic fields and the way the fluid approximation breaks down. This paper represents a first step towards developing a self-consistent model of the ICM by characterizing the statistical properties of magnetic fields in cosmological simulations of galaxy clusters. We find that plasma conditions are largely homogeneous across a range of cluster masses and relaxation states. We also find that the magnetic field length scales are resolution dependent and not based on any particular physical process. Energy transfer mechanisms and scales are also identified, and imply the existence of small scale dynamo action. The scales of the small scale dynamo are resolution limited and driven by numerical resistivity and viscosity.
Energy Technology Data Exchange (ETDEWEB)
Li, Pak Shing; Klein, Richard I. [Astronomy Department, University of California, Berkeley, CA 94720 (United States); Martin, Daniel F. [Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720 (United States); McKee, Christopher F., E-mail: psli@astron.berkeley.edu, E-mail: klein@astron.berkeley.edu, E-mail: DFMartin@lbl.gov, E-mail: cmckee@astro.berkeley.edu [Physics Department and Astronomy Department, University of California, Berkeley, CA 94720 (United States)
2012-02-01
Performing a stable, long-duration simulation of driven MHD turbulence with a high thermal Mach number and a strong initial magnetic field is a challenge to high-order Godunov ideal MHD schemes because of the difficulty in guaranteeing positivity of the density and pressure. We have implemented a robust combination of reconstruction schemes, Riemann solvers, limiters, and constrained transport electromotive force averaging schemes that can meet this challenge, and using this strategy, we have developed a new adaptive mesh refinement (AMR) MHD module of the ORION2 code. We investigate the effects of AMR on several statistical properties of a turbulent ideal MHD system with a thermal Mach number of 10 and a plasma {beta}{sub 0} of 0.1 as initial conditions; our code is shown to be stable for simulations with higher Mach numbers (M{sub rms}= 17.3) and smaller plasma beta ({beta}{sub 0} = 0.0067) as well. Our results show that the quality of the turbulence simulation is generally related to the volume-averaged refinement. Our AMR simulations show that the turbulent dissipation coefficient for supersonic MHD turbulence is about 0.5, in agreement with unigrid simulations.
Energy Technology Data Exchange (ETDEWEB)
Cannon, Bradford E. [Physics Department, Florida State University, Tallahassee, FL 32306 (United States); Smith, Charles W.; Isenberg, Philip A.; Vasquez, Bernard J.; Joyce, Colin J. [Physics Department and Space Science Center, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824 (United States); Murphy, Neil [Jet Propulsion Laboratory, Mail Stop 180-600, 4800 Oak Grove Drive, Pasadena, CA 91109 (United States); Nuno, Raquel G., E-mail: bc13h@my.fsu.edu, E-mail: Charles.Smith@unh.edu, E-mail: Phil.Isenberg@unh.edu, E-mail: Bernie.Vasquez@unh.edu, E-mail: cjl46@wildcats.unh.edu, E-mail: Neil.Murphy@jpl.nasa.gov, E-mail: raquel.nuno@asu.edu [School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287 (United States)
2014-06-01
The low-frequency magnetic waves that arise from the isotropization of newborn interstellar pickup ions (PUIs) are reasonably well described by linear and quasi-linear kinetic theory in so far as those theories predict the wave frequency and polarization in the spacecraft frame. Those theories fail to describe the scarce observability of the waves. Quasilinear theory predicts that the wave power should accumulate over long periods of time as the relatively weak kinetic instability slowly adds power to the observed spectrum. At the same time it has been argued that the same wave energy must serve as a secondary source of thermal ion heating in the outer heliosphere once the initial turbulence is depleted. To the extent that turbulent transport of the wave energy acts against the spectrally confined accumulation of wave energy, turbulence should be a limiting factor in observability. We argue that turbulence does limit the observability of the waves and we use turbulence theory to predict the observed wave energy. We compare this prediction against a database of 502 wave observations attributed to newborn interstellar PUIs observed by the Ulysses spacecraft.
Scale size of magnetic turbulence in tokamaks probed with 30-MeV electrons
Entrop; Lopes Cardozo NJ; Jaspers; Finken
2000-04-17
Measurements of synchrotron radiation emitted by 30-MeV runaway electrons in the TEXTOR-94 tokamak show that the runaway population decays after switching on neutral beam injection (NBI). The decay starts only with a significant delay, which decreases with increasing NBI heating power. This delay provides direct evidence of the energy dependence of runaway confinement, which is expected if magnetic modes govern the loss of runaways. Application of the theory by Mynick and Strachan [Phys. Fluids 24, 695 (1981)] yields estimates for the "mode width" (delta) of magnetic perturbations: delta<0.5 cm in Ohmic discharges, increasing to delta = 4.4 cm for 0. 6 MW NBI.
Van Borm, C.; Spaans, M.
2013-01-01
Context. The seeds of the supermassive black holes with masses of ~109M⊙ observed already at z ~ 6 may have formed through the direct collapse of primordial gas in Tvir ≳ 104 K halos, whereby the gas must stay hot (~104 K) in order to avoid fragmentation. Aims: The interplay between magnetic fields,
Van Borm, C.; Spaans, M.
2013-01-01
Context. The seeds of the supermassive black holes with masses of ~109 M⊙ observed already at z ~ 6 may have formed through the direct collapse of primordial gas in Tvir ≳ 104 K halos, whereby the gas must stay hot (~104 K) in order to avoid fragmentation. Aims: The interplay between magnetic field
Bias driven coherent carrier dynamics in a two-dimensional aperiodic potential
de Moura, F. A. B. F.; Viana, L. P.; Lyra, M. L.; Malyshev, Victor; Dominguez-Adame, F.
2008-01-01
We study the dynamics of an electron wave-packet in a two-dimensional square lattice with an aperiodic site potential in the presence of an external uniform electric field. The aperiodicity is described by epsilon(m) = V cos(pi alpha m(x)(nu x)) cos(pi alpha m(y)(nu y)) at lattice sites (m(x),m(y)),
Analytical and numerical studies of disordered spin-1 Heisenberg chains with aperiodic couplings
Grande, H. L. Casa; Laflorencie, N.; Alet, F.; Vieira, A. P.
2013-01-01
We investigate the low-temperature properties of the one-dimensional spin-1 Heisenberg model with geometric fluctuations induced by aperiodic but deterministic coupling distributions, involving two parameters. We focus on two aperiodic sequences, the Fibonacci sequence and the 6-3 sequence. Our goal is to understand how these geometric fluctuations modify the physics of the (gapped) Haldane phase, which corresponds to the ground state of the uniform spin-1 chain. We make use of different adap...
Three-dimensional Iroshnikov-Kraichnan turbulence in a mean magnetic field
Grappin, Roland; Verdini, Andrea; Gürcan, Özgür
2013-01-01
As shown by M\\"uller et al. (2003), the 1D energy spectra deduced from structure functions in simulations of MHD turbulence with mean field $B_0$ show varying scalings, passing from $k_\\bot^{-5/3}$ to $k_\\bot^{-3/2}$ when $B_0$ increases, while the parallel scaling remains $k_\\|^{-2}$. To explain this, Boldyrev (2005, 2006) proposed, as an alternative to the Iroshnikov-Kraichnan (IK) scenario which predicts an isotropic $k^{-3/2}$ scaling, a scenario with a strong perpendicular $k_\\bot^{-3/2}$ cascade constrained by small-scale dynamic alignment, and critical balance controlling the parallel extent of the spectrum (Goldreich and Sridhar, 1995). Since then however, a more complete analysis of the 3D energy spectra by Grappin and M\\"uller (2010) showed that the spectral anisotropy is in fact in between the two predictions, with a scaling as $k^{-3/2}$ independent of the direction with respect to the mean field. We reanalyze the data and show that the isotropic scaling is not a consequence of measuring the energ...
Chakraborty Thakur, S.; Adriany, K.; Gosselin, J. J.; McKee, J.; Scime, E. E.; Sears, S. H.; Tynan, G. R.
2016-11-01
We report experimental measurements of the axial plasma flow and the parallel ion temperature in a magnetized linear plasma device. We used laser induced fluorescence to measure Doppler resolved ion velocity distribution functions in argon plasma to obtain spatially resolved axial velocities and parallel ion temperatures. We also show changes in the parallel velocity profiles during the transition from resistive drift wave dominated plasma to a state of weak turbulence driven by multiple plasma instabilities.
Turbulence in the Interstellar Medium
Falceta-Goncalves, D; Falgarone, E; Chian, A C -L
2014-01-01
Turbulence is ubiquitous in the insterstellar medium and plays a major role in several processes such as the formation of dense structures and stars, the stability of molecular clouds, the amplification of magnetic fields, and the re-acceleration and diffusion of cosmic rays. Despite its importance, interstellar turbulence, alike turbulence in general, is far from being fully understood. In this review we present the basics of turbulence physics, focusing on the statistics of its structure and energy cascade. We explore the physics of compressible and incompressible turbulent flows, as well as magnetized cases. The most relevant observational techniques that provide quantitative insights of interstellar turbulence are also presented. We also discuss the main difficulties in developing a three-dimensional view of interstellar turbulence from these observations. Finally, we briefly present what could be the the main sources of turbulence in the interstellar medium.
Turbulent convection in a horizontal duct with strong axial magnetic field
Zhang, Xuan; Zikanov, Oleg
2014-11-01
Convection in a horizontal duct with one heated wall is studied computationally. The work is motivated by the concept of a blanket for fusion reactors, according to which liquid metal slowly flows in toroidal ducts aligned with the main component of the magnetic field. We first assume that the magnetic field is sufficiently strong for the flow to be purely two-dimensional and analyze chaotic flow regimes at very high Grashof numbers. Furthermore, three-dimensional perturbations are considered and the relation between the length of the duct and the critical Hartmann number, below which the flow becomes three-dimensional, is determined. Financial support was provided by the US NSF (Grant CBET 1232851).
Particle Acceleration at a Flare Termination Shock: Effect of Large-scale Magnetic Turbulence
Guo, Fan
2012-01-01
We investigate the acceleration of charged particles (both electrons and protons) at collisionless shocks predicted to exist in the vicinity of solar flares. The existence of standing termination shocks has been examined by flare models and numerical simulations e.g., Shibata,Forbes. We study electron energization by numerically integrating the equations of motion of a large number of test-particle electrons in the time-dependent two-dimensional electric and magnetic fields generated from hybrid simulations (kinetic ions and fluid electron) using parameters typical of the solar flare plasma environment. The shock is produced by injecting plasma flow toward a rigid piston. Large-scale magnetic fluctuations -- known to exist in plasmas and known to have important effects on the nonthermal electron acceleration at shocks -- are also included in our simulations. For the parameters characteristic of the flaring region, our calculations suggest that the termination shock formed in the reconnection outflow region (a...
Poidevin, Frederick; Matthews, Brenda C
2010-01-01
The SCUBA polarized 850 microns thermal emission data of the region OMC-2 in Orion A are added to and homogeneously reduced with data already available in the region OMC-3. The data set shows that OMC-2 is a region generally less polarized than OMC-3. Where coincident, most of the 850 microns polarization pattern is similar to that measured in 350 microns polarization data. Only 850 microns polarimetry data have been obtained in and around MMS7, FIR1 & FIR2, and in the region south of FIR6. A realignment of the polarization vectors with the filament can be seen near FIR1 in the region south of OMC-3. An analysis shows that the energy injected by CO outflows and H2 jets associated to OMC-2 and OMC-3 does not appear to alter the polarization patterns at a scale of the 14'' resolution beam. A second order structure function analysis of the polarization position angles shows that OMC-2 is a more turbulent region than OMC-3. OMC-3 appears to be a clear case of a magnetically dominated region with respect to th...
PERIODIC AND APERIODIC VARIABILITY IN THE MOLECULAR CLOUD ρ OPHIUCHUS
Energy Technology Data Exchange (ETDEWEB)
Parks, J. Robert; Plavchan, Peter; Gee, Alan H. [Infrared Processing and Analysis Center, California Institute of Technology, Mail Code 100-22, 770 South Wilson Avenue, Pasadena, CA 91125 (United States); White, Russel J., E-mail: parksj@chara.gsu.edu [Georgia State University, Department of Physics and Astronomy, 25 Park Place, Room 605, Atlanta, GA 30303 (United States)
2014-03-01
Presented are the results of a near-IR photometric survey of 1678 stars in the direction of the ρ Ophiuchus (ρ Oph) star forming region using data from the 2MASS Calibration Database. For each target in this sample, up to 1584 individual J-, H-, and K{sub s} -band photometric measurements with a cadence of ∼1 day are obtained over three observing seasons spanning ∼2.5 yr; it is the most intensive survey of stars in this region to date. This survey identifies 101 variable stars with ΔK{sub s} -band amplitudes from 0.044 to 2.31 mag and Δ(J – K{sub s} ) color amplitudes ranging from 0.053 to 1.47 mag. Of the 72 young ρ Oph star cluster members included in this survey, 79% are variable; in addition, 22 variable stars are identified as candidate members. Based on the temporal behavior of the K{sub s} time-series, the variability is distinguished as either periodic, long time-scale or irregular. This temporal behavior coupled with the behavior of stellar colors is used to assign a dominant variability mechanism. A new period-searching algorithm finds periodic signals in 32 variable stars with periods between 0.49 to 92 days. The chief mechanism driving the periodic variability for 18 stars is rotational modulation of cool starspots while 3 periodically vary due to accretion-induced hot spots. The time-series for six variable stars contains discrete periodic ''eclipse-like'' features with periods ranging from 3 to 8 days. These features may be asymmetries in the circumstellar disk, potentially sustained or driven by a proto-planet at or near the co-rotation radius. Aperiodic, long time-scale variations in stellar flux are identified in the time-series for 31 variable stars with time-scales ranging from 64 to 790 days. The chief mechanism driving long time-scale variability is variable extinction or mass accretion rates. The majority of the variable stars (40) exhibit sporadic, aperiodic variability over no discernable time-scale. No chief
Influence of magnetic configuration on edge turbulence and transport in the H-1 Heliac
Michael, C. A.; Zhao, F.; Blackwell, B.; Vos, M. F. J.; Brotankova, J.; Haskey, S. R.; Seiwald, B.; Howard, J.
2017-02-01
The role of the rotational transform (ι) profile on fluctuations and transport is investigated in the H-1 Heliac by means of dynamic (i.e. changing during a shot) and static (fixed during a shot) scans of rotational transform through a range of values where the electron density drops markedly and which correspond to having the point of {\\iota }{{\\min }} located near r/a=0.75 in a region of magnetic well (such that the surface averaged magnetic field strength increases with radius). The gap is near the \\iota =4/3 resonance, but as the resonance is not in the plasma for more than half the gap it is not clear that this is relevant. Although this drop is clearly driven by the variation of helical current, under particular circumstances, similar density changes occur spontaneously. Plasma currents are measured throughout the scan and are found to slightly affect the rotational transform profile, and reverse about the configuration of minimum confinement, while induced currents through a toroidal loop voltage in the dynamical scans are not found to be significant. The confinement and fluctuation properties are studied by means of 2D movable Langmuir probes. Large near edge-localised dithering quasi-coherent fluctuations at ∼ 6 kHz develop in a strong density gradient region with low magnetic shear as ι is scanned up to a point where the density collapses in the outer region. This dithering corresponds to an m = 3 mode comprising of standing and propagating components. The net and fluctuation-induced transport components are measured near the plasma edge in a similar discharge, and it is found that fluctuation-induced transport driven by these low frequency coherent modes dominates the particle balance during the low density phase but is only a small component of the net flux when the density is higher.
