Cosmic Magnetization: From Spontaneously Emitted Aperiodic Turbulent to Ordered Equipartition Fields
Schlickeiser, R.
2012-12-01
It is shown that an unmagnetized nonrelativistic thermal electron-proton plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of strength |δB|=3.5βeg1/3We1/2G, where βe is the normalized thermal electron temperature, We 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 2×10-16G in cosmic voids and 2×10-10G in protogalaxies, both too weak to affect the dynamics of the plasma. Accounting for simultaneous viscous damping reduces these estimates to 2×10-21G in cosmic voids and 2×10-12G 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.
Farmer, B.; Bhat, V. S.; Sklenar, J.; Teipel, E.; Woods, J.; Ketterson, J. B.; Hastings, J. T.; De Long, L. E.
2015-05-01
The static and dynamic magnetic responses of patterned ferromagnetic thin films are uniquely altered in the case of aperiodic patterns that retain long-range order (e.g., quasicrystals). We have fabricated permalloy wire networks based on periodic square antidot lattices (ADLs) distorted according to an aperiodic Fibonacci sequence applied to two lattice translations, d1 = 1618 nm and d2 = 1000 nm. The wire segment thickness is fixed at t = 25 nm, and the width W varies from 80 to 510 nm. We measured the DC magnetization between room temperature and 5 K. Room-temperature, narrow-band (9.7 GHz) ferromagnetic resonance (FMR) spectra were acquired for various directions of applied magnetic field. The DC magnetization curves exhibited pronounced step anomalies and plateaus that signal flux closure states. Although the Fibonacci distortion breaks the fourfold symmetry of a finite periodic square ADL, the FMR data exhibit fourfold rotational symmetry with respect to the applied DC magnetic field direction.
Inhomogeneous turbulence in magnetic reconnection
Yokoi, Nobumitsu
2016-07-01
Turbulence is expected to play an essential role in enhancing magnetic reconnection. Turbulence associated with magnetic reconnection is highly inhomogeneous: it is generated by inhomogeneities of the field configuration such as the velocity shear, temperature gradient, density stratification, magnetic shear, etc. This self-generated turbulence affects the reconnection through the turbulent transport. In this reconnection--turbulence interaction, localization of turbulent transport due to dynamic balance between several turbulence effects plays an essential role. For investigating inhomogeneous turbulence in a strongly nonlinear regime, closure or turbulence modeling approaches provide a powerful tool. A turbulence modeling approach for the magnetic reconnection is introduced. In the model, the mean-field equations with turbulence effects incorporated are solved simultaneously with the equations of turbulent statistical quantities that represent spatiotemporal properties of turbulence under the effect of large-scale field inhomogeneities. Numerical simulations of this Reynolds-averaged turbulence model showed that self-generated turbulence enhances magnetic reconnection. It was pointed out that reconnection states may be divided into three category depending on the turbulence level: (i) laminar reconnection; (ii) turbulent reconnection, and (iii) turbulent diffusion. Recent developments in this direction are also briefly introduced, which includes the magnetic Prandtl number dependence, spectral evolution, and guide-field effects. Also relationship of this fully nonlinear turbulence approach with other important approaches such as plasmoid instability reconnection will be discussed.
Magnetic turbulence in Tokamaks
From a discussion of the disruption process, it is concluded that this process plausibly consists of the onset of a fine grain turbulence. This turbulence must be able to produce the large values of the inductive electric field which are associated with the reorganization of the poloidal flux and the current density on the magnetic surfaces. It is then plausible that the turbulence belongs to a class of 'rippling' modes, that may explain the experimental values for the magnetic perturbations corresponding to a substantial radial ergodicity of the flux lines. The stability of the modes in the presence of such an ergodicity is accordingly considered. It is found that the modes may be unstable even in collisionless regime, the ergodicity playing a role similar to the resistivity to partially remove the M.H.D. constraint
Surface Magnetization of Aperiodic Ising Systems: a Comparative Study of the Bond and Site Problems
Turban, L.; Berche, P. E.; Berche, B.
1994-01-01
We investigate the influence of aperiodic perturbations on the critical behaviour at a second order phase transition. The bond and site problems are compared for layered systems and aperiodic sequences generated through substitution. In the bond problem, the interactions between the layers are distributed according to an aperiodic sequence whereas in the site problem, the layers themselves follow the sequence. A relevance-irrelevance criterion introduced by Luck for the bond problem is extend...
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 General Magnetic Reconnection
Eyink, G. L.
2015-07-01
Plasma flows with a magnetohydrodynamic (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 per unit arclength of field line, the ratio of the curl of the non-ideal electric field in the generalized Ohm’s Law and magnetic field strength. It diverges at magnetic nulls, unifying GMR with null-point reconnection. Only under restrictive assumptions is the slip velocity related to the gradient of quasi-potential (which is the integral of parallel electric field along magnetic field lines). In a turbulent inertial range, the non-ideal field becomes tiny while its curl is large, so that line slippage occurs even while ideal MHD becomes accurate. The resolution is that ideal MHD is valid for a turbulent inertial range only in a weak sense that does not imply magnetic line freezing. The notion of weak solution is explained in terms of renormalization group (RG) type theory. The weak validity of the ideal Ohm’s law in the inertial range is shown via rigorous estimates of the terms in the generalized Ohm’s Law. All non-ideal terms are irrelevant in the RG sense and large-scale reconnection is thus governed solely by ideal dynamics. We discuss the implications for heliospheric reconnection, in particular for deviations from the Parker spiral model. Solar wind observations show that reconnection in a turbulence-broadened heliospheric current sheet, which is consistent with Lazarian-Vishniac theory, leads to slip velocities that cause field lines to lag relative to the spiral model.
Turbulent Dynamos and Magnetic Helicity
Ji, H
1999-01-01
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 electrostatic or due to the electron diamagnetic effect. In all cases, however, the dynamo effect strictly conserves the total helicity except for resistive effects and a small battery effect. Implications for astrophysical situations, especially for the solar dynamo, are discussed.
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...
Magnetized Turbulent Dynamo in Protogalaxies
Leonid Malyshkin; Russell M. Kulsrud
2002-01-28
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 to field-turbulence energy equipartition, and the turbulent dynamo becomes what we call the magnetized turbulent dynamo. In this paper we lay the theoretical groundwork for the magnetized turbulent dynamo. In particular, we predict that the magnetic energy growth rate in the magnetized dynamo theory is up to ten times larger than that in the kinematic dynamo theory. We also briefly discuss how the Braginskii viscosity can aid the development of the inverse cascade of magnetic energy after the energy equipartition is reached.
Magnetized Turbulent Dynamo in Protogalaxies
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 to field-turbulence energy equipartition, and the turbulent dynamo becomes what we call the magnetized turbulent dynamo. In this paper we lay the theoretical groundwork for the magnetized turbulent dynamo. In particular, we predict that the magnetic energy growth rate in the magnetized dynamo theory is up to ten times larger than that in the kinematic dynamo theory. We also briefly discuss how the Braginskii viscosity can aid the development of the inverse cascade of magnetic energy after the energy equipartition is reached
Wave turbulence in magnetized plasmas
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 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.
Relation of Astrophysical Turbulence and Magnetic Reconnection
Lazarian, A; Vishniac, E
2011-01-01
Astrophysical fluids are generically turbulent and this must be taken into account for most transport processes. We discuss how the preexisting turbulence modifies magnetic reconnection and how magnetic reconnection affects the MHD turbulent cascade. We show the intrinsic interdependence and interrelation of magnetic turbulence and magnetic reconnection, in particular, that strong magnetic turbulence in 3D requires reconnection and 3D magnetic turbulence entails fast reconnection. We follow the approach in Eyink, Lazarian & Vishniac 2011 to show that the expressions of fast magnetic reconnection in Lazarian & Vishniac 1999 can be recovered if Richardson diffusion of turbulent flows is used instead of ordinary Ohmic diffusion. This does not revive, however, the concept of magnetic turbulent diffusion which assumes that magnetic fields can be mixed up in a passive way down to a very small dissipation scales. On the contrary, we are dealing the reconnection of dynamically important magnetic field bundles...
Mixing in Magnetized Turbulent Medium
Sur, Sharanya; Scannapieco, Evan
2014-01-01
Turbulent motions are essential to the mixing of entrained fluids and are also capable of amplifying weak initial magnetic fields by small-scale dynamo action. Here we perform a systematic study of turbulent mixing in magnetized media, using three-dimensional magnetohydrodynamic simulations that include a scalar concentration field. We focus on how mixing depends on the magnetic Prandtl number, Pm, from 1 to 4 and the Mach number, M}, from 0.3 to 2.4. For all subsonic flows, we find that the velocity power spectrum has a k^-5/3 slope in the early, kinematic phase, but steepens due to magnetic back reactions as the field saturates. The scalar power spectrum, on the other hand, flattens compared to k^-5/3 at late times, consistent with the Obukohov-Corrsin picture of mixing as a cascade process. At higher Mach numbers, the velocity power spectrum also steepens due to the presence of shocks, and the scalar power spectrum again flattens accordingly. Scalar structures are more intermittent than velocity structures...
Magnetized Turbulent Dynamo in Protogalaxies; TOPICAL
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 to field-turbulence energy equipartition, and the turbulent dynamo becomes what we call the magnetized turbulent dynamo. In this paper we lay the theoretical groundwork for the magnetized turbulent dynamo. In particular, we predict that the magnetic energy growth rate in the magnetized dynamo theory is up to ten times larger than that in the kinematic dynamo theory. We also briefly discuss how the Braginskii viscosity can aid the development of the inverse cascade of magnetic energy after the energy equipartition is reached
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.
Scaling laws in magnetized plasma turbulence
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
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.
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 ...
Turbulent transport and dynamo in sheared MHD turbulence with a non-uniform magnetic field
Leprovost, Nicolas; Kim, Eun-Jin
2009-01-01
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, α and β effect) in Reynolds-averaged equations. ...
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...
Magnetic presheath in a turbulent plasma
Fluid model of the magnetic presheath in a turbulent boundary plasma is presented. Turbulent transport corrections of the classical three-dimensional fluid transport equations, which can be used to study magnetic presheaths in various geometries, are derived by means of the ensemble averaging procedure from the statistical theory of plasma turbulence. The magnetic presheath in front of an infinite plane surface is then analysed in detail, by using linearised planar magnetic presheath equations for studying the plasma presheath-magnetic presheath boundary, i.e., the magnetic presheath edge, and the original non-linear planar magnetic presheath equations for studying the entire magnetic presheath when various sets of experimentally relevant free input parameters of the model are applied. Important new results of this study are, among others, new expressions for the fluid approximation of the Bohm criterion at the electrostatic sheath edge and for the ion flux density perpendicular to the wall, which include corrections due to the turbulent charged particle transport. These results can qualitatively explain electric currents measured by Langmuir probes in the boundary regions of nuclear fusion devices and in various low-temperature plasmas, which are anomalously enhanced in comparison with those expected or predicted by other theoretical models, when the angle between the magnetic field and the wall is very small (i.e., several degrees), or when the magnetic field is parallel to the wall. The boundary conditions of the fluid transport codes, which are used for tokamak boundary plasma modelling, can be improved by using the results of this study. (author)
Drift Wave Turbulence and Magnetic Reconnection
Price, L.; Drake, J. F.; Swisdak, M.
2015-12-01
An important feature in collisionless magnetic reconnection is the development of sharp discontinuities along the separatrices bounding the Alfvenic outflow. The typical scale length of these features is ρs (the Larmor radius based on the sound speed) for guide field reconnection. Temperature gradients in the inflowing plasma (as might be found in the magnetopause and the magnetotail) can lead to instabilities at these separatrices, specifically drift wave turbulence. We present standalone 2D and 3D PIC simulations of drift wave turbulence to investigate scaling properties and growth rates. We specifically consider stabilization of the lower hybrid drift instability (LHDI) and the development of this instability in the presence of a sheared magnetic field. Further investigations of the relative importance of drift wave turbulence in the development of reconnection will also be considered.
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...
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. PMID:19792244
Turbulent reconnection of magnetic bipoles in stratified turbulence
Jabbari, S.; Brandenburg, A.; Mitra, Dhrubaditya; Kleeorin, N.; Rogachevskii, I.
2016-07-01
We consider strongly stratified forced turbulence in a plane-parallel layer with helicity and corresponding large-scale dynamo action in the lower part and non-helical 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 focus on the formation of a 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 large Lundquist numbers, S > 103, the reconnection rate is nearly independent of S in agreement with results of recent numerical simulations performed by other groups in simpler settings.
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.
Electrostatic turbulence in strongly magnetized plasmas
Turbulence in plasmas has been investigated experimentally and numerically. On the experimental side the turbulent nature of the Kelvin-Helmholtz instability has been studied in a single-ended Q-machine. The development of coherent structures in the background of the turbulent flow has been demonstrated and the capability of structures of transporting plasma across the magnetic field-lines is explained in detail. The numerical investigations are divided into two parts: Numerical simulations of the dynamics from the Q-machine experiments using spectral methods to solve the two-dimensional Navier-Stokes equations in a cylindrical geometry. A numerical study of the Eulerian-Lagrangian transformation in a two-dimensional flow. Here the flow is made up by a large number of structures, where each individual structure is convected by the superposed flow field of all the others. (au) (33 ills., 67 refs.)
Small-scale magnetic buoyancy and magnetic pumping effects in a turbulent convection
Rogachevskii, I.; Kleeorin, N.
2006-01-01
We determine the nonlinear drift velocities of the mean magnetic field and nonlinear turbulent magnetic diffusion in a turbulent convection. We show that the nonlinear drift velocities are caused by the three kinds of the inhomogeneities, i.e., inhomogeneous turbulence; the nonuniform fluid density and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear drift velocities of the mean magnetic...
Magnetic reconnection as an element of turbulence
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.
Microtearing turbulence: Magnetic braiding and disruption limit
Firpo, Marie-Christine [Laboratoire de Physique des Plasmas, CNRS-Ecole Polytechnique, 91128 Palaiseau cedex (France)
2015-12-15
A realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement: the mean residence time of magnetic field lines. Their computation in the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of δB/B above which stochasticity is no longer benign yet, leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence δB/B has been measured to grow with the plasma electron density, this would also be a density limit.
Microtearing turbulence: Magnetic braiding and disruption limit
A realistic reduced model involving a large poloidal spectrum of microtearing modes is used to probe the existence of some stochasticity of magnetic field lines. Stochasticity is shown to occur even for the low values of the magnetic perturbation δB/B devoted to magnetic turbulence that have been experimentally measured. Because the diffusion coefficient may strongly depend on the radial (or magnetic-flux) coordinate, being very low near some resonant surfaces, and because its evaluation implicitly makes a normal diffusion hypothesis, one turns to another indicator appropriate to diagnose the confinement: the mean residence time of magnetic field lines. Their computation in the microturbulence frame points to the existence of a disruption limit, namely of a critical order of magnitude of δB/B above which stochasticity is no longer benign yet, leads to a macroscopic loss of confinement in some tens to hundred of electron toroidal excursions. Since the level of magnetic turbulence δB/B has been measured to grow with the plasma electron density, this would also be a density limit
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 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 ...
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.
Yokoi, Nobumitsu; Higashimori, Katsuaki; Hoshino, Masahiro
2014-01-01
Through the enhancement of transport, turbulence is expected to contribute to the fast reconnection. However the effects of turbulence are not so straightforward. In addition to the enhancement of transport, turbulence under some environment shows effects that suppress the transport. In the presence of turbulent cross helicity, such a dynamic balance between the transport enhancement and suppression occurs. As this result of dynamic balance, the region of effective enhanced magnetic diffusivi...
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
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, ...
Effect of Externally Driven Magnetic Islands on Resistive Ballooning Turbulence
NISHIMURA, Seiya; Yagi, Masatoshi
2011-01-01
Turbulent transport in the edge region of tokamak plasmas is simulated using a reduced set of magnetohydrodynamic equations. Repetitive and intermittent transport bursts driven by resistive ballooning turbulence with external heating are observed. The effect of a resonant magnetic perturbation (RMP) on turbulent heat transport is examined, where the electromagnetic response of the plasma to the RMP is solved consistently. The penetration of the RMP excites a magnetic island chain and damps th...
Magnetic Discontinuities in Magnetohydrodynamic Turbulence and in the Solar Wind
Zhdankin, Vladimir; Boldyrev, Stanislav; Mason, Joanne; Perez, Jean Carlos
2012-01-01
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, Delta theta, across fixed spatial increments Delta x in direct numerical simulations of MHD turbulence with an imposed uniform guide field...
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.
Toward the Theory of Turbulence in Magnetized Plasmas
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 kllel)). 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
Magnetic flux concentrations from turbulent stratified convection
Käpylä, P. J.; Brandenburg, A.; Kleeorin, N.; Käpylä, M. J.; Rogachevskii, I.
2016-04-01
Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims: We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods: We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8.5-24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 × 103 or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results: We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to 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 near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows 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 derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative
Effect of externally driven magnetic islands on resistive ballooning turbulence
Turbulent transport in the edge region of tokamak plasmas is simulated using a reduced set of magnetohydrodynamic equations. Repetitive and intermittent transport bursts driven by resistive ballooning turbulence with external heating are observed. The effect of a resonant magnetic perturbation (RMP) on turbulent heat transport is examined, where the electromagnetic response of the plasma to the RMP is solved consistently. The penetration of the RMP excites a magnetic island chain and damps the poloidal flow near the magnetic islands. The transport bursts are found to be replaced by more moderate and continuous transport. The change in the transport pattern is associated with the effect of the RMP on nonlinear coupling of fluctuations. (author)
Turbulent particle transport in magnetized fusion plasma
The understanding of the mechanisms responsible for particle transport is of the utmost importance for magnetized fusion plasmas. Indeed, a peaked density profile is attractive to improve the fusion rate, which is proportional to the square of the density, and to self-generate a large fraction of non-inductive current required for continuous operation. Experiments in various tokamak devices have indicated the existence of an anomalous inward particle pinch. Recently, such an anomalous pinch has been unambiguously identified in Tore Supra very long discharges, in absence of toroidal electric field and of central particle source, for more than 3 minutes. This anomalous particle pinch is predicted by a quasilinear theory of particle transport, and confirmed by non-linear turbulence simulations and general considerations based on the conservation of motion invariants. Experimentally, the particle pinch is found to be sensitive to the magnetic field gradient in many cases, to the temperature gradient and also to the collisionality that changes the nature of the microturbulence. The consistency of some of the observed dependences with the theoretical predictions gives us a clearer understanding of the particle pinch in tokamaks, allowing us to predict more accurately the density profile in ITER. (authors)
Turbulence Reduces Magnetic Diffusivity in DTS Liquid Sodium Experiment
Cabanes, Simon; Nataf, Henri-Claude; Schaeffer, Nathanael
2015-04-01
Earth, Sun and many other astrophysical bodies produce their own magnetic field by dynamo action, where induction of magnetic field by fluid motion overcomes the Joule dissipation when the magnetic Reynolds number Rm = UL/η is large enough (U and L are characteristic velocity and length-scale and η the magnetic diffusivity). Large scale motion of a conducting medium shearing pre-existing magnetic field lines is a well known process to produce large scale magnetic field by omega-effect. However, such a process cannot sustain a self-excited dynamo and small-scale turbulent motions are usually invoked as the appropriate mechanism to dynamo action. The contribution of turbulent fluctuations to the induction of mean magnetic field is investigated in our liquid sodium spherical Couette experiment, with an imposed magnetic field. Many measurements are used through an inversion technique to obtain a radial profile of α and β effects together with the mean flow at magnetic Reynolds number Rm = 100. It appears that the small scale turbulent fluctuations can be modeled as a strong contribution to the magnetic diffusivity which is negative in the interior region and positive close to the outer shell.Direct numerical simulations of our experiment support these results. The lowering of the effective magnetic diffusivity by small scale fluctuations implies that turbulence can actually help to achieve self-generation of large scale magnetic fields.
The inverse cascade of magnetic helicity in magnetohydrodynamic turbulence
Müller, W.; Malapaka, S.; Busse, A.
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. Fl...
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 reconnection in 2D and 3D
Lazarian, A; Vishniac, E; Kulpa-Dubel, K; Otmianowska-Mazur, K
2010-01-01
Magnetic field embedded in a perfectly conducting fluid preserves its topology for all time. Although ionized astrophysical objects, like stars and galactic disks, are almost perfectly conducting, they show indications of changes in topology, `magnetic reconnection', on dynamical time scales. Reconnection can be observed directly in the solar corona, but can also be inferred from the existence of large scale dynamo activity inside stellar interiors. Solar flares and gamma ray busts are usually associated with magnetic reconnection. Previous work has concentrated on showing how reconnection can be rapid in plasmas with very small collision rates. Here we present numerical evidence, based on three dimensional simulations, that reconnection in a turbulent fluid occurs at a speed comparable to the rms velocity of the turbulence, regardless of the value of the resistivity. In particular, this is true for turbulent pressures much weaker than the magnetic field pressure so that the magnetic field lines are only slig...
Magnetic flux concentration and zonal flows in magnetorotational instability turbulence
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.
Asymmetric diffusion of magnetic field lines in turbulence
Stochasticity of magnetic field lines is paramount to understanding particle transport and mixing (Rechester and Rosenbluth 1978 Phys. Rev. Lett. 40 38–41). In this paper we study magnetic field separation in turbulent plasmas, which feature so-called superdiffusion, where field lines separate faster than diffusively. We discovered that turbulent superdiffusion can be pronouncedly asymmetric, so that the separation of field lines will be different if one follows magnetic field lines along the direction of the field or follows them in the opposite direction. Particle transport perpendicular to the magnetic field is primarily due to the field line separation in collisionless, well-magnetized plasmas. The asymmetry in diffusion, however, was certainly unexpected, as it will create free energy in particles that follow field lines, e.g. it will create large-scale streaming out of particle gradients perpendicular to the mean magnetic field. While the symmetry of the flow is broken by the so-called imbalance or cross-helicity, the difference between forward and backward diffusion is not directly due to imbalance, but a non-trivial consequence of both imbalance and non-reversibility of turbulence. It therefore follows that this peculiar property of field lines is directly related to turbulence being a dissipative phenomenon, and that turbulence can directly channel part of its free energy to particles. (invited comment)
Turbulent Diffusion of Magnetic Fields in Weakly Ionized Gas
Kim, Eun-Jin; P. H. Diamond
2002-01-01
The diffusion of uni-directional magnetic fields by two dimensional turbulent flows in a weakly ionized gas is studied. The fields here are orthogonal to the plane of fluid motion. This simple model arises in the context of the decay of the mean magnetic flux to mass ratio in the interstellar medium. When ions are strongly coupled to neutrals, the transport of a large--scale magnetic field is driven by both turbulent mixing and nonlinear, ambipolar drift. Using a standard homogeneous and Gaus...
Wavelet analysis of magnetic turbulence in the Earth's plasma sheet
Baumjohann, W; Runov, A; Volwerk, M; Zhang, T L; Balogh, A
2004-01-01
Recent studies provide evidence for the multi-scale nature of magnetic turbulence in the plasma sheet. Wavelet methods represent modern time series analysis techniques suitable for the description of statistical characteristics of multi-scale turbulence. Cluster FGM (fluxgate magnetometer) magnetic field high-resolution (~67 Hz) measurements are studied during an interval in which the spacecraft are in the plasma sheet. As Cluster passes through different plasma regions, physical processes exhibit non-steady properties on magnetohydrodynamic (MHD) and small, possibly kinetic scales. As a consequence, the implementation of wavelet-based techniques becomes complicated due to the statistically transitory properties of magnetic fluctuations and finite size effects. Using a supervised multi-scale technique which allows existence test of moments, the robustness of higher-order statistics is investigated. On this basis the properties of magnetic turbulence are investigated for changing thickness of the plasma sheet.
The Inherently Three-Dimensional Nature of Magnetized Plasma Turbulence
Howes, Gregory G
2013-01-01
It is often asserted or implicitly assumed, without justification, that the results of two-dimensional investigations of plasma turbulence are applicable to the three-dimensional plasma environments of interest. A projection method is applied to derive two scalar equations that govern the nonlinear evolution of the Alfvenic and pseudo-Alfvenic components of ideal incompressible magnetohydrodynamic (MHD) plasma turbulence. The mathematical form of these equations makes clear the inherently three-dimensional nature of plasma turbulence, enabling an analysis of the nonlinear properties of two-dimensional limits often used to study plasma turbulence. In the anisotropic limit k_perp >>k_parallel that naturally arises in magnetized plasma systems, the perpendicular 2D limit retains the dominant nonlinearities that are mediated only by the Alfvenic fluctuations but lacks the wave physics associated with the linear term that is necessary to capture the anisotropic cascade of turbulent energy. In the in-plane 2D limit...
INVERSE CASCADE OF NONHELICAL MAGNETIC TURBULENCE IN A RELATIVISTIC FLUID
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.
Turbulent Amplification and Structure of the Intracluster Magnetic Field
Beresnyak, Andrey; Miniati, Francesco
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.
Aperiodic Subshifts on Polycyclic Groups
Jeandel, Emmanuel
2016-01-01
Previous version had a mistake in the proof of the polycyclic case. The new proof needs a very strong new result by Barbieri and Sablik, that the authors hopes is avoidable We prove that every polycyclic group of nonlinear growth admits a strongly aperiodic SFT and has an undecidable domino problem. This answers a question of [4] and generalizes the result of [2].
Hall MHD Stability and Turbulence in Magnetically Accelerated Plasmas
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; 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...
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.
Turbulence-induced magnetic fields in shock precursors
del Valle, M. V.; Lazarian, A.; Santos-Lima, R.
2016-05-01
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. 1015 eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic ray non-resonant 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. Key ingredients for the mechanism to operate are the inhomogeneities present in the interstellar medium. These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different interstellar medium conditions through 3D magnetohydrodynamic numerical simulations.
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. PMID:20866731
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.
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...
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
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...
Exploring phase space turbulence in magnetic fusion plasmas
Plasma turbulence accompanied with fluctuations of the distribution function and the electromagnetic fields develops on the phase space composed of the configuration space and the velocity space. Detailed structures of the distribution function in magnetic fusion plasmas are investigated by means of gyrokinetic simulations performed on massively parallel supercomputers. The gyrokinetic simulations of drift wave turbulence have demonstrated entropy transfer in the phase space, zonal flow enhancement by helical fields and the resultant transport reduction. The state-of-the-art high performance computing is utilized for a multi-scale turbulence simulation covering ion- and electron-scales and for a global-scale simulation of turbulent transport in a sub-ITER sized plasma.
BIPOLAR MAGNETIC STRUCTURES DRIVEN BY STRATIFIED TURBULENCE WITH A CORONAL ENVELOPE
We report the spontaneous formation of bipolar magnetic structures in direct numerical simulations of stratified forced turbulence with an outer coronal envelope. The turbulence is forced with transverse random waves only in the lower (turbulent) part of the domain. Our initial magnetic field is either uniform in the entire domain or confined to the turbulent layer. After about 1-2 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion times. The resulting magnetic field strengths inside the bipolar region are comparable to the equipartition value with respect to the turbulent kinetic energy. The bipolar magnetic region forms a loop-like structure in the upper coronal layer. We associate the magnetic structure formation with the negative effective magnetic pressure instability in the two-layer model
Thermal Instability and Magnetic Pressure in the Turbulent Interstellar Medium
Vázquez-Semadeni, E; Passot, T; Sánchez-Salcedo, F J; Vazquez-Semadeni, Enrique; Gazol, Adriana; Passot, Thierry; Sanchez-Salcedo, Javier
2003-01-01
We review recent results on the nonlinear development of thermal instability (TI) in the context of the turbulent atomic interstellar medium (ISM). First, we discuss the growth of entropy perturbations in isolation, as a function of the ratio \\eta of the cooling time to the dynamical crossing time. For \\eta~ 0.3) and occurs at scales with \\eta>1. We then consider the behavior of magnetic pressure in turbulent regimes. We propose that the reported lack of correlation between the magnetic pressure and the density is a consequence of the different scaling of the magnetic pressure with density for the slow and fast modes of nonlinear MHD waves. This implies that magnetic ``pressure'' is not a suitable candidate for supplementing thermal pressure in the presence of TI, and that polytropic descriptions of it are probably not adequate in the fully turbulent regime. Finally, we consider TI in a turbulent ISM-like medium. We find that the flow does not exhibit sharp phase transitions, as would be expected in classical...
Fast Diffusion of Magnetic Field in Turbulence and Origin of Cosmic Magnetism
Cho, Jungyeon
2013-01-01
Turbulence is believed to play important roles in the origin of cosmic magnetism. While it is well known that turbulence can efficiently amplify a uniform or spatially homogeneous seed magnetic field, it is not clear whether or not we can draw a similar conclusion for a localized seed magnetic field. The main uncertainty is the rate of magnetic field diffusion on scales larger than the outer scale of turbulence. To measure the diffusion rate of magnetic field on those large scales, we perform a numerical simulation in which the outer scale of turbulence is much smaller than the size of the system. We numerically compare diffusion of a localized seed magnetic field and a localized passive scalar. We find that diffusion of the magnetic field can be much faster than that of the passive scalar and that turbulence can efficiently amplify the localized seed magnetic field. Based on the simulation result, we construct a model for fast diffusion of magnetic field. Our model suggests that a localized seed magnetic fie...
DIFFUSION OF MAGNETIC FIELD AND REMOVAL OF MAGNETIC FLUX FROM CLOUDS VIA TURBULENT RECONNECTION
The diffusion of astrophysical magnetic fields in conducting fluids in the presence of turbulence depends on whether magnetic fields can change their topology via reconnection in highly conducting media. Recent progress in understanding fast magnetic reconnection in the presence of turbulence reassures that the magnetic field behavior in computer simulations and turbulent astrophysical environments is similar, as far as magnetic reconnection is concerned. This makes it meaningful to perform MHD simulations of turbulent flows in order to understand the diffusion of magnetic field in astrophysical environments. Our studies of magnetic field diffusion in turbulent medium reveal interesting new phenomena. First of all, our three-dimensional MHD simulations initiated with anti-correlating magnetic field and gaseous density exhibit at later times a de-correlation of the magnetic field and density, which corresponds well to the observations of the interstellar media. While earlier studies stressed the role of either ambipolar diffusion or time-dependent turbulent fluctuations for de-correlating magnetic field and density, we get the effect of permanent de-correlation with one fluid code, i.e., without invoking ambipolar diffusion. In addition, in the presence of gravity and turbulence, our three-dimensional simulations show the decrease of the magnetic flux-to-mass ratio as the gaseous density at the center of the gravitational potential increases. We observe this effect both in the situations when we start with equilibrium distributions of gas and magnetic field and when we follow the evolution of collapsing dynamically unstable configurations. Thus, the process of turbulent magnetic field removal should be applicable both to quasi-static subcritical molecular clouds and cores and violently collapsing supercritical entities. The increase of the gravitational potential as well as the magnetization of the gas increases the segregation of the mass and magnetic flux in the
Turbulent magnetic field amplification from the smallest to the largest magnetic Prandtl numbers
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, Pm ∼ 1 and Pm ≫ 1. We derive scaling laws of the growth rate as a function of hydrodynamic and magnetic Reynolds number for Pm ≪ 1 and Pm ≫ 1 for all types of turbulence. A central result concerns the regime of Pm ∼ 1, where the magnetic field amplification rate increases rapidly as a function of Pm. This phenomenon occurs for all types of turbulence we have explored. We further find that the dynamo growth rate can be decreased by a few orders of magnitude for turbulence spectra steeper than Kolmogorov. We calculate the critical magnetic Reynolds number Rmc for magnetic field amplification, which is highest for the Burgers case. As expected, our calculation shows a linear behaviour of the amplification rate close to the threshold proportional to (Rm − Rmc). On the basis of the Kazantsev model, we therefore expect the existence of the small-scale dynamo for a given value of Pm as long as the magnetic Reynolds number is above the critical threshold. (paper)
Aperiodic and semi-periodic perfect maps
Mitchell, Chris
1995-01-01
Paterson [1] has recently shown that the trivial necessary conditions are sufficient for the existence of a (binary) perfect map. These periodic structures can be transformed very simply into corresponding aperiodic and semi-periodic perfect maps. However, aperiodic and semi-periodic perfect maps can exist for parameter sets for which the corresponding periodic perfect maps cannot. In this paper it is shown, by construction, that (binary) aperiodic and semi-periodic perfect maps exist for all...
Bondi-like Accretion in Magnetized Supersonic Isothermal Turbulence
Burleigh, Kaylan J.; McKee, Christopher F.; Klein, Richard I.
2016-01-01
The Bondi and Bondi-Hoyle-Lytlleton formulas give the order of magnitude steady-accretion rate onto a point mass at rest or moving, respectively, in a uniform density gas in the limit of negligible gas self-gravity. This applies in star-forming clouds where self-gravity is negligible near protostars and new-born stars, but instead of being uniform the gas is supersonically turbulent and threaded by dynamically important (Alven Mach number ˜ 1) large-scale magnetic fields. To determine the Bondi-like accretion rate in these environments, we used the ORION2 code to carry out grid-based 3D adaptive mesh refinement (AMR) magnetohydrodynamic (MHD) simulations of accretion onto sink particles embedded in an environment of fully developed, magnetized supersonic isothermal turbulence. We evolved the models until the median and mean accretion rates, over particles, became steady. We present a simple semi-analytic model that predicts the median and mean accretion rate from the turbulent properties of the background medium, such as the 3D Mach number and RMS plasma-β, and show that it is highly consistent with our simulations. Numerical codes can use our semi-analytic model as an accurate sub-grid model for accretion in magnetized supersonic isothermal turbulence.
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...
MAGNETIC ENERGY PRODUCTION BY TURBULENCE IN BINARY NEUTRON STAR MERGERS
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 moment non-conservation in magnetohydrodynamic turbulence models
Dalena, S; Rappazzo, A F; Mace, R L; Matthaeus, W H
2012-01-01
The fundamental assumptions of the adiabatic theory do not apply in presence of sharp field gradients as well as in presence of well developed magnetohydrodynamic turbulence. For this reason in such conditions the magnetic moment $\\mu$ 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 $\\Delta \\mu$ (defined as the half peak-to-peak difference in the particle magnetic moment) and the bounce frequency $\\omega_b$. We perform test-particle simulations to investigate magnetic moment behavior when resonances overlapping occurs and during the interaction of a ring-beam particle distribution with a broad-band slab spectrum. We find that magnetic moment dynamics is strictly related to pitch angle $\\alpha$ for a low level of magnetic fluctuation, $\\delta B/B_0...
Anisotropic Formation of Magnetized Cores in Turbulent Clouds
Chen, Che-Yu
2015-01-01
In giant molecular clouds (GMCs), shocks driven by converging turbulent flows create high-density, strongly-magnetized regions that are locally sheetlike. In previous work, we showed that within these layers, dense filaments and embedded self-gravitating cores form by gathering material along the magnetic field lines. Here, we extend the parameter space of our three-dimensional, turbulent MHD core formation simulations. We confirm the anisotropic core formation model we previously proposed, and quantify the dependence of median core properties on the pre-shock inflow velocity and upstream magnetic field strength. Our results suggest that bound core properties are set by the total dynamic pressure (dominated by large-scale turbulence) and thermal sound speed c_s in GMCs, independent of magnetic field strength. For models with Mach number between 5 and 20, the median core masses and radii are comparable to the critical Bonnor-Ebert mass and radius defined using the dynamic pressure for P_ext. Our results corres...
Radiation from Relativistic Shocks with Turbulent Magnetic Fields
Nishkawa, K.; Medvedev, M.; Zhang, B.; Hardee, P.; Niemiec, J.; Mizuno, A.; Nordlund, A.; Frederiksen, J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Oka, M.; Fishman, J.
2009-01-01
Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked region. The simulation results show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields. These magnetic fields contribute to the electron's transverse deflection behind the shock. The "jitter" radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation which is calculated in a uniform magnetic field. This jitter radiation may be important to understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets, and supernova remnants. New recent calculation of spectra with various different Lorentz factors of jets (two electrons) and initial magnetic fields. New spectra based on small simulations will be presented.
Transport of solar electrons in the turbulent interplanetary magnetic field
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.
Turbulence and intermittent transport at the boundary of magnetized plasmas
Garcia, O.E.; Naulin, V.; Nielsen, A.H.;
2005-01-01
Numerical fluid simulations of interchange turbulence for geometry and parameters relevant to the boundary region of magnetically confined plasmas are shown to result in intermittent transport qualitatively similar to recent experimental measurements. The two-dimensional simulation domain features...... 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...... formation of blob structures is thus related to profile variations, which are here triggered in a quasiperiodic manner by a global dynamical regulation due to the self-sustained sheared flows. (C) 2005 American Institute of Physics....
Bacterial turbulence reduction by passive magnetic particle chains
Liu, Kuo-An; I, Lin
2013-09-01
We report the experimental observation of the bacterial turbulence reduction in dense E. coli suspensions by increasing the coupling of passive particle additives (paramagnetic particles). Applying an external magnetic field induces magnetic dipoles for particles and causes the formation of vertical chain bundles, which are hard for bacterial flows to tilt and break. The larger effective drag coefficient of chains causes slow horizontal motion of chains, which in turn form obstacles to suppress bacterial flows through the strong correlation in coherent bacterial clusters and intercluster interaction. The interruption of the upward energy flow from individual self-propelling bacteria to the larger scale in the bacterial turbulence with multiscaled coherent flow by the chain bundle leads to more severe suppression in the low frequency (wave number) regimes of the power spectra.
Transport of solar electrons in the turbulent interplanetary magnetic field
Ablaßmayer, J.; Tautz, R. C.; Dresing, N.
2016-01-01
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.
Transport of solar electrons in the turbulent interplanetary magnetic field
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
Current filaments in turbulent magnetized plasmas
Martines, E; Vianello, N; Spolaore, M; Zuin, M; Agostini, M; Antoni, V; Cavazzana, R; Scarin, P; Serianni, G; Spada, E [Consorzio RFX, Associazione Euratom-ENEA sulla Fusione, Padova (Italy); Sundkvist, D [Space Sciences Laboratory, University of California at Berkeley, Berkeley, CA (United States); Ionita, C; Mehlmann, F; Schrittwieser, R [Association EURATOM/OeAW, Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck (Austria); Maraschek, M; Mueller, H W; Rohde, V [Max-Planck-Institut fuer Plasmaphysik, EURATOM Association, Garching (Germany); Naulin, V; Rasmussen, J J, E-mail: emilio.martines@igi.cnr.i [Association EURATOM/RISOe-Technical University of Denmark, Roskilde (Denmark)
2009-12-15
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-Alfven vortices. Current structures associated with reconnection events measured in a reversed field pinch plasma and in the magnetosheath are detected and compared. Evidence of current filaments occurring during ELMs in an H-mode tokamak plasma is displayed.
Current filaments in turbulent magnetized plasmas
Martines, E.; Vianello, N.; Sundkvist, D.;
2009-01-01
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......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...... measured in a reversed field pinch plasma and in the magnetosheath are detected and compared. Evidence of current filaments occurring during ELMs in an H-mode tokamak plasma is displayed....
Turbulent transport of impurities in a magnetized plasma
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.)
Homological aperiodic tilings of 3-dimensional geometries
Nowak, Piotr W
2012-01-01
We construct the first aperiodic tiles for two amenable 3-dimensional Lie groups: Sol and the Heisenberg group. Our construction relies on the use of higher-dimensional uniformly finite homology. In particular, we settle completely the existence of aperiodic tiles for all of the non-compact geometries of 3-manifolds appearing in the geometrization conjecture.
Wave turbulence on the surface of a ferrofluid submitted to a magnetic field
Boyer, François; Falcon, Eric
2008-01-01
We report the observation of wave turbulence on the surface of a ferrofluid mechanically forced and submitted to a static normal magnetic field. We show that magnetic surface waves arise only above a critical field. The power spectrum of their amplitudes displays a frequency-power law leading to the observation of a magnetic wave turbulence regime which is experimentally shown to involve a 4-wave interaction process. The existence of the regimes of gravity, magnetic and capillary wave turbule...
Turbulence in Global Simulations of Magnetized Thin Accretion Disks
Beckwith, Kris; Simon, Jacob B
2011-01-01
We use a global magnetohydrodynamic simulation of a geometrically thin accretion disk to investigate the locality and detailed structure of turbulence driven by the magnetorotational instability (MRI). The model disk has an aspect ratio $H / R \\simeq 0.07$, and is computed using a higher-order Godunov MHD scheme with accurate fluxes. We focus the analysis on late times after the system has lost direct memory of its initial magnetic flux state. The disk enters a saturated turbulent state in which the fastest growing modes of the MRI are well-resolved, with a relatively high efficiency of angular momentum transport $ > \\approx 2.5 \\times 10^{-2}$. The accretion stress peaks at the disk midplane, above and below which exists a moderately magnetized corona with patches of superthermal field. By analyzing the spatial and temporal correlations of the turbulent fields, we find that the spatial structure of the magnetic and kinetic energy is moderately well-localized (with correlation lengths along the major axis of ...
Magnetic shear-driven instability and turbulent mixing in magnetized protostellar disks
Bonanno, Alfio
2008-01-01
Observations of protostellar disks indicate the presence of the magnetic field of thermal (or superthermal) strength. In such a strong magnetic field, many MHD instabilities responsible for turbulent transport of the angular momentum are suppressed. We consider the shear-driven instability that can occur in protostellar disks even if the field is superthermal. This instability is caused by the combined influence of shear and compressibility in a magnetized gas and can be an efficient mechanism to generate turbulence in disks. The typical growth time is of the order of several rotation periods.
Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
Schekochihin, A A [Blackett Laboratory, Imperial College, London SW7 2BW (United Kingdom); Iskakov, A B [Department of Physics and Astronomy, UCLA, Los Angeles CA 90095-1547 (United States); Cowley, S C [Blackett Laboratory, Imperial College, London SW7 2BW (United Kingdom); McWilliams, J C [Department of Atmospheric Sciences, UCLA, Los Angeles CA 90095-1565 (United States); Proctor, M R E [DAMTP, University of Cambridge, Cambridge CB3 0WA (United Kingdom); Yousef, T A [DAMTP, University of Cambridge, Cambridge CB3 0WA (United Kingdom)
2007-08-15
This paper is a detailed report on a programme of direct numerical simulations of incompressible nonhelical randomly forced magnetohydrodynamic (MHD) turbulence that are used to settle a long-standing issue in the turbulent dynamo theory and demonstrate that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm >> 1 and small magnetic Prandtl number Pm << 1. The dependence of the critical Rm{sub c} for dynamo versus the hydrodynamic Reynolds number Re is obtained for 1 {approx}< Re {approx}< 6700. In the limit Pm >> 1, Rm{sub c} is at most three times larger than for the previously well established dynamo at large and moderate Prandtl numbers: Rm{sub c} {approx}< 200 for Re {approx}> 6000 compared to Rm{sub c} {approx} 60 for Pm{>=}1. The stability curve Rm{sub c}(Re) (and, it is argued, the nature of the dynamo) is substantially different from the case of the simulations and liquid-metal experiments with a mean flow. It is not as yet possible to determine numerically whether the growth rate of the magnetic energy is {proportional_to}Rm{sup 1/2} in the limit Re >> Rm >> 1, as should be the case if the dynamo is driven by the inertial-range motions at the resistive scale, or tends to an Rm-independent value comparable to the turnover rate of the outer-scale motions. The magnetic-energy spectrum in the low-Pm regime is qualitatively different from the Pm {>=} 1 case and appears to develop a negative spectral slope, although current resolutions are insufficient to determine its asymptotic form. At Rm element of (1,Rm{sub c}), the magnetic fluctuations induced via the tangling by turbulence of a weak mean field are investigated and the possibility of a k{sup -1} spectrum above the resistive scale is examined. At low Rm < 1, the induced fluctuations are well described by the quasistatic approximation; the k{sup -11/3} spectrum is confirmed for the first time in direct numerical simulations. Applications of the results on turbulent induction to
Li, Zhi-Yun; Shang, Hsien; Zhao, Bo
2014-01-01
The formation of rotationally supported protostellar disks is suppressed in ideal MHD in non-turbulent cores with aligned magnetic field and rotation axis. A promising way to resolve this so-called "magnetic braking catastrophe" is through turbulence. The reason for the turbulence-enabled disk formation is usually attributed to the turbulence-induced magnetic reconnection, which is thought to reduce the magnetic flux accumulated in the disk-forming region. We advance an alternative interpretation, based on magnetic decoupling-triggered reconnection of severely pinched field lines close to the central protostar and turbulence-induced warping of the pseudodisk of Galli and Shu. Such reconnection weakens the central split magnetic monopole that lies at the heart of the magnetic braking catastrophe under flux freezing. We show, through idealized numerical experiments, that the pseudodisk can be strongly warped, but not completely destroyed, by a subsonic or sonic turbulence. The warping decreases the rates of ang...
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 $\\alpha$.
Turbulence, transport and confinement: from tokamaks to star magnetism
This thesis is part of the general study of self-organization in hot and magnetized plasmas. We focus our work on two specific objects: stars and tokamaks. We use first principle numerical simulations to study turbulence, transport and confinement in these plasmas. The first part of this thesis introduces the main characteristics of stellar and tokamak plasmas. The reasons for studying them together are properly detailed. The second part is focused on stellar aspects. We study the interactions between the 3D turbulent motions in the solar convection zone with an internal magnetic field in the tachocline (the transition region between the instable and stable zones in the Sun). The tachocline is a very thin layer (less than five percent of the solar radius) that acts as a transport barrier of angular momentum. We show that such an internal magnetic field is not likely to explain the observed thickness of the tachocline and we give some insights on how to find alternative mechanisms to constrain it. We also explore the effect of the environment of star on its structure. We develop a methodology to study the influence of stellar wind and of the magnetic coupling of a star with its orbiting planets. We use the same methodology to analyse the magnetic interaction between a stellar wind and a planetary magnetosphere that acts as a transport barrier of matter. Then, the third part is dedicated to fusion oriented research. We present a numerical investigation on the experimental mechanisms that lead to the development of transport barriers in the plasma. These barriers are particularly important for the design of high performance fusion devices. The creation of transport barriers is obtained in turbulent first principle simulations for the very first time. The collaboration between the two scientific teams lead to the results presented in the fourth part of this thesis. An original spectral method is developed to analyse the saturation of stellar convective dynamos and of
Magnetic self-organisation in Hall-dominated magnetorotational turbulence
Kunz, Matthew W
2013-01-01
The magnetorotational instability (MRI) is the most promising mechanism by which angular momentum is efficiently transported outwards in astrophysical discs. However, its application to protoplanetary discs remains problematic. These discs are so poorly ionised that they may not support magnetorotational turbulence in regions referred to as `dead zones'. It has recently been suggested that the Hall effect, a non-ideal magnetohydrodynamic (MHD) effect, could revive these dead zones by enhancing the magnetically active column density by an order of magnitude or more. We investigate this idea by performing local, three-dimensional, resistive Hall-MHD simulations of the MRI in situations where the Hall effect dominates over Ohmic dissipation. As expected from linear stability analysis, we find an exponentially growing instability in regimes otherwise linearly stable in resistive MHD. However, instead of vigorous and sustained magnetorotational turbulence, we find that the MRI saturates by producing large-scale, l...
Wave turbulence on the surface of a ferrofluid in a horizontal magnetic field
Dorbolo, Stéphane; Falcon, Eric
2011-01-01
We report observations of wave turbulence on the surface of a ferrofluid submitted to a magnetic field parallel to the fluid surface. The magnetic wave turbulence shows several differences compared to the normal field case reported recently. The inertial zone of the magnetic wave turbulence regime is notably found to be strongly increased with respect to the normal field case, and to be well described by our theoretical predictions. The dispersion relation of linear waves is also measured and...
Dynamics of fluctuating magnetic fields in turbulent dynamos incorporating ambipolar drifts
Subramanian, K.(Srikumar)
1997-01-01
Turbulence with a large magnetic Reyonolds number, generically leads to rapidly growing magnetic noise over and above any mean field. We revisit the dynamics of this fluctuating field, in homogeneous, isotropic, helical turbulence. Assuming the turbulence to be Markovian, we first rederive, in a fairly transparent manner, the equation for the mean field, and corrected Fokker-Plank type equations for the magnetic correlations. In these equations, we also incorporate the effects of ambipolar dr...
Parity of solar global magnetic field determined by turbulent diffusivity
Hotta, H.; Yokoyama, T.
We investigate the criterion for the solar dipole-field in a kinematic flux-transport dynamo model. The sun has a dipole-like global magnetic field. This field is thought to be generated by the dynamo action of the solar internal plasma. The flux-transport dynamo succeeds to reproduce some features of solar cycle, e.g. poleward the migration of the general magnetic field and the butterfly diagram. The parity, however, of the global magnetic field significantly depends on parameters in the flux-transport dynamo. It is known that the coupling of the magnetic field between hemispheres due to turbulent diffusivity is an important factor for the solar parity issue, but the detailed criterion for the generation of the dipole field has not been investigated. Our conclusions are as follows. (1) The stronger diffusivity near the surface is more likely to cause the magnetic field to be a dipole. (2) The thinner layer of the strong diffusivity near the surface is also more apt to generate a dipolar magnetic field. (3) The faster meridional flow is more prone to cause the magnetic field to be a quadrupole, i.e., symmetric about the equator. The result (1) is consistent with our previous work tep{hotta2010a}, which is on the effect of the surface diffusivity for the observed weak polar field.
Electrostatic instabilities and turbulence in a toroidal magnetized plasma
This Thesis aims at characterizing the linear properties of electrostatic drift instabilities arising in a toroidal plasma and the mechanisms leading to their development into turbulence. The experiments are performed on the TORoidal Plasma EXperiment (TORPEX) at CRPP-EPFL, Lausanne. The first part of the Thesis focuses on the identification of the nature of the instabilities observed in TORPEX, using a set of electrostatic probes, designed and built for this purpose. The global features of fluctuations, analyzed for different values of control parameters such as the magnetic field, the neutral gas pressure and the injected microwave power, are qualitatively similar in different experimental scenarios. The maximum of fluctuations is observed on the low field side, where the pressure gradient and the gradient of the magnetic field are co-linear, indicating that the curvature of the magnetic field lines has an important role in the destabilization of the waves. The power spectrum is dominated by electrostatic fluctuations with frequencies much lower than the ion cyclotron frequency. Taking advantage of the extended diagnostics coverage, the spectral properties of fluctuations are measured over the whole poloidal cross-section. Both drift and interchange instabilities develop and propagate on TORPEX, with the stability of both being affected by the curvature of the magnetic field. It is shown that modes of different nature are driven at separate locations over the plasma cross-section and that the wavenumber and frequency spectra, narrow at the location where the instabilities are generated, broaden during convection, suggesting an increase in the degree of turbulence. The transition from coherent to turbulent spectral features and the role of nonlinear coupling between modes in the development of turbulence are treated in the second part of this work. It is found that nonlinear mode-mode coupling is responsible for the redistribution of spectral energy from the
Semiconductor Laser with Aperiodic Photonic Lattice
Subhasish Chakraborty
2008-01-01
A semiconductor laser and method for selecting laser frequency emission from the semiconductor laser are disclosed. The semiconductor laser provides selectable frequency emission and includes an aperiodic photonic lattice.
Garbage Collection Scheduling of Aperiodic Tasks
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.
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.)
Multi-scale structures of turbulent magnetic reconnection
Nakamura, T. K. M.; Nakamura, R.; Narita, Y.; Baumjohann, W.; Daughton, W.
2016-05-01
We have analyzed data from a series of 3D fully kinetic simulations of turbulent magnetic reconnection with a guide field. A new concept of the guide filed reconnection process has recently been proposed, in which the secondary tearing instability and the resulting formation of oblique, small scale flux ropes largely disturb the structure of the primary reconnection layer and lead to 3D turbulent features [W. Daughton et al., Nat. Phys. 7, 539 (2011)]. In this paper, we further investigate the multi-scale physics in this turbulent, guide field reconnection process by introducing a wave number band-pass filter (k-BPF) technique in which modes for the small scale (less than ion scale) fluctuations and the background large scale (more than ion scale) variations are separately reconstructed from the wave number domain to the spatial domain in the inverse Fourier transform process. Combining with the Fourier based analyses in the wave number domain, we successfully identify spatial and temporal development of the multi-scale structures in the turbulent reconnection process. When considering a strong guide field, the small scale tearing mode and the resulting flux ropes develop over a specific range of oblique angles mainly along the edge of the primary ion scale flux ropes and reconnection separatrix. The rapid merging of these small scale modes leads to a smooth energy spectrum connecting ion and electron scales. When the guide field is sufficiently weak, the background current sheet is strongly kinked and oblique angles for the small scale modes are widely scattered at the kinked regions. Similar approaches handling both the wave number and spatial domains will be applicable to the data from multipoint, high-resolution spacecraft observations such as the NASA magnetospheric multiscale (MMS) mission.
Instabilities, turbulence and transport in a magnetized plasma
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.)
Basic Investigation of Turbulent Structures and Blobs of Relevance for Magnetic Fusion Plasmas
Theiler, Christian Gabriel
2011-01-01
Similarly to neutral fluids, plasmas often exhibit turbulent behavior. Turbulence in plasmas is usually more complex than in neutral fluids due to long range interactions via electric and magnetic fields, and kinetic effects. It gives rise to many interesting phenomena such as self-generated magnetic fields (dynamos), zonal-flows, transport barriers, or particle pinches. Plasma turbulence plays a crucial role for the success of nuclear fusion as a ...
Gyrokinetic particle simulation for thermonuclear plasma turbulence studies in magnetic confinement
Janhunen, Salomon
2013-01-01
Thermal transport in a magnetised plasma is believed to be substantially enhanced due to turbulence. The ELMFIRE code has been developed for tokamak plasma turbulence studies in high temperature magnetized plasmas. ELMFIRE calculates the evolution of the Boltzmann equation in a magnetized plasma, including long scale interactions between particles calculated through field equations. In this work we concentrate on benchmarking the ELMFIRE against published results from other turbulence code...
Fundamental Statistical Descriptions of Plasma Turbulence in Magnetic Fields
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
Investigation of turbulent structures in the edge of magnetized plasmas
Rising energy cost and progressing climate change will exacerbate existing and give birth to new conflicts. Energy savings and the development of new technologies can counteract the reasons for these conflicts. Beside renewable energy sources, nuclear fusion can help to meet this challenge. To build future fusion power plants smaller and more efficient, the magnetic confinement must be improved and the load on plasma facing components reduced. To this end, better understanding is required of turbulent transport processes in magnetized plasmas. Within the frame of the present work, the properties and dynamics of turbulent density structures (''blobs'') have been investigated, as well as their interaction with shear flows. Langmuir-probe measurements have been conducted in the tokamak ASDEX Upgrade and in the stellarator TJ-K, and compared with GEMR plasma turbulence simulations. It has been shown, that blobs are generated at the last closed flux surface (LCFS) of ASDEX Upgrade. They propagate perpendicular to the magnetic field lines in the radial and poloidal directions. The poloidal E x B-drift depends on the radial variation of the plasma potential. The latter is given by the electron temperature profile in front of the electrically conducting wall. Experimental results show, that this can lead to a shear layer inside the scrape-off layer (SOL) of a divertor tokamak due to inhomogeneous connection lengths to the wall. Blobs can hardly cross such a shear layer unchanged. This investigation shows how blobs can exchange particles and energy across a shear layer without changing their shapes and velocities substantially. However, the dynamics of the structures are different between both sides of the shear layer. Parallel drift-wave dynamics are dominant on the plasma core side, i.e. density and potential of the blobs are in phase. Outside of the shear layer, the interchange mechanism dominates due to shorter parallel connection lengths to the wall. The poloidal
Effect of the external magnetic field on the MHD turbulence spectra
The turbulent properties of the conducting liquids at external constant magnetic field change with increase of magnetic field strength. The behavior of the second order structure function for the turbulent field velocity in the homogenous incompressible liquid is studied in the presence of the external homogeneous magnetic field. It is shown that different determining parameters may occur in system as well as in the inertial and dissipative turbulence intervals depending on magnetic field strength. It gives rise to appearance of the new spectral scalings which correspond to those observed at experiments
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 (...
Dynamics of fluctuating magnetic fields in turbulent dynamos incorporating ambipolar drifts
Subramanian, K
1997-01-01
Turbulence with a large magnetic Reyonolds number, generically leads to rapidly growing magnetic noise over and above any mean field. We revisit the dynamics of this fluctuating field, in homogeneous, isotropic, helical turbulence. Assuming the turbulence to be Markovian, we first rederive, in a fairly transparent manner, the equation for the mean field, and corrected Fokker-Plank type equations for the magnetic correlations. In these equations, we also incorporate the effects of ambipolar drift which would obtain if the turbulent medium has a significant neutral component. We apply these equations to discuss a number of astrophysically interesting problems: (a) the small scale dynamo in galactic turbulence with a model Kolmogorov spectrum, incorporating the effect of ambipolar drift; (b) current helicity dynamics and the quasilinear corrections to the alpha effect; (c) growth of the current helicity and large-scale magnetic fields due to nonlinear effects.
This thesis is devoted to two studies of low-frequency turbulence in toroidally confined plasma. Low-frequency turbulence is believed to play an important role in anomalous transport in toroidal confinement devices. The first study pertains the development of an analytic theory of ion-temperature-gradient-driven turbulence in tokamaks. Energy conserving, renormalized spectrum equations are derived and solved in order to obtain the spectra of stationary ion temperature gradient driven turbulence. Corrections to mixing length estimates are calculated explicitly. The resulting anomalous ion thermal diffusivity is derived and is found to be consistent with experimentally-deduced ion thermal diffusivities. The associated electron thermal diffusivity, particle and heat-pinch velocities are also calculated. The second study is devoted to the role of multiple helicity nonlinear interactions of tearing modes and dynamics of magnetic relaxation in a high-temperature current carrying plasma. To extend the resistive MHD theory of magnetic fluctuations and dynamo activity observed in the reversed field pinch, the fluid equations for high temperature regime are derived and basic nonlinear interaction mechanism and the effects of diamagnetic corrections to the MHD turbulence theory are studied for the case of fully developed, densely packed turbulence
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. (author)
Kunz, M. W.; Schekochihin, A. A.; 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 microsca...
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. PMID:24784044
Effect of the external magnetic field on the MHD turbulence spectra
The turbulent properties of conducting fluids in an external constant magnetic field are known to change with increasing field strength. A study is made of the behavior of the second-order structural function of the velocity field in a homogeneous incompressible turbulent fluid in the presence of an external uniform magnetic field. It is shown that, depending on the magnetic field strength, there may be different governing parameters of the system in both the inertial and dissipative intervals of turbulence. This leads to new spectral scalings that are consistent with experimental ones
Wave turbulence on the surface of a ferrofluid in a horizontal magnetic field
Dorbolo, Stéphane
2011-01-01
We report observations of wave turbulence on the surface of a ferrofluid submitted to a magnetic field parallel to the fluid surface. The magnetic wave turbulence shows several differences compared to the normal field case reported recently. The inertial zone of the magnetic wave turbulence regime is notably found to be strongly increased with respect to the normal field case, and to be well described by our theoretical predictions. The dispersion relation of linear waves is also measured and differs from the normal field case due to the absence of the Rosensweig instability.
Turbulent Amplification and Structure of the Intracluster Magnetic Field
Beresnyak, Andrey; Miniati, Francesco
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 ar...
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...
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.
Enhanced Dissipation Rate of Magnetic Field in Striped Pulsar Winds by the Effect of Turbulence
Takamoto, Makoto; Inutsuka, Shu-ichiro
2012-01-01
In this letter we report on turbulent acceleration of the dissipation of magnetic field in the postshock re- gion 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 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 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
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
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 ($\
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.
Zhang, Bing
2010-01-01
The recent Fermi observation of GRB 080916C shows that the bright photosphere emission associated with a putative fireball is missing, which suggests a Poynting-flux-dominated outflow. We propose a model of gamma-ray burst (GRB) prompt emission in the Poynting-flux-dominated regime, namely, the Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART) model. It is envisaged that the GRB central engine launches an intermittent, magnetically-dominated wind, and that in the GRB emission region, the ejecta is still moderately magnetized. Similar to the internal shock (IS) model, the mini-shells interact internally at the traditional internal shock radius. Most of these early collision have little energy dissipation, but serve to distort the ordered magnetic field lines. At a certain point, the distortion of magnetic field configuration reaches the critical condition to allow fast reconnection seeds to occur, which induce relativistic MHD turbulence in the interaction regions. The turbulence further...
Blackman, Eric G
2012-01-01
The extent to which large scale magnetic fields are susceptible to turbulent diffusion is important for interpreting the need for in situ large scale dynamos in astrophysics and for observationally inferring field strengths compared to kinetic energy. By solving coupled equations for magnetic energy and magnetic helicity in a system initiated with isotropic turbulence and an arbitrarily helical large scale field, we quantify the decay rate of the latter for a bounded or periodic system. The energy associated with the non-helical magnetic field rapidly decays by turbulent diffusion, but the decay rate of the helical component depends on whether its magnetic energy exceeds E_C =(k_1/k_f)^2 E_V, where E_V is the kinetic energy per mass of turbulence and k_1 and k_f are the wave numbers of the large and forcing scales. Turbulently diffusing helical fields to small scales while conserving magnetic helicity requires a rapid increase in total magnetic energy. As such, only when the helical fields are sub-critical ca...
Turbulence and transport in a magnetized argon plasma
An experimental study on turbulence and transport in the highly ionized argon plasma of a hollow cathode discharge is described. In order to determine the plasma parameters three standard diagnostics have been used, whilst two diagnostics have been developed to study the plasma turbulence. (Auth.)
Sub-grid-scale description of turbulent magnetic reconnection in magnetohydrodynamics
Widmer, F.; Büchner, J.; Yokoi, N.
2016-04-01
Magnetic reconnection requires, at least locally, a non-ideal plasma response. In collisionless space and astrophysical plasmas, turbulence could transport energy from large to small scales where binary particle collisions are rare. We have investigated the influence of small scale magnetohydrodynamics (MHD) turbulence on the reconnection rate in the framework of a compressible MHD approach including sub-grid-scale (SGS) turbulence. For this sake, we considered Harris-type and force-free current sheets with finite guide magnetic fields directed out of the reconnection plane. The goal is to find out whether unresolved by conventional simulations MHD turbulence can enhance the reconnection process in high-Reynolds-number astrophysical plasmas. Together with the MHD equations, we solve evolution equations for the SGS energy and cross-helicity due to turbulence according to a Reynolds-averaged turbulence model. The SGS turbulence is self-generated and -sustained through the inhomogeneities of the mean fields. By this way, the feedback of the unresolved turbulence into the MHD reconnection process is taken into account. It is shown that the turbulence controls the regimes of reconnection by its characteristic timescale τt. The dependence on resistivity was investigated for large-Reynolds-number plasmas for Harris-type as well as force-free current sheets with guide field. We found that magnetic reconnection depends on the relation between the molecular and apparent effective turbulent resistivity. We found that the turbulence timescale τt decides whether fast reconnection takes place or whether the stored energy is just diffused away to small scale turbulence. If the amount of energy transferred from large to small scales is enhanced, fast reconnection can take place. Energy spectra allowed us to characterize the different regimes of reconnection. It was found that reconnection is even faster for larger Reynolds numbers controlled by the molecular resistivity η, as
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.)
Simulation of turbulent magnetic reconnection in the small-scale solar wind
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.
Wide-stopband aperiodic phononic filters
Rostem, K.; Chuss, D. T.; Denis, K. L.; Wollack, E. J.
2016-06-01
We demonstrate that a phonon stopband can be synthesized from an aperiodic structure comprising a discrete set of phononic filter stages. Each element of the set has a dispersion relation that defines a complete bandgap when calculated under a Bloch boundary condition. Hence, the effective stopband width in an aperiodic phononic filter (PnF) may readily exceed that of a phononic crystal with a single lattice constant or a coherence scale. With simulations of multi-moded phononic waveguides, we discuss the effects of finite geometry and mode-converting junctions on the phonon transmission in PnFs. The principles described may be utilized to form a wide stopband in acoustic and surface wave media. Relative to the quantum of thermal conductance for a uniform mesoscopic beam, a PnF with a stopband covering 1.6–10.4 GHz is estimated to reduce the thermal conductance by an order of magnitude at 75 mK.
Optimized aperiodic highly directional narrowband infrared emitters
Granier, Christopher H.; Afzal, Francis O.; Min, Changjun; Dowling, Jonathan P.; Veronis, Georgios
2014-09-01
Bulk thermal emittance sources possess incoherent, isotropic, and broadband radiation spectra that vary from material to material. However, these radiation spectra can be drastically altered by modifying the geometry of the structures. In particular, several approaches have been proposed to achieve narrowband, highly directional thermal emittance based on photonic crystals, gratings, textured metal surfaces, metamaterials, and shock waves propagating through a crystal. Here we present optimized aperiodic structures for use as narrowband, highly directional thermal infrared emitters for both TE and TM polarizations. One-dimensional layered structures without texturing are preferable to more complex two- and three-dimensional structures because of the relative ease and low cost of fabrication. These aperiodic multilayer structures designed with alternating layers of silicon and silica on top of a semi-infinite tungsten substrate exhibit extremely high emittance peaked around the wavelength at which the structures are optimized. Structures were designed by a genetic optimization algorithm coupled to a transfer matrix code which computed thermal emittance. First, we investigate the properties of the genetic-algorithm optimized aperiodic structures and compare them to a previously proposed resonant cavity design. Second, we investigate a structure optimized to operate at the Wien wavelength corresponding to a near-maximum operating temperature for the materials used in the aperiodic structure. Finally, we present a structure that exhibits nearly monochromatic and highly directional emittance for both TE and TM polarizations at the frequency of one of the molecular resonances of carbon monoxide (CO); hence, the design is suitable for a detector of CO via absorption spectroscopy.
The first nonlinear gyrokinetic simulations of electron internal transport barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed magnetic shear can suppress thermal transport by increasing the nonlinear critical gradient for electron-temperature-gradient-driven turbulence to three times its linear critical value. An interesting feature of this turbulence is non- linearly driven off-midplane radial streamers. This work reinforces the experimental observation that magnetic shear is likely an effective way of triggering and sustaining e-ITBs in magnetic fusion devices.
Peterson, J. L.; Hammet, G. W.; Mikkelsen, D. R.; Yuh, H. Y.; Candy, J.; Guttenfelder, W.; Kaye, S. M.; LeBlanc, B.
2011-05-11
The first nonlinear gyrokinetic simulations of electron internal transport barriers (e-ITBs) in the National Spherical Torus Experiment show that reversed magnetic shear can suppress thermal transport by increasing the nonlinear critical gradient for electron-temperature-gradient-driven turbulence to three times its linear critical value. An interesting feature of this turbulence is non- linearly driven off-midplane radial streamers. This work reinforces the experimental observation that magnetic shear is likely an effective way of triggering and sustaining e-ITBs in magnetic fusion devices.
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...
Wave turbulence on the surface of a ferrofluid submitted to a magnetic field
Boyer, François
2008-01-01
We report the observation of wave turbulence on the surface of a ferrofluid mechanically forced and submitted to a static normal magnetic field. We show that magnetic surface waves arise only above a critical field. The power spectrum of their amplitudes displays a frequency-power law leading to the observation of a magnetic wave turbulence regime which is experimentally shown to involve a 4-wave interaction process. The existence of the regimes of gravity, magnetic and capillary wave turbulence is reported in the phase space parameters as well as a triple point of coexistence of these three regimes. Most of these features are understood using dimensional analysis or the dispersion relation of the ferrohydrodynamics surface waves.
Turbulence in ferrofluids in channel flow with steady and oscillating magnetic fields.
Schumacher, Kristopher R; Riley, James J; Finlayson, Bruce A
2011-01-01
The turbulent flow of a ferrofluid in channel flow is studied using direct numerical simulation. The method of analysis is an extension of that used for Newtonian fluids, with additional features necessary to model the ferrofluid. The analysis is applied to low Reynolds number turbulence in the range of existing experimental data in a capillary. For steady and oscillating magnetic fields, comparisons are made between a Newtonian fluid and a ferrofluid by comparing the pressure drop, turbulent Reynolds number, turbulent kinetic energy (k), Reynolds stress, velocity, and spin profiles. The results are also compared with predictions of a k-ɛ model to show the accuracy of that model when applied to ferrofluids, where ɛ is the rate of viscous dissipation of turbulent kinetic energy. PMID:21405774
Sub-Grid-Scale Description of Turbulent Magnetic Reconnection in Magnetohydrodynamics
Widmer, Fabien; Yokoi, Nobumitsu
2015-01-01
Magnetic reconnection requires, at least locally, a non-ideal plasma response. In collisionless space and astrophysical plasmas, turbulence could permit this instead of the too rare binary collisions. We investigated the influence of turbulence on the reconnection rate in the framework of a single fluid compressible MHD approach. The goal is to find out, whether unresolved, sub-grid for MHD simulations, turbulence can enhance the reconnection process in high Reynolds number astrophysical plasma. We solve, simultaneously with the grid-scale MHD equations, evolution equations for the sub-grid turbulent energy and cross helicity according to Yokoi's model (Yokoi (2013)) where turbulence is self-generated and -sustained through the inhomogeneities of the mean fields. Simulations of Harris and force free sheets confirm the results of Higashimori et al. (2013) and new results are obtained about the dependence on resistivity for large Reynolds number as well as guide field effects. The amount of energy transferred f...
Turbulent Generation of Flows and Magnetic Field at the Rational Magnetic Surfaces of a Tokamak
Full text: Comparative analysis of generation of large-scale structures, zonal flows and streamers, by drift wave turbulence is conducted for periodic systems with magnetic shear such as a tokamak. In a strong magnetic field dynamics of quasi two-dimensional perturbations strongly depends on the value of the wave vector along the magnetic field. When the parallel wave vector is significantly large, so that the parallel phase velocity of perturbation is small compared to electron thermal velocity, the parallel electron motion results in a finite electron density perturbation. It follows the Boltzmann distribution. However, for large-scale structures with poloidal and toroidal symmetry m = n = 0, and the parallel wave vector is zero. This results in strong reduction of density perturbation for m = n = 0. This difference has profound consequences for generation of large-scale zonal flows and streamers due to different structure of the nonlinear interaction matrix. The interaction term has a structure similar to the standard convective nonlinearity for zonal flows, while for streamers it has the structure of the Hasegawa-Mima nonlinearity (which is the higher order due to a small parameter associated with a finite ion Larmor radius). Respectively, zonal flows have the larger growth rate gamma(ZF) compared to that of the streamers. It is shown that 3D electromagnetic helical perturbations will have the growth rate comparable to that of zonal flows if their symmetry coincides with the symmetry of rational magnetic surface, m = nq. The field line bending provides a stabilizing effect and thus determines the radial localization of such structures. Therefore, it is expected that three-dimensional structures of flows and magnetic field will be preferentially generated at the rational magnetic surfaces of a tokamak with a growth rate of order gamma(ZF). This theoretical result may corroborate existing experimental correlations of large-scale shear flow structures with
Turbulent transport across shear layers in magnetically confined plasmas
Shear layers modify the turbulence in diverse ways and do not only suppress it. A spatial-temporal investigation of gyrofluid simulations in comparison with experiments allows to identify further details of the transport process across shear layers. Blobs in and outside a shear layer merge, thereby exchange particles and heat and subsequently break up. Via this mechanism particles and heat are transported radially across shear layers. Turbulence spreading is the immanent mechanism behind this process
Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks
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...... emission. Our results suggest that a significant fraction of the magnetic energy in accretion disks resides in large scale fields, which in turn provides circumstantial evidence for significant non-local transport phenomena and the need for large scale magnetic field generation. For the example of Seyfert...
The Complex Structure of Magnetic Field Discontinuities in the Turbulent Solar Wind
Greco, A; Servidio, S; Yordanova, E; Veltri, P
2015-01-01
Using high resolution Cluster satellite observations, we show that the turbulent solar wind is populated by magnetic discontinuities at different scales, going from proton down to electron scales. The structure of these layers resembles the Harris equilibrium profile in plasmas. Using a multi-dimensional intermittency technique, we show that these structures are connected through the scales. Supported by numerical simulations of magnetic reconnection, we show that observations are consistent with a scenario where many current layers develop in turbulence, and where the outflow of these reconnection events are characterized by complex sub-proton networks of secondary islands, in a self-similar way. The present work establishes that the picture of "reconnection in turbulence" and "turbulent reconnection", separately invoked as ubiquitous, coexist in space plasmas.
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.
Dispersion blue-shift in an aperiodic Bragg reflection waveguide
Fesenko, Volodymyr I.; Tuz, Vladimir R.
2016-04-01
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.
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.
Turbulence-Level Variations and Magnetic Field Orientations in the Fast Solar Wind
Ragot, B R
2013-01-01
The turbulent magnetic fields of a large set of fast solar wind streams measured onboard ACE and STEREO A and B are analyzed in an effort to identify the effects of the turbulence-level broad variations on the orientations of the local, time-averaged magnetic fields. The power level of turbulence, roughly defined as the power in the transverse field fluctuations normalized to the medium-scale average background field, tightly orders the location of the peaks in the probability distribution functions (PDFs) of the angles between local fields and Parker spiral. As a result, the broad variations in the power level of turbulence cause a steep dependence of the average power level of turbulence on the angle of the local field to the Parker spiral, with the highest turbulence levels found near the normal to the Parker spiral and the lowest levels near the Parker spiral direction. Generalized quasilinear estimates of the mean cross-field displacements adapted to intermittent time-varying turbulence lead to accurate ...
DNS of turbulent channel flow at high Reynolds number under a uniform magnetic field
A direct numerical simulation (DNS) of turbulent channel flow with high Reynolds number has been carried out to show the effects of magnetic field. In this study, the Reynolds number for channel flow based on bulk velocity ub, kinematic viscosity ν, and channel width 2δ was set to be constant; Reb=2δub/ν=45818. A uniform magnetic field was applied in the direction of the wall normal. The Hartmann number was Ha=2δB0√σ/ρν=32.5 and 65. Turbulent quantities such as the mean flow, turbulent stress, and the turbulent statistics were obtained by DNS. Although the influence of the magnetohydrodynamic dissipation terms in the turbulent kinetic energy budget was small, large-scale turbulent structures, such as vertical structures, low-speed streaks, ejection, and sweep, disappear at the central region of the channel. Consequently, the difference between production and dissipation in the turbulent kinetic energy decreases with increasing Hartmann number at the central region and large-scale structures at this region disappear. (author)
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.
Blackman, Eric G.; Subramanian, Kandaswamy
2013-02-01
The extent to which large-scale magnetic fields are susceptible to turbulent diffusion is important for interpreting the need for in situ large-scale dynamos in astrophysics and for observationally inferring field strengths compared to kinetic energy. By solving coupled evolution equations for magnetic energy and magnetic helicity in a system initialized with isotropic turbulence and an arbitrarily helical large-scale field, we quantify the decay rate of the latter for a bounded or periodic system. The magnetic energy associated with the non-helical large-scale field decays at least as fast as the kinematically estimated turbulent diffusion rate, but the decay rate of the helical part depends on whether the ratio of its magnetic energy to the turbulent kinetic energy exceeds a critical value given by M1, c = (k1/k2)2, where k1 and k2 are the wavenumbers of the large and forcing scales. Turbulently diffusing helical fields to small scales while conserving magnetic helicity requires a rapid increase in total magnetic energy. As such, only when the helical field is subcritical can it so diffuse. When supercritical, it decays slowly, at a rate determined by microphysical dissipation even in the presence of macroscopic turbulence. In effect, turbulent diffusion of such a large-scale helical field produces small-scale helicity whose amplification abates further turbulent diffusion. Two curious implications are that (1) standard arguments supporting the need for in situ large-scale dynamos based on the otherwise rapid turbulent diffusion of large-scale fields require re-thinking since only the large-scale non-helical field is so diffused in a closed system. Boundary terms could however provide potential pathways for rapid change of the large-scale helical field. (2) Since M1, c ≪ 1 for k1 ≪ k2, the presence of long-lived ordered large-scale helical fields as in extragalactic jets do not guarantee that the magnetic field dominates the kinetic energy.
Theoretical study of anisotropic MHD turbulence with low magnetic Reynolds number
Sukoriansky, Semion; Zemach, Efi
2016-03-01
Flows of electrically conducting fluids under the action of external magnetic field present an example of strongly anisotropic turbulence. Such flows are not only important for different engineering applications, but also provide an interesting framework for studies of quasi-two-dimensional turbulence with strongly modified transport properties in easily controllable laboratory experiments. We present theoretical results that advance our understanding of magnetohydrodynamic (MHD) flows with low magnetic Reynolds number by treating this phenomenon within the quasi-normal scale elimination (QNSE) theory. Previous applications of the theory to turbulent flows with stable stratification and solid body rotation have demonstrated that QNSE is a powerful tool for studies of anisotropic turbulent flows. We derive expressions for scale-dependent eddy viscosities and eddy diffusivities in the directions parallel and normal to the external magnetic field and investigate progressive anisotropization of turbulent transport of momentum and passive scalar. The theory yields analytical expressions for anisotropic one-dimensional spectra of MHD turbulence. In particular, the theory sheds light upon the modification of the Kolmogorov k-5/3 spectrum by anisotropic Ohmic (Joule) dissipation.
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.
Intermittent magnetic field excitation by a turbulent flow of liquid sodium
Nornberg, M. D.; Spence, E. J.; 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 ...
DNS of turbulent heat transfer under a uniform magnetic field at high Reynolds number
In recent developments in nuclear fusion research, certain design concepts for liquid breeder blankets for nuclear fusion reactors use molten salts, such as FLiBe, as coolant material. The mean velocity of liquid coolant material in a reactor is strongly influenced by magnetic field, and hence it is important to study the turbulent magnetohydrodynamic (MHD) flow behavior for an applied magnetic field perpendicular to the main flow. Furthermore, because the flow characteristics of coolant at high Reynolds number are assumed to be different from the usual turbulent MHD flow, it is important to investigate the flow under a magnetic field where the Reynolds number is high. A direct numerical simulation (DNS) of turbulent heat transfer with high Reynolds number has been carried out to show the effects of magnetic field. In this study, the Reynolds number for channel flow based on bulk velocity, viscosity, and channel width was set to be constant; Reb=45818. A uniform magnetic field was applied in the direction of the wall normal. The values of Hartmann number Ha were 32.5 and 65. A constant temperature was applied to the wall as a thermal boundary condition. Prandtl number of the working fluid was assumed to be 0.06. The number of computational grids used in this study was 1024 x 1024 x 768 in the x-, y- and z- directions, respectively. The turbulent quantities such as the mean flow, mean temperature, turbulent stress, and turbulent statistics were obtained by DNS. Moreover, the large-scale turbulent structure about temperature field will be presented at final paper. (orig.)
Non-Collisional Ion Heating and Magnetic Turbulence in the RFP
Full text: Strong non-collisional ion heating occurs during sawtooth-like magnetic reconnection events in MST Reversed Field Pinch (RFP) plasma, where ions are transiently heated to as high as 3 keV, often exceeding the electron temperature. The source for this ion heating is the mean magnetic field energy. This transient ion heating has been utilized to enhance plasma beta and energy confinement in MST by timing the application of current profile control just after a reconnection event. The high frequency magnetic turbulence broadens and develops anisotropy with respect to the background magnetic field as expected in magnetized plasma. Magnetic fluctuations are measured over a broad range of scales down to the ion gyroradius. These fluctuations exhibit an exponential spectral density consistent with dissipative nonlinear turbulent cascade, originating from unstable MHD tearing fluctuations, believed to be related to the observed non-collisional ion heating (which is not understood). The non-collisional ion heating and anisotropic magnetic turbulence are also observed in astrophysical plasmas, heightening the interest in these observations. MST provides a laboratory setting to examine this ion heating mechanism and magnetic turbulence. Our key results are 1) achieved high ion temperature through a non-collisional heating and then sustained by parallel current profile control helping to establish MST's record energy confinement time of 12 ms, 2) the resulting ion temperature has a mass dependence where heavier ions heat up to higher temperature 3) the high frequency magnetic turbulence is anisotropic in wave number, and 4) we observe a dissipative cascade in kperp with kdis =0.22/cm. (author)
Asymptotical theory of runaway electron diffusion due to magnetic turbulence in Tokamak plasmas
Asymptotic theory of transport of runaway electrons in a toroidal plasma in the presence of small-scale magnetic turbulence is proposed. It is based on relativistic Hamiltonian guiding center equations for runaway electrons in toroidal plasmas. Using the asymptotical analysis the explicit relation between the spectral (m, n)- components of perturbation Hamiltonian and the corresponding spectrum of the magnetic turbulence is found. This relation depends only on a few parameters of runaway orbits and magnetic surfaces. The radial profiles of runaway diffusion coefficients are found employing two methods, the quasilinear approximation and the direct calculations using a fast running mapping. The dependence of the shielding factor of the runaway electron parameters and the turbulence spectra is discussed (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)
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.
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...
Magnetic turbulence and particle dynamics in the Earth’s magnetotail
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
DNS of turbulent heat transfer under a uniform magnetic field at high Reynolds number
A low Pr number fluid flow, such as liquid-metals, has relatively less than turbulent heat transport capability because of the liquid-metals' high thermal conductivity and its very large thermal boundary layer. Liquid-metals as coolant material in fusion reactor have a significant role in the design of advanced reactors. This is true since the investigation of thermal behavior in the actual facility environment such as in the case of low Pr number fluid flow, is needed at high Reynolds number under a magnetic field. In the present study, a direct numerical simulation (DNS) for the low Pr number fluid flow of turbulent heat transfer with high Reynolds number has been carried out to show the effects of magnetic field. The Reynolds number for channel flow based on bulk velocity Ub, viscosity ν, and channel width 2δ was set to be constant as Reb = 2δUb/ν = 45,818. A uniform magnetic field was applied in a direction perpendicular to the wall of the channel. The values of Hartmann number Ha were 0 and 65 (where Ha=2δB0√(σ/ρν)), and Prandtl number was 0.06. The turbulent quantities such as the mean temperature, turbulent heat flux, and temperature variant were obtained by DNS. Although large-scale turbulent structures of both velocity and temperature fields are found at the central region of the channel, the mean temperature profiles near wall region show up as laminar profile, that is, the thermal efficiency of the transport is less than that of turbulent flow. This means that it is necessary to consider the fusion reactor design of thermal mixing argumentation for low Pr number fluid flow because the heat transfer enhancement at turbulent flow cannot be acceptable even where the flow state happens to be a turbulent flow at high Reynolds number
Transport of magnetic field by a turbulent flow of liquid sodium
We study the effect of a turbulent flow of liquid sodium generated in the von Karman geometry, on the localized field of a magnet placed close to the frontier of the flow. We observe that the field can be transported by the flow on distances larger than its integral length scale. In the most turbulent configurations, the mean value of the induced field at large distance vanishes. However, the root-mean-square (rms) value of the fluctuations increases linearly with the magnetic Reynolds number. The induced field is strongly intermittent. (authors)
Transport of magnetic field by a turbulent flow of liquid sodium
Volk, R.; Ravelet, F.; Monchaux, R.; Berhanu, M.; Chiffaudel, A.; Daviaud, F; Odier, Ph.; Pinton, J. F.; Fauve, S.; Mordant, N.; Petrelis, F.
2006-01-01
We study the effect of a turbulent flow of liquid sodium generated in the von K\\'arm\\'an geometry, on the localized field of a magnet placed close to the frontier of the flow. We observe that the field can be transported by the flow on distances larger than its integral length scale. In the most turbulent configurations, the mean value of the field advected at large distance vanishes. However, the rms value of the fluctuations increases linearly with the magnetic Reynolds number. The advected...
Time Aperiodic Perturbations of Integrable Hamiltonian Systems
Martinez, A; Wiggins, S
2000-01-01
We consider a Hamiltonian $H=H^{0}(p)+\\kappa H^{1}(p,q,t)$, $(p,q)\\in {\\mathbb{R}}^{n} \\times {\\mathbb{T}}^n$, $t\\in{\\mathbb{R}}$ where $\\kappa \\in {\\mathbb{R}}$ is a small perturbation parameter and $p$, $q$ are the action and angle variables respectively. The Hamiltonian generates an autonomous vector field obtained by extending the phase space making $t$ a dependent variable and adding its conjugate variable $\\tau$. In this paper we look at a time aperiodic perturbation $H^{1}(p,q,t)$ whic...
The effect of magnetic islands on Ion Temperature Gradient turbulence driven transport
In this work, we address the question of the influence of magnetic islands on the perpendicular transport due to steady-state ITG turbulence on the energy transport time scale. We demonstrate that turbulence can cross the separatrix and enhance the perpendicular transport across magnetic islands. As the perpendicular transport in the interior of the island sets the critical island size needed for growth of neoclassical tearing modes, this increased transport leads to a critical island size larger than that predicted from considering collisional conductivities, but smaller than that using anomalous effective conductivities. We find that on Bohm time scales, the turbulence is able to re-establish the temperature gradient across the island for islands widths w ≲ λturb, the turbulence correlation length. The reduction in the island flattening is estimated by comparison with simulations retaining only the perpendicular temperature and no turbulence. At intermediate island widths, comparable to λturb, turbulence is able to maintain finite temperature gradients across the island
Aperiodic behaviour of a non-linear oscillator.
Baker, N. H.; Moore, D. W.; Spiegel, E. A.
1971-01-01
The aperiodic behavior of the solution of the equation of motion derived previously (1966) when considering a model thermomechanical oscillator is examined. Periodic solutions of this equation are studied numerically and analytically. Conditions for the instability of the solutions are determined. This instability seems to be the cause of the observed aperiodicity.
Density effects on tokamak edge turbulence and transport with magnetic X-points
Results are presented from the 3D electromagnetic turbulence code BOUT, the 2D transport code UEDGE, and theoretical analysis of boundary turbulence and transport in a real divertor-plasma geometry and its relationship to the density limit. Key results include: (1) a transition of the boundary turbulence from resistive X-point to resistive-ballooning as a critical plasma density is exceeded; (2) formation of an X-point MARFE in 2D UEDGE transport simulations for increasing outboard radial transport as found by BOUT for increasing density; (3) identification of convective transport by localized plasma 'blobs' in the SOL at high density during neutral fueling, and decorrelation of turbulence between the midplane and the divertor leg due to strong X-point magnetic shear; (4) a new divertor-leg instability driven at high plasma beta by a radial tilt of the divertor plate. (author)
Magnetic Turbulence and Line Broadening in Simulations of Lyman-Alpha Absorption
Gurvich, Alex; Burkhart, Blakesley K.; Bird, Simeon
2016-01-01
We use the Illustris cosmological AREPO simulations to study the effects of gas turbulence and magnetic fields on measurements from the Lyman-Alpha forest. We generate simulated Lyman-Alpha spectra and plot the distributions of Column Density (CDD) and Doppler Width (b) both by adhering to the canonical method of fitting Voigt profiles to absorption lines and by directly measuring the column density and equivalent widths from snapshot data .We investigate the effects of additional unresolved gas turbulence in Illustris by adding an additional broadening term to the line profiles to mimic turbulent broadening. When we do this, we find a measurable effect in the CDD and an offset in the mean of the b distribution corresponding to the additional turbulence. We also compare different MHD runs in AREPO we find that the CDD can measurably differentiate between magnetic seed field at redshifts as low as z=0.1, but we do not find that the b distribution is affected at a detectable level. Our work suggests that the effects of turbulence and magnetic fields from z=2-0.1 can potentially be measured with these diagnostics. This work was supported in part by the NSF REU and DoD ASSURE programs under NSF grant no. 1262851 and by the Smithsonian Institution.
Optimization of Aperiodic Waveguide Mode Converters
Burke, G J; White, D A; Thompson, C A
2004-12-09
Previous studies by Haq, Webb and others have demonstrated the design of aperiodic waveguide structures to act as filters and mode converters. These aperiodic structures have been shown to yield high efficiency mode conversion or filtering in lengths considerably shorter than structures using gradual transitions and periodic perturbations. The design method developed by Haq and others has used mode-matching models for the irregular, stepped waveguides coupled with computer optimization to achieve the design goal using a Matlab optimization routine. Similar designs are described here, using a mode matching code written in Fortran and with optimization accomplished with the downhill simplex method with simulated annealing using an algorithm from the book Numerical Recipes in Fortran. Where Haq et al. looked mainly for waveguide shapes with relatively wide cavities, we have sought lower profile designs. It is found that lower profiles can meet the design goals and result in a structure with lower Q. In any case, there appear to be very many possible configurations for a given mode conversion goal, to the point that it is unlikely to find the same design twice. Tolerance analysis was carried out for the designs to show edge sensitivity and Monte Carlo degradation rate. The mode matching code and mode conversion designs were validated by comparison with FDTD solutions for the discontinuous waveguides.
Periodic and aperiodic synchronization in skilled action
Fred Cummins
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.
Kubo number and magnetic field line diffusion coefficient for anisotropic magnetic turbulence
The magnetic field line diffusion coefficients Dx and Dy are obtained by numerical simulations in the case that all the magnetic turbulence correlation lengths lx, ly, and lz 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=(δB/B0)(l#parallel#/l#perpendicular#) to R=(δB/B0)(lz/lx), for lx≥ly. Here, l#parallel# (l#perpendicular#) is the correlation length along (perpendicular to) the average field B0=B0{cflx e}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, Di=D(δBlz/B0lx)μ(li/lx)νlx2/lz, which well describes the numerical simulation results in the quasilinear, intermediate, and percolative regimes
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...
Väisälä, M S; Mitra, Dhrubaditya; Käpylä, P J; Mantere, M J
2013-01-01
In astrophysics, turbulent diffusion is often used in place of microphysical diffusion to avoid resolving the small scales. However, we expect this approach to break down when time and length scales of the turbulence become comparable with other relevant time and length scales in the system. Turbulent diffusion has previously been applied to the magneto-rotational instability (MRI), but no quantitative comparison of growth rates at different turbulent intensities has been performed. We investigate to what extent turbulent diffusion can be used to model the effects of small-scale turbulence on the kinematic growth rates of the MRI, and how this depends on angular velocity and magnetic field strength. We use direct numerical simulations in three-dimensional shearing boxes with periodic boundary conditions in the spanwise direction and additional random plane-wave volume forcing to drive a turbulent flow at a given length scale. We estimate the turbulent diffusivity using a mixing length formula and compare with...
Production of Magnetic Turbulence by Cosmic Rays Drifting Upstream of Supernova Remnant Shocks
Stroman, Thomas; Niemiec, Jacek; Pohl, Martin; Nishikawa, Ken-ichi
2008-01-01
I will present results of our recent two- and three-dimensional Particle-In-Cell simulations of magnetic-turbulence production by cosmic-ray ions drifting upstream of supernova remnant shocks. These studies' aim is twofold: test recent predictions of strong amplification in short wavelength, non-resonant wave modes, and study the subsequent evolution of the magnetic turbulence, including its backreaction on cosmic-ray trajectories. We confirm that the drifting cosmic rays give rise to a turbulent magnetic field, but show that an oblique filamentary mode grows more rapidly than the non-resonant parallel modes found in analytical theory. The field perturbations grow more slowly than estimated using a quasi-linear analytical approach for the parallel plane-wave mode, and saturate in amplitude at deltaB/B approximately equal to 1. The backreaction of the magnetic turbulence on the particles leads to an alignment of the bulk-flow velocities of the cosmic rays and the background medium. This is an essential characteristic of cosmic ray-modified shocks: the upstream flow speed is continuously changed by the cosmic rays. The reduction of relative drift between cosmic rays and background medium accounts for the saturation of the instability at only moderate magnetic-field amplitudes. It is possible that the prolonged magnetic field growth observed in recent MHD simulations results from a cosmic-ray current assumed to be constant and thus immune to the backreaction from the turbulent field. We speculate that the parallel plane-wave mode found in analytical treatments very quickly leads co filamentation, which we observe in our PIC modeling and is also apparent in the MHD simulations.
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....
Parallel and perpendicular structure of the edge turbulence in a three-dimensional magnetic field
Edge turbulence and blobs are studied in the three-dimensional magnetic topology of the RFX-mod reversed field pinch. The edge of the RFX-mod shows a three-dimensional structure dominated by a helical equilibrium with (1, −7) symmetry, which gives the same space-time modulation to all of the kinetic properties. The interaction between the edge turbulence and this magnetic topology is studied. It is shown that the edge blobs are current-carrying filaments aligned with the magnetic field, and in the perpendicular plane each blob is a positive peak of electron density and a valley of temperature. The inner nature of these blobs is not affected by the presence of the O and X points of the (1, −7) island; however, the statistical properties are sensitive to them, pointing to the influence of the magnetic topology on the edge fluctuations. (paper)
Parallel and perpendicular structure of the edge turbulence in a three-dimensional magnetic field
Agostini, M.; Scarin, P.; Spizzo, G.; Vianello, N.; Carraro, L.
2014-09-01
Edge turbulence and blobs are studied in the three-dimensional magnetic topology of the RFX-mod reversed field pinch. The edge of the RFX-mod shows a three-dimensional structure dominated by a helical equilibrium with (1, -7) symmetry, which gives the same space-time modulation to all of the kinetic properties. The interaction between the edge turbulence and this magnetic topology is studied. It is shown that the edge blobs are current-carrying filaments aligned with the magnetic field, and in the perpendicular plane each blob is a positive peak of electron density and a valley of temperature. The inner nature of these blobs is not affected by the presence of the O and X points of the (1, -7) island; however, the statistical properties are sensitive to them, pointing to the influence of the magnetic topology on the edge fluctuations.
Turbulence simulations of blob formation and radial propagation in toroidally magnetized plasmas
Garcia, O.E.; Naulin, V.; Nielsen, A.H.; Juul Rasmussen, J.
Two- dimensional numerical fluid turbulence simulations demonstrating the formation and radial propagation of blob structures in toroidally magnetized plasmas are presented and analysed in detail. A salient feature of the model is a linearly unstable edge plasma region with localized sources 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 to a...... dynamical regulation by self- sustained differential rotation of the plasma layer. Radial propagation of the blob structures follows from a vertical charge polarization due to magnetic guiding centre drifts in the toroidally magnetized plasma. Statistical analysis of the particle density, radial electric...
Effects of Magnetic Turbulence on the Dynamics of Pickup Ions in the Ionosheath of Mars
Aceves, H.; Reyes-Ruiz, M.; C. E. Chávez
2011-01-01
We study some of the effects that magnetic turbulent fluctuations have on the dynamics of pickup O+ ions in the magnetic polar regions of the Mars ionosheath. In particular we study their effect on the bulk velocity profiles of ions as a function of altitude over the magnetic poles, in order to compare them with recent Mars Express data; that indicate that their average velocity is very low and essentially in the anti-sunward direction. We find that, while magnetic field fluctuations do give ...
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.
2016-04-01
We investigate the migration of low-mass planets (1M⊕, 5M⊕ and 20M⊕) 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 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 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 investigated.
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.
ON THE AMPLIFICATION OF MAGNETIC FIELD BY A SUPERNOVA BLAST SHOCK WAVE IN A TURBULENT MEDIUM
We have performed extensive two-dimensional magnetohydrodynamic simulations to study the amplification of magnetic fields when a supernova blast wave propagates into a turbulent interstellar plasma. The blast wave is driven by injecting high pressure in the simulation domain. The interstellar magnetic field can be amplified by two different processes, occurring in different regions. One is facilitated by the fluid vorticity generated by the 'rippled' shock front interacting with the background turbulence. The resulting turbulent flow keeps amplifying the magnetic field, consistent with earlier work. The other process is facilitated by the growth of the Rayleigh-Taylor instability at the contact discontinuity between the ejecta and the shocked medium. This can efficiently amplify the magnetic field and tends to produce the highest magnetic field. We investigate the dependence of the amplification on numerical parameters such as grid-cell size and on various physical parameters. We show that the magnetic field has a characteristic radial profile such that the downstream magnetic field gets progressively stronger away from the shock. This is because the downstream magnetic field needs a finite time to reach the efficient amplification, and will get further amplified in the Rayleigh-Taylor region. In our simulation, we do not observe a systematic strong magnetic field within a small distance to the shock. This indicates that if the magnetic-field amplification in supernova remnants indeed occurs near the shock front, other processes such as three-dimensional instabilities, plasma kinetics, and/or cosmic ray effect may need to be considered to explain the strong magnetic field in supernova remnants.
Cosmic-ray parallel and perpendicular transport in turbulent magnetic fields
A correct description of cosmic-ray (CR) diffusion in turbulent plasma is essential for many astrophysical and heliospheric problems. This paper aims to present the physical diffusion behavior of CRs in actual turbulent magnetic fields, a model of which has been numerically tested. We perform test particle simulations in compressible magnetohydrodynamic turbulence. We obtain scattering and spatial diffusion coefficients by tracing particle trajectories. We find no resonance gap for pitch-angle scattering at 90°. Our result confirms the dominance of mirror interaction with compressible modes for most pitch angles, as revealed by the nonlinear theory. For cross-field transport, our results are consistent with normal diffusion predicted earlier for large scales. The diffusion behavior strongly depends on the Alfvénic Mach number and the particle's parallel mean free path. We, for the first time, numerically derive the dependence of MA4 for the perpendicular diffusion coefficient with respect to the mean magnetic field. We conclude that CR diffusion coefficients are spatially correlated to the local turbulence properties. On scales smaller than the injection scale, we find that CRs are superdiffusive. We emphasize the importance of our results in a wide range of astrophysical processes, including magnetic reconnection.
Correlation Reflectometry for Turbulence and Magnetic Field Measurements in Fusion Plasmas
For the interpretation of correlation reflectometry data a fast two-dimensional full wave code has been developed in which realistic plasma geometries are used. Results of this code are compared with experiments and turbulence correlation lengths and fluctuation levels are extracted with statistical optics methods. It is shown that in general the measured reflectometer correlation length is not equal to the turbulence correlation length. The code is also used to study the possibility of O-X correlation reflectometry in FIRE for the determination of the local magnetic field strength. It was found that this is only possible at very low fluctuation levels
Generation of Magnetic Field by Combined Action of Turbulence and Shear
Yousef, T A; Schekochihin, A A; Kleeorin, N; Rogachevskii, I; Iskakov, A B; Cowley, S C; McWilliams, J C
2007-01-01
The possibility of a mean-field dynamo in nonhelical turbulence with superimposed linear shear is studied numerically in elongated shearing boxes. Exponential growth of magnetic field at scales much larger than the outer scale of the turbulence is found. The charateristic scale of the field is ~ S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly discovered form of large-scale dynamo action may have an extremely broad range of applications to astrophysical systems with spatially coherent mean flows.
Turbulent amplification of magnetic field driven by dynamo effect at rippled shocks
Fraschetti, Federico
2013-01-01
We derive analytically the vorticity generated downstream of a two-dimensional rippled hydromagnetic shock neglecting fluid viscosity and resistivity. The growth of the turbulent component of the downstream magnetic field is driven by the vortical eddies motion. We determine an analytic time-evolution of the magnetic field amplification at shocks, so far described only numerically, until saturation occurs due to seed-field reaction to field lines whirling. The explicit expression of the amplification growth rate and of the non-linear field back-reaction in terms of the parameters of shock and interstellar density fluctuations is derived from MHD jump conditions at rippled shocks. A magnetic field saturation up to the order of milligauss and a short-time variability in the $X$-ray observations of supernova remnants can be obtained by using reasonable parameters for the interstellar turbulence.
Stochastic field-line wandering in magnetic turbulence with shear. I. Quasi-linear theory
Shalchi, A.; Negrea, M.; Petrisor, I.
2016-07-01
We investigate the random walk of magnetic field lines in magnetic turbulence with shear. In the first part of the series, we develop a quasi-linear theory in order to compute the diffusion coefficient of magnetic field lines. We derive general formulas for the diffusion coefficients in the different directions of space. We like to emphasize that we expect that quasi-linear theory is only valid if the so-called Kubo number is small. We consider two turbulence models as examples, namely, a noisy slab model as well as a Gaussian decorrelation model. For both models we compute the field line diffusion coefficients and we show how they depend on the aforementioned Kubo number as well as a shear parameter. It is demonstrated that the shear effect reduces all field line diffusion coefficients.
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.
Magnetic Turbulence and Thermodynamics in the Inner Region of Protoplanetary Discs
Hirose, Shigenobu
2015-01-01
Using radiation magnetohydrodynamics simulations with realistic opacities and equation of state, and zero net magnetic flux, we have explored thermodynamics in the inner part of protoplanetary discs where magnetic turbulence is expected. The thermal equilibrium curve consists of the upper, lower, and middle branches. The upper (lower) branch corresponds to hot (cool) and optically very (moderately) thick discs, respectively, while the middle branch is characterized by convective energy transport near the midplane. Convection is also the major energy transport process near the low surface density end of the upper branch. There, convective motion is fast with Mach numbers reaching $\\gtrsim 0.01$, and enhances both magnetic turbulence and cooling, raising the ratio of vertically-integrated shear stress to vertically-integrated pressure by a factor of several. This convectively enhanced ratio seems a robust feature in accretion discs having an ionization transition. We have also examined causes of the S-shaped th...
A Lagrangian model for the evolution of turbulent magnetic and passive scalar fields
Hater, T; Grauer, R
2010-01-01
In this paper we present an extension of the \\emph{Recent Fluid Deformation (RFD)} closure introduced by Chevillard and Meneveau (2006) which was developed for modeling the time evolution of Lagrangian fluctuations in incompressible Navier-Stokes turbulence. We apply the RFD closure to study the evolution of magnetic and passive scalar fluctuations. This comparison is especially interesting since the stretching term for the magnetic field and for the gradient of the passive scalar are similar but differ by a sign such that the effect of stretching and compression by the turbulent velocity field is reversed. Probability density functions (PDFs) of magnetic fluctuations and fluctuations of the gradient of the passive scalar obtained from the RFD closure are compared against PDFs obtained from direct numerical 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...
Phase-coded pulse aperiodic transmitter coding
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.
Schaffner, David
2015-11-01
A typical signature of dissipation in conventional fluid turbulence is the steepening power spectrum of velocity fluctuations, signaling the transition from the inertial range to the dissipation range where scales become small enough for fluid viscosity effects to be dominant and convert flow energy into thermal energy. In MHD fluids, resistivity can play an analogous role to viscosity for magnetic field fluctuations, where collisional scales determine the onset of dissipation. However, turbulent plasmas can exhibit other mechanisms for converting magnetic energy into thermal energy such as through the generation of current sheets and magnetic reconnection or through coupling to kinetic scale fluctuations such as Kinetic Alfven waves or Whistler waves. In collisionless plasmas such as the solar wind, only these alternative dissipation mechanisms are likely active. Recent experiments with MHD turbulence generated in the wind-tunnel configuration of the Swarthmore Spheromak Experiment (SSX) provide an environment in which various potential non-resistive signatures of magnetic turbulent energy dissipation can be studied. SSX plasma is magnetically dynamic with no background field. Previous work has demonstrated that a steepening in the magnetic fluctuation spectrum is observed which can be roughly interpreted as a transition from inertial range to a dissipation range magnetic turbulence. The frequency range at which this steepening occurs can be correlated to the ion inertial scale of the plasma, a length which is characteristic of the size of current sheets in MHD plasmas. Detailed intermittency and structure function analysis presented here coupled with appeals to fractal scaling models support the hypothesis that the observed turbulence is being affected by a global dissipation mechanism such as the generation of current sheets. Information theory based analysis techniques using permutation entropy and statistical complexity are also applied to seek dissipation
Fractal dissipation of small-scale magnetic fluctuations in solar wind turbulence as seen by CLUSTER
A robust multiscale analysis of high-frequency magnetic field measurements from the Cluster spacecraft is presented using complimentary measurements from the magnetometer and search-coil instruments; in a stationary interval of fast ambient solar wind. We show a surprising and novel result: that for time-scales smaller than the ion-gyro period the magnetic field fluctuations exhibit non-Gaussian monoscaling i.e. are globally scale invariant and can be characterized by a single scaling exponent and thus a single universal scaling function for the probability density. This presents new questions, as well as theoretical constraints, for determining how magnetic turbulent energy is dissipated in collisionless plasmas.
Transport and turbulence in a magnetized argon plasma
Three aspects of the longitudinal motion of ionized and neutral particles in a hollow cathode arc are investigated. The longitudinal plasma momentum balance of the column has been investigated, we have studied the momentum balance in relation to turbulence and we have investigated the source properties of the cathode. The study of the plasma momentum balance contains two aspects: (1) to collect experimental data on ion drift velocities and temperatures with Fabry-Perot interferometry, on electron densities and temperatures with Thomson scattering or optical spectroscopy and on neutral densities with a collisional radiative model combined with the ion energy balance; (2) to check the (classical) theory of the momentum balance with these data. The coupling between these aspects has been investigated and found to be in good agreement. (Auth.)
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...
The Complex Structure of Magnetic Field Discontinuities in the Turbulent Solar Wind
Greco, A.; Perri, S.; Servidio, S.; Yordanova, E.; Veltri, P.
2016-06-01
Using high-resolution Cluster satellite observations and a multi-dimensional intermittency technique, we show that the magnetic discontinuities in the turbulent solar wind are connected through the spatial scales, going from proton down to electron scales. In some circumstances, their structure resembles the Harris equilibrium profile in plasmas. Observations are consistent with a scenario where many current layers develop in turbulence and where the outflow of these reconnection events are characterized by complex sub-proton networks of secondary islands, in a self-similar way. Although in the past these pictures have been speculated to be separately ubiquitous, through theories and simulations, the present work confirms that “reconnection in turbulence” and “turbulent reconnection” coexist in space plasmas.
Magnetic field amplification and saturation in two-dimensional magnetohydrodynamic turbulence
Two-dimensional (2-D) magnetohydrodynamic turbulence is investigated for weak initial magnetic fields using numerical simulation. It is found that the magnetic field is amplified owing to the formation of flux sheets with saturation due either to resistive diffusion (kinematic regime) or to nonlinear effects (dynamic regime). In the kinematic regime, which corresponds to the problem of passive scalar convection by 2-D Navier--Stokes turbulence, the saturation value of the magnetic energy is observed to scale as EMmax∝η-0.8 in approximate agreement with a simple theoretical estimate, EMmax/EM(0)congruent Rm, where Rm is the magnetic Reynolds number. Because of the strongly disparate kinetic and magnetic energy spectra in the kinematic regime, roughly EVk∼k-3, EMk∼k, dynamic interaction on small scales already occurs at very small global energy ratios EM/EV, giving rise to strongly enhanced kinetic energy dissipation. In the fully dynamic regime (reached for EM/EV|t=0>R-1m) global magnetic and kinetic energies become tightly coupled, with EM/EV being approximately constant in time and the energy dissipation rates being independent of the collisional diffusion coefficients. Finally, the effect of the magnetic Prandtl number Pr=μ/η is discussed
2D turbulence structure observed by a fast framing camera system in linear magnetized device PANTA
Mesoscale structure, such as the zonal flow and the streamer plays important role in the drift-wave turbulence. The interaction of the mesoscale structure and the turbulence is not only interesting phenomena but also a key to understand the turbulence driven transport in the magnetically confined plasmas. In the cylindrical magnetized device, PANTA, the interaction of the streamer and the drift wave has been found by the bi-spectrum analysis of the turbulence. In order to study the mesoscale physics directly, the 2D turbulence is studied by a fast-framing visible camera system view from a window located at the end plate of the device. The parameters of the plasma is the following; Te∼3eV, n ∼ 1x1019 m-3, Ti∼0.3eV, B=900G, Neutral pressure Pn=0.8 mTorr, a∼ 6cm, L=4m, Helicon source (7MHz, 3kW). Fluctuating component of the visible image is decomposed by the Fourier-Bessel expansion method. Several rotating mode is observed simultaneously. From the images, m = 1 (f∼0.7 kHz) and m = 2, 3 (f∼-3.4 kHz) components which rotate in the opposite direction can be easily distinguished. Though the modes rotate constantly in most time, there appear periods where the radially complicated node structure is formed (for example, m=3 component, t = 142.5∼6 in the figure) and coherent mode structures are disturbed. Then, a new rotating period is started again with different phase of the initial rotation until the next event happens. The typical time interval of the event is 0.5 to 1.0 times of the one rotation of the slow m = 1 mode. The wave-wave interaction might be interrupted occasionally. Detailed analysis of the turbulence using imaging technique will be discussed. (author)
Dispersion relation analysis of turbulent magnetic field fluctuations in fast solar wind
Perschke, C. [Technische Univ. Braunschweig (Germany). Inst. fuer Theoretische Physik; Technische Univ. Braunschweig (Germany). Inst. fuer Geophysik und extraterrestrische Physik; Narita, Y. [Technische Univ. Braunschweig (Germany). Inst. fuer Geophysik und extraterrestrische Physik; Austrian Academy of Sciences, Graz (Austria). Space Research Inst.; Gary, S.P. [Space Science Institute, Boulder, CO (United States); Motschmann, U. [Technische Univ. Braunschweig (Germany). Inst. fuer Theoretische Physik; Deutsches Zentrum fuer Luft- und Raumfahrt, Berlin (Germany). Inst. fuer Planetenforschung; Glassmeier, K.H. [Technische Univ. Braunschweig (Germany). Inst. fuer Geophysik und extraterrestrische Physik; Max-Planck-Institut fuer Sonnensystemforschung, Katlenburg-Lindau (Germany)
2013-07-01
Physical processes of the energy transport in solar wind turbulence are a subject of intense studies, and different ideas exist to explain them. This manuscript describes the investigation of dispersion properties in short-wavelength magnetic turbulence during a rare high-speed solar wind event with a flow velocity of about 700 km s{sup -1} using magnetic field and ion data from the Cluster spacecraft. Using the multipoint resonator technique, the dispersion relations (i.e., frequency versus wave-number values in the solar wind frame) of turbulent magnetic fluctuations with wave numbers near the inverse ion inertial length are determined. Three major results are shown: (1) the wave vectors are uniformly quasi-perpendicular to the mean magnetic field; (2) the fluctuations show a broad range of frequencies at wavelengths around the ion inertial length; and (3) the direction of propagation at the observed wavelengths is predominantly in the sunward direction. These results suggest the existence of high-frequency dispersion relations partly associated with normal modes on small scales. Therefore nonlinear energy cascade processes seem to be acting that are not described by wave-wave interactions.
Nonlinear competition of turbulent structures and their roles in transport are investigated by using three-dimensional simulation code of resistive drift wave turbulence in magnetized cylindrical plasmas. Selective formation of zonal flows and streamers has been obtained by controlling the strength of damping of the zonal flow. In addition, there is an energy path from the drift waves to a flute type structure, which is linearly stable, and it becomes effective just below the stability boundary of the zonal flow. The flute structure directly induces transport effectively, and affects the drift waves and the zonal flow. A large amplitude zonal flow is formed selectively even with existence of the flute structure. The property of the particle confinement is investigated by changing the particle source intensity, which controls the strength of driving of the drift waves. The characteristic of the particle confinement changes according to turbulent states, and an improved confinement regime is obtained in the zonal flow dominant state. Study on cylindrical plasmas reveals the fundamental mechanism of improved confinement in the magnetized plasma with influence of turbulent structural formation. (paper)
Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
Schekochihin, A A; Cowley, S C; McWilliams, J C; Proctor, M R E; Yousef, T A
2007-01-01
This paper is a detailed report on a programme of simulations used to settle a long-standing issue in the dynamo theory and demonstrate that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm>>1 and small magnetic Prandtl number Pm1. The dependence of the critical Rm_c vs. the hydrodynamic Reynolds number Re is obtained for 11. The stability curve Rm_c(Re) (and, it is argued, the nature of the dynamo) is substantially different from the case of the simulations and liquid-metal experiments with a mean flow. It is not as yet possible to determine numerically whether the growth rate is ~Rm^{1/2} in the limit ReRm>>1, as should be the case if the dynamo is driven by the inertial-range motions. The magnetic-energy spectrum in the low-Pm regime is qualitatively different from the Pm>1 case and appears to develop a negative spectral slope, although current resolutions are insufficient to determine its asymptotic form. At 1
New construction of hybrid and aperiodic ordered PBG cavity
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.
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...
Reduction of compressibility and parallel transfer by Landau damping in turbulent magnetized plasmas
Hunana, P; Passot, T; Sulem, P L; Borgogno, D
2011-01-01
Three-dimensional numerical simulations of decaying turbulence in a magnetized plasma are performed using a so-called FLR-Landau fluid model which incorporates linear Landau damping and finite Larmor radius (FLR) corrections. It is shown that compared to simulations of compressible Hall-MHD, linear Landau damping is responsible for significant damping of magnetosonic waves, which is consistent with the linear kinetic theory. Compressibility of the fluid and parallel energy cascade along the ambient magnetic field are also significantly inhibited when the beta parameter is not too small. In contrast with Hall-MHD, the FLR-Landau fluid model can therefore correctly describe turbulence in collisionless plasmas such as the solar wind, providing an interpretation for its nearly incompressible behavior.
M. El-Alaoui; R. L. Richard; 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 ...
Landry, Russell; Dodson-Robinson, Sarah E.; 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...
Interaction of supra-thermal ions with turbulence in a magnetized toroidal plasma
This thesis addresses the interaction of a supra-thermal ion beam with turbulence in the simple magnetized toroidal plasma of TORPEX. The first part of the Thesis deals with the ohmic assisted discharges on TORPEX. The aim of these discharges is the investigation of the open to closed magnetic field line transition. The relevant magnetic diagnostics were developed. Ohmic assisted discharges with a maximum plasma current up to 1 kA are routinely obtained. The equilibrium conditions on the vacuum magnetic field configuration were investigated. In the second part of the Thesis, the design of the fast ion source and detector are discussed. The accelerating electric field needed for the fast ion source was optimized. The fast ion source was constructed and commissioned. To detect the fast ions a specially designed gridded energy analyzer was used. The electron energy distribution function was obtained to demonstrate the efficiency of the detector. The experiments with the fast ion beam were conducted in different plasma regions of TORPEX. In the third part of the Thesis, numerical simulations are used to interpret the measured fast ion beam behavior. It is shown that a simple single particle equation of motion explains the beam behavior in the experiments in the absence of plasma. To explain the fast ion beam experiments with the plasma a turbulent electric field must be used. The model that takes into account this turbulent electrical field qualitatively explains the shape of the fast ion current density profile in the different plasma regions of TORPEX. The vertically elongated fast ion current density profiles are explained by a spread in the fast ion velocity distribution. The theoretically predicted radial fast ion beam spreading due to the turbulent electric field was observed in the experiment. (author)
The role of turbulence, magnetic fields and feedback for star formation
Federrath, Christoph
2016-05-01
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 (SFRff) 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/outflow feedback. These simulations produce nearly realistic star formation rates consistent with observations, but only if turbulence, magnetic fields and feedback are included simultaneously.
The Effects of Velocity Correlation Times on the Turbulent Amplification of Magnetic Energy
Chandran, Benjamin D. G.
1997-06-01
This paper extends the quasilinear theory of Kulsrud & Anderson to assess the effects of realistically long velocity correlation times on the turbulent amplification of a very weak magnetic field. A computer simulation is presented that tracks the growth of the magnetic energy in a turbulent plasma at a single point moving with the turbulent flow. The velocities are assumed to conform to the ideas of Kraichnan concerning Lagrangian correlation times, and are modeled as a set of randomly generated pulses chosen to reproduce the correct two-time Lagrangian correlation tensor. The model is simple computationally and can be used to calculate the growth rate of the magnetic energy for arbitrarily high magnetic Reynolds numbers. The simulations show that the magnetic energy grows roughly half as fast as predicted in the short correlation time approximation of Kulsrud & Anderson's quasilinear theory. In a separate analysis, the effects of nonzero correlation times are considered using an analytic method developed by van Kampen. The growth rate is expanded, roughly speaking, in powers of the correlation time divided by the time required for the energy to exponentiate once. The first two terms in the series are calculated. In themselves, these two terms do not exactly determine the growth rate, but they are consistent with the numerical results. The analytic treatment is included mostly for completeness and because it offers some physical understanding of the problem. The main conclusion of the paper is that velocity correlation times do not play an important role in the growth of the magnetic energy. As a result, Kulsrud & Anderson's short correlation time analysis of the spectrum of amplified small-scale fields should be approximately correct.
Fast electron generation and transport in a turbulent, magnetized plasma
The nature of fast electron generation and transport in the Madison Symmetric Torus (MST) reversed field pinch (RFP) is investigated using two electron energy analyzer (EEA) probes and a thermocouple calorimeter. The parallel velocity distribution of the fast electron population is well fit by a drifted Maxwellian distribution with temperature of about 100 eV and drift velocity of about 2 x 106 m/s. Cross-calibration of the EEA with the calorimeter provides a measurement of the fast electron perpendicular temperature of 30 eV, much lower than the parallel temperature, and is evidence that the kinetic dynamo mechanism (KDT) is not operative in MST. The fast electron current is found to match to the parallel current at the edge, and the fast electron density is about 4 x 1011 cm-3 independent of the ratio of the applied toroidal electric field to the critical electric field for runaways. First time measurements of magnetic fluctuation induced particle transport are reported. By correlating electron current fluctuations with radial magnetic fluctuations the transported flux of electrons is found to be negligible outside r/a∼0.9, but rises the level of the expected total particle losses inside r/a∼0.85. A comparison of the measured diffusion coefficient is made with the ausilinear stochastic diffusion coefficient. Evidence exists that the reduction of the transport is due to the presence of a radial ambipolar electric field of magnitude 500 V/m, that acts to equilibrate the ion and electron transport rates. The convective energy transport associated with the measured particle transport is large enough to account for the observed magnetic fluctuation induced energy transport in MST
Artificial ionospheric turbulence (review)
This study is an analysis of artificial ionospheric turbulence (AIT) arising near the level at which a powerful wave is reflected with ordinary polarization. AIT is an inhomogeneous structure in the ionosphere with a size on the order of centimeters or tens of kilometers and with characteristic frequencies from a fraction of a hertz (aperiodic inhomogeneity) to several megahertz (plasma waves). The authors are primarily concerned with small-scale artificial ionospheric turbulence (SAIT), i.e., with inhomogeneities that are greatly extended along the geomagnetic field with transverse dimensions that are less than the wavelengths of the perturbing waves - the pumping waves (PW) - in a vacuum
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 ...
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.
Simulation of Relativistic Shocks and Associated Radiation from Turbulent Magnetic Fields
Nishikawa, K.-I.; Mizuno, Y.; Niemiec, J.; Medvedev, M.; Zhang, B.; Hardee, P.; Frederiksen, J.; Sol, H.; Pohl, M.; Hartmann, D. H.; Fishman, G. J.
2010-01-01
Recent PIC simulations of relativistic electron-positron (electron-ion) jets injected into a stationary medium show that particle acceleration occurs at shocked regions. Simulations show that the Weibel instability is responsible for generating and amplifying highly nonuniform, small-scale magnetic fields and particle acceleration. These magnetic fields contribute to the electron's transverse deflection behind the shock. The jitter'' radiation from deflected electrons in turbulent magnetic fields has different properties than synchrotron radiation, which is calculated in a uniform magnetic field. This jitter radiation may be important for understanding the complex time evolution and/or spectral structure in gamma-ray bursts, relativistic jets in general, and supernova remnants. We will present detailed spectra for conditions relevant of various astrophysical sites of shock formation via the Weibel instability. In particular we will discuss the application to GRBs and SNRs
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...
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.
Nonlinear magnetic induction by helical motion in a liquid sodium turbulent flow
We report an experimental study of the magnetic field B(→) induced by a turbulent swirling flow of liquid sodium submitted to a transverse magnetic field B(→)0. We show that the induced field can behave nonlinearly as a function of the magnetic Reynolds number, Rm. At low Rm, the induced mean field along the axis of the flow, x>, and the one parallel to B(→)0, y>, first behave like Rm2, whereas the third component, z>, is linear in Rm. The sign of x> is determined by the flow helicity. At higher Rm, B(→) strongly depends on the local geometry of the mean flow: x> decreases to zero in the core of the swirling flow but remains finite outside. We compare the experimental results with the computed magnetic induction due to the mean flow alone
Wavelet characterization of 2D turbulence and intermittency in magnetized electron plasmas
Romé, M.; Chen, S.; Maero, G.
2016-06-01
A study of the free relaxation of turbulence in a two-dimensional (2D) flow is presented, with a focus on the role of the initial vorticity conditions. Exploiting a well-known analogy with 2D inviscid incompressible fluids, the system investigated here is a magnetized pure electron plasma. The dynamics of this system are simulated by means of a 2D particle-in-cell code, starting from different spiral density (vorticity) distributions. A wavelet multiresolution analysis is adopted, which allows the coherent and incoherent parts of the flow to be separated. Comparison of the turbulent evolution in the different cases is based on the investigation of the time evolution of statistical properties, including the probability distribution functions and structure functions of the vorticity increments. It is also based on an analysis of the enstrophy evolution and its spectrum for the two components. In particular, while the statistical features assess the degree of flow intermittency, spectral analysis allows us not only to estimate the time required to reach a state of fully developed turbulence, but also estimate its dependence on the thickness of the initial spiral density distribution, accurately tracking the dynamics of both the coherent structures and the turbulent background. The results are compared with those relevant to annular initial vorticity distributions (Chen et al 2015 J. Plasma Phys. 81 495810511).
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
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.
Magnetic turbulent spectra in the magnetosheath: new insights
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.
Kolmogorov-Sinai entropy in field line diffusion by anisotropic magnetic turbulence
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 = (δB/B0) (ξ||/ξperpendicular), where δB/B0 is the ratio of the rms magnetic fluctuation field to the magnetic field strength, and ξperpendicular and ξ|| 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 → ∞, 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.
Turbulent, megagauss magnetic fields in intense, ultrashort laser pulse interaction with solids
Intense laser-plasma interactions provide a novel and fascinating platform to simulate astrophysical scenarios. Giant magnetic fields (102 - 103 megagauss) are created when a relativistic intensity >1018 W/cm2, ultrashort laser pulse interacts with plasma created on a solid. Here we present snapshots of these megagauss magnetic fields, capturing their picosecond-scale evolution with micron-precision. The plasma created by an 800 nm laser is probed at density of ∼1022 electrons/cc at 266 nm. This density is so far the highest at which plasma probing has been performed. The Fourier spectrum of these megagauss magnetic fields shows a power-law behaviour for the magnetic energy, which provides the signature of magnetic turbulence. Detailed particle-in-cell simulations have shown that the relativistic hot electron transport in a hot dense laser-generated plasma suffers from several instabilities including the Weibel instability, which leads to the spatial separation of forward and return currents and eventually lead to the filamentary structure. The currents subsequently get Weibel-separated, followed by the tearing and coalescence instabilities, which produce current channels and thereby filamentary magnetic field structures. These results are fundamentally interesting in the context of fast ignition of laser fusion, laser-based acceleration of protons, ions and neutral particles, the feasibility of experimentally verifying such instability mechanisms in astrophysical magnetic fields, mimic observations of kinetic Alfven wave turbulence in the earth's magneto-sheath, solar flares and solar wind and simulating intra-planetary matter existing at ultrahigh pressures. (author)
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.
Radio Synchrotron Fluctuation Statistics as a Probe of Magnetized Interstellar Turbulence
Herron, C. A.; Burkhart, Blakesley; Lazarian, A.; Gaensler, B. M.; McClure-Griffiths, N. M.
2016-05-01
We investigate how observations of synchrotron intensity fluctuations can be used to probe the sonic and Alfvénic 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 Alfvénic 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.
Magnetic fields and Turbulence in Star Formation using Smoothed Particle Hydrodynamics
Price, Daniel J
2010-01-01
Firstly, we give a historical overview of attempts to incorporate magnetic fields into the Smoothed Particle Hydrodynamics method by solving the equations of Magnetohydrodynamics (MHD), leading an honest assessment of the current state-of-the-art in terms of the limitations to performing realistic calculations of the star formation process. Secondly, we discuss the results of a recent comparison we have performed on simulations of driven, supersonic turbulence with SPH and Eulerian techniques. Finally we present some new results on the relationship between the density variance and the Mach number in supersonic turbulent flows, finding sigma^2_{ln rho} = ln (1 + b^2 M^2) with b=0.33 up to Mach~20, consistent with other numerical results at lower Mach number (Lemaster and Stone 2008) but inconsistent with observational constraints on sigma_rho and M in Taurus and IC5146.
Scattering of polarized radio waves by Langmuir turbulence in a plasma in a magnetic field
The effects of the radio waves scattering in the plasma by the Langmuir turbulent pulses and their impact on the radiation polarized characteristics for the radiowave radiation frequencies above the electron one and close to the plasma one are considered. It is shown that these effects may essentially change the radiation polarization characteristics. The degree of polarization by optical thickness relative to the scattering process of the order of one unit may change by the value of 30% both on high frequencies and on frequencies close to the plasma one, whereby the sign of the annular radiation polarization may change on the contrary one. It is noted that scattering on the Langmuir turbulence may lead to appearance of dependence of the annular radiation polarization degree on the radio wave length even in a homogeneous magnetic field
Sturm und Drang: The turbulent, magnetic tempest in the Galactic center
Lacki, Brian C.
2014-05-01
The Galactic center central molecular zone (GCCMZ) bears similarities with extragalactic starburst regions, including a high supernova (SN) rate density. As in other starbursts like M82, the frequent SNe can heat the ISM until it is filled with a hot (˜ 4 × 107 K) superwind. Furthermore, the random forcing from SNe stirs up the wind, powering Mach 1 turbulence. I argue that a turbulent dynamo explains the strong magnetic fields in starbursts, and I predict an average B ˜70 μG in the GCCMZ. I demonstrate how the SN driving of the ISM leads to equipartition between various pressure components in the ISM. The SN-heated wind escapes the center, but I show that it may be stopped in the Galactic halo. I propose that the Fermi bubbles are the wind's termination shock.
Experimental Investigation of Active Feedback Control of Turbulent Transport in a Magnetized Plasma
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.
A kinetic theory of trapped electron driven drift wave turbulence in a sheared magnetic field
A kinetic theory of collisionless and dissipative trapped electron driven drift wave turbulence in a sheared magnetic field is presented. Weak turbulence theory is employed to calculate the nonlinear electron and ion responses and to derive a wave kinetic equation that determines the nonlinear evolution of trapped electron mode turbulence. Saturated fluctuation spectrum is calculated using the condition of nonlinear saturation. The turbulent transport coefficients are in turn calculated using saturated fluctuation spectrum. Due to the disparity in the three different radial scale lengths of the slab-like eigenmode: Δ (trapped electron layer width), xt (turning point width) and xi (Landau damping point), Δ t i, we find that ion Compton scattering rather than trapped electron Compton scattering is the dominant nonlinear saturation mechanism. Ion Compton scattering transfers wave energy from short to long wavelengths where the wave energy is shear damped. As a consequence, a saturated fluctuation spectrum |φ|2(kθ) ∼ kθ-α (α = 2 and 3 for the dissipative and collisionless regime, respectively) occurs for kθρs θρs > 1. The predicted fluctuation level and transport coefficients are well below the ''mixing length'' estimate. This is due to the contribution of radial wavenumbers xt-1 r ≤ ρi-1 to the nonlinear couplings, the effect of radial localization of trapped electron response to a layer of width, Δ, and the weak turbulence factor left-angle(γel)/(ωrvecκ)right-angle rveck < 1, which enters the saturation level. 18 refs., 1 tab
Turbulent Origin of the Galactic-Center Magnetic Field: Nonthermal Radio Filaments
Boldyrev, S; Boldyrev, Stanislav; Yusef-Zadeh, Farhad
2006-01-01
A great deal of study has been carried out over the last twenty years on the origin of the magnetic activity in the Galactic center. One of the most popular hypotheses assumes milli-Gauss magnetic field with poloidal geometry, pervading the inner few hundred parsecs of the Galactic-center region. However, there is a growing observational evidence for the large-scale distribution of a much weaker field of B \\lesssim 10 micro G in this region. Here, we propose that the Galactic-center magnetic field originates from turbulent activity that is known to be extreme in the central hundred parsecs. In this picture the spatial distribution of the magnetic field energy is highly intermittent, and the regions of strong field have filamentary structures. We propose that the observed nonthermal radio filaments appear in (or, possibly, may be identified with) such strongly magnetized regions. At the same time, the large-scale diffuse magnetic field is weak. Both results of our model can explain the magnetic field measureme...
Alfv\\'en-dynamo balance and magnetic excess in MHD turbulence
Grappin, Roland; Verdini, Andrea
2016-01-01
3D Magnetohydrodynamic (MHD) turbulent flows with initially magnetic and kinetic energies at equipartition spontaneously develop a magnetic excess (or residual energy), as well in numerical simulations and in the solar wind. Closure equations obtained in 1983 describe the residual spectrum as being produced by a dynamo source proportional to the total energy spectrum, balanced by a linear Alfv\\'en damping term. A good agreement was found in 2005 with incompressible simulations; however, recent solar wind measurements disagree with these results. The previous dynamo-Alfv\\'en theory is generalized to a family of models, leading to simple relations between residual and total energy spectra. We want to assess these models in detail against MHD simulations and solar wind data. The family of models is tested against compressible decaying MHD simulations with low Mach number, low cross-helicity, zero mean magnetic field, without or with expansion terms (EBM or expanding box model). A single dynamo-Alfv\\'en model is ...
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...
We estimated the total radiation losses from argon Z-pinches. Radiation losses due to excitation, di-electronic recombination, Bremsstrahlung, radiative recombination and also ionization, were considered. Each separate ion is analyzed in detail and the influence of density is taken into account. Our pinch dynamics includes Joule heating, anomalous plasma resistance, plasma outflow in Z-direction, electron beam generation, magnetic field pressure, radiation losses. Main novelty of the given model is generation of chaotic/turbulent magnetic fields. It is shown, that turbulent magnetic fields affect the dynamics of plasma parameters. The influence of turbulent magnetic field is analyzed on the dynamics of plasma temperature, density, radiation losses, line emission. (author)
Influence of magnetic well on electromagnetic turbulence in the TJ-II stellarator
Castejón, F.; de Aguilera, M.; Ascasíbar, E.; Estrada, T.; Hidalgo, C.; López-Fraguas, A.; Ochando, M. A.; Yamamoto, S.; Melnikov, A. V.; Eliseev, L. G.; Krupnik, L. I.; the HIBP Team; the TJ-II Team
2016-09-01
A magnetic well scan has been performed in the TJ-II stellarator to investigate the confinement properties with different values of the well, or even of the hill, and to explore the properties of electromagnetic turbulence. Stable plasmas have been obtained in theoretically Mercier-unstable configurations, and the electrostatic turbulence levels in the edge are increased. Three families of modes appear during the experiments: (1) a family of modes of Alfvénic nature with high frequencies; (2) a second set of modes of middle frequencies (tens of kHz) and (3) an oscillation at f ≈ 10–20 kHz happens in several cases. In spite of the fact that the vacuum rotational transform is very similar in all of the cases, the Alfvénic mode family changes drastically when decreasing the magnetic well, showing a non-monotonic behaviour of the amplitude, and a decrease of the typical frequencies. This behaviour cannot be explained only by current or density variations, so the effect of the modification of the configuration is playing a key role. Regarding the intermediate frequencies, a coherent mode appears with decreasing frequency as the magnetic well decreases. This mode is a candidate for a GAM, which can survive in these TJ-II plasmas, despite of the strong damping these modes should suffer in this device.
Band structures and localization properties of aperiodic layered phononic crystals
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.
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...
The Effect of Magnetic Turbulence Energy Spectra on the Heating of the Solar Wind
Ng, C. S.; Bhattacharjee, A.; Isenberg, P. A.; Munsi, D.; Smith, C. W.
2008-11-01
Recently, a phenomenological solar wind heating model based on a turbulent energy cascade prescribed by the Kolmogorov theory has produced reasonably good agreement with observations on proton temperatures out to distances around 70 AU, provided the effect of turbulence generation due to pickup ions is included in the model. In the present study, we have incorporated in the heating model the energy cascade rate based on Iroshnikov-Kraichnan (IK) scaling, derivable from incompressible magnetohydrodynamics. We show that the IK cascade rate can also produce good agreement with observations, with or without the inclusion of pickup ions. This effect is confirmed both by integrating the model using average boundary conditions at 1 AU, and by applying a method [Smith et al., Astrophys. J. 638, 508 (2006)] that uses directly observed values as boundary conditions. These results suggest that if the observed proton heating rates are used to constrain theories of turbulence, there is room in the model to include spectral scalings of magnetic fluctuations varying from IK to Kolmogorov.
Scaling and anisotropy in magnetohydrodynamic turbulence in a strong mean magnetic field
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 kd(θ). The three-dimensional spectrum for k≥k0 is described by a single θ-independent functional form F(k/kd), 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=10brms, 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.
This paper presents a theoretical framework for understanding plasma turbulence in astrophysical plasmas. It is motivated by observations of electromagnetic and density fluctuations in the solar wind, interstellar medium and galaxy clusters, as well as by models of particle heating in accretion disks. All of these plasmas and many others have turbulent motions at weakly collisional and collisionless scales. The paper focuses on turbulence in a strong mean magnetic field. The key assumptions are that the turbulent fluctuations are small compared to the mean field, spatially anisotropic with respect to it and that their frequency is low compared to the ion cyclotron frequency. The turbulence is assumed to be forced at some system-specific outer scale. The energy injected at this scale has to be dissipated into heat, which ultimately cannot be accomplished without collisions. A kinetic cascade develops that brings the energy to collisional scales both in space and velocity. The nature of the kinetic cascade in various scale ranges depends on the physics of plasma fluctuations that exist there. There are four special scales that separate physically distinct regimes: the electron and ion gyroscales, the mean free path and the electron diffusion scale. In each of the scale ranges separated by these scales, the fully kinetic problem is systematically reduced to a more physically transparent and computationally tractable system of equations, which are derived in a rigorous way. In the 'inertial range' above the ion gyroscale, the kinetic cascade separates into two parts: a cascade of Alfvenic fluctuations and a passive cascade of density and magnetic-field strength fluctuations. The former are governed by the Reduced Magnetohydrodynamic (RMHD) equations at both the collisional and collisionless scales; the latter obey a linear kinetic equation along the (moving) field lines associated with the Alfvenic component (in the collisional limit, these compressive fluctuations
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.
Piecewise Parabolic Method on a Local Stencil for Magnetized Supersonic Turbulence Simulation
Ustyugov, Sergey D; Kritsuk, Alexei G; Norman, Michael L
2009-01-01
Stable, accurate, divergence-free simulation of magnetized supersonic turbulence is a severe test of numerical MHD schemes and has been surprisingly difficult to achieve due to the range of flow conditions present. Here we present a new, higher order-accurate, low dissipation numerical method which requires no additional dissipation or local "fixes" for stable execution. We describe PPML, a local stencil variant of the popular PPM algorithm for solving the equations of compressible ideal magnetohydrodynamics. The principal difference between PPML and PPM is that cell interface states are evolved rather that reconstructed at every timestep, resulting in a compact stencil. Interface states are evolved using Riemann invariants containing all transverse derivative information. The conservation laws are updated in an unsplit fashion, making the scheme fully multidimensional. Divergence-free evolution of the magnetic field is maintained using the higher order-accurate constrained transport technique of Gardiner and...
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...
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...
Shockwave-turbulent boundary layer interaction control using magnetically driven surface discharges
Kalra, Chiranjeev S.; Zaidi, Sohail H.; Miles, Richard B.; Macheret, Sergey O.
2011-03-01
This study demonstrates the potential for shockwave-turbulent boundary layer interaction control in air using low current DC constricted surface discharges forced by moderate strength magnetic fields. An analytical model describing the physics of magnetic field forced discharge interaction with boundary layer flow is developed and compared to experiments. Experiments are conducted in a Mach 2.6 indraft air tunnel with discharge currents up to 300 mA and magnetic field strengths up to 5 Tesla. Separation- and non-separation-inducing shocks are generated with diamond-shaped shockwave generators located on the wall opposite to the surface electrodes, and flow properties are measured with schlieren imaging, static wall pressure probes and acetone flow visualization. The effect of plasma control on boundary layer separation depends on the direction of the Lorentz force ( j × B). It is observed that by using a Lorentz force that pushes the discharge upstream, separation can be induced or further strengthened even with discharge currents as low as 30 mA in a 3-Tesla magnetic field. If shock-induced separation is present, it is observed that by using Lorentz force that pushes the discharge downstream, separation can be suppressed, but this required higher currents, greater than 80 mA. Acetone planar laser scattering is used to image the flow structure in the test section and the reduction in the size of recirculation bubble and its elimination are observed experimentally as a function of actuation current and magnetic field strength.