Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models
Jouve, L.; Proctor, M. R. E.; Lesur, G.
2010-09-01
Context. The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. Aims: We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods: To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. Results: We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cycle appear after a sequence of period doubling bifurcations typical of non-linear systems. The strong effects introduced even by small delays is found to be due to the dependence of the delays on the magnetic field strength at the base of the convection zone, the modulation being much less when time delays remain constant. We do not find any significant influence on the cycle period except when the delays are made artificially strong. Conclusions: A possible new origin of the solar cycle variability is here revealed. This modulated activity and the resulting butterfly diagram are then more compatible with observations than what the standard Babcock-Leighton model produces.
Buoyancy-induced time delays in Babcock-Leighton flux-transport dynamo models
Jouve, L; Lesur, G
2010-01-01
The Sun is a magnetic star whose cyclic activity is thought to be linked to internal dynamo mechanisms. A combination of numerical modelling with various levels of complexity is an efficient and accurate tool to investigate such intricate dynamical processes. We investigate the role of the magnetic buoyancy process in 2D Babcock-Leighton dynamo models, by modelling more accurately the surface source term for poloidal field. Methods. To do so, we reintroduce in mean-field models the results of full 3D MHD calculations of the non-linear evolution of a rising flux tube in a convective shell. More specifically, the Babcock-Leighton source term is modified to take into account the delay introduced by the rise time of the toroidal structures from the base of the convection zone to the solar surface. We find that the time delays introduced in the equations produce large temporal modulation of the cycle amplitude even when strong and thus rapidly rising flux tubes are considered. Aperiodic modulations of the solar cy...
3D Babcock-Leighton Solar Dynamo Models
Miesch, Mark S.; Hazra, Gopal; Karak, Bidya Binay; Teweldebirhan, Kinfe; Upton, Lisa
2016-05-01
We present results from the new STABLE (Surface flux Transport and Babcock Leighton) Dynamo Model. STABLE is a 3D Babcock-Leighton/Flux Transport dynamo model in which the source of poloidal field is the explicit emergence, distortion, and dispersal of bipolar magnetic regions (BMRs). In this talk I will discuss initial results with axisymmetric flow fields, focusing on the operation of the model, the general features of the cyclic solutions, and the challenge of achieving supercritical dynamo solutions using only the Babcock-Leighton source term. Then I will present dynamo simulations that include 3D convective flow fields based on the observed velocity power spectrum inferred from photospheric Dopplergrams. I'll use these simulations to assess how the explicit transport and amplification of fields by surface convection influences the operation of the dynamo. I will also discuss the role of surface magnetic fields in regulating the subsurface toroidal flux budget.
Karak, Bidya Binay; Cameron, Robert
2016-01-01
The key elements of the Babcock-Leighton dynamo are the generation of poloidal field through the decay of tilted bipolar active regions and the generation of toroidal field through the observed differential rotation. There are two classes of Babcock-Leighton models: flux transport dynamos where an equatorward flow at the bottom of the convection zone (CZ) causes the equatorial propagation of the butterfly wings, and dynamo waves where the radial shear and the $\\alpha$ effect act in conjunctio...
A 3D Babcock-Leighton Solar Dynamo Model
Miesch, Mark S
2014-01-01
We present a 3D kinematic solar dynamo model in which poloidal field is generated by the emergence and dispersal of tilted sunspot pairs (more generally Bipolar Magnetic Regions, or BMRs). The axisymmetric component of this model functions similarly to previous 2D Babcock-Leighton (BL) dynamo models that employ a double-ring prescription for poloidal field generation but we generalize this prescription into a 3D flux emergence algorithm that places BMRs on the surface in response to the dynamo-generated toroidal field. In this way, the model can be regarded as a unification of BL dynamo models (2D in radius/latitude) and surface flux transport models (2D in latitude/longitude) into a more self-consistent framework that captures the full 3D structure of the evolving magnetic field. The model reproduces some basic features of the solar cycle including an 11-yr periodicity, equatorward migration of toroidal flux in the deep convection zone, and poleward propagation of poloidal flux at the surface. The poleward-p...
Magnetic flux transport and the sun's dipole moment - New twists to the Babcock-Leighton model
Wang, Y.-M.; Sheeley, N. R., Jr.
1991-01-01
The mechanisms that give rise to the sun's large-scale poloidal magnetic field are explored in the framework of the Babcock-Leighton (BL) model. It is shown that there are in general two quite distinct contributions to the generation of the 'alpha effect': the first is associated with the axial tilts of the bipolar magnetic regions as they erupt at the surface, while the second arises through the interaction between diffusion and flow as the magnetic flux is dispersed over the surface. The general relationship between flux transport and the BL dynamo is discussed.
Karak, Bidya Binay
2016-01-01
The key elements of the Babcock-Leighton dynamo are the generation of poloidal field through the decay of tilted bipolar active regions and the generation of toroidal field through the observed differential rotation. There are two classes of Babcock-Leighton models: flux transport dynamos where an equatorward flow at the bottom of the convection zone (CZ) is responsible for the equatorial propagation of the butterfly wings, and dynamo waves where the radial gradient of differential rotation and the $\\alpha$ effect act in conjunction to produce the equatorial propagation. Here we investigate the role of downward magnetic pumping near the surface using a kinematic Babcock-Leighton model. We find that the pumping causes the poloidal field to become predominately radial in the near-surface shear layer. This allows the negative radial shear in the near-surface layer to effectively act on the radial field to produce a toroidal field. Consequently, we observe a clear equatorward migration of the toroidal field at lo...
A coupled $2\\times2$D Babcock-Leighton solar dynamo model. II. Reference dynamo solutions
Lemerle, Alexandre
2016-01-01
In this paper we complete the presentation of a new hybrid $2\\times2$D flux transport dynamo (FTD) model of the solar cycle based on the Babcock-Leighton mechanism of poloidal magnetic field regeneration via the surface decay of bipolar magnetic regions (BMRs). This hybrid model is constructed by allowing the surface flux transport (SFT) simulation described in Lemerle et al. 2015 to provide the poloidal source term to an axisymmetric FTD simulation defined in a meridional plane, which in turn generates the BMRs required by the SFT. A key aspect of this coupling is the definition of an emergence function describing the probability of BMR emergence as a function of the spatial distribution of the internal axisymmetric magnetic field. We use a genetic algorithm to calibrate this function, together with other model parameters, against observed cycle 21 emergence data. We present a reference dynamo solution reproducing many solar cycle characteristics, including good hemispheric coupling, phase relationship betwe...
A Three-Dimensional Babcock-Leighton Solar Dynamo Model: Initial Results with Axisymmetric Flows
Miesch, Mark S
2015-01-01
The main objective of this paper is to introduce the STABLE (Surface flux Transport And Babcock-LEighton) solar dynamo model. STABLE is a 3D Babcock-Leighton/Flux Transport dynamo model in which the source of poloidal field is the explicit emergence, distortion, and dispersal of bipolar magnetic regions (BMRs). Here we describe the STABLE model in more detail than we have previously and we verify it by reproducing a 2D mean-field benchmark. We also present some representative dynamo simulations, focusing on the special case of kinematic magnetic induction and axisymmetric flow fields. Not all solutions are supercritical; it can be a challenge for the BL mechanism to sustain the dynamo when the turbulent diffusion near the surface is $\\geq 10^{12}$ cm$^2$ s$^{-1}$. However, if BMRs are sufficiently large, deep, and numerous, then sustained, cyclic, dynamo solutions can be found that exhibit solar-like features. Furthermore, we find that the shearing of radial magnetic flux by the surface differential rotation ...
Karak, Bidya Binay; Cameron, Robert
2016-05-01
We investigate the role of downward magnetic pumping near the surface using a kinematic Babcock-Leighton model. We find that the pumping causes the poloidal field to become predominately radial in the near-surface shear layer. This allows the negative radial shear in the near-surface layer to effectively act on the radial field to produce a toroidal field. Consequently, we observe a clear equatorward migration of the toroidal field at low latitudes even when there is no meridional flow in the deep CZ. We show a case where the period of a dynamo wave solution is approximately 11 years. Flux transport models are also shown with periods close to 11 years. Both the dynamo wave and flux transport dynamo are thus able to reproduce some of the observed features of solar cycle. The main difference between the two types of dynamo is the value of $\\alpha$ required to produce dynamo action. In both types of dynamo, the surface meridional flow helps to advect and build the polar field in high latitudes, while in flux transport dynamo the equatorward flow near the bottom of CZ advects toroidal field to cause the equatorward migration in butterfly wings and this advection makes the dynamo easier by transporting strong toroidal field to low latitudes where $\\alpha$ effect works. Another conclusion of our study is that the magnetic pumping suppresses the diffusion of fields through the photospheric surface which helps to achieve the 11-year dynamo cycle at a moderately larger value of magnetic diffusivity than has previously been used.
Hazra, Soumitra
2016-01-01
At present, Babcock-Leighton flux transport solar dynamo models appear as the most promising model for explaining diverse observational aspects of the sunspot cycle. The success of these flux transport dynamo models is largely dependent upon a single-cell meridional circulation with a deep equatorward component at the base of the Sun's convection zone. However, recent observations suggest that the meridional flow may in fact be very shallow (confined to the top 10 % of the Sun) and more complex than previously thought. Taken together these observations raise serious concerns on the validity of the flux transport paradigm. By accounting for the turbulent pumping of magnetic flux as evidenced in magnetohydrodynamic simulations of solar convection, we demonstrate that flux transport dynamo models can generate solar-like magnetic cycles even if the meridional flow is shallow. Solar-like periodic reversals is recovered even when meridional circulation is altogether absent, however, in this case the solar surface m...
The Role of Magnetic Buoyancy in a Babcock-Leighton Type Solar Dynamo
Indian Academy of Sciences (India)
Dibyendu Nandy; Arnab Rai Choudhuri
2000-09-01
We study the effects of incorporating magnetic buoyancy in a model of the solar dynamo—which draws inspiration from the Babcock-Leighton idea of surface processes generating the poloidal field. We present our main results here.
An example of Ensemble Kalman Filter data assimilation in a Babcock-Leighton solar dynamo model
Dikpati, Mausumi; Anderson, Jeffrey L.
2016-05-01
Atmospheric and oceanic prediction models have been greatly advanced over the past 40 years by using modern data assimilation techniques. Application of similar techniques in solar models started about 7 years ago. However, acceptance of such techniques by the solar community has been slow to develop. In order to make accurate predictions of solar activity as well as reconstruction of certain model parameters that cannot be directly measured, it will be essential to implement sophisticated data assimilation techniques as used by atmospheric and oceanic models. We will present here an example of parameter reconstruction, namely the time variation in meridional flow-speed, done by assimilating data into a Babcock-Leighton solar dynamo model in the framework of NCAR's Data Assimilation Research Testbed (NCAR-DART). By performing many 'Observing System Simulation Experiments' (OSSEs) we find that an optimally good reconstruction in time series of meridional circulation can be obtained by using 16 ensemble members and assimilating one magnetic observation with less than 40 percent observational error. However, the RMS error in reconstruction reduces with increase in ensemble size, increase in number of observations and decrease in observational error. We also find that assimilation of magnetic field observations taken from low-to-mid latitudes at the surface compared to any other locations produces the best reconstruction. We will close by showing that assimilation cycle of 15 days is optimal; generally a longer assimilation cycle deteriorates the results, but the Dynamo DART system needs a minimum time to develop the dynamics.
Belucz, Bernadett; Forgacs-Dajka, Emese
2015-01-01
Babcock-Leighton type solar dynamo models with single-celled meridional circulation are successful in reproducing many solar cycle features. Recent observations and theoretical models of meridional circulation do not indicate a single-celled flow pattern. We examine the role of complex multi-cellular circulation patterns in a Babcock-Leighton solar dynamo in advection- and diffusion-dominated regimes. We show from simulations that presence of a weak, second, high-latitude reverse cell speeds up the cycle and slightly enhances the poleward branch in butterfly diagram, whereas the presence of a second cell in depth reverses the tilt of butterfly wing to an anti-solar type. A butterfly diagram constructed from middle of convection zone yields a solar-like pattern, but this may be difficult to realize in the Sun because of magnetic buoyancy effects. Each of the above cases behaves similarly in higher and lower magnetic diffusivity regimes. However, our dynamo with a meridional circulation containing four cells in...
The treatment of magnetic buoyancy in flux transport dynamo models
Choudhuri, Arnab Rai; Hazra, Gopal
2015-01-01
One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method have been used widely by different groups in constructing 2D kinematic models of the flux transpo...
A coupled 2$\\times$2D Babcock-Leighton solar dynamo model. I. Surface magnetic flux evolution
Lemerle, Alexandre; Carignan-Dugas, Arnaud
2015-01-01
The need for reliable predictions of the solar activity cycle motivates the development of dynamo models incorporating a representation of surface processes sufficiently detailed to allow assimilation of magnetographic data. In this series of papers we present one such dynamo model, and document its behavior and properties. This first paper focuses on one of the model's key components, namely surface magnetic flux evolution. Using a genetic algorithm, we obtain best-fit parameters of the transport model by least-squares minimization of the differences between the associated synthetic synoptic magnetogram and real magnetographic data for activity cycle 21. Our fitting procedure also returns Monte Carlo-like error estimates. We show that the range of acceptable surface meridional flow profiles is in good agreement with Doppler measurements, even though the latter are not used in the fitting process. Using a synthetic database of bipolar magnetic region (BMR) emergences reproducing the statistical properties of ...
The treatment of magnetic buoyancy in flux transport dynamo models
Choudhuri, Arnab Rai
2015-01-01
One important ingredient of flux transport dynamo models is the rise of the toroidal magnetic field through the convection zone due to magnetic buoyancy to produce bipolar sunspots and then the generation of the poloidal magnetic field from these bipolar sunspots due to the Babcock-Leighton mechanism. Over the years, two methods of treating magnetic buoyancy, a local method and a non-local method have been used widely by different groups in constructing 2D kinematic models of the flux transport dynamo. We review both these methods and conclude that neither of them is fully satisfactory, presumably because magnetic buoyancy is an inherently 3D process. We also point out so far we do not have proper understanding of why sunspot emergence is restricted to rather low latitudes.
International Nuclear Information System (INIS)
The emergence of tilted bipolar active regions (ARs) and the dispersal of their flux, mediated via processes such as diffusion, differential rotation, and meridional circulation, is believed to be responsible for the reversal of the Sun's polar field. This process (commonly known as the Babcock-Leighton mechanism) is usually modeled as a near-surface, spatially distributed α-effect in kinematic mean-field dynamo models. However, this formulation leads to a relationship between polar field strength and meridional flow speed which is opposite to that suggested by physical insight and predicted by surface flux-transport simulations. With this in mind, we present an improved double-ring algorithm for modeling the Babcock-Leighton mechanism based on AR eruption, within the framework of an axisymmetric dynamo model. Using surface flux-transport simulations, we first show that an axisymmetric formulation-which is usually invoked in kinematic dynamo models-can reasonably approximate the surface flux dynamics. Finally, we demonstrate that our treatment of the Babcock-Leighton mechanism through double-ring eruption leads to an inverse relationship between polar field strength and meridional flow speed as expected, reconciling the discrepancy between surface flux-transport simulations and kinematic dynamo models.
Exploring the $P_{cyc}$ vs $P_{rot}$ relation with flux transport dynamo models of solar-like stars
Jouve, L; Brun, A S
2009-01-01
Aims: To understand stellar magnetism and to test the validity of the Babcock-Leighton flux transport mean field dynamo models with stellar activity observations Methods: 2-D mean field dynamo models at various rotation rates are computed with the STELEM code to study the sensitivity of the activity cycle period and butterfly diagram to parameter changes and are compared to observational data. The novelty is that these 2-D mean field dynamo models incorporate scaling laws deduced from 3-D hydrodynamical simulations for the influence of rotation rate on the amplitude and profile of the meridional circulation. These models make also use of observational scaling laws for the variation of differential rotation with rotation rate. Results: We find that Babcock-Leighton flux transport dynamo models are able to reproduce the change in topology of the magnetic field (i.e. toward being more toroidal with increasing rotation rate) but seem to have difficulty reproducing the cycle period vs activity period correlation o...
Flux Transport Solar Dynamos with Shallow Meridional Flow and Turbulent Pumping
Nandi, Dibyendu; Hazra, Soumitra
2016-07-01
The large-scale solar magnetic cycle is sustained by a dynamo mechanism in which the induction of the toroidal component of the magnetic field by differential rotation and the regeneration of the poloidal component are crucial processes. In the Sun, the Babcock-Leighton mechanism, i.e., the (near-surface) redistribution of the flux of tilted bipolar sunspot pairs is thought to be the main source of the solar poloidal field. The poloidal field so generated must be transported to the solar interior where the toroidal field is generated and stored - presumably near the base of the solar convection zone. Traditionally, flux transport dynamo models have relied on a deep meridional circulation to achieve this transport. However, recent observations claim that the meridional circulation could be much shallower that previously thought. We explore the question whether flux transport dynamos can function with a shallow meridional flow and present an alternative paradigm for flux transport dynamics in solar-stellar interiors sustained by turbulent pumping.
On the role of meridional flows in flux transport dynamo models
Jouve, L
2007-01-01
The Sun is a magnetic star whose magnetism and cyclic activity is linked to the existence of an internal dynamo. We aim to understand the establishment of the solar magnetic 22-yr cycle, its associated butterfly diagram and field parity selection through numerical simulations of the solar global dynamo. Inspired by recent observations and 3D simulations that both exhibit multicellular flows in the solar convection zone, we seek to characterise the influence of various profiles of circulation on the behaviour of solar mean-field dynamo models. We are using 2-D mean field flux transport Babcock-Leighton numerical models in which we test several types of meridional flows: 1 large single cell, 2 cells in radius and 4 cells per hemisphere. We confirm that adding cells in latitude tends to speed up the dynamo cycle whereas adding cells in radius more than triples the period. We find that the cycle period in the four cells model is less sensitive to the flow speed than in the other simpler meridional circulation pro...
Resonance in Forced Flux Transport Dynamos
Gilman, Peter A.; Dikpati, Mausumi
2011-01-01
We show that simple 2 and 3-layer flux-transport dynamos, when forced at the top by a poloidal source term, can produce a widely varying amplitude of toroidal field at the bottom, depending on how close the meridional flow speed of the bottom layer is to the propagation speed of the forcing applied above the top layer, and how close the amplitude of the $\\alpha$-effect is to two values that give rise to a resonant response. This effect should be present in this class of dynamo model no matter...
Does the butterfly diagram indicate asolar flux-transport dynamo?
Schuessler, M.; Schmitt, D
2004-01-01
We address the question whether the properties of the observed latitude-time diagram of sunspot occurence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnet...
Does the butterfly diagram indicate asolar flux-transport dynamo?
Schüssler, M
2004-01-01
We address the question whether the properties of the observed latitude-time diagram of sunspot occurence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnetic flux by a flow. These properties seem to be generic for an oscillatory and migratory field of dipole parity and thus do not permit an observational distinction between different dynamo approaches.
A Critical Assessment of the Flux Transport Dynamo
Indian Academy of Sciences (India)
Arnab Rai Choudhuri
2015-03-01
We first discuss how the flux transport dynamo with reasonably high diffusion can explain both the regular and the irregular features of the solar cycle quite well. Then, we critically examine the inadequacies of the model and the challenge posed by some recent observational data about meridional circulation, arriving at a conclusion that this model can still work within the bounds of observational data.
Solar cycle prediction using precursors and flux transport models
Cameron, R
2006-01-01
We study the origin of the predictive skill of some methods to forecast the strength of solar activity cycles. A simple flux transport model for the azimuthally averaged radial magnetic field at the solar surface is used, which contains a source term describing the emergence of new flux based on observational sunspot data. We consider the magnetic flux diffusing over the equator as a predictor, since this quantity is directly related to the global dipole field from which a Babcock-Leighton dynamo generates the toroidal field for the next activity cycle. If the source is represented schematically by a narrow activity belt drifting with constant speed over a fixed range of latitudes between activity minima, our predictor shows considerable predictive skill with correlation coefficients up to 0.95 for past cycles. However, the predictive skill is completely lost when the actually observed emergence latitudes are used. This result originates from the fact that the precursor amplitude is determined by the sunspot ...
Does the butterfly diagram indicate a solar flux-transport dynamo?
Schüssler, M.; Schmitt, D.
2004-07-01
We address the question whether the properties of the observed latitude-time diagram of sunspot occurrence (the butterfly diagram) provide evidence for the operation of a flux-transport dynamo, which explains the migration of the sunspot zones and the period of the solar cycle in terms of a deep equatorward meridional flow. We show that the properties of the butterfly diagram are equally well reproduced by a conventional dynamo model with migrating dynamo waves, but without transport of magnetic flux by a flow. These properties seem to be generic for an oscillatory and migratory field of dipole parity and thus do not permit an observational distinction between different dynamo approaches.
Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?
Hazra, Gopal; Karak, Bidya Binay; Choudhuri, Arnab Rai
2013-01-01
The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still r...
A flux-transport dynamo with a multi-cell meridional circulation
Bonanno, A.; Elstner, D.; Belvedere, G.; Rüdiger, G.
2005-04-01
We discuss the effect of a non-trivial meridional circulation pattern on a flux-transport type of solar dynamo. The critical value of the turbulent helicity and the periods are calculated as a function of the meridional flow strength. We found that the dynamo mechanism is mainly determined by the global topology of the meridional flow. In particular the equatorwards migration in the butterfly diagram can be easily obtained by the combined action of two cells of meridional circulation.
Solar and Stellar Dynamos Saas-Fee Advanced Course 39 Swiss Society for Astrophysics and Astronomy
2013-01-01
Astrophysical dynamos are at the heart of cosmic magnetic fields of a wide range of scales, from planets and stars to entire galaxies. This book presents a thorough, step-by-step introduction to solar and stellar dynamos. Looking first at the ultimate origin of cosmic seed magnetic fields, the antagonists of field amplification are next considered: resistive decay, flux expulsion, and flows ruled out by anti-dynamo theorems. Two kinematic flows that can act as dynamos are then studied: the Roberts cell and the CP-flow. Mean-field electrodynamics and derivation of the mean-field dynamo equations lead to the alpha Omega-dynamo, the flux transport dynamo, and dynamos based on the Babcock-Leighton mechanism. Alternatives to the mean-field theory are also presented, as are global MHD dynamo simulations. Fluctuations and grand minima in the solar cycle are discussed in terms of dynamo modulations through stochastic forcing and nonlinear effects. The book concludes with an overview of the major challenges in underst...
Solar cycle prediction using precursors and flux transport models
Cameron, R; Schuessler, M.
2006-01-01
We study the origin of the predictive skill of some methods to forecast the strength of solar activity cycles. A simple flux transport model for the azimuthally averaged radial magnetic field at the solar surface is used, which contains a source term describing the emergence of new flux based on observational sunspot data. We consider the magnetic flux diffusing over the equator as a predictor, since this quantity is directly related to the global dipole field from which a Babcock-Leighton dy...
International Nuclear Information System (INIS)
Fluctuations in the Sun's magnetic activity, including episodes of grand minima such as the Maunder minimum have important consequences for space and planetary environments. However, the underlying dynamics of such extreme fluctuations remain ill-understood. Here, we use a novel mathematical model based on stochastically forced, non-linear delay differential equations to study solar cycle fluctuations in which time delays capture the physics of magnetic flux transport between spatially segregated dynamo source regions in the solar interior. Using this model, we explicitly demonstrate that the Babcock-Leighton poloidal field source based on dispersal of tilted bipolar sunspot flux, alone, cannot recover the sunspot cycle from a grand minimum. We find that an additional poloidal field source effective on weak fields—e.g., the mean-field α effect driven by helical turbulence—is necessary for self-consistent recovery of the sunspot cycle from grand minima episodes.
Karak, Bidya Binay
2010-01-01
Meridional circulation is an important ingredient in flux transport dynamo model. We have studied its importance on the period, amplitude of solar cycle and also on producing Maunder-like grand minima in this model. First, we model the periods of last 23 sunspot cycles by varying the meridional circulation speed. We find that most of the cycle amplitudes also get modeled up to some extent when the dynamo is in diffusion-dominated regime. Next, we propose that at the beginning of the Maunder minimum the amplitude of meridional circulation dropped to a low value and then after a few years it increased again. Several independent studies also favor this assumption. With this assumption, a diffusion-dominated dynamo is able to reproduce many important features of Maunder minimum remarkably well. If the dynamo is in diffusion-dominated regime, then the slower meridional circulation means that the poloidal field gets more time to diffuse away from the tachocline, making the dynamo weaker. This consequence helps to m...
Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?
Hazra, Gopal; Choudhuri, Arnab Rai
2014-01-01
The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as proper butterfly diagram, proper phase relation between the toroidal and poloidal fields). We consider additional cells of the meridional circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. Howeve...
Is a deep one-cell meridional circulation essential for the flux transport solar dynamo?
International Nuclear Information System (INIS)
The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as a proper butterfly diagram, the proper phase relation between the toroidal and poloidal fields). We consider additional cells of the meridional circulation below the shallow return flow—both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. However, if there is either no flow or a poleward flow at the bottom of the convection zone, then we cannot reproduce solar behavior. On making the turbulent diffusivity low, we still find periodic behavior, although the period of the cycle becomes unrealistically large. In addition, with a low diffusivity, we do not get the observed correlation between the polar field at the sunspot minimum and the strength of the next cycle, which is reproduced when diffusivity is high. On introducing radially downward pumping, we get a more reasonable period and more solar-like behavior even with low diffusivity.
Karak, Bidya Binay
2010-01-01
Meridional circulation is an important ingredient in flux transport dynamo models. We have studied its importance on the period, the amplitude of the solar cycle, and also in producing Maunder-like grand minima in these models. First, we model the periods of the last 23 sunspot cycles by varying the meridional circulation speed. If the dynamo is in a diffusion-dominated regime, then we find that most of the cycle amplitudes also get modeled up to some extent when we model the periods. Next, w...
Cameron, R H; Brandenburg, A
2016-01-01
A brief summary of the various observations and constraints that underlie solar dynamo research are presented. The arguments that indicate that the solar dynamo is an alpha-omega dynamo of the Babcock-Leighton type are then shortly reviewed. The main open questions that remain are concerned with the subsurface dynamics, including why sunspots emerge at preferred latitudes as seen in the familiar butterfly wings, why the cycle is about 11 years long, and why the sunspot groups emerge tilted with respect to the equator (Joy's law). Next, we turn to magnetic helicity, whose conservation property has been identified with the decline of large-scale magnetic fields found in direct numerical simulations at large magnetic Reynolds numbers. However, magnetic helicity fluxes through the solar surface can alleviate this problem and connect theory with observations, as will be discussed.
Cameron, R. H.; Dikpati, M.; Brandenburg, A.
2016-02-01
A brief summary of the various observations and constraints that underlie solar dynamo research are presented. The arguments that indicate that the solar dynamo is an alpha-omega dynamo of the Babcock-Leighton type are then shortly reviewed. The main open questions that remain are concerned with the subsurface dynamics, including why sunspots emerge at preferred latitudes as seen in the familiar butterfly wings, why the cycle is about 11 years long, and why the sunspot groups emerge tilted with respect to the equator (Joy's law). Next, we turn to magnetic helicity, whose conservation property has been identified with the decline of large-scale magnetic fields found in direct numerical simulations at large magnetic Reynolds numbers. However, magnetic helicity fluxes through the solar surface can alleviate this problem and connect theory with observations, as will be discussed.
Research in Solar Dynamo Theories%太阳发电机理论研究
Institute of Scientific and Technical Information of China (English)
吴建清; 杨志良
2011-01-01
太阳活动主要是由磁场产生的，因此，对太阳磁场性质和起源的研究具有重要意义．太阳发电机理论主要研究的是太阳上观测到的与太阳活动相关的磁场起源、磁场特征、各种活动现象之间的相关性及其变化规律．其是太阳物理学中有待解决的最基本、最重要的问题．根据太阳黑子及太阳周期的相关观测，介绍了构成发电机的基本要素，具体描述了各种典型发电机模型，并对其分别进行评述，进而探讨了目前存在的问题及发展方向．%Various activities of the Sun, such as flare, coronal mass ejections, solar wind, which will affect the normal work of satellites, radio communication and electric power system. Therefore, researching the activities of the Sun has great significance for human. Moreover, these activities are dominated by the magnetic field. The purpose of study the solar dynamo theories is not only to unpuzzle the correlation and variation among various activities, but also to explain the origin and character of the magnetic fields on the Sun. The paper briefly reviews the relevant observational data pertaining to sunspots and solar cycle first and researches evolution if the solar dynamo theories. Then the three basic elements of the dynamo, the differential rotation, meridional circulation and magnetic diffusivity, with the fundamental equations of magnetic field generation are introduced. Magnetic induced equation is the first basic equation for dynamo theories and the first item of induced equation is from Ohm Law. Moreover, mean field dynamos which contain Babcock-Leighton dynamo, interface dynamo and flux transport dynamo, non-axisymmetric dynamo, MHD simulation and small scale dynamo are described. The advantages and shortcomings of them are presented respectively. Finally, the existing problems and possible development in the future for solar dynamo theories are suggested.
A dynamo model of magnetic activity in solar-like stars with different rotational velocities
Energy Technology Data Exchange (ETDEWEB)
Karak, Bidya Binay; Choudhuri, Arnab Rai [Department of Physics, Indian Institute of Science, Bangalore 560012 (India); Kitchatinov, Leonid L. [Institute for Solar-Terrestrial Physics, Lermontov Str. 126A, Irkutsk 664033 (Russian Federation)
2014-08-10
We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of 1 M{sub ☉} stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude f{sub m} of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of f{sub m}. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of the magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes rise radially from the bottom of the convection zone. Taking into account the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively.
The Hemispheric Asymmetry of Solar Activity During the Twentieth Century and the Solar Dynamo
Goel, Ashish; Choudhuri, Arnab Rai
2007-01-01
We believe the Babcock--Leighton process of poloidal field generation to be the main source of irregularity in the solar cycle. The random nature of this process may make the poloidal field in one hemisphere stronger than that in the other hemisphere at the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We look for evidence of this in the observational data and then model it theoretically with our dynamo code. Since actual polar field measurements exist only ...
Kinematic Solar Dynamo with Spot Deposition
Karak, Bidya Binay; Miesch, Mark S.
2016-05-01
We have recently developed a kinematic dynamo model by including the observed differential rotation and the meridional flow. This model includes the emergence of sunspots from the deep-seated toroidal field and their subsequent decay at the surface, i.e., the Babcock-Leighton process for the generation of poloidal field.We shall show that this model reproduces most of the basic features of the solar magnetic cycle including the polarity reversals, 11 years periodicity, equatorward migration of sunspots at low latitudes and the poleward migration of the radial field at the surface. This model also produces the observed cycle variations when the fluctuations in the active-region tilt are included. North-south asymmetries of cycles from this model will also be demonstrated.
Surface Flux Transport and the Evolution of the Sun's Polar Fields
Wang, Y.-M.
2016-04-01
The evolution of the polar fields occupies a central place in flux transport (Babcock-Leighton) models of the solar cycle. We discuss the relationship between surface flux transport and polar field evolution, focusing on two main issues: the latitudinal profile of the meridional flow and the axial tilts of active regions. Recent helioseismic observations indicate that the poleward flow speed peaks at much lower latitudes than inferred from magnetic feature tracking, which includes the effect of supergranular diffusion and thus does not represent the actual bulk flow. Employing idealized simulations, we demonstrate that flow profiles that peak at mid latitudes give rise to overly strong and concentrated polar fields. We discuss the differences between magnetic and white-light measurements of tilt angles, noting the large uncertainties inherent in the sunspot group measurements and their tendency to underestimate the actual tilts. We find no clear evidence for systematic cycle-to-cycle variations in Joy's law during cycles 21-23. Finally, based on the observed evolution of the Sun's axial dipole component and polar fields up to the end of 2015, we predict that cycle 25 will be similar in amplitude to cycle 24.
Solar activity forecast with a dynamo model
Jiang, Jie; Choudhuri, Arnab Rai
2007-01-01
Although systematic measurements of the solar polar magnetic field exist only from mid 1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock Leighton mechanism of poloidal field generation from decaying sunspots involves randomness, whereas the other aspects of the dynamo process must be reasonably ordered and deterministic. Only if the magnetic diffusivity within the convection zone is assumed to be high, we can explain the correlation between the polar field at a minimum and the next cycle. We give several independent arguments that the diffusivity must be of this order. In a dynamo model with diffusivity like this, the poloidal field generated at the mid latitudes is advected toward the poles by the meridional circulation an...
Solar Cycle #24 and the Solar Dynamo
Pesnell, W. Dean; Schatten, Kenneth
2007-01-01
We focus on two solar aspects related to flight dynamics. These are the solar dynamo and long-term solar activity predictions. The nature of the solar dynamo is central to solar activity predictions, and these predictions are important for orbital planning of satellites in low earth orbit (LEO). The reason is that the solar ultraviolet (UV) and extreme ultraviolet (EUV) spectral irradiances inflate the upper atmospheric layers of the Earth, forming the thermosphere and exosphere through which these satellites orbit. Concerning the dynamo, we discuss some recent novel approaches towards its understanding. For solar predictions we concentrate on a solar precursor method, in which the Sun s polar field plays a major role in forecasting the next cycle s activity based upon the Babcock- Leighton dynamo. With a current low value for the Sun s polar field, this method predicts that solar cycle #24 will be one of the lowest in recent times, with smoothed F10.7 radio flux values peaking near 130+ 30 (2 4, in the 2013 timeframe. One may have to consider solar activity as far back as the early 20th century to find a cycle of comparable magnitude. Concomitant effects of low solar activity upon satellites in LEO will need to be considered, such as enhancements in orbital debris. Support for our prediction of a low solar cycle #24 is borne out by the lack of new cycle sunspots at least through the first half of 2007. Usually at the present epoch in the solar cycle (-7+ years after the last solar maximum), for a normal size following cycle, new cycle sunspots would be seen. The lack of their appearance at this time is only consistent with a low cycle #24. Polar field observations of a weak magnitude are consistent with unusual structures seen in the Sun s corona. Polar coronal holes are the hallmarks of the Sun s open field structures. At present, it appears that the polar coronal holes are relatively weak, and there have been many equatorial coronal holes. This appears
Modulated cycles in an illustrative solar dynamo model with competing alpha effects
Cole, Laura
2014-01-01
The large-scale magnetic field in the Sun varies with a period of approximately 22 years, although the amplitude of the cycle is subject to long-term modulation with recurrent phases of significantly reduced magnetic activity. It is believed that a hydromagnetic dynamo is responsible for producing this large-scale field, although this dynamo process is not well understood. Within the framework of mean-field dynamo theory, our aim is to investigate how competing mechanisms for poloidal field regeneration (namely a time delayed Babcock-Leighton surface alpha-effect and an interface-type alpha-effect), can lead to the modulation of magnetic activity in a deep-seated solar dynamo model. We solve the standard alpha-omega dynamo equations in one spatial dimension, including source terms corresponding to both of the the competing alpha-effects in the evolution equation for the poloidal field. This system is solved using two different methods. In addition to solving the one-dimensional partial differential equations ...
Choudhuri, Arnab Rai
2010-01-01
The most promising model for explaining the origin of solar magnetism is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock--Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. After discussing how this model explains the regular periodic features of the solar cycle, we come to the questions of what causes irregularities of solar cycles and w...
The Hemispheric Asymmetry of Solar Activity During the Twentieth Century and the Solar Dynamo
Goel, Ashish
2007-01-01
We believe the Babcock--Leighton process of poloidal field generation to be the main source of irregularity in the solar cycle. The random nature of this process may make the poloidal field in one hemisphere stronger than that in the other hemisphere at the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We look for evidence of this in the observational data and then model it theoretically with our dynamo code. Since actual polar field measurements exist only from 1970s, we use the polar faculae number data recorded by Sheeley (1991) as a proxy of the polar field and estimate the hemispheric asymmetry of the polar field in different solar minima during the major part of the twentieth century. This asymmetry is found to have a reasonable correlation with the asymmetry of the next cycle. We then run our dynamo code by feeding information about this asymmetry at the successive minima and compare with observational data. We find that the theoretically computed asymmetries of diffe...
Global Solar Dynamo Models: Simulations and Predictions
Indian Academy of Sciences (India)
Mausumi Dikpati; Peter A. Gilman
2008-03-01
Flux-transport type solar dynamos have achieved considerable success in correctly simulating many solar cycle features, and are now being used for prediction of solar cycle timing and amplitude.We first define flux-transport dynamos and demonstrate how they work. The essential added ingredient in this class of models is meridional circulation, which governs the dynamo period and also plays a crucial role in determining the Sun’s memory about its past magnetic fields.We show that flux-transport dynamo models can explain many key features of solar cycles. Then we show that a predictive tool can be built from this class of dynamo that can be used to predict mean solar cycle features by assimilating magnetic field data from previous cycles.
Solar activity forecast with a dynamo model
Jiang, Jie; Chatterjee, Piyali; Choudhuri, Arnab Rai
2007-01-01
Although systematic measurements of the solar polar magnetic field exist only from mid 1970s, other proxies can be used to infer the polar field at earlier times. The observational data indicate a strong correlation between the polar field at a sunspot minimum and the strength of the next cycle, although the strength of the cycle is not correlated well with the polar field produced at its end. This suggests that the Babcock Leighton mechanism of poloidal field generation from decaying sunspot...
The hemispheric asymmetry of solar activity during the last century and the solar dynamo
Institute of Scientific and Technical Information of China (English)
Ashish Goel; Arnab Rai Choudhuri
2009-01-01
We believe the Babcock-Leighton process of poloidal field generation to be the main source of irregularity in the solar cycle. The random nature of this process may make the poloidal field in one hemisphere stronger than that in the other hemisphere at the end of a cycle. We expect this to induce an asymmetry in the next sunspot cycle. We look for evidence of this in the observational data and then model it theoretically with our dynamo code. Since actual polar field measurements exist only from the 1970s, we use the polar faculae number data recorded by Sheeley (1991, 2008) as a proxy of the polar field and estimate the hemispheric asymmetry of the polar field in different solar minima during the major part of the twentieth century. This asymmetry is found to have a reasonable correlation with the asymmetry of the next cycle. We then run our dynamo code by feeding information about this asymmetry at the successive minima and compare the results with observational data. We find that the theoretically computed asymmetries of different cycles compare favorably with the observational data, with the correlation co-efficient being 0.73. Due to the coupling between the two hemispheres, any hemispheric asymmetry tends to get attenuated with time. The hemispheric asymmetry of a cycle ei-ther from observational data or from theoretical calculations statistically tends to be less than the asymmetry in the polar field (as inferred from the faculae data) in the preceding minimum. This reduction factor turns out to be 0.43 and 0.51 respectively in observational data and theoretical simulations.
North-south asymmetry of solar dynamo in the current activity cycle
Kitchatinov, L L
2014-01-01
An explanation is suggested for the north-south asymmetry of the polar magnetic field reversal in the current cycle of solar activity. The contribution of the Babcock-Leighton mechanism to the poloidal field generation is estimated using sunspot data for the current activity cycle. Estimations are performed separately for the northern and southern hemispheres. The contribution of the northern hemisphere exceeded considerably that of the southern hemisphere during the initial stage of the cycle. This is the probable reason for the earlier reversal of the northern polar field. The estimated contributions of the Babcock-Leighton mechanism are considerably smaller than similar estimations for the previous activity cycles. A relatively weak (<1G) large-scale polar field can be expected for the next activity minimum.
North-south asymmetry of solar dynamo in the current activity cycle
Kitchatinov, L. L.; Khlystova, A. I.
2014-01-01
An explanation is suggested for the north-south asymmetry of the polar magnetic field reversal in the current cycle of solar activity. The contribution of the Babcock-Leighton mechanism to the poloidal field generation is estimated using sunspot data for the current activity cycle. Estimations are performed separately for the northern and southern hemispheres. The contribution of the northern hemisphere exceeded considerably that of the southern hemisphere during the initial stage of the cycl...
Choudhuri, Arnab Rai
2010-01-01
The most promising model for explaining the origin of solar magnetism is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock--Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. After discussing how this model explains the regular periodic features of the solar cycle, we come to the questions of what causes irregularities of solar cycles and whether we can predict future cycles. Only if the diffusivity within the convection zone is sufficiently high, the polar field at the sunspot minimum is correlated with strength of the next cycle. This is in conformity with the limited available observational data.
The solar dynamo and prediction of sunspot cycles
Dikpati, Mausumi
2012-07-01
Much progress has been made in understanding the solar dynamo since Parker first developed the concepts of dynamo waves and magnetic buoyancy around 1955, and the German school first formulated the solar dynamo using the mean-field formalism. The essential ingredients of these mean-field dynamos are turbulent magnetic diffusivity, a source of lifting of flux, or 'alpha-effect', and differential rotation. With the advent of helioseismic and other observations at the Sun's photosphere and interior, as well as theoretical understanding of solar interior dynamics, solar dynamo models have evolved both in the realm of mean-field and beyond mean-field models. After briefly discussing the status of these models, I will focus on a class of mean-field model, called flux-transport dynamos, which include meridional circulation as an essential additional ingredient. Flux-transport dynamos have been successful in simulating many global solar cycle features, and have reached the stage that they can be used for making solar cycle predictions. Meridional circulation works in these models like a conveyor-belt, carrying a memory of the magnetic fields from 5 to 20 years back in past. The lower is the magnetic diffusivity, the longer is the model's memory. In the terrestrial system, the great-ocean conveyor-belt in oceanic models and Hadley, polar and Ferrel circulation cells in the troposphere, carry signatures from the past climatological events and influence the determination of future events. Analogously, the memory provided by the Sun's meridional circulation creates the potential for flux-transport dynamos to predict future solar cycle properties. Various groups in the world have built flux-transport dynamo-based predictive tools, which nudge the Sun's surface magnetic data and integrated forward in time to forecast the amplitude of the currently ascending cycle 24. Due to different initial conditions and different choices of unknown model-ingredients, predictions can vary; so
Rincon, F; Schekochihin, A A; Valentini, F
2015-01-01
Magnetic fields pervade the entire Universe and, through their dynamical interactions with matter, affect the formation and evolution of astrophysical systems from cosmological to planetary scales. How primordial cosmological seed fields arose and were further amplified to $\\mu$Gauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions and on scales of at least tens of kiloparsecs, is a major theoretical puzzle still largely unconstrained by observations. Extragalactic plasmas are weakly collisional (as opposed to collisional magnetohydrodynamic fluids), and whether magnetic-field growth and its sustainment through an efficient dynamo instability driven by chaotic motions is possible in such plasmas is not known. Fully kinetic numerical simulations of the Vlasov equation in a six-dimensional phase space necessary to answer this question have until recently remained beyond computational capabilities. Here, we show by means of such simulations that magnetic-field a...
Predicting cycle 24 using various dynamo-based tools
Directory of Open Access Journals (Sweden)
M. Dikpati
2008-02-01
Full Text Available Various dynamo-based techniques have been used to predict the mean solar cycle features, namely the amplitude and the timings of onset and peak. All methods use information from previous cycles, including particularly polar fields, drift-speed of the sunspot zone to the equator, and remnant magnetic flux from the decay of active regions. Polar fields predict a low cycle 24, while spot zone migration and remnant flux both lead to predictions of a high cycle 24. These methods both predict delayed onset for cycle 24. We will describe how each of these methods relates to dynamo processes. We will present the latest results from our flux-transport dynamo, including some sensitivity tests and how our model relates to polar fields and spot zone drift methods.
Forecasting the solar activity cycle: new insights
Nandy, Dibyendu; Karak, Bidya Binay
2013-01-01
Having advanced knowledge of solar activity is important because the Sun's magnetic output governs space weather and impacts technologies reliant on space. However, the irregular nature of the solar cycle makes solar activity predictions a challenging task. This is best achieved through appropriately constrained solar dynamo simulations and as such the first step towards predictions is to understand the underlying physics of the solar dynamo mechanism. In Babcock-Leighton type dynamo models, ...
Solar-cycle precursors and predictions
Jiang, Jie
2013-01-01
The sunspot number data during the past 400 years indicates that both the profile and the amplitude of the solar cycle have large variations. Some precursors of the solar cycle were identified aiming to predict the solar cycle. The polar field and the geomagnetic index are two precursors which are received the most attention. The geomagnetic variations during the solar minima are potentially caused by the solar polar field by the connection of the solar open flux. The robust prediction skill of the polar field indicates that the memory of the dynamo process is less than 11 yrs within the framework of the Babcock-Leighton flux transport dynamo. One possible reason to get the short magnetic memory is the high magnetic diffusivity in the convective zone. Our recent studies show that the radial downward pumping is another possible reason. Based upon the mechanism, we well simulate the cycle irregularities during RGO time period. This opens the possibility to set up a standard dynamo based model to predict the sol...
Wang, Z; Barnes, C W; Barnes, D C; Wang, Zhehui; Pariev, Vladimir I.; Barnes, Cris W.; Barnes, Daniel C.
2002-01-01
A new kind of dynamo utilizing flowing laboratory plasmas has been identified. Conversion of plasma kinetic energy to magnetic energy is verified numerically by kinematic dynamo simulations for magnetic Reynolds numbers above 210. As opposed to intrinsically-turbulent liquid-sodium dynamos, the proposed plasma dynamos correspond to laminar flow topology. Modest plasma parameters, 1-20 eV temperatures, 10^{19}-10^{20} m^{-3} densities in 0.3-1.0 m scale-lengths driven by velocities on the order of the Alfven Critical Ionization Velocity (CIV), self-consistently satisfy the conditions needed for the magnetic field amplication. Growth rates for the plasma dynamos are obtained numerically with different geometry and magnetic Reynolds numbers. Magnetic-field-free coaxial plasma guns can be used to sustain the plasma flow and the dynamo.
Wang, Zhehui; Pariev, Vladimir I.; Barnes, Cris W.; Barnes, Daniel C.
2002-01-01
A new kind of dynamo utilizing flowing laboratory plasmas has been identified. Conversion of plasma kinetic energy to magnetic energy is verified numerically by kinematic dynamo simulations for magnetic Reynolds numbers above 210. As opposed to intrinsically-turbulent liquid-sodium dynamos, the proposed plasma dynamos correspond to laminar flow topology. Modest plasma parameters, 1-20 eV temperatures, 10^{19}-10^{20} m^{-3} densities in 0.3-1.0 m scale-lengths driven by velocities on the orde...
Inflows towards active regions and the modulation of the solar cycle: a parameter study
Martin-Belda, David
2016-01-01
Aims: We aim to investigate how converging flows towards active regions affect the surface transport of magnetic flux, as well as their impact on the generation of the Sun's poloidal field. The inflows constitute a potential non-linear mechanism for the saturation of the global dynamo and may contribute to the modulation of the solar cycle in the Babcock-Leighton framework. Methods: We build a surface flux transport code incorporating a parametrized model of the inflows and run simulations spanning several cycles. We carry out a parameter study to assess how the strength and extension of the inflows affect the build-up of the global dipole field. We also perform simulations with different levels of activity to investigate the potential role of the inflows in the saturation of the global dynamo. Results: We find that the interaction of neighbouring active regions can lead to the occasional formation of single-polarity magnetic flux clumps inconsistent with observations. We propose the darkening caused by pores...
Simulations of astrophysical dynamos
Brandenburg, Axel
2010-01-01
Numerical aspects of dynamos in periodic domains are discussed. Modifications of the solutions by numerically motivated alterations of the equations are being reviewed using the examples of magnetic hyperdiffusion and artificial diffusion when advancing the magnetic field in its Euler potential representation. The importance of using integral kernel formulations in mean-field dynamo theory is emphasized in cases where the dynamo growth rate becomes comparable with the inverse turnover time. Finally, the significance of microscopic magnetic Prandtl number in controlling the conversion from kinetic to magnetic energy is highlighted.
Indian Academy of Sciences (India)
Mahendra K Verma; Bidya Binay Karak; Rohit Kumar
2013-12-01
In this paper, we estimate the magnetic Reynolds number of a typical protostar before and after deuterium burning, and claim for the existence of dynamo process in both the phases, because the magnetic Reynolds number of the protostar far exceeds the critical magnetic Reynolds number for dynamo action. Using the equipartition of kinetic and magnetic energies, we estimate the steady-state magnetic field of the protostar to be of the order of kilogauss, which is in good agreement with observations.
Brandenburg, Axel; Tuominen, Ilkka
The traditional -dynamo as a model for the solar cycle has been successful in explaining the butterfly diagram, phase relations between poloidal and toroidal field, and polar branch migration features. Observational and theoretical achievements in recent years have however shaken this picture. The current trend is towards dynamos operating in the overshoot region of the convection zone. Nevertheless, there are many open questions and a consistent picture has not been established. In this paper we compare recent approaches and discuss remaining problems.
Strong Field Spherical Dynamos
Dormy, Emmanuel
2014-01-01
Numerical models of the geodynamo are usually classified in two categories: those denominated dipolar modes, observed when the inertial term is small enough, and multipolar fluctuating dynamos, for stronger forcing. I show that a third dynamo branch corresponding to a dominant force balance between the Coriolis force and the Lorentz force can be produced numerically. This force balance is usually referred to as the strong field limit. This solution co-exists with the often described viscous branch. Direct numerical simulations exhibit a transition from a weak-field dynamo branch, in which viscous effects set the dominant length scale, and the strong field branch in which viscous and inertial effects are largely negligible. These results indicate that a distinguished limit needs to be sought to produce numerical models relevant to the geodynamo and that the usual approach of minimizing the magnetic Prandtl number (ratio of the fluid kinematic viscosity to its magnetic diffusivity) at a given Ekman number is mi...
Jiang, J; Schuessler, M
2014-01-01
The tilt angles of sunspot groups represent the poloidal field source in Babcock-Leighton-type models of the solar dynamo and are crucial for the build-up and reversals of the polar fields in Surface Flux Transport (SFT) simulations. The evolution of the polar field is a consequence of Hale's polarity rules, together with the tilt angle distribution which has a systematic component (Joy