Simard, Corinne; Charbonneau, Paul; Dubé, Caroline
2016-10-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos and Charbonneau (2014). The turbulent electromotive force (emf) operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. (2011), we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed α -tensor shows good agreement with that obtained by Racine et al. (2011), who did not include derivatives of the mean-field in their fit, as well as with the α -tensor extracted by Augustson et al. (2015) from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor β is positive definite and reaches values of 5.0 ×107 m2 s-1 in the middle of the convecting fluid layers. The spatial variations of both αϕϕ and βϕϕ component are well reproduced by expressions obtained under the Second Order Correlation Approximation, with a good matching of amplitude requiring a turbulent correlation time about five times smaller than the estimated turnover time of the small-scale turbulent flow. By segmenting the simulation data into epochs of magnetic cycle minima and maxima, we also measure α - and β -quenching. We find the magnetic quenching of the α -effect to be driven primarily by a reduction of the small-scale flow's kinetic helicity, with variations of the current helicity playing a lesser role in most locations in the simulation domain. Our measurements of turbulent diffusivity quenching are restricted to the βϕϕ component, but indicate a weaker quenching, by a factor of ≃ 1.36, than of the α -effect, which in our simulation drops by a factor of three between
Global simulations of magnetorotational turbulence II: turbulent energetics
Parkin, E R
2013-01-01
Magnetorotational turbulence draws its energy from gravity and ultimately releases it via dissipation. However, the quantitative details of this energy flow have not been assessed for global disk models. In this work we examine the energetics of a well-resolved, three-dimensional, global magnetohydrodynamic accretion disk simulation by evaluating statistically-averaged mean-field equations for magnetic, kinetic, and internal energy using simulation data. The results reveal that turbulent magnetic (kinetic) energy is primarily injected by the correlation between Maxwell (Reynolds) stresses and shear in the (almost Keplerian) mean flow, and removed by dissipation. This finding differs from previous work using local (shearing-box) models, which indicated that turbulent kinetic energy was primarily sourced from the magnetic energy reservoir. Lorentz forces provide the bridge between the magnetic and kinetic energy reservoirs, converting ~ 1/5 of the total turbulent magnetic power input into turbulent kinetic ener...
Radiation-disorder and aperiodicity in irradiated ceramics
Energy Technology Data Exchange (ETDEWEB)
Hobbs, L.W.
1992-01-01
This final technical report documents the accomplishments of the program of research entitled Radiation Disorder and Aperiodicity in Irradiated Ceramics'' for the period June 22, 1989--June 21, 1992. This research forms the latest part on an on-going program, begun at MIT in 1983 under DOE support, which has had as its objectives investigation of the responses in radiation environments of ceramics heavily-irradiated with electrons, neutrons and ions, with potential applications to fusion energy technology and high-level nuclear waste storage. Materials investigated have included SiO{sub 2}, MgAl{sub 2}O{sub 4}, Al{sub 23}O{sub 27}N{sub 5}, SiC, BeO, LiAlO{sub 2}, Li{sub 2}ZrO{sub 3}, CaTiO{sub 3}KTaO{sub 3} and Ca(Zr, Pu)Ti{sub 2}O{sub 7}. The program initially proposed for 1989 had as its major objectives two main thrusts: (1) research on defect aggregation in irradiated non-oxide ceramics, and (2) research on irradiation-induced amorphization of network silicas and phosphates.
Accretion of Chondrules formed by Impact Jetting in Magnetically Induced Turbulent Solar Nebula
Hasegawa, Yasuhiro; Masiero, Joseph; Wakita, Shigeru; Matsumoto, Yuji; Oshino, Shoichi
2016-01-01
Chondritic meteorites provide valuable opportunities to investigate origins of the solar system. We explore impact jetting as a mechanism to form chondrules and subsequent pebble accretion as a mechanism to accrete them onto parent bodies of chondrites, and investigate how these two processes can account for the currently available meteoritic data. We find that when the solar nebula is $\\le 5$ times more massive than the minimum-mass solar nebula at $a \\simeq 2-3$ AU and parent bodies of chondrites are $\\le 10^{24}$ g ($\\le$ 500 km in radius) there, impact jetting and subsequent pebble accretion can reproduce a number of properties of the meteoritic data. The properties include the present asteroid belt mass, formation timescale of chondrules, and the magnetic field strength of the nebula derived from chondrules in Semarkona. Since this scenario requires a first generation of planetesimals that trigger impact jetting and serve as parent bodies to accrete chondrules, the upper limit of parent bodies' mass lead...
Numerical study of Potts models with aperiodic modulations: influence on first-order transitions
Branco, Nilton; Girardi, Daniel
2012-02-01
We perform a numerical study of Potts models on a rectangular lattice with aperiodic interactions along one spatial direction. The number of states q is such that the transition is a first-order one for the uniform model. The Wolff algorithm is employed, for many lattice sizes, allowing for a finite-size scaling analyses to be carried out. Three different self-dual aperiodic sequences are employed, such that the exact critical temperature is known: this leads to precise results for the exponents. We analyze models with q=6 and 15 and show that the Harris-Luck criterion, originally introduced in the study of continuous transitions, is obeyed also for first-order ones. The new universality class that emerges for relevant aperiodic modulations depends on the number of states of the Potts model, as obtained elsewhere for random disorder, and on the aperiodic sequence. We determine the occurrence of log-periodic behavior, as expected for models with aperiodic modulated interactions.
Reclaiming Spare Capacity and Improving Aperiodic Response Times in Real-Time Environments
Directory of Open Access Journals (Sweden)
Liu Xue
2011-01-01
Full Text Available Abstract Scheduling recurring task sets that allow some instances of the tasks to be skipped produces holes in the schedule which are nonuniformly distributed. Similarly, when the recurring tasks are not strictly periodic but are sporadic, there is extra processor bandwidth arising because of irregular job arrivals. The additional computation capacity that results from skips or sporadic tasks can be reclaimed to service aperiodic task requests efficiently and quickly. We present techniques for improving the response times of aperiodic tasks by identifying nonuniformly distributed spare capacity—because of skips or sporadic tasks—in the schedule and adding such extra capacity to the capacity queue of a BASH server. These gaps can account for a significant portion of aperiodic capacity, and their reclamation results in considerable improvement to aperiodic response times. We present two schemes: NCLB-CBS, which performs well in periodic real-time environments with firm tasks, and NCLB-CUS, which can be deployed when the basic task set to schedule is sporadic. Evaluation via simulations and implementation suggests that performance improvements for aperiodic tasks can be obtained with limited additional overhead.
Analytical and numerical studies of disordered spin-1 Heisenberg chains with aperiodic couplings
Casa Grande, H. L.; Laflorencie, N.; Alet, F.; Vieira, A. P.
2014-04-01
We investigate the low-temperature properties of the one-dimensional spin-1 Heisenberg model with geometric fluctuations induced by aperiodic but deterministic coupling distributions, involving two parameters. We focus on two aperiodic sequences, the Fibonacci sequence and the 6-3 sequence. Our goal is to understand how these geometric fluctuations modify the physics of the (gapped) Haldane phase, which corresponds to the ground state of the uniform spin-1 chain. We make use of different adaptations of the strong-disorder renormalization-group (SDRG) scheme of Ma, Dasgupta, and Hu, widely employed in the study of random spin chains, supplemented by quantum Monte Carlo and density-matrix renormalization-group numerical calculations, to study the nature of the ground state as the coupling modulation is increased. We find no phase transition for the Fibonacci chain, while we show that the 6-3 chain exhibits a phase transition to a gapless, aperiodicity-dominated phase similar to the one found for the aperiodic spin-1/2 XXZ chain. Contrary to what is verified for random spin-1 chains, we show that different adaptations of the SDRG scheme may lead to different qualitative conclusions about the nature of the ground state in the presence of aperiodic coupling modulations.
Transition to turbulence in ferrofluids
Altmeyer, Sebastian; Lai, Ying-Cheng
2015-01-01
It is known that in classical fluids turbulence typically occurs at high Reynolds numbers. But can turbulence occur at low Reynolds numbers? Here we investigate the transition to turbulence in the classic Taylor-Couette system in which the rotating fluids are manufactured ferrofluids with magnetized nanoparticles embedded in liquid carriers. We find that, in the presence of a magnetic field turbulence can occur at Reynolds numbers that are at least one order of magnitude smaller than those in conventional fluids. This is established by extensive computational ferrohydrodynamics through a detailed bifurcation analysis and characterization of behaviors of physical quantities such as the energy, the wave number, and the angular momentum through the bifurcations. A striking finding is that, as the magnetic field is increased, the onset of turbulence can be determined accurately and reliably. Our results imply that experimental investigation of turbulence can be greatly facilitated by using ferrofluids, opening up...
Turbulence measurements in fusion plasmas
Conway, G. D.
2008-12-01
Turbulence measurements in magnetically confined toroidal plasmas have a long history and relevance due to the detrimental role of turbulence induced transport on particle, energy, impurity and momentum confinement. The turbulence—the microscopic random fluctuations in particle density, temperature, potential and magnetic field—is generally driven by radial gradients in the plasma density and temperature. The correlation between the turbulence properties and global confinement, via enhanced diffusion, convection and direct conduction, is now well documented. Theory, together with recent measurements, also indicates that non-linear interactions within the turbulence generate large scale zonal flows and geodesic oscillations, which can feed back onto the turbulence and equilibrium profiles creating a complex interdependence. An overview of the current status and understanding of plasma turbulence measurements in the closed flux surface region of magnetic confinement fusion devices is presented, highlighting some recent developments and outstanding problems.
Suppression of turbulent resistivity in turbulent Couette flow
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Si, Jiahe, E-mail: jsi@nmt.edu; Sonnenfeld, Richard G.; Colgate, Arthur S.; Westpfahl, David J.; Romero, Van D.; Martinic, Joe [New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801 (United States); Colgate, Stirling A.; Li, Hui [Los Alamos National Laboratory, Los Alamos, New Mexico 87544 (United States); Nornberg, Mark D. [University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
2015-07-15
Turbulent transport in rapidly rotating shear flow very efficiently transports angular momentum, a critical feature of instabilities responsible both for the dynamics of accretion disks and the turbulent power dissipation in a centrifuge. Turbulent mixing can efficiently transport other quantities like heat and even magnetic flux by enhanced diffusion. This enhancement is particularly evident in homogeneous, isotropic turbulent flows of liquid metals. In the New Mexico dynamo experiment, the effective resistivity is measured using both differential rotation and pulsed magnetic field decay to demonstrate that at very high Reynolds number rotating shear flow can be described entirely by mean flow induction with very little contribution from correlated velocity fluctuations.
Deterministic aperiodic composite lattice-structured silicon thin films for photon management
Xavier, Jolly; Becker, Christiane
2016-01-01
Exotic manipulation of the flow of photons in nanoengineered semiconductor materials with an aperiodic distribution of nanostructures plays a key role in efficiency-enhanced and industrially viable broadband photonic technologies. Through a generic deterministic nanotechnological route, in addition to periodic, transversely quasicrystallographic or disordered random photonic lattices, here we show scalable nanostructured semiconductor thin films on large area nanoimprinted substrates up to 4cm^2 with advanced functional features of aperiodic composite nanophotonic lattices having tailorable supercell tiles. The richer Fourier spectra of the presented artificially nanostructured materials with well-defined lattice point morphologies are designed functionally akin to two-dimensional incommensurate intergrowth aperiodic lattices-comprising periodic photonic crystals and in-plane quasicrystals as subgroups. The composite photonic lattice-structured crystalline silicon thin films with tapered nanoholes or nanocone...
Structural color of a lycaenid butterfly: analysis of an aperiodic multilayer structure.
Yoshioka, S; Shimizu, Y; Kinoshita, S; Matsuhana, B
2013-12-01
We investigated the structural color of the green wing of the lycaenid butterfly Chrysozephyrus brillantinus. Electron microscopy revealed that the bottom plate of the cover scale on the wing consists of an alternating air-cuticle multilayer structure. However, the thicknesses of the layers were not constant but greatly differed depending on the layer, unlike the periodic multilayer designs often adopted for artificial laser-reflecting mirrors. The agreement between the experimentally determined and theoretically calculated reflectance spectra led us to conclude that the multilayer interference in the aperiodic system is the primary origin of the structural color. We analyzed optical interference in this aperiodic system using a simple analytical model and found that two spectral peaks arise from constructive interference among different parts of the multilayer structure. We discuss the advantages and disadvantages of the aperiodic system over a periodic one.
Focusing properties of diffractive lenses constructed with the aperiodic m-bonacci sequence
Furlan, Walter D.; Ferrando, Vicente; Monsoriu, Juan A.
2015-01-01
In this contribution we present a new family of diffractive lenses which are designed using the m-bonacci sequence. These lenses are a generalization of the Fibonacci Zone Plates previously reported. Diffractive elements of this type are called aperiodic zone plates because they are characterized by a radial profile that follows a given deterministic aperiodic sequence (Cantor set, Thue-Morse, Fibonacci...). Aperiodic lenses have demonstrated new interesting focusing and imaging properties that have found applications in different fields such as soft X-ray microscopy and spectral domain optical coherence tomography. Here, we show that m-bonacci zone plates are inherently bifocal lenses. We demonstrate that the relative separation of their foci depends on the m-value of the sequence and also can be correlated with the generalized golden ratio. As a particular case, the properties of the m-bonacci sequence with m=2 and m=3, called Fibonacci and Tribonacci Zone Plates respectively are discussed.
Nonclassical properties of electronic states of aperiodic chains in a homogeneous electric field
Spisak, B. J.; Wołoszyn, M.
2009-07-01
The electronic energy levels of one-dimensional aperiodic systems driven by a homogeneous electric field are studied by means of a phase-space description based on the Wigner distribution function. The formulation provides physical insight into the quantum nature of the electronic states for the aperiodic systems generated by the Fibonacci and Thue-Morse sequences. The nonclassical parameter for electronic states is studied as a function of the magnitude of homogeneous electric field to achieve the main result of this work, which is to prove that the nonclassical properties of the electronic states in the aperiodic systems determine the transition probability between electronic states in the region of anticrossings. The localization properties of electronic states and the uncertainty product of momentum and position variables are also calculated as functions of the electric field.
Girardi, D.; Branco, N. S.
2011-06-01
We study the Potts model on a rectangular lattice with aperiodic modulations in its interactions along one direction. Numerical results are obtained using the Wolff algorithm and for many lattice sizes, allowing for a finite-size scaling analyses to be carried out. Three different self-dual aperiodic sequences are employed, which leads to more precise results, since the exact critical temperature is known. We analyze two models, with 6 and 15 number of states: both present first-order transitions on their uniform versions. We show that the Harris-Luck criterion, originally introduced in the study of continuous transitions, is obeyed also for first-order ones. Also, we show that the new universality class that emerges for relevant aperiodic modulations depends on the number of states of the Potts model, as obtained elsewhere for random disorder, and on the aperiodic sequence. We determine the occurrence of log-periodic behavior, as expected for models with aperiodic modulated interactions.
Saturation of the turbulent dynamo.
Schober, J; Schleicher, D R G; Federrath, C; Bovino, S; Klessen, R S
2015-08-01
The origin of strong magnetic fields in the Universe can be explained by amplifying weak seed fields via turbulent motions on small spatial scales and subsequently transporting the magnetic energy to larger scales. This process is known as the turbulent dynamo and depends on the properties of turbulence, i.e., on the hydrodynamical Reynolds number and the compressibility of the gas, and on the magnetic diffusivity. While we know the growth rate of the magnetic energy in the linear regime, the saturation level, i.e., the ratio of magnetic energy to turbulent kinetic energy that can be reached, is not known from analytical calculations. In this paper we present a scale-dependent saturation model based on an effective turbulent resistivity which is determined by the turnover time scale of turbulent eddies and the magnetic energy density. The magnetic resistivity increases compared to the Spitzer value and the effective scale on which the magnetic energy spectrum is at its maximum moves to larger spatial scales. This process ends when the peak reaches a characteristic wave number k☆ which is determined by the critical magnetic Reynolds number. The saturation level of the dynamo also depends on the type of turbulence and differs for the limits of large and small magnetic Prandtl numbers Pm. With our model we find saturation levels between 43.8% and 1.3% for Pm≫1 and between 2.43% and 0.135% for Pm≪1, where the higher values refer to incompressible turbulence and the lower ones to highly compressible turbulence.
Optimized aperiodic broadband thermal emitters for use as light bulb filaments
Granier, Christopher H; Veronis, Georgios; Dowling, Jonathan P
2016-01-01
We present optimized aperiodic structures for use as broadband thermal incandescent emitters which are capable of increasing the emittance by nearly a factor of two over the visible wavelength range when compared to bulk tungsten. These aperiodic multilayer structures are designed with alternating layers of tungsten and air or tungsten and silicon carbide on top of a tungsten substrate. We investigate the properties of these structures for use as lightbulb filaments. We find that these structures greatly enhance the emittance over the visible wavelength range, while also increasing the overall efficiency of the bulb and could lead to a decrease in incandescent lightbulb power consumption by nearly 50%.
Co-universal C*-algebras associated to aperiodic k-graphs
Kang, Sooran; Sims, Aidan
2011-01-01
We construct a representation of each finitely aligned aperiodic k-graph \\Lambda\\ on the Hilbert space H^{ap} with basis indexed by aperiodic boundary paths in \\Lambda. We show that the canonical expectation on B(H^{ap}) restricts to an expectation of the image of this representation onto the subalgebra spanned by the final projections of the generating partial isometries. We then show that every quotient of the Toeplitz algebra of the k-graph admits an expectation compatible with this one. U...
An aperiodic phenomenon of the unscented Kalman filter in filtering noisy chaotic signals
Institute of Scientific and Technical Information of China (English)
无
2007-01-01
A non-periodic oscillatory behavior of the unscented Kalman filter (UKF) when used to filter noisy contaminated chaotic signals is reported. We show both theoretically and experimentally that the gain of the UKF may not converge or diverge but oscillate aperiodically. More precisely, when a nonlinear system is periodic, the Kalman gain and error covariance of the UKF converge to zero. However, when the system being considered is chaotic, the Kalman gain either converges to a fixed point with a magnitude larger than zero or oscillates aperiodically.
Extended field-of-view in a lensless endoscope using an aperiodic multicore fiber
Sivankutty, Siddharth; Bouwmans, Géraud; Kogan, Dani; Oron, Dan; Andresen, Esben Ravn; Rigneault, Hervé
2016-01-01
We investigate lensless endoscopy using coherent beam combining and aperiodic multicore fibers (MCF). We show that diffracted orders, inherent to MCF with periodically arranged cores, dramatically reduce the field of view (FoV) and that randomness in MCF core positions can increase the FoV up to the diffraction limit set by a single fiber core, while maintaining MCF experimental feasibility. We demonstrate experimentally pixelation-free lensless endoscopy imaging over a 120 micron FoV with an aperiodic MCF designed with widely spaced cores. We show that this system is suitable to perform beam scanning imaging by simply applying a tilt to the proximal wavefront.
Iteration of Differentiable Functions under m-Modal Maps with Aperiodic Kneading Sequences
Directory of Open Access Journals (Sweden)
Maria F. Correia
2012-01-01
Full Text Available We consider the dynamical system (, , where is a class of differentiable functions defined on some interval and : → is the operator ∶=∘, where is a differentiable m-modal map. Using an algorithm, we obtained some numerical and symbolic results related to the frequencies of occurrence of critical values of the iterated functions when the kneading sequences of are aperiodic. Moreover, we analyze the evolution as well as the distribution of the aperiodic critical values of the iterated functions.
Zhang, Wei; Li, Chuandong; Huang, Tingwen; Xiao, Mingqing
2015-11-01
In this paper, the synchronization problem for neural networks with stochastic perturbation is studied with intermittent control via adaptive aperiodicity. Under the framework of stochastic theory and Lyapunov stability method, we develop some techniques of intermittent control with adaptive aperiodicity to achieve the synchronization of a class of neural networks, modeled by stochastic systems. Some effective sufficient conditions are established for the realization of synchronization of the underlying network. Numerical simulations of two examples are provided to illustrate the theoretical results obtained in the paper.
Energy Technology Data Exchange (ETDEWEB)
Bershadskii, A.G.
1985-06-01
An exact solution for the nonlinear problem of the spectral energy function of a homogeneous turbulence is derived under the assumption that energy transfer under the effect of inertial forces is determined mainly by the interactions among vortices whose wavenumbers are only slightly different from each other. The results are experimentally verified for turbulence behind grids. Similar problems are solved for MHD turbulence and for a nonstationary spectral energy function. It is shown that at the initial stage of degeneration, the spectral energy function is little influenced by the Stewart number; this agrees with experimental data for the damping of longitudinal velocity pulsations behind a grid in a longitudinal magnetic field. 15 references.
Li, Pak Shing; Klein, Richard I; McKee, Christopher F
2011-01-01
Performing a stable, long duration simulation of driven MHD turbulence with a high thermal Mach number and a strong initial magnetic field is a challenge to high-order Godunov ideal MHD schemes because of the difficulty in guaranteeing positivity of the density and pressure. We have implemented a robust combination of reconstruction schemes, Riemann solvers, limiters, and Constrained Transport EMF averaging schemes that can meet this challenge, and using this strategy, we have developed a new Adaptive Mesh Refinement (AMR) MHD module of the ORION2 code. We investigate the effects of AMR on several statistical properties of a turbulent ideal MHD system with a thermal Mach number of 10 and a plasma $\\beta_0$ of 0.1 as initial conditions; our code is shown to be stable for simulations with higher Mach numbers ($M_rms = 17.3$) and smaller plasma beta ($\\beta_0 = 0.0067$) as well. Our results show that the quality of the turbulence simulation is generally related to the volume-averaged refinement. Our AMR simulati...
Bruno, Roberto
2016-01-01
This book provides an overview of solar wind turbulence from both the theoretical and observational perspective. It argues that the interplanetary medium offers the best opportunity to directly study turbulent fluctuations in collisionless plasmas. In fact, during expansion, the solar wind evolves towards a state characterized by large-amplitude fluctuations in all observed parameters, which resembles, at least at large scales, the well-known hydrodynamic turbulence. This text starts with historical references to past observations and experiments on turbulent flows. It then introduces the Navier-Stokes equations for a magnetized plasma whose low-frequency turbulence evolution is described within the framework of the MHD approximation. It also considers the scaling of plasma and magnetic field fluctuations and the study of nonlinear energy cascades within the same framework. It reports observations of turbulence in the ecliptic and at high latitude, treating Alfvénic and compressive fluctuations separately in...
Hazra, Soumitra
2016-01-01
At present, Babcock-Leighton flux transport solar dynamo models appear as the most promising model for explaining diverse observational aspects of the sunspot cycle. The success of these flux transport dynamo models is largely dependent upon a single-cell meridional circulation with a deep equatorward component at the base of the Sun's convection zone. However, recent observations suggest that the meridional flow may in fact be very shallow (confined to the top 10 % of the Sun) and more complex than previously thought. Taken together these observations raise serious concerns on the validity of the flux transport paradigm. By accounting for the turbulent pumping of magnetic flux as evidenced in magnetohydrodynamic simulations of solar convection, we demonstrate that flux transport dynamo models can generate solar-like magnetic cycles even if the meridional flow is shallow. Solar-like periodic reversals is recovered even when meridional circulation is altogether absent, however, in this case the solar surface m...
Jouve, Laurene
2009-01-01
We present the first 3D MHD study in spherical geometry of the non-linear dynamical evolution of magnetic flux tubes in a turbulent rotating convection zone. We study numerically the rise of magnetic toroidal flux ropes from the base of a modelled convection zone up to the top of our computational domain where bipolar patches are formed. We compare the dynamical behaviour of flux tubes in a fully convective shell possessing self-consistently generated mean flows such as meridional circulation and differential rotation, with reference calculations done in a quiet isentropic zone. We find that two parameters influence the tubes during their rise through the convection zone: the initial field strength and amount of twist, thus confirming previous findings in Cartesian geometry. Further, when the tube is sufficiently strong with respect to the equipartition field, it rises almost radially independently of the initial latitude (either low or high). By contrast, weaker field cases indicate that downflows and upflow...
Kiyani, K.; Sahraoui, F.; Hnat, B.; Chapman, S. C.; Fauvarque, O.; Khotyaintsev, Y. V.
2012-12-01
The anisotropic nature of solar wind magnetic turbulence fluctuations is investigated scale-by-scale using high cadence in-situ magnetic field measurements from the Cluster and ACE spacecraft missions. The data span five decades in scales from the inertial range to the electron Larmor radius. In contrast to the inertial range, there is a successive increase towards isotropy between parallel and transverse power at scales below the ion Larmor radius, with isotropy being achieved at the electron Larmor radius. In the context of wave-mediated theories of turbulence, we show that this enhancement in magnetic fluctuations parallel to the local mean background field is qualitatively consistent with the magnetic compressibility signature of kinetic Alfvén wave solutions of the linearized Vlasov equation. More generally, we discuss how these results may arise naturally due to the prominent role of the Hall term at sub-ion Larmor scales. Furthermore, computing higher-order statistics, we show that the full statistical signature of the fluctuations at scales below the ion Larmor radius is that of a single isotropic globally scale-invariant process distinct from the anisotropic statistics of the inertial range.(Upper panel) PSD (from Cluster) of the transverse and parallel components spanning the inertial and dissipation ranges. (Lower panel) Ratio of parallel over transverse PSD. Horizontal dot-dashed line indicates a ratio of 1/3 where isotropy in power occurs. Vertical dashed and dashed-dotted lines indicate the ion and electron gyro-radii respectively, Doppler-shifted to spacecraft frequency using the Taylor hypothesis.
Simard, C; Dube, C
2016-01-01
We perform a mean-field analysis of the EULAG-MHD millenium simulation of global magnetohydrodynamical convection presented in Passos et al. 2014. The turbulent electromotive force operating in the simulation is assumed to be linearly related to the cyclic axisymmetric mean magnetic field and its first spatial derivatives. At every grid point in the simulation's meridional plane, this assumed relationship involves 27 independent tensorial coefficients. Expanding on Racine et al. 2011, we extract these coefficients from the simulation data through a least-squares minimization procedure based on singular value decomposition. The reconstructed alpha-tensor shows good agreement with that obtained by Racine et al. 2011, who did not include derivatives of the mean-field in their fit, as well as with the alpha-tensor extracted by Augustson et al. 2015 from a distinct ASH MHD simulation. The isotropic part of the turbulent magnetic diffusivity tensor beta is positive definite and reaches values of 5.0x10^7 m2s-1 in t...
Dynamic multiscaling in magnetohydrodynamic turbulence
Ray, Samriddhi Sankar; Pandit, Rahul
2016-01-01
We present the first study of the multiscaling of time-dependent velocity and magnetic-field structure functions in homogeneous, isotropic magnetohydrodynamic (MHD) turbulence in three dimensions. We generalize the formalism that has been developed for analogous studies of time-dependent structure functions in fluid turbulence to MHD. By carrying out detailed numerical studies of such time-dependent structure functions in a shell model for three-dimensional MHD turbulence, we obtain both equal-time and dynamic scaling exponents.
Dynamic multiscaling in magnetohydrodynamic turbulence.
Ray, Samriddhi Sankar; Sahoo, Ganapati; Pandit, Rahul
2016-11-01
We present a study of the multiscaling of time-dependent velocity and magnetic-field structure functions in homogeneous, isotropic magnetohydrodynamic (MHD) turbulence in three dimensions. We generalize the formalism that has been developed for analogous studies of time-dependent structure functions in fluid turbulence to MHD. By carrying out detailed numerical studies of such time-dependent structure functions in a shell model for three-dimensional MHD turbulence, we obtain both equal-time and dynamic scaling exponents.
Shell Models of Magnetohydrodynamic Turbulence
Plunian, Franck; Frick, Peter
2012-01-01
Shell models of hydrodynamic turbulence originated in the seventies. Their main aim was to describe the statistics of homogeneous and isotropic turbulence in spectral space, using a simple set of ordinary differential equations. In the eighties, shell models of magnetohydrodynamic (MHD) turbulence emerged based on the same principles as their hydrodynamic counter-part but also incorporating interactions between magnetic and velocity fields. In recent years, significant improvements have been made such as the inclusion of non-local interactions and appropriate definitions for helicities. Though shell models cannot account for the spatial complexity of MHD turbulence, their dynamics are not over simplified and do reflect those of real MHD turbulence including intermittency or chaotic reversals of large-scale modes. Furthermore, these models use realistic values for dimensionless parameters (high kinetic and magnetic Reynolds numbers, low or high magnetic Prandtl number) allowing extended inertial range and accu...
Tamain, P.; Bufferand, H.; Ciraolo, G.; Colin, C.; Galassi, D.; Ghendrih, Ph.; Schwander, F.; Serre, E.
2016-09-01
The new code TOKAM3X simulates plasma turbulence in full torus geometry including the open field lines of the Scrape-off Layer (SOL) and the edge closed field lines region in the vicinity of the separatrix. Based on drift-reduced Braginskii equations, TOKAM3X is able to simulate both limited and diverted plasmas. Turbulence is flux driven by incoming particles from the core plasma and no scale separation between the equilibrium and the fluctuations is assumed so that interactions between large scale flows and turbulence are consistently treated. Based on a domain decomposition, specific numerical schemes are proposed using conservative finite-differences associated to a semi-implicit time advancement. The process computation is multi-threaded and based on MPI and OpenMP libraries. In this paper, fluid model equations are presented together with the proposed numerical methods. The code is verified using the manufactured solution technique and validated through documented simple experiments. Finally, first simulations of edge plasma turbulence in X-point geometry are also introduced in a JET geometry.
Magnetohydrodynamic simulation of reconnection in turbulent astrophysical plasmas
Energy Technology Data Exchange (ETDEWEB)
Widmer, Fabien
2016-07-19
Turbulence is ubiquitous at large-Reynolds-number astrophysical plasmas like in the Solar corona. In such environments, the turbulence is thought to enhance the energy conversion rate by magnetic reconnection above the classical model predictions. Since turbulence cannot be simulated together with the large scale behaviour of the plasma, magnetic reconnection is studied through the average properties of turbulence. A Reynolds-averaged turbulence model is explored in which turbulence is self-sustained and -generated by the large scales (mean-) field inhomogeneities. Employing that model, the influence of turbulence is investigated by large-scale MHD numerical simulations solving evolution equations of the energy and cross-helicity of the turbulence together with the MHD equations. Magnetic reconnection is found to be either rapidly enhanced or suppressed by turbulence depending on the turbulence timescale. If the turbulence timescale is self-consistently calculated, reconnection is always strongly enhanced. Since the solar corona bears strong guide magnetic fields perpendicular to the reconnecting magnetic fields, the influences of a strong guide field on turbulent reconnection is separately investigated. A slow down of reconnection, obtained in the presence of a finite guide field, can be understood by a finite residual helicity working against the enhancement of reconnection by the turbulence. The influence of turbulence on magnetic reconnection is further studied by means of high resolution simulations of plasmoid-unstable current sheets. These simulations revealed the importance of turbulence for reaching fast reconnection.
Turbulent reconnection and its implications
Lazarian, A.; Eyink, G.; Vishniac, E.; Kowal, G.
2015-01-01
Magnetic reconnection is a process of magnetic field topology change, which is one of the most fundamental processes happening in magnetized plasmas. In most astrophysical environments, the Reynolds numbers corresponding to plasma flows are large and therefore the transition to turbulence is inevitable. This turbulence, which can be pre-existing or driven by magnetic reconnection itself, must be taken into account for any theory of magnetic reconnection that attempts to describe the process in the aforementioned environments. This necessity is obvious as three-dimensional high-resolution numerical simulations show the transition to the turbulence state of initially laminar reconnecting magnetic fields. We discuss ideas of how turbulence can modify reconnection with the focus on the Lazarian & Vishniac (Lazarian & Vishniac 1999 Astrophys. J. 517, 700–718 ()) reconnection model. We present numerical evidence supporting the model and demonstrate that it is closely connected to the experimentally proven concept of Richardson dispersion/diffusion as well as to more recent advances in understanding of the Lagrangian dynamics of magnetized fluids. We point out that the generalized Ohm's law that accounts for turbulent motion predicts the subdominance of the microphysical plasma effects for reconnection for realistically turbulent media. We show that one of the most dramatic consequences of turbulence is the violation of the generally accepted notion of magnetic flux freezing. This notion is a cornerstone of most theories dealing with magnetized plasmas, and therefore its change induces fundamental shifts in accepted paradigms, for instance, turbulent reconnection entails reconnection diffusion process that is essential for understanding star formation. We argue that at sufficiently high Reynolds numbers the process of tearing reconnection should transfer to turbulent reconnection. We discuss flares that are predicted by turbulent reconnection and relate this process to
Guszejnov, Dávid; Zoletnik, Sándor; Lazányi, Nóra
2014-01-01
This paper is aimed to contribute to the scientific discussions that have been triggered by the experimental observation of a quadratic relation between the kurtosis and skewness of turbulent fluctuations present in fusion plasmas and other nonlinear physical systems. In this paper we offer a general statistical model which attributes the observed $K = aS^2 + b$ relation to the varying intermittency of the experimental signals. The model is a two random variable model constructed to catch the essential intermittent feature of the real signal. One of the variables is the amplitude of the underlying intermittent event (e.g. turbulent structure) while the other is connected to the intermittency level of the system. This simple model can attribute physical meaning to the $a$ and $b$ coefficients, as they characterize the spatio-temporal statistics of intermittent events. By constructing a particle-conserving Gaussian model for the underlying coherent structures the experimentally measured $a$ and $b$ coefficients...
Magnetohydrodynamic Turbulence and the Geodynamo
Shebalin, John V.
2016-01-01
Recent research results concerning forced, dissipative, rotating magnetohydrodynamic (MHD) turbulence will be discussed. In particular, we present new results from long-time Fourier method (periodic box) simulations in which forcing contains varying amounts of magnetic and kinetic helicity. Numerical results indicate that if MHD turbulence is forced so as to produce a state of relatively constant energy, then the largest-scale components are dominant and quasistationary, and in fact, have an effective dipole moment vector that aligns closely with the rotation axis. The relationship of this work to established results in ideal MHD turbulence, as well as to models of MHD turbulence in a spherical shell will also be presented. These results appear to be very pertinent to understanding the Geodynamo and the origin of its dominant dipole component. Our conclusion is that MHD turbulence, per se, may well contain the origin of the Earth's dipole magnetic field.
Interstellar Turbulence II: Implications and Effects
Scalo, J
2004-01-01
Interstellar turbulence has implications for the dispersal and mixing of the elements, cloud chemistry, cosmic ray scattering, and radio wave propagation through the ionized medium. This review discusses the observations and theory of these effects. Metallicity fluctuations are summarized, and the theory of turbulent transport of passive tracers is reviewed. Modeling methods, turbulent concentration of dust grains, and the turbulent washout of radial abundance gradients are discussed. Interstellar chemistry is affected by turbulent transport of various species between environments with different physical properties and by turbulent heating in shocks, vortical dissipation regions, and local regions of enhanced ambipolar diffusion. Cosmic rays are scattered and accelerated in turbulent magnetic waves and shocks, and they generate turbulence on the scale of their gyroradii. Radio wave scintillation is an important diagnostic for small scale turbulence in the ionized medium, giving information about the power spe...
Bian, Nicolas H; Emslie, A Gordon
2016-01-01
The transport of the energy contained in electrons, both thermal and suprathermal, in solar flares plays a key role in our understanding of many aspects of the flare phenomenon, from the spatial distribution of hard X-ray emission to global energetics. Motivated by recent {\\em RHESSI} observations that point to the existence of a mechanism that confines electrons to the coronal parts of flare loops more effectively than Coulomb collisions, we here consider the impact of pitch-angle scattering off turbulent magnetic fluctuations on the parallel transport of electrons in flaring coronal loops. It is shown that the presence of such a scattering mechanism in addition to Coulomb collisional scattering can significantly reduce the parallel thermal and electrical conductivities relative to their collisional values. We provide illustrative expressions for the resulting thermoelectric coefficients that relate the thermal flux and electrical current density to the temperature gradient and the applied electric field. We...
Energy Technology Data Exchange (ETDEWEB)
Kolberg, U.; Schlickeiser, R. [Institut für Theoretische Physik, Lehrstuhl IV: Weltraum- and Astrophysik, Ruhr-Universität, Bochum (Germany); Yoon, P. H., E-mail: uk@tp4.rub.de, E-mail: rsch@tp4.rub.de, E-mail: yoonp@umd.edu [IPST, University of Maryland, College Park, Maryland 20742-2431 (United States)
2016-02-01
Highly relativistic electron–positron pair beams considerably affect the spontaneously emitted field fluctuations in the unmagnetized intergalactic medium (IGM). In view of the considered small density ratio of beam and background plasma, a perturbative treatment is employed in order to derive the spectral balance equations for the fluctuating fields from first principles of plasma kinetic theory that are covariantly correct within the limits of special relativity. They self-consistently account for the competing effects of spontaneous and induced emission and absorption in the perturbed thermal plasma. It is found that the presence of the beam transforms the growth rate of the dominating transverse damped aperiodic mode into an effective growth rate that displays positive values in certain spectral regions if beam velocity and wave vector are perpendicular or almost perpendicular to each other. This corresponds to a quasi-instability that induces an amplification of the fluctuations for these wavenumbers. Such an effect can greatly influence the cosmic magnetogenesis as it affects the strengths of the spontaneously emitted magnetic seed fields in the IGM, thereby possibly lowering the required growth time and effectivity of any further amplification mechanism such as an astrophysical dynamo.
Kolberg, U.; Schlickeiser, R.; Yoon, P. H.
2016-02-01
Highly relativistic electron-positron pair beams considerably affect the spontaneously emitted field fluctuations in the unmagnetized intergalactic medium (IGM). In view of the considered small density ratio of beam and background plasma, a perturbative treatment is employed in order to derive the spectral balance equations for the fluctuating fields from first principles of plasma kinetic theory that are covariantly correct within the limits of special relativity. They self-consistently account for the competing effects of spontaneous and induced emission and absorption in the perturbed thermal plasma. It is found that the presence of the beam transforms the growth rate of the dominating transverse damped aperiodic mode into an effective growth rate that displays positive values in certain spectral regions if beam velocity and wave vector are perpendicular or almost perpendicular to each other. This corresponds to a quasi-instability that induces an amplification of the fluctuations for these wavenumbers. Such an effect can greatly influence the cosmic magnetogenesis as it affects the strengths of the spontaneously emitted magnetic seed fields in the IGM, thereby possibly lowering the required growth time and effectivity of any further amplification mechanism such as an astrophysical dynamo.
Semena, Andrey N; Buckley, David A H; Kotze, Marissa M; Khabibullin, Ildar I; Breytenbach, Hannes; Gulbis, Amanda A S; Coppejans, Rocco; Potter, Stephen B
2014-01-01
We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at timescales shorter than the cooling time of hot plasma in the post shock region of the accretion curtain near the WD surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely SALT and the SAAO 1.9m telescope. We place upper limits on the plasma cooling timescale $\\tau<$0.3 sec, on the fractional area of the accretion curtain footprint $f<1.6\\times10^{-4}$, and a lower limit on the specific mass accretion rate $\\dot{M}/A \\gtrsim $3 g/sec/cm$^{-2}$. We show that measurements of accretion column footprints v...
Directory of Open Access Journals (Sweden)
Xuefeng Li
2014-04-01
Full Text Available Based on solving numerically the generalized nonlinear Langevin equation describing the nonlinear dynamics of stochastic resonance by Fourth-order Runge-Kutta method, an aperiodic stochastic resonance based on an optical bistable system is numerically investigated. The numerical results show that a parameter-tuning stochastic resonance system can be realized by choosing the appropriate optical bistable parameters, which performs well in reconstructing aperiodic signals from a very high level of noise background. The influences of optical bistable parameters on the stochastic resonance effect are numerically analyzed via cross-correlation, and a maximum cross-correlation gain of 8 is obtained by optimizing optical bistable parameters. This provides a prospective method for reconstructing noise-hidden weak signals in all-optical signal processing systems.
Energy Technology Data Exchange (ETDEWEB)
Li, Xuefeng, E-mail: lixfpost@163.com [School of Science, Xi' an University of Post and Telecommunications, Xi' an, 710121 (China); Cao, Guangzhan; Liu, Hongjun [Xi' an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi' an, 710119 (China)
2014-04-15
Based on solving numerically the generalized nonlinear Langevin equation describing the nonlinear dynamics of stochastic resonance by Fourth-order Runge-Kutta method, an aperiodic stochastic resonance based on an optical bistable system is numerically investigated. The numerical results show that a parameter-tuning stochastic resonance system can be realized by choosing the appropriate optical bistable parameters, which performs well in reconstructing aperiodic signals from a very high level of noise background. The influences of optical bistable parameters on the stochastic resonance effect are numerically analyzed via cross-correlation, and a maximum cross-correlation gain of 8 is obtained by optimizing optical bistable parameters. This provides a prospective method for reconstructing noise-hidden weak signals in all-optical signal processing systems.
Remarks on disorder and aperiodicity in a model for interacting polymers
Haddad, T. A. S.; Andrade, R. F. S.; Salinas, S. R.
2004-12-01
We present a comparative study of the effects of random and aperiodically distributed interactions on the critical behavior of a model for two interacting polymers on a diamond hierarchical lattice. The problem is formulated in terms of exact renormalization-group (RG) recursion relations. In the disordered case, it is possible to develop a perturbative treatment in order to obtain the fixed points of the moments associated with the random distribution of interactions. Fully uncorrelated disorder may become relevant, driving the system away from a homogeneous fixed point. Layered disorder may lead to a breakdown of the perturbative treatment. In the case of aperiodic interactions, we also show some examples of relevance and irrelevance of geometric fluctuations, and further investigate the models by resorting to an independent transfer-matrix (TM) analysis, which fully corroborates the scaling results.
Electronic spectrum and localization of electronic states in aperiodic quantum dot chains
Korotaev, P. Yu.; Vekilov, Yu. Kh.; Kaputkina, N. E.
2014-02-01
The electronic energy spectra of aperiodic Thue-Morse, Rudin-Shapiro, and double-periodic quantum dot chains are investigated in the tight-binding approximation. The dependence of the spectrum on all parameters of a "mixed" aperiodic chain model is studied: the electronic energy at quantum dots and the hopping integrals. The electronic degree of localization in the chains under consideration is determined by analyzing the inverse participation ratio. Its spectral distribution and the dependence of the band-averaged degree of localization on these model parameters have been calculated. It is shown that a transition of the system's sites to a resonant state in which the degree of electron localization decreases, while an overlap between the subbands occurs in the spectrum is possible when the parameters are varied.
Lazo, E.; Castro, C. E.; Cortés-Cortés, F.
2016-09-01
In this work we study the relationship existing between the localization properties of the diluted and non-diluted direct electrical transmission lines with the overlap amplitude Cijω = 2 | Iiω Ijω | , where Ijω is the amplitude of the electric current function at jth cell of the transmission line for the state with frequency ω. We distribute two values of inductances LA and LB, according to the generalized aperiodic Thue-Morse m-tupling sequence. We find that the behavior of Ci,jω is directly related to the localization properties of the aperiodic sequences measured by the ξ normalized participation number, the Rq Rényi entropies and the μq moments. In addition, we generalize the scaling relationship for the overlap amplitude Ci,jω, i.e., =(2/N) 2 q.
Can fractal-like spectra be experimentally observed in aperiodic superlattices?
Maciá, Enrique; Domínguez-Adame, Francisco
1996-07-01
We numerically investigate the effects of inhomogeneities in the energy spectrum of aperiodic semiconductor superlattices, focusing our attention on Thue-Morse and Fibonacci sequences. In the absence of disorder, the corresponding electronic spectra are self-similar. The presence of a certain degree of randomness, due to imperfections occurring during the growth processes, gives rise to a progressive loss of quantum coherence, smearing out the finer details of the energy spectra predicted for perfect aperiodic superlattices and spurring the onset of electron localization. However, depending on the degree of disorder introduced, a critical size for the system exists, below which peculiar transport properties, related to the pre-fractal nature of the energy spectrum, may be measured.
Multifluid magnetohydrodynamic turbulent decay
Downes, Turlough P
2011-01-01
It is generally believed that turbulence has a significant impact on the dynamics and evolution of molecular clouds and the star formation which occurs within them. Non-ideal magnetohydrodynamic effects are known to influence the nature of this turbulence. We present the results of a suite of 512-cubed resolution simulations of the decay of initially super-Alfvenic and supersonic fully multifluid MHD turbulence. We find that ambipolar diffusion increases the rate of decay of the turbulence while the Hall effect has virtually no impact. The decay of the kinetic energy can be fitted as a power-law in time and the exponent is found to be -1.34 for fully multifluid MHD turbulence. The power spectra of density, velocity and magnetic field are all steepened significantly by the inclusion of non-ideal terms. The dominant reason for this steepening is ambipolar diffusion with the Hall effect again playing a minimal role except at short length scales where it creates extra structure in the magnetic field. Interestingl...
Turbulent Plasmoid Reconnection
Widmer, Fabien; Yokoi, Nobumitsu
2016-01-01
The plasmoid instability may lead to fast magnetic reconnection through long current sheets(CS). It is well known that large-Reynolds-number plasmas easily become turbulent. We address the question whether turbulence enhances the energy conversion rate of plasmoid-unstable current sheets. We carry out appropriate numerical MHD simulations, but resolving simultaneously the relevant large-scale (mean-) fields and the corresponding small-scale, turbulent, quantities by means of direct numerical simulations (DNS) is not possible. Hence we investigate the influence of small scale turbulence on large scale MHD processes by utilizing a subgrid-scale (SGS) turbulence model. We verify the applicability of our SGS model and then use it to investigate the influence of turbulence on the plasmoid instability. We start the simulations with Harris-type and force-free CS equilibria in the presence of a finite guide field in the direction perpendicular to the reconnection plane. We use the DNS results to investigate the growt...
Aperiodicity in one-way Markov cycles and repeat times of large earthquakes in faults
Tejedor, Alejandro; Pacheco, Amalio F
2011-01-01
A common use of Markov Chains is the simulation of the seismic cycle in a fault, i.e. as a renewal model for the repetition of its characteristic earthquakes. This representation is consistent with Reid's elastic rebound theory. Here it is proved that in {\\it any} one-way Markov cycle, the aperiodicity of the corresponding distribution of cycle lengths is always lower than one. This fact concurs with observations of large earthquakes in faults all over the world.
Deterministic Aperiodic Structures for on-chip Nanophotonics and Nanoplasmonics Device Applications
2013-04-01
nanosphere (50nm radius) and for periodic, Fibonacci, Thue-Morse, and Rudin -Shapiro aperiodic arrays of nano-spheres with 50 nm minimum interparticle...where we show the calculated near-field spectra for Ag nanosphere arrays with periodic, Fibonacci, Thue-Morse, and Rudin -Shapiro sequences. The case of...versus wavelength (x-axis) and field-enhancement (y-axis) for (a) Periodic, (b) Fibonacci, (c) Thue-Morse, (d) Rudin -Shapiro. The arrays consist of 50 nm
Tyc, Michał H.; Salejda, Włodzimierz; Klauzer-Kruszyna, Agnieszka; Tarnowski, Karol
2007-05-01
The dispersion relation for polarized light transmitting through a one-dimensional superlattice composed of aperiodically arranged layers made of ordinary dielectric and negative refraction metamaterials is calculated with finite element method. Generalized Fibonacci, generalized Thue-Morse, double-periodic and Rudin-Shapiro superlattices are investigated, using their periodic approximants. Strong dispersion of metamaterials is taken into account. Group velocities and effective refraction indices in the structures are calculated. The self-similar structure of the transmission spectra is observed.
Institute of Scientific and Technical Information of China (English)
KONG Yan; CHEN Xian-Feng; XIA Yu-Xing
2008-01-01
@@ In order to minimize the effect of the unconsidered frequency components on the generated compression pulse,the phasing-blanking effect is taken into account of designing the one-dimensional aperiodic domain reversal structure. Hierarchic genetic algorithm for the design of a domain reversal grating to modulate the spectrum and phase of the generated SH pulse simultaneously are presented. Our simulation shows that the quality of an output pulse is fairly improved.
Real-time remedial action against aperiodic small signal rotor angle instability
DEFF Research Database (Denmark)
Weckesser, Johannes Tilman Gabriel; Jóhannsson, Hjörtur; Østergaard, Jacob
2016-01-01
This paper presents a method that in real-time determines remedial actions, which restore stable operation with respect to aperiodic small signal rotor angle stability (ASSRAS) when insecure or unstable operation has been detected. An ASSRAS assessment method is used to monitor the stability boun...... on the IEEE 14-bus and the Nordic32 test systems where results show that the method can efficiently determine the required active power redispatch to avoid an imminent instability....
Finite current stationary states of random walks on one-dimensional lattices with aperiodic disorder
Miki, Hiroshi
2016-11-01
Stationary states of random walks with finite induced drift velocity on one-dimensional lattices with aperiodic disorder are investigated by scaling analysis. Three aperiodic sequences, the Thue-Morse (TM), the paperfolding (PF), and the Rudin-Shapiro (RS) sequences, are used to construct the aperiodic disorder. These are binary sequences, composed of two symbols A and B, and the ratio of the number of As to that of Bs converges to unity in the infinite sequence length limit, but their effects on diffusional behavior are different. For the TM model, the stationary distribution is extended, as in the case without current, and the drift velocity is independent of the system size. For the PF model and the RS model, as the system size increases, the hierarchical and fractal structure and the localized structure, respectively, are broken by a finite current and changed to an extended distribution if the system size becomes larger than a certain threshold value. Correspondingly, the drift velocity is saturated in a large system while in a small system it decreases as the system size increases.
Baili, Amira; Cherif, Rim; Heidt, Alexander; Zghal, Mourad
2014-05-01
We describe in detail a new procedure of maximizing the bandwidth of mid-infrared (mid-IR) supercontinuum (SC) in highly nonlinear microstructured As2Se3 and tellurite aperiodic nanofibers. By introducing aperiodic rings of first and secondary air holes into the cross-sections of our microstructured fiber designs, we achieve flattened and all-normal dispersion profiles over much broader bandwidths than would be possible with simple periodic designs. These fiber designs are optimized for efficient, broadband, and coherent SC generation in the mid-IR spectral region. Numerical simulations show that these designs enable the generation of a SC spanning over 2290 nm extending from 1140 to 3430 nm in 8 cm length of tellurite nanofiber with input energy of E = 200 pJ and a SC bandwidth of over 4700 nm extending from 1795 to 6525 nm generated in only 8 mm-length of As2Se3-based nanofiber with input energy as low as E = 100 pJ. This work provides a new type of broadband mid-IR SC source with flat spectral shape as well as excellent coherence and temporal properties by using aperiodic nanofibers with all-normal dispersion suitable for applications in ultrafast science, metrology, coherent control, non-destructive testing, spectroscopy, and optical coherence tomography in the mid-IR region.
Turbulence and diffusion fossil turbulence
Gibson, C H
2000-01-01
Fossil turbulence processes are central to turbulence, turbulent mixing, and turbulent diffusion in the ocean and atmosphere, in astrophysics and cosmology, and in most other natural flows. George Gamov suggested in 1954 that galaxies might be fossils of primordial turbulence produced by the Big Bang. John Woods showed that breaking internal waves on horizontal dye sheets in the interior of the stratified ocean form highly persistent remnants of these turbulent events, which he called fossil turbulence. The dark mixing paradox of the ocean refers to undetected mixing that must exist somewhere to explain why oceanic scalar fields like temperature and salinity are so well mixed, just as the dark matter paradox of galaxies refers to undetected matter that must exist to explain why rotating galaxies don't fly apart by centrifugal forces. Both paradoxes result from sampling techniques that fail to account for the extreme intermittency of random variables involved in self-similar, nonlinear, cascades over a wide ra...
Linkmann, Moritz; McKay, Mairi; Berera, Arjun; Biferale, Luca
2016-01-01
We present a numerical and analytical study of incompressible homogeneous conducting fluids using a Fourier-helical representation. We analytically study both small- and large-scale dynamo properties, as well as the inverse cascade of magnetic helicity, in the most general minimal subset of interacting velocity and magnetic fields on a closed Fourier triad. We mainly focus on the dependency of magnetic field growth as a function of the distribution of kinetic and magnetic helicities among the three interacting wavenumbers. By combining direct numerical simulations of the full magnetohydrodynamics (MHD) equations with the Fourier-helical decomposition we numerically confirm that in the kinematic dynamo regime the system develops a large-scale magnetic helicity with opposite sign compared to the small-scale kinetic helicity, a sort of triad-by-triad $\\alpha$-effect in Fourier space. Concerning the small-scale perturbations, we predict theoretically and confirm numerically that the largest instability is achived...
Anisotropic turbulence in weakly stratified rotating magnetoconvection
Giesecke, A
2010-01-01
Numerical simulations of the 3D MHD-equations that describe rotating magnetoconvection in a Cartesian box have been performed using the code NIRVANA. The characteristics of averaged quantities like the turbulence intensity and the turbulent heat flux that are caused by the combined action of the small-scale fluctuations are computed. The correlation length of the turbulence significantly depends on the strength and orientation of the magnetic field and the anisotropic behavior of the turbulence intensity induced by Coriolis and Lorentz force is considerably more pronounced for faster rotation. The development of isotropic behavior on the small scales -- as it is observed in pure rotating convection -- vanishes even for a weak magnetic field which results in a turbulent flow that is dominated by the vertical component. In the presence of a horizontal magnetic field the vertical turbulent heat flux slightly increases with increasing field strength, so that cooling of the rotating system is facilitated. Horizont...
Aperiodic topological order in the domain configurations of functional materials
Huang, Fei-Ting; Cheong, Sang-Wook
2017-03-01
In numerous functional materials, such as steels, ferroelectrics and magnets, new functionalities can be achieved through the engineering of the domain structures, which are associated with the ordering of certain parameters within the material. The recent progress in technologies that enable imaging at atomic-scale spatial resolution has transformed our understanding of domain topology, revealing that, along with simple stripe-like or irregularly shaped domains, intriguing vortex-type topological domain configurations also exist. In this Review, we present a new classification scheme of 'Zm Zn domains with Zl vortices' for 2D macroscopic domain structures with m directional variants and n translational antiphases. This classification, together with the concepts of topological protection and topological charge conservation, can be applied to a wide range of materials, such as multiferroics, improper ferroelectrics, layered transition metal dichalcogenides and magnetic superconductors, as we discuss using selected examples. The resulting topological considerations provide a new basis for the understanding of the formation, kinetics, manipulation and property optimization of domains and domain boundaries in functional materials.
Energy Technology Data Exchange (ETDEWEB)
Gary, S Peter [Los Alamos National Laboratory; Narita, Y [UNIV OF BRAUNSCHWEIG; Glassmeier, K H [UNIV OF BRAUNSCHWEIG; Goldstein, M L [NGSFC; Safraoui, F [NGSFC; Treumann, R A [UNIV. MUNICH
2009-01-01
Using four-point measurements of the CLUSTER spacecraft, the energy distribution of magnetic field fluctuations in the solar wind is determined directly in the three-dimensional wave vector domain in the range 3 x 10{sup -4} rad/km < k < 3 x 10{sup -3} rad/km. The analysis method takes account of a regular tetrahedron configuration of CLUSTER and the Doppler effect. The energy distribution in the flow rest frame is anisotropic, characterized by two distinct extended structures perpendicular to the mean magnetic field and furthermore perpendicular to the flow direction. The three-dimensional distribution is averaged around the direction of the mean magnetic field direction, and then is further reduced to one-dimensional distributions in the wave number domain parallel and perpendicular to the mean magnetic field. The one-dimensional energy spectra are characterized by the power law with the index -5/3 and furthermore very close energy density between parallel and perpendicular directions to the mean magnetic field at the same wave numbers. Though the distributions and the spectra are not covered in a wide range of wave vectors, our measurements suggest that the solar wind fluctuation is anisotropic in the three-dimensional wave vector space. It is, however, rather isotropic when reduced into the parallel and perpendicular wave vector geometries due to the second anisotropy imposed by the flow direction.
Quasilinear saturation of the aperiodic ordinary mode streaming instability
Energy Technology Data Exchange (ETDEWEB)
Stockem Novo, A., E-mail: anne@tp4.rub.de; Schlickeiser, R. [Institut für Theoretische Physik, Lehrstuhl IV: Weltraum-und Astrophysik, Ruhr-Universität Bochum, D-44780 Bochum (Germany); Yoon, P. H. [Institute for Physical Science & Technology, University of Maryland, College Park, Maryland 20742 (United States); School of Space Research, Kyung Hee University, Yongin-Si, Gyeonggi-Do 446-701 (Korea, Republic of); Lazar, M. [Institut für Theoretische Physik, Lehrstuhl IV: Weltraum-und Astrophysik, Ruhr-Universität Bochum, D-44780 Bochum (Germany); Centre for Mathematical Plasma Astrophysics, Celestijnenlaan 200B, 3001 Leuven (Belgium); Poedts, S. [Centre for Mathematical Plasma Astrophysics, Celestijnenlaan 200B, 3001 Leuven (Belgium); Seough, J. [Faculty of Human Development, University of Toyama, 3190, Gofuku, Toyama City, Toyama 930-8555 (Japan); International Research Fellow of the Japan Society for the Promotion of Science, Tokyo (Japan)
2015-09-15
In collisionless plasmas, only kinetic instabilities and fluctuations are effective in reducing the free energy and scatter plasma particles, preventing an increase of their anisotropy. Solar energetic outflows into the interplanetary plasma give rise to important thermal anisotropies and counterstreaming motions of plasma shells, and the resulting instabilities are expected to regulate the expansion of the solar wind. The present paper combines quasilinear theory and kinetic particle-in-cell simulations in order to study the weakly nonlinear saturation of the ordinary mode in hot counter-streaming plasmas with a temperature anisotropy as a follow-up of the paper by Seough et al. [Phys. Plasmas 22, 082122 (2015)]. This instability provides a plausible mechanism for the origin of dominating, two-dimensional spectrum of transverse magnetic fluctuations observed in the solar wind. Stimulated by the differential motion of electron counterstreams the O mode instability may convert their free large-scale energy by nonlinear collisionless dissipation on plasma particles.
Statistics of Reconnection-Driven Turbulence
Kowal, Grzegorz; Lazarian, Alex; Vishniac, Ethan T
2016-01-01
Magnetic reconnection is a process that changes magnetic field topology in highly conducting fluids. Within the standard Sweet-Parker model, this process would be too slow to explain observations (e.g. solar flares). In reality, the process must be ubiquitous as astrophysical fluids are magnetized and motions of fluid elements necessarily entail crossing of magnetic frozen-in field lines and magnetic reconnection. In the presence of turbulence, the reconnection is independent of microscopic plasma properties, and may be much faster than previously thought, as proposed in Lazarian & Vishniac (1999) and tested in Kowal et al. (2009, 2012). However, the considered turbulence in the Lazarian-Vishniac model was imposed externally. In this work we consider reconnection-driven magnetized turbulence in realistic three-dimensional geometry initiated by stochastic noise. We demonstrate through numerical simulations that the stochastic reconnection is able to self-generate turbulence through interactions between the...
Energy Technology Data Exchange (ETDEWEB)
Horton, W. [Univ. of Texas, Austin, TX (United States). Inst. for Fusion Studies; Hu, G. [Globalstar LP, San Jose, CA (United States)
1998-07-01
The origin of plasma turbulence from currents and spatial gradients in plasmas is described and shown to lead to the dominant transport mechanism in many plasma regimes. A wide variety of turbulent transport mechanism exists in plasmas. In this survey the authors summarize some of the universally observed plasma transport rates.
EuHIT, Collaboration
2015-01-01
As a member of the EuHIT (European High-Performance Infrastructures in Turbulence - see here) consortium, CERN is participating in fundamental research on turbulence phenomena. To this end, the Laboratory provides European researchers with a cryogenic research infrastructure (see here), where the first tests have just been performed.
Turbulence optimisation in stellarator experiments
Energy Technology Data Exchange (ETDEWEB)
Proll, Josefine H.E. [Max-Planck/Princeton Center for Plasma Physics (Germany); Max-Planck-Institut fuer Plasmaphysik, Wendelsteinstr. 1, 17491 Greifswald (Germany); Faber, Benjamin J. [HSX Plasma Laboratory, University of Wisconsin-Madison, Madison, WI 53706 (United States); Helander, Per; Xanthopoulos, Pavlos [Max-Planck/Princeton Center for Plasma Physics (Germany); Lazerson, Samuel A.; Mynick, Harry E. [Plasma Physics Laboratory, Princeton University, P.O. Box 451 Princeton, New Jersey 08543-0451 (United States)
2015-05-01
Stellarators, the twisted siblings of the axisymmetric fusion experiments called tokamaks, have historically suffered from confining the heat of the plasma insufficiently compared with tokamaks and were therefore considered to be less promising candidates for a fusion reactor. This has changed, however, with the advent of stellarators in which the laminar transport is reduced to levels below that of tokamaks by shaping the magnetic field accordingly. As in tokamaks, the turbulent transport remains as the now dominant transport channel. Recent analytical theory suggests that the large configuration space of stellarators allows for an additional optimisation of the magnetic field to also reduce the turbulent transport. In this talk, the idea behind the turbulence optimisation is explained. We also present how an optimised equilibrium is obtained and how it might differ from the equilibrium field of an already existing device, and we compare experimental turbulence measurements in different configurations of the HSX stellarator in order to test the optimisation procedure.
Turbulent transport in hydromagnetic flows
Brandenburg, A; Del Sordo, F; Hubbard, A; Käpylä, P J; Rheinhardt, M
2010-01-01
The predictive power of mean-field theory is emphasized by comparing theory with simulations under controlled conditions. The recently developed test-field method is used to extract turbulent transport coefficients both in kinematic as well as nonlinear and quasi-kinematic cases. A striking example of the quasi-kinematic method is provided by magnetic buoyancy-driven flows that produce an alpha effect and turbulent diffusion.
Turbulence spreading in gyro-kinetic theory
Migliano, P.; Buchholz, R.; Grosshauser, S. R.; Hornsby, W. A.; Peeters, A. G.; Stauffert, O.
2016-01-01
In this letter a new operative definition for the turbulence intensity in connection with magnetized plasmas is given. In contrast to previous definitions the new definition satisfies a Fisher-Kolmogorov-Petrovskii-Piskunov type equation. Furthermore, explicit expressions for the turbulence intensity and the turbulence intensity flux, that allow for the first time direct numerical evaluation, are derived. A carefully designed numerical experiment for the case of a tokamak is performed to study the impact of turbulence spreading. The effective turbulence diffusion coefficient is measured to be smaller than the heat conduction coefficient and the turbulence spreading length is found to be of the order of the turbulence correlation length. The results show that turbulence spreading can play a role in the non-local flux gradient relation, or in the scaling of transport coefficients with the normalized Larmor radius, only over lengths scale of the order of the turbulence correlation length. A new turbulence convection mechanism, due to the drift connected with the magnetic field inhomogeneities, is described. The convective flux integrates to zero under the flux surface average unless there is an up-down asymmetry in the tubulence intensity. The latter asymmetry can be generated through a radial inhomogeneity or plasma rotation. It is shown that the turbulence convection can lead to a spreading of the order of the correlation length.
Wide-Area Assessment of Aperiodic Small Signal Rotor Angle Stability in Real-Time
DEFF Research Database (Denmark)
Jóhannsson, Hjörtur; Nielsen, Arne Hejde; Østergaard, Jacob
2013-01-01
This paper presents the details of a new real-time stability assessment method. The method assesses a particular mechanism of stability: each generator’s capability to generate sufficient steady state electromechanical torque. The lack of sufficient steady state torque causes aperiodic increase...... of multiple operating points is derived in the paper. Finally, results from time-domain simulation of instability scenarios in the Nordic32 test system are presented and results used for testing the assessment method. The results illustrate the method’s capability to efficiently identify the location...
Wide-Area Assessment of Aperiodic Small Signal Rotor Angle Stability in Real-Time
DEFF Research Database (Denmark)
Jóhannsson, Hjörtur; Nielsen, Arne Hejde; Østergaard, Jacob
2014-01-01
This paper presents the details of a new real-time stability assessment method. The method assesses a particular mechanism of stability: each generator's capability to generate sufficient steady state electromechanical torque. The lack of sufficient steady state torque causes aperiodic increase...... of multiple operating points is derived in the paper. Finally, results from timedomain simulation of instability scenarios in the Nordic32 test system are presented and results used for testing the assessment method. The results illustrate the method's capability to efficiently identify the location...
Bendahma, F.; Djelti, R.; Bentata, S.
2016-08-01
The aperiodic GaAs/AlxGa1-xAs superlattices (SL) with trimer disorder have been studied in this paper. The transfer-matrix technique and the exact Airy function formalism have been used to determine the miniband structure, the transmission coefficient, the resonance energy and resonant tunneling time (RTT). Although the disorder localizes the states on average, our numerical calculations show that the localization length of the states becomes more extended when the disorder is correlated (trimer case). We have also found that the RTT is of the order of several femtoseconds.
INVERSE CORRECTION OF FOURIER TRANSFORMS FOR ONE-DIMENSIONAL STRONGLY SCATTERING APERIODIC LATTICES
Directory of Open Access Journals (Sweden)
Y. F. Hsin
2016-05-01
Full Text Available The accuracy of the Fourier transform (FT, advantageous for the aperiodic lattice (AL design, is significantly improved for strongly scattering periodic lattices (PLs and ALs. The approach is to inversely obtain corrected parameters from an accurate transfer matrix method for the FT. We establish a corrected FT in order to improve the spectral inaccuracy of strongly scattering PLs by redefining wave numbers and reflective intensity. We further correct the FT for strongly scattering ALs by implementing improvements applied to strongly scattering PLs and then making detailed wave number adjustments in the main band spectral region. Silicon lattice simulations are presented.
Critical behavior of the Ising model on a hierarchical lattice with aperiodic interactions
Pinho, S. T. R.; Haddad, T. A. S.; Salinas, S. R.
We write the exact renormalization-group recursion relations for nearest-neighbor ferromagnetic Ising models on Migdal-Kadanoff hierarchical lattices with a distribution of aperiodic exchange interactions according to a class of substitutional sequences. For small geometric fluctuations, the critical behavior is unchanged with respect to the uniform case. For large fluctuations, as in the case of the Rudin-Shapiro sequence, the uniform fixed point in the parameter space cannot be reached from any physical initial conditions. We derive a criterion to check the relevance of the geometric fluctuations.
Scaling in the Optical Characteristics of Aperiodic Structures with Self-Similarity Symmetry
Energy Technology Data Exchange (ETDEWEB)
Zotov, A. M.; Korolenko, P. V., E-mail: pvkorolenko@rambler.ru; Mishin, A. Yu. [Moscow State University (Russian Federation)
2010-11-15
The properties of diffraction gratings and multilayered systems constructed using 1D models of quasicrystals are considered based on numerical simulation. It is shown that there is a direct relationship between the self-similarity symmetry of quasicrystals and scaling in the characteristics of the above-mentioned optical devices. The degree of structural correspondence between the graphical representations of the geometric properties of crystals, light diffraction patterns of gratings, and the transmission spectra of multilayered systems is estimated. It is shown that certain types of self-similarity symmetry make the characteristics of aperiodic diffraction gratings highly stable to a change in the size ratio of forming elements.
Vecchio, A.; Primavera, L.; Carbone, V.
2006-03-01
The dynamics of the occurrence of the dengue hemorrhagic fever in the 72 provinces of Thailand is investigated by performing a proper orthogonal decomposition (POD) on spatiotemporal data. Using this technique, we are able to identify and select the contribution of different modes, selected according to the energy content, to the evolution of the epidemic during 14 years. We found that the phenomenon is characterized by periodic cycles of yearly occurrence characterized by spatial scales of about 420 km. Superimposed on this basic mode, POD analysis is able to reveal the presence of high-energetic aperiodic traveling pulses of the epidemic, which extend spatially for about 510 km from Bangkok.
Bloch-like surface waves in Fibonacci quasi-crystals and Thue-Morse aperiodic dielectric multilayers
Koju, Vijay; Robertson, William M.
2016-09-01
Bloch surface waves (BSWs) in periodic dielectric multilayer structures with surface defect have been extensively studied. However, it has recently been recognized that quasi-crystals and aperiodic dielectric multilayers also support Bloch-like surface waves (BLSWs). In this work, we numerically show the existence of BLSWs in Fibonacci quasi-crystals and Thue-Morse aperiodic dielectric multilayers using the prism coupling technique. We compare the surface field enhancement and penetration depth of BLSWs in these structures with that of BSWs in their periodic counterparts.
Recent developments in plasma turbulence and turbulent transport
Energy Technology Data Exchange (ETDEWEB)
Terry, P.W. [Univ. of Wisconsin, Madison, WI (United States)
1997-09-22
This report contains viewgraphs of recent developments in plasma turbulence and turbulent transport. Localized nonlinear structures occur under a variety of circumstances in turbulent, magnetically confined plasmas, arising in both kinetic and fluid descriptions, i.e., in either wave-particle or three-wave coupling interactions. These structures are non wavelike. They cannot be incorporated in the collective wave response, but interact with collective modes through their shielding by the plasma dielectric. These structures are predicted to modify turbulence-driven transport in a way that in consistent with, or in some cases are confirmed by recent experimental observations. In kinetic theory, non wavelike structures are localized perturbations of phase space density. There are two types of structures. Holes are self-trapped, while clumps have a self-potential that is too weak to resist deformation and mixing by ambient potential fluctuations. Clumps remain correlated in turbulence if their spatial extent is smaller than the correlation length of the scattering fields. In magnetic turbulence, clumps travel along stochastic magnetic fields, shielded by the plasma dielectric. A drag on the clump macro-particle is exerted by the shielding, inducing emission into the collective response. The emission in turn damps back on the particle distribution via Landau dampling. The exchange of energy between clumps and particles, as mediated by the collective mode, imposes constraints on transport. For a turbulent spectrum whose mean wavenumber along the equilibrium magnetic field is nonzero, the electron thermal flux is proportional to the ion thermal velocity. Conventional predictions (which account only for collective modes) are larger by the square root of the ion to electron mass ratio. Recent measurements are consistent with the small flux. In fluid plasma,s localized coherent structures can occur as intense vortices.
Energy Technology Data Exchange (ETDEWEB)
Oishi, Jeffrey S.; /KIPAC, Menlo Park; Low, Mordecai-Mark Mac; /Amer. Museum Natural Hist.
2012-02-14
The magnetorotational instability (MRI) may dominate outward transport of angular momentum in accretion disks, allowing material to fall onto the central object. Previous work has established that the MRI can drive a mean-field dynamo, possibly leading to a self-sustaining accretion system. Recently, however, simulations of the scaling of the angular momentum transport parameter {alpha}{sub SS} with the magnetic Prandtl number Pm have cast doubt on the ability of the MRI to transport astrophysically relevant amounts of angular momentum in real disk systems. Here, we use simulations including explicit physical viscosity and resistivity to show that when vertical stratification is included, mean field dynamo action operates, driving the system to a configuration in which the magnetic field is not fully helical. This relaxes the constraints on the generated field provided by magnetic helicity conservation, allowing the generation of a mean field on timescales independent of the resistivity. Our models demonstrate the existence of a critical magnetic Reynolds number Rm{sub crit}, below which transport becomes strongly Pm-dependent and chaotic, but above which the transport is steady and Pm-independent. Prior simulations showing Pm-dependence had Rm < Rm{sub crit}. We conjecture that this steady regime is possible because the mean field dynamo is not helicity-limited and thus does not depend on the details of the helicity ejection process. Scaling to realistic astrophysical parameters suggests that disks around both protostars and stellar mass black holes have Rm >> Rm{sub crit}. Thus, we suggest that the strong Pm dependence seen in recent simulations does not occur in real systems.
Hanratty, Thomas J.
1980-01-01
This paper gives an account of research on the structure of turbulence close to a solid boundary. Included is a method to study the flow close to the wall of a pipe without interferring with it. (Author/JN)
Turbulence and cooling in cluster cores
Banerjee, Nilanjan
2014-01-01
We study the interplay between turbulent heating, mixing, and radiative cooling in an idealized model of cool cluster cores. Active galactic nuclei (AGN) jets are expected to drive turbulence and heat cluster cores. Cooling of the intracluster medium (ICM) and stirring by AGN jets are tightly coupled in a feedback loop. We impose the feedback loop by balancing radiative cooling with turbulent heating. In addition to heating the plasma, turbulence also mixes it, suppressing the formation of cold gas at small scales. In this regard, the effect of turbulence is analogous to thermal conduction. For uniform plasma in thermal balance (turbulent heating balancing radiative cooling), cold gas condenses only if the cooling time is shorter than the mixing time. This condition requires the turbulent kinetic energy to be $\\gtrsim$ the plasma internal energy; such high velocities in cool cores are ruled out by observations. The results with realistic magnetic fields and thermal conduction are qualitatively similar to the ...
Directory of Open Access Journals (Sweden)
Trunev A. P.
2014-05-01
Full Text Available In this article we have investigated the solutions of Maxwell's equations, Navier-Stokes equations and the Schrödinger associated with the solutions of Einstein's equations for empty space. It is shown that in some cases the geometric instability leading to turbulence on the mechanism of alternating viscosity, which offered by N.N. Yanenko. The mechanism of generation of matter from dark energy due to the geometric turbulence in the Big Bang has been discussed
Scale locality of magnetohydrodynamic turbulence.
Aluie, Hussein; Eyink, Gregory L
2010-02-26
We investigate the scale locality of cascades of conserved invariants at high kinetic and magnetic Reynold's numbers in the "inertial-inductive range" of magnetohydrodynamic (MHD) turbulence, where velocity and magnetic field increments exhibit suitable power-law scaling. We prove that fluxes of total energy and cross helicity-or, equivalently, fluxes of Elsässer energies-are dominated by the contributions of local triads. Flux of magnetic helicity may be dominated by nonlocal triads. The magnetic stretching term may also be dominated by nonlocal triads, but we prove that it can convert energy only between velocity and magnetic modes at comparable scales. We explain the disagreement with numerical studies that have claimed conversion nonlocally between disparate scales. We present supporting data from a 1024{3} simulation of forced MHD turbulence.
Mitra-Delmotte, G.; Mitra, A. N.
2014-04-01
A non-biological analog undergoing Darwinian-like evolution could have enhanced the probability of many crucial independent bottom-up emergent steps, engendered within its premises, and smoothen the inanimate-animate transition. Now, the higher-level environment-mutable DNA sequences influence the lower-level pattern of oriented templates (enzymes, lipid membranes, RNA) in the organized cell matrix and hence their associated substrate-dynamics; note how templates are akin to local fields, kinetically constraining reactant orientations. Since the lowerlevel is likely the more primitive of the two (rather than Cairns-Smith's "readily available" rigid clay crystal sequence-based replicators as a memory-like basis for slowly mutating predecessor-patterns enroute to complex RNA-based Darwinian evolution), a gradual thermodynamic-to-kinetic transition in an isotropic medium, is proposed as driven by some order-parameter --via "available" field-responsive dipolar colloid networks, as apart from bio-organics, mineral colloids also can display liquid crystal (LC) phases (see [1]). An access to solid-like orientational order in a fluid matrix suggests how aperiodic patterns can be influenced and sustained (a la homeostasis) via external inhomogeneous fields (e.g. magnetic rocks); this renders these cooperative networks with potential as confining host-media, whose environment-sensitivity can not only influence their sterically-coupled guest-substrates but also their network properties (the latter can enable 'functions' like spontaneous transport under non-equilibrium suggesting a natural basis for their selection by the environment). In turn LC systems could have been preceded by even simpler anisotropic fluid hosts, viz., external field-induced mineral magnetic nanoparticle (MNP) aggregates. Indeed, the capacity of an MNP to couple its magnetic and rotational d.o.f.s suggests how an environment-sensitive field-influenced network of interacting dipolar colloids close to
Branco, N S; de Sousa, J Ricardo; Ghosh, Angsula
2008-03-01
Using a real-space renormalization-group approximation, we study the anisotropic quantum Heisenberg model on hierarchical lattices, with interactions following aperiodic sequences. Three different sequences are considered, with relevant and irrelevant fluctuations, according to the Luck-Harris criterion. The phase diagram is discussed as a function of the anisotropy parameter Delta (such that Delta=0 and 1 correspond to the isotropic Heisenberg and Ising models, respectively). We find three different types of phase diagrams, with general characteristics: the isotropic Heisenberg plane is always an invariant one (as expected by symmetry arguments) and the critical behavior of the anisotropic Heisenberg model is governed by fixed points on the Ising-model plane. Our results for the isotropic Heisenberg model show that the relevance or irrelevance of aperiodic models, when compared to their uniform counterpart, is as predicted by the Harris-Luck criterion. A low-temperature renormalization-group procedure was applied to the classical isotropic Heisenberg model in two-dimensional hierarchical lattices: the relevance criterion is obtained, again in accordance with the Harris-Luck criterion.
Directory of Open Access Journals (Sweden)
Carlos Santos
2015-05-01
Full Text Available One of the main challenges in wireless cyber-physical systems is to reduce the load of the communication channel while preserving the control performance. In this way, communication resources are liberated for other applications sharing the channel bandwidth. The main contribution of this work is the design of a remote control solution based on an aperiodic and adaptive triggering mechanism considering the current network delay of multiple robotics units. Working with the actual network delay instead of the maximum one leads to abandoning this conservative assumption, since the triggering condition is fixed depending on the current state of the network. This way, the controller manages the usage of the wireless channel in order to reduce the channel delay and to improve the availability of the communication resources. The communication standard under study is the widespread IEEE 802.11g, whose channel delay is clearly uncertain. First, the adaptive self-triggered control is validated through the TrueTime simulation tool configured for the mentioned WiFi standard. Implementation results applying the aperiodic linear control laws on four P3-DX robots are also included. Both of them demonstrate the advantage of this solution in terms of network accessing and control performance with respect to periodic and non-adaptive self-triggered alternatives.
Ye, Junye; le Roux, Jakobus A.; Arthur, Aaron D.
2016-08-01
We study the physics of locally born interstellar pickup proton acceleration at the nearly perpendicular solar wind termination shock (SWTS) in the presence of a random magnetic field spiral angle using a focused transport model. Guided by Voyager 2 observations, the spiral angle is modeled with a q-Gaussian distribution. The spiral angle fluctuations, which are used to generate the perpendicular diffusion of pickup protons across the SWTS, play a key role in enabling efficient injection and rapid diffusive shock acceleration (DSA) when these particles follow field lines. Our simulations suggest that variation of both the shape (q-value) and the standard deviation (σ-value) of the q-Gaussian distribution significantly affect the injection speed, pitch-angle anisotropy, radial distribution, and the efficiency of the DSA of pickup protons at the SWTS. For example, increasing q and especially reducing σ enhances the DSA rate.
Sanchez, R.; Newman, D. E.
2015-12-01
The high plasma temperatures expected at reactor conditions in magnetic confinement fusion toroidal devices suggest that near-marginal operation could be a reality in future devices and reactors. By near-marginal it is meant that the plasma profiles might wander around the local critical thresholds for the onset of instabilities. Self-organized criticality (SOC) was suggested in the mid 1990s as a more proper paradigm to describe the dynamics of tokamak plasma transport in near-marginal conditions. It advocated that, near marginality, the evolution of mean profiles and fluctuations should be considered simultaneously, in contrast to the more common view of a large separation of scales existing between them. Otherwise, intrinsic features of near-marginal transport would be missed, that are of importance to understand the properties of energy confinement. In the intervening 20 years, the relevance of the idea of SOC for near-marginal transport in fusion plasmas has transitioned from an initial excessive hype to the much more realistic standing of today, which we will attempt to examine critically in this review paper. First, the main theoretical ideas behind SOC will be described. Secondly, how they might relate to the dynamics of near-marginal transport in real magnetically confined plasmas will be discussed. Next, we will review what has been learnt about SOC from various numerical studies and what it has meant for the way in which we do numerical simulation of fusion plasmas today. Then, we will discuss the experimental evidence available from the several experiments that have looked for SOC dynamics in fusion plasmas. Finally, we will conclude by identifying the various problems that still remain open to investigation in this area. Special attention will be given to the discussion of frequent misconceptions and ongoing controversies. The review also contains a description of ongoing efforts that seek effective transport models better suited than traditional
Turbulence and turbulent mixing in natural fluids
Gibson, Carl H
2010-01-01
Turbulence and turbulent mixing in natural fluids begins with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretion on a spinning pair releases 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscosity and negative turbulence stresses work against gravity, creating mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until a quark-gluon strong-force SF freeze-out. Gluon-viscosity anti-gravity ({\\Lambda}SF) exponentially inflates the fireball to preserve big bang turbulence information at scales larger than ct as the first fossil turbulence. Cosmic microwave background CMB temperature anisotropies show big bang turbulence fossils along with fossils of weak plasma turbulence triggered (10^12 s) as plasma viscous forces permit gravitational fragmentation on supercluster to galaxy mass scales (10^13 s). Turbulent morphologies and viscous-turbulent lengths a...
TEM turbulence optimisation in stellarators
Proll, J H E; Xanthopoulos, P; Lazerson, S A; Faber, B J
2015-01-01
With the advent of neoclassically optimised stellarators, optimising stellarators for turbulent transport is an important next step. The reduction of ion-temperature-gradient-driven turbulence has been achieved via shaping of the magnetic field, and the reduction of trapped-electron mode (TEM) turbulence is adressed in the present paper. Recent analytical and numerical findings suggest TEMs are stabilised when a large fraction of trapped particles experiences favourable bounce-averaged curvature. This is the case for example in Wendelstein 7-X [C.D. Beidler $\\textit{et al}$ Fusion Technology $\\bf{17}$, 148 (1990)] and other Helias-type stellarators. Using this knowledge, a proxy function was designed to estimate the TEM dynamics, allowing optimal configurations for TEM stability to be determined with the STELLOPT [D.A. Spong $\\textit{et al}$ Nucl. Fusion $\\bf{41}$, 711 (2001)] code without extensive turbulence simulations. A first proof-of-principle optimised equilibrium stemming from the TEM-dominated stella...
Energy dissipation processes in solar wind turbulence
Wang, Y; Feng, X S; Xu, X J; Zhang, J; Sun, T R; Zuo, P B
2015-01-01
Turbulence is a chaotic flow regime filled by irregular flows. The dissipation of turbulence is a fundamental problem in the realm of physics. Theoretically, dissipation cannot be ultimately achieved without collisions, and so how turbulent kinetic energy is dissipated in the nearly collisionless solar wind is a challenging problem. Wave particle interactions and magnetic reconnection are two possible dissipation mechanisms, but which mechanism dominates is still a controversial topic. Here we analyze the dissipation region scaling around a solar wind magnetic reconnection region. We find that the magnetic reconnection region shows a unique multifractal scaling in the dissipation range, while the ambient solar wind turbulence reveals a monofractal dissipation process for most of the time. These results provide the first observational evidences for the intermittent multifractal dissipation region scaling around a magnetic reconnection site, and they also have significant implications for the fundamental energy...
Turbulence and turbulent mixing in natural fluids
2010-01-01
Turbulence and turbulent mixing in natural fluids begins with big bang turbulence powered by spinning combustible combinations of Planck particles and Planck antiparticles. Particle prograde accretions on a spinning pair releases 42% of the particle rest mass energy to produce more fuel for turbulent combustion. Negative viscous stresses and negative turbulence stresses work against gravity, extracting mass-energy and space-time from the vacuum. Turbulence mixes cooling temperatures until str...
Woods, Justin; Bhat, Vinayak; Farmer, Barry; Sklenar, Joseph; Teipel, Eric; Ketterson, John; Hastings, J. Todd; de Long, Lance
2015-03-01
Artificial spin ice (ASI) systems are composed of nanoscale ferromagnetic segments whose shape anisotropy dictates they behave as mesoscopic Ising spins. Most ASI have segments patterned on periodic lattices and a single vertex topology. We have continuously distorted 2D honeycomb and square lattices such that the pattern vertex spacings follow a Fibonacci chain sequence along primitive lattice directions. The Fibonacci distortion is related to the aperiodic translational symmetry of 2D artificial quasicrystals1 that cannot be viewed as continuous distortions of periodic lattices due to their forbidden (e.g., fivefold) rotational symmetries. In contrast, Fibonacci distortions of 2D periodic lattices can be ``turned on'' by control of the ratio of two lattice parameters d1 and d2. Distortions alter film segments such that pattern vertices are no longer equivalent and traditional spin ice rules are no longer strictly valid. We have performed OOMMF simulations of magnetization reversal for samples having different levels of distortion, and found the magnetic reversal to be dramatically slowed by small distortions (d1/d2 ~ 1). Research at Kentucky is supported by U.S. DoE Grant DE-FG02-97ER45653 and NSF Grant EPS-0814194.
Achieving Fast Reconnection in Resistive MHD Models via Turbulent Means
Lapenta, Giovanni
2011-01-01
Astrophysical fluids are generally turbulent and this preexisting turbulence must be taken into account for the models of magnetic reconnection which are attepmted to be applied to astrophysical, solar or heliospheric environments. In addition, reconnection itself induces turbulence which provides an important feedback on the reconnection process. In this paper we discuss both theoretical model and numerical evidence that magnetic reconnection gets fast in the approximation of resistive MHD. We consider the relation between the Lazarian & Vishniac turbulent reconnection theory and Lapenta's numerical experiments testifying of the spontaneous onset of turbulent reconnection in systems which are initially laminar.
Controlled-Turbulence Bioreactors
Wolf, David A.; Schwartz, Ray; Trinh, Tinh
1989-01-01
Two versions of bioreactor vessel provide steady supplies of oxygen and nutrients with little turbulence. Suspends cells in environment needed for sustenance and growth, while inflicting less damage from agitation and bubbling than do propeller-stirred reactors. Gentle environments in new reactors well suited to delicate mammalian cells. One reactor kept human kidney cells alive for as long as 11 days. Cells grow on carrier beads suspended in liquid culture medium that fills cylindrical housing. Rotating vanes - inside vessel but outside filter - gently circulates nutrient medium. Vessel stationary; magnetic clutch drives filter cylinder and vanes. Another reactor creates even less turbulence. Oxygen-permeable tubing wrapped around rod extending along central axis. Small external pump feeds oxygen to tubing through rotary coupling, and oxygen diffuses into liquid medium.
Turbulence and self-oscillations. [In magnetohydrodynamics
Energy Technology Data Exchange (ETDEWEB)
Bershadskii, A.G.
1985-12-01
The paper is concerned with the generation of turbulence by attenuating subcritical self-oscillations and with the evolution of this turbulence behind gratings, behind a cylinder, in ducts, and in a boundary layer (both in a magnetic field and in the absence of a magnetic field). Expressions are obtained for the spectral density of the energy, and the results are found to be in good agreement with experimental data. 15 references.
Intrinsic rotation drive by collisionless trapped electron mode turbulence
Wang, Lu; Diamond, P H
2016-01-01
Both the parallel residual stress and parallel turbulent acceleration driven by electrostatic collisionsless trapped electron mode (CTEM) turbulence are calculated analytically using gyrokinetic theory. Quasilinear results show that the parallel residual stress contributes an outward flux of co-current rotation for normal magnetic shear and turbulence intensity profile increasing outward. This may induce intrinsic counter-current rotation or flattening of the co-current rotation profile. The parallel turbulent acceleration driven by CTEM turbulence vanishes, due to the absence of a phase shift between density fluctuation and ion pressure fluctuation. This is different from the case of ion temperature gradient (ITG) turbulence, for which the turbulent acceleration can provide co-current drive for normal magnetic shear and turbulence intensity profile increasing outward. Its order of magnitude is predicted to be the same as that of the divergence of the residual stress [Lu Wang and P.H. Diamond, Phys. Rev. Lett...
Molybdenum-silicon aperiodic multilayer broadband polarizer for 13-30 nm wavelength range
Energy Technology Data Exchange (ETDEWEB)
Tan, M.Y.; Zhu, J.T.; Chen, L.Y. [Institute of Precision Optical Engineering, Physics Department, Tongji University, 1239 Siping Road, Shanghai 200092 (China); Wang, Z.S., E-mail: wangzs@tongji.edu.cn [Institute of Precision Optical Engineering, Physics Department, Tongji University, 1239 Siping Road, Shanghai 200092 (China); Le Guen, K.; Jonnard, P. [Laboratoire Chimie Physique-Matiere Rayonnement, UPMC Univ Paris 06, CNRS UMR 7614, 11 rue Pierre et Marie Curie, F-75231 Paris cedex 05 (France); Giglia, A.; Mahne, N. [IOM CNR Laboratorio TASC, SS. 14 Km 163.5, 34149 Basovizza (TS) (Italy); Nannarone, S. [Dipartimento di Ingegneria dei Materiali e dell Ambiente, Universita di Modena e Reggio Emilia, Via Vignolese 905, I-41100 Modena (Italy)
2011-10-21
Aperiodic molybdenum/silicon (Mo/Si) multilayer designed as a broadband polarizer with reflectivity over a wide wavelength range of 13-30 nm at a fixed incidence angle of 45{sup o} was developed by a numerical method. The multilayer was prepared using direct current magnetron sputtering. The reflectivity and polarizing properties were characterized using synchrotron radiation. In 13-30 nm wavelength region, the measured s-reflectivity (R{sub s}) with standard deviation is 13.5%{+-}3.1%, p-reflectivity (R{sub p}) with standard deviation is 1.4%{+-}0.5%. The mean of polarization degree is 81.2%. This broadband multilayer polarizer can be used in extreme ultraviolet polarization measurements, and will greatly simplify experimental arrangements.
The DNA electronic specific heat at low temperature: The role of aperiodicity
Energy Technology Data Exchange (ETDEWEB)
Sarmento, R.G. [Departamento de Física, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN (Brazil); Mendes, G.A. [Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN (Brazil); Albuquerque, E.L., E-mail: eudenilson@gmail.com [Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN (Brazil); Fulco, U.L. [Departamento de Biofísica e Farmacologia, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN (Brazil); Vasconcelos, M.S. [Escola de Ciências e Tecnologia, Universidade Federal do Rio Grande do Norte, 59072-970, Natal, RN (Brazil); Ujsághy, O. [Department of Theoretical Physics and Condensed Matter Research Group of the Hungarian Academy of Sciences, Budapest University of Technology and Economics, Budafoki út 8, H-1521 Budapest (Hungary); Freire, V.N. [Departamento de Física, Universidade Federal do Ceará, 60455-760, Fortaleza, CE (Brazil); Caetano, E.W.S. [Instituto Federal de Educação, Ciência e Tecnologia do Ceará, 60040-531, Fortaleza, CE (Brazil)
2012-07-16
The electronic specific heat spectra at constant volume (C{sub V}) of a long-range correlated extended ladder model, mimicking a DNA molecule, is theoretically analyzed for a stacked array of a double-stranded structure made up from the nucleotides guanine G, adenine A, cytosine C and thymine T. The role of the aperiodicity on C{sub V} is discussed, considering two different nucleotide arrangements with increasing disorder, namely the Fibonacci and the Rudin–Shapiro quasiperiodic structures. Comparisons are made for different values of the band fillings, considering also a finite segment of natural DNA, as part of the human chromosome Ch22. -- Highlights: ► Quasiperiodic sequence to mimic the DNA nucleotides arrangement. ► Electronic tight-binding Hamiltonian model. ► Electronic density of states. ► Electronic specific heat spectra.
Stability Analysis of Networked Control Systems With Aperiodic Sampling and Time-Varying Delay.
Chen, Jie; Meng, Su; Sun, Jian
2016-12-01
This paper addresses the stability of networked control systems with aperiodic sampling and time-varying network-induced delay. The sampling intervals are assumed to vary within a known interval. The transmission delay is assumed to belong to a given interval. The closed-loop system is first converted to a discrete-time system with multiple time-varying delays and norm-bounded uncertainties resulting from the variation of the sampling intervals. And then, it is transformed into a delay-free system being form of an interconnection of two subsystems. By utilizing scaled small gain theorem, an asymptotic stability criterion for the closed-loop system is proposed in terms of linear matrix inequality. Finally, numerical examples demonstrate the effectiveness of the proposed method and its advantages over existing methods.
Evidence of parameter-induced aperiodic stochastic resonance with fixed noise
Institute of Scientific and Technical Information of China (English)
Li Jian-Long
2007-01-01
Stochastic resonance (SR) is based on the cooperative effect between the stochastic dynamical system and the external forcing. As is well known, the cooperative effect is produced by adding noises. In this paper, we show the evidence that by changing the system parameters and the signal intensity, a nonlinear system in the presence of an input aperiodic signal can yield the cooperative effect, with the noise fixed. To quantify the nonlinear system output,we determine the theoretical bit error rate (BER). By numerical simulation, the validity of the theoretical derivation is checked. Besides, we show that parameter-induced SR is more realizable than SR via adding noises, especially when the noise intensity exceeds the resonance level, or when the characteristic of the noise is not known.
Short term aperiodic variability of X-ray binaries: its origin and implications
Revnivtsev, M
2008-01-01
In this review I briefly describe the latest advances in studies of aperiodic variability of accreting X-ray binaries and outline the model which currently describe the majority of observational appearances of variability of accreting sources in the best way. Then I concentrate on the case of luminous accreting neutron star binaries (in the soft/high spectral state), where study of variability of X-ray emission of sources allowed us to resolve long standing problem of disentangling the contribution of accretion disk and boundary/spreading layer components to the time average spectrum of sources. The obtained knowledge of the shape of the spectrum of the boundary layer allowed us to make estimates of the mass and radii of accreting neutron stars.
Applied optics. Gain modulation by graphene plasmons in aperiodic lattice lasers.
Chakraborty, S; Marshall, O P; Folland, T G; Kim, Y-J; Grigorenko, A N; Novoselov, K S
2016-01-15
Two-dimensional graphene plasmon-based technologies will enable the development of fast, compact, and inexpensive active photonic elements because, unlike plasmons in other materials, graphene plasmons can be tuned via the doping level. Such tuning is harnessed within terahertz quantum cascade lasers to reversibly alter their emission. This is achieved in two key steps: first, by exciting graphene plasmons within an aperiodic lattice laser and, second, by engineering photon lifetimes, linking graphene's Fermi energy with the round-trip gain. Modal gain and hence laser spectra are highly sensitive to the doping of an integrated, electrically controllable, graphene layer. Demonstration of the integrated graphene plasmon laser principle lays the foundation for a new generation of active, programmable plasmonic metamaterials with major implications across photonics, material sciences, and nanotechnology.
Gate-controlled mid-infrared light bending with aperiodic graphene nanoribbons array
Carrasco, Eduardo; Mosig, Juan R; Low, Tony; Perruisseau-Carrier, Julien
2014-01-01
Graphene plasmonic nanostructures enable subwavelength confinement of electromagnetic energy from the mid-infrared down to the terahertz frequencies. By exploiting the spectrally varying light scattering phase at vicinity of the resonant frequency of the plasmonic nanostructure, it is possible to control the angle of reflection of an incoming light beam. We demonstrate, through full-wave electromagnetic simulations based on Maxwell equations, the electrical control of the angle of reflection of a mid-infrared light beam by using an aperiodic array of graphene nanoribbons, whose widths are engineered to produce a spatially varying reflection phase profile that allows for the construction of a far-field collimated beam towards a predefined direction.
Generation of broadband and multiple-peak THz radiation in aperiodically poled lithium niobate
Institute of Scientific and Technical Information of China (English)
Fucheng Chen; Xianfeng Chen; Yuping Chen; Yuxing Xia
2005-01-01
@@ We theoretically analyze the generation of broadened and multi-peak terahertz (THz) radiation in aperiodically poled lithium niobate (APPLN), whose sequence of opposite domains is optimized by simulated annealing (SA) algorithm. The full-width at half maximum (FWHM) of the broadened THz radiation in our simulation is 0.26 THz. Both of the central wavelength and FWHM can be easily tuned by choosing proper objective functions. THz radiation with wider and flatter FWHM can be achieved by increasing the length of the lithium niobate crystal. The two-peak THz generation is also provided as an example of multi-peak with the central wavelengths at 1.68 and 1.80 THz, respectively.
Wave propagation in one-dimensional solid-fluid quasi-periodic and aperiodic phononic crystals
Energy Technology Data Exchange (ETDEWEB)
Chen Ali, E-mail: alchen@bjtu.edu.cn [Institute of Engineering Mechanics, Beijing Jiaotong University, Beijing 100044 (China); Wang Yuesheng [Institute of Engineering Mechanics, Beijing Jiaotong University, Beijing 100044 (China); Zhang Chuanzeng [Department of Civil Engineering, University of Siegen, D-57068 Siegen (Germany)
2012-02-01
The propagation of the elastic waves in one-dimensional (1D) solid-fluid quasi-periodic phononic crystals is studied by employing the concept of the localization factor, which is calculated by the transfer matrix method. The solid-fluid interaction effect at the interfaces between the solid and the fluid components is considered. For comparison, the periodic systems and aperiodic Thue-Morse sequence are also analyzed in this paper. The splitting phenomenon of the pass bands and bandgaps are discussed for these 1D solid-fluid systems. At last the influences of the material impedance ratios on the band structures of the 1D solid-fluid quasi-periodic phononic crystals arranged as Fibonacci sequence are discussed.
Nonuniversal behavior for aperiodic interactions within a mean-field approximation.
Faria, Maicon S; Branco, N S; Tragtenberg, M H R
2008-04-01
We study the spin-1/2 Ising model on a Bethe lattice in the mean-field limit, with the interaction constants following one of two deterministic aperiodic sequences, the Fibonacci or period-doubling one. New algorithms of sequence generation were implemented, which were fundamental in obtaining long sequences and, therefore, precise results. We calculate the exact critical temperature for both sequences, as well as the critical exponents beta, gamma, and delta . For the Fibonacci sequence, the exponents are classical, while for the period-doubling one they depend on the ratio between the two exchange constants. The usual relations between critical exponents are satisfied, within error bars, for the period-doubling sequence. Therefore, we show that mean-field-like procedures may lead to nonclassical critical exponents.
Loiko, V. A.; Miskevich, A. A.
2017-01-01
The spectra of the coherent transmission and reflection coefficients of multilayers consisting of the periodic, Fibonacci (quasi-periodic), and Thue-Morse (aperiodic) sequences of plane-parallel monolayers of monodisperse spherical alumina and silica particles are investigated using the quasi-crystalline approximation (QCA) and the transfer matrix method (TMM). The additional opportunities for the transmission and reflection spectra manipulation in comparison with the periodic sequence of monolayers are demonstrated. Photonic band gaps in the spectra of the particulate structures are shifted to the short-wavelength range in comparison with those for systems of homogeneous layers. The shift is larger for the Thue-Morse sequence. The widths of the photonic band gaps for particulate systems are narrower than the ones for multilayers consisting of homogeneous layers of an equivalent volume of matter. The results can be used to create optical, optoelectronics, and photonics devices—for example, multispectral filters, light emitting diodes, solar cells, displays.
Coherence in Turbulence: New Perspective
Levich, Eugene
2009-07-01
. In particular, theoretical and numerical evidence is given indicating that BCC in turbulent channel/pipe flows have the depth at the walls proportional to the square root of the Reynolds number in wall units, Ly ∝ √Re, which is equivalent to the fractal dimension in normal to the walls y direction DyF = 0, 5, and the total dimension DF = Dx, zF + DyF = 2 + 0.5 = 2.5. Similar BCC structure and the same fractal dimension are suggested for geophysical turbulence, in near agreement with the recent comprehensive analysis of experimental and observational data. It is asserted that the atmospheric and oceanic events, e.g., tropical hurricanes, tornadoes and other mesoscale phenomena, and probably ocean currents are manifestations of BCC and their environs. Generally BCC should be rather seen as the turbulence core, while the whole surrounding 3D flow as being created and sustained by the intense vorticity of BCC by means of induction, in a manner similar to that for an electric current generating magnetic field. It is further argued that BCC is not only a theoretical concept important for fundamental grasp on turbulence, but may be a practical asset furnishing tools for turbulence management in regular fluids and plasmas. The concept of helical fluctuations in turbulence goes 25 years back in time, and while never totally abandoned nevertheless has been residing on the fringes of research activity. Experiment and numerical simulations had not been able to either validate or repudiate decisively the concept. However, recent large scale direct numerical simulations and proliferation of experimental and observational data showed convincingly how ubiquitous is the phenomenon of helicity fluctuations in various turbulent flows, from hurricanes and tornadoes to turbulent jets to solar wind plasma turbulence to turbulent flows in compressible fluids. This allowed a fresh look at the concept and led to a quantitative theory exposed in this paper. The paper concludes with a
Energy Technology Data Exchange (ETDEWEB)
Bec, Jeremie [Laboratoire Cassiopee UMR6202, CNRS, OCA, BP4229, 06304 Nice Cedex 4 (France)]. E-mail: jeremie.bec@obs-nice.fr; Khanin, Konstantin [Department of Mathematics, University of Toronto, Toronto, Ont., M5S 3G3 (Canada)]. E-mail: khanin@math.toronto.edu
2007-08-15
The last decades witnessed a renewal of interest in the Burgers equation. Much activities focused on extensions of the original one-dimensional pressureless model introduced in the thirties by the Dutch scientist J.M. Burgers, and more precisely on the problem of Burgers turbulence, that is the study of the solutions to the one- or multi-dimensional Burgers equation with random initial conditions or random forcing. Such work was frequently motivated by new emerging applications of Burgers model to statistical physics, cosmology, and fluid dynamics. Also Burgers turbulence appeared as one of the simplest instances of a nonlinear system out of equilibrium. The study of random Lagrangian systems, of stochastic partial differential equations and their invariant measures, the theory of dynamical systems, the applications of field theory to the understanding of dissipative anomalies and of multiscaling in hydrodynamic turbulence have benefited significantly from progress in Burgers turbulence. The aim of this review is to give a unified view of selected work stemming from these rather diverse disciplines.
1991-10-01
and complexity of thermochemistry . Accordingly a practical viewpoint is required to meet near-term work required for use in advanced CFD codes...teachers the opportunity to learn/explore/ teach turbulence issues. While such a product could be an invaluable eductaional tool (university), it also
Energy Technology Data Exchange (ETDEWEB)
Talbot, L.; Cheng, R.K. [Lawrence Berkeley Laboratory, CA (United States)
1993-12-01
Turbulent combustion is the dominant process in heat and power generating systems. Its most significant aspect is to enhance the burning rate and volumetric power density. Turbulent mixing, however, also influences the chemical rates and has a direct effect on the formation of pollutants, flame ignition and extinction. Therefore, research and development of modern combustion systems for power generation, waste incineration and material synthesis must rely on a fundamental understanding of the physical effect of turbulence on combustion to develop theoretical models that can be used as design tools. The overall objective of this program is to investigate, primarily experimentally, the interaction and coupling between turbulence and combustion. These processes are complex and are characterized by scalar and velocity fluctuations with time and length scales spanning several orders of magnitude. They are also influenced by the so-called {open_quotes}field{close_quotes} effects associated with the characteristics of the flow and burner geometries. The authors` approach is to gain a fundamental understanding by investigating idealized laboratory flames. Laboratory flames are amenable to detailed interrogation by laser diagnostics and their flow geometries are chosen to simplify numerical modeling and simulations and to facilitate comparison between experiments and theory.
Observational Tests of Recent MHD Turbulence Perspectives
Ghosh, Sanjoy
2001-06-01
This grant seeks to analyze the Heliospheric Missions data to test current theories on the angular dependence (with respect to mean magnetic field direction) of magnetohydrodynamic (MHD) turbulence in the solar wind. Solar wind turbulence may be composed of two or more dynamically independent components. Such components include magnetic pressure-balanced structures, velocity shears, quasi-2D turbulence, and slab (Alfven) waves. We use a method, developed during the first two years of this grant, for extracting the individual reduced spectra of up to three separate turbulence components from a single spacecraft time series. The method has been used on ISEE-3 data, Pioneer Venus Orbiter, Ulysses, and Voyager data samples. The correlation of fluctuations as a function of angle between flow direction and magnetic-field direction is the focus of study during the third year.
Simulation and optimisation of turbulence in stellarators
Energy Technology Data Exchange (ETDEWEB)
Xanthopoulos, Pavlos; Helander, Per; Turkin, Yuriy; Plunk, Gabriel G.; Bird, Thomas; Proll, Josefine H.E. [Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Wendelsteinstr. 1, 17491 Greifswald (Germany); Mynick, Harry [Plasma Physics Laboratory, Princeton University, Princeton, New Jersey 08543 (United States); Jenko, Frank; Goerler, Tobias; Told, Daniel [Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching (Germany)
2014-07-01
In tokamaks and stellarators - two leading types of devices used in fusion research - magnetic field lines trace out toroidal surfaces on which the plasma density and temperature are constant, but turbulent fluctuations carry energy across these surfaces to the wall, thus degrading the plasma confinement. Using petaflop-scale simulations, we calculate for the first time the pattern of turbulent structures forming on stellarator magnetic surfaces, and find striking differences relative to tokamaks. The observed sensitivity of the turbulence to the magnetic geometry suggests that there is room for further confinement improvement, in addition to measures already taken to minimise the laminar transport. With an eye towards fully optimised stellarators, we present a proof-of-principle configuration with substantially reduced turbulence compared to an existing design.
Low-Energy Electron Diffraction (LEED) Study of an Aperiodic Thin Film of Cu on 5-fold i-Al-Pd-Mn
Pussi, Katariina; Reid, Dennis; Ferralis, Nicola; McGrath, Ronan; Lograsso, Thomas; Ross, Amy; Diehl, Renee
2008-01-01
Abstract Thin films of copper grown on 5-fold i-AlPdMn at room temperature consist of domains that are rotationally aligned with the five primary symmetry directions of the substrate and which have one-dimensional aperiodic order. This aperiodic order is evident in scanning tunneling microscopy (STM) images as wide and narrow rows that are spaced according to a Fibonacci sequence. A low-energy electron diffraction (LEED) study of this film indicates that the structure within the ...
DEFF Research Database (Denmark)
Nielsen, Mogens Peter; Shui, Wan; Johansson, Jens
2011-01-01
In this report a new turbulence model is presented.In contrast to the bulk of modern work, the model is a classical continuum model with a relatively simple constitutive equation. The constitutive equation is, as usual in continuum mechanics, entirely empirical. It has the usual Newton or Stokes...... term with stresses depending linearly on the strain rates. This term takes into account the transfer of linear momentum from one part of the fluid to another. Besides there is another term, which takes into account the transfer of angular momentum. Thus the model implies a new definition of turbulence....... The model is in a virgin state, but a number of numerical tests have been carried out with good results. It is published to encourage other researchers to study the model in order to find its merits and possible limitations....
Magnetic Reconnection in Astrophysical Environments
Lazarian, A; Vishniac, E; Kowal, G
2014-01-01
Magnetic reconnection is a process that changes magnetic field topology in highly conducting fluids. Traditionally, magnetic reconnection was associated mostly with solar flares. In reality, the process must be ubiquitous as astrophysical fluids are magnetized and motions of fluid elements necessarily entail crossing of magnetic frozen in field lines and magnetic reconnection. We consider magnetic reconnection in realistic 3D geometry in the presence of turbulence. This turbulence in most astrophysical settings is of pre-existing nature, but it also can be induced by magnetic reconnection itself. In this situation turbulent magnetic field wandering opens up reconnection outflow regions, making reconnection fast. We discuss Lazarian \\& Vishniac (1999) model of turbulent reconnection, its numerical and observational testings, as well as its connection to the modern understanding of the Lagrangian properties of turbulent fluids. We show that the predicted dependences of the reconnection rates on the level of...
Kühnen, Jakob; Hof, Björn
2015-11-01
We show that a simple modification of the velocity profile in a pipe can lead to a complete collapse of turbulence and the flow fully relaminarises. The annihilation of turbulence is achieved by a steady manipulation of the streamwise velocity component alone, greatly reducing control efforts. Several different control techniques are presented: one with a local modification of the flow profile by means of a stationary obstacle, one employing a nozzle injecting fluid through a small gap at the pipe wall and one with a moving wall, where a part of the pipe is shifted in the streamwise direction. All control techniques act on the flow such that the streamwise velocity profile becomes more flat and turbulence gradually grows faint and disappears. In a smooth straight pipe the flow remains laminar downstream of the control. Hence a reduction in skin friction by a factor of 8 and more can be accomplished. Stereoscopic PIV-measurements and movies of the development of the flow during relaminarisation are presented.
Guo, Min; Xie, Keyu; Wang, Yu; Zhou, Limin; Huang, Haitao
2014-09-23
Bandgap engineering of a photonic crystal is highly desirable for photon management in photonic sensors and devices. Aperiodic photonic crystals (APCs) can provide unprecedented opportunities for much more versatile photon management, due to increased degrees of freedom in the design and the unique properties brought about by the aperiodic structures as compared to their periodic counterparts. However, many efforts still remain on conceptual approaches, practical achievements in APCs are rarely reported due to the difficulties in fabrication. Here, we report a simple but highly controllable current-pulse anodization process to design and fabricate TiO2 nanotube APCs. By coupling an APC into the photoanode of a dye-sensitized solar cell, we demonstrate the concept of using APC to achieve nearly full-visible-spectrum light harvesting, as evidenced by both experimental and simulated results. It is anticipated that this work will lead to more fruitful practical applications of APCs in high-efficiency photovoltaics, sensors and optoelectronic devices.
Solar Wind Turbulence and the Role of Ion Instabilities
Alexandrova, Olga; Sorriso-Valvo, Luca; Horbury, Timothy S; Bale, Stuart D
2013-01-01
Solar wind is probably the best laboratory to study turbulence in astrophysical plasmas. In addition to the presence of magnetic field, the differences with neutral fluid isotropic turbulence are: weakness of collisional dissipation and presence of several characteristic space and time scales. In this paper we discuss observational properties of solar wind turbulence in a large range from the MHD to the electron scales. At MHD scales, within the inertial range, turbulence cascade of magnetic fluctuations develops mostly in the plane perpendicular to the mean field. Solar wind turbulence is compressible in nature. The spectrum of velocity fluctuations do not follow magnetic field one. Probability distribution functions of different plasma parameters are not Gaussian, indicating presence of intermittency. At the moment there is no global model taking into account all these observed properties of the inertial range. At ion scales, turbulent spectra have a break, compressibility increases and the density fluctuat...
Yan, Zhi-Zhong; Zhang, Chuanzeng; Wang, Yue-Sheng
2011-03-01
The band structures of in-plane elastic waves propagating in two-dimensional phononic crystals with one-dimensional random disorder and aperiodicity are analyzed in this paper. The localization of wave propagation is discussed by introducing the concept of the localization factor, which is calculated by the plane-wave-based transfer-matrix method. By treating the random disorder and aperiodicity as the deviation from the periodicity in a special way, three kinds of aperiodic phononic crystals that have normally distributed random disorder, Thue-Morse and Rudin-Shapiro sequence in one direction and translational symmetry in the other direction are considered and the band structures are characterized using localization factors. Besides, as a special case, we analyze the band gap properties of a periodic planar layered composite containing a periodic array of square inclusions. The transmission coefficients based on eigen-mode matching theory are also calculated and the results show the same behaviors as the localization factor does. In the case of random disorders, the localization degree of the normally distributed random disorder is larger than that of the uniformly distributed random disorder although the eigenstates are both localized no matter what types of random disorders, whereas, for the case of Thue-Morse and Rudin-Shapiro structures, the band structures of Thue-Morse sequence exhibit similarities with the quasi-periodic (Fibonacci) sequence not present in the results of the Rudin-Shapiro sequence.
Graphical Turbulence Guidance - Composite
National Oceanic and Atmospheric Administration, Department of Commerce — Forecast turbulence hazards identified by the Graphical Turbulence Guidance algorithm. The Graphical Turbulence Guidance product depicts mid-level and upper-level...
Intrinsic Turbulence Stabilization in a Stellarator
Xanthopoulos, P.; Plunk, G. G.; Zocco, A.; Helander, P.
2016-04-01
The magnetic surfaces of modern stellarators are characterized by complex, carefully optimized shaping and exhibit locally compressed regions of strong turbulence drive. Massively parallel computer simulations of plasma turbulence reveal, however, that stellarators also possess two intrinsic mechanisms to mitigate the effect of this drive. In the regime where the length scale of the turbulence is very small compared to the equilibrium scale set by the variation of the magnetic field, the strongest fluctuations form narrow bandlike structures on the magnetic surfaces. Thanks to this localization, the average transport through the surface is significantly smaller than that predicted at locations of peak turbulence. This feature results in a numerically observed upshift of the onset of turbulence on the surface towards higher ion temperature gradients as compared with the prediction from the most unstable regions. In a second regime lacking scale separation, the localization is lost and the fluctuations spread out on the magnetic surface. Nonetheless, stabilization persists through the suppression of the large eddies (relative to the equilibrium scale), leading to a reduced stiffness for the heat flux dependence on the ion temperature gradient. These fundamental differences with tokamak turbulence are exemplified for the QUASAR stellarator [G. H. Neilson et al., IEEE Trans. Plasma Sci. 42, 489 (2014)].
Magnetohydrodynamic turbulence: Observation and experiment
Energy Technology Data Exchange (ETDEWEB)
Brown, M. R.; Schaffner, D. A.; Weck, P. J. [Department of Physics and Astronomy, Swarthmore College, 500 College Avenue, Swarthmore, Pennsylvania 19081 (United States)
2015-05-15
We provide a tutorial on the paradigms and tools of magnetohydrodynamic (MHD) turbulence. The principal paradigm is that of a turbulent cascade from large scales to small, resulting in power law behavior for the frequency power spectrum for magnetic fluctuations E{sub B}(f). We will describe five useful statistical tools for MHD turbulence in the time domain: the temporal autocorrelation function, the frequency power spectrum, the probability distribution function of temporal increments, the temporal structure function, and the permutation entropy. Each of these tools will be illustrated with an example taken from MHD fluctuations in the solar wind. A single dataset from the Wind satellite will be used to illustrate all five temporal statistical tools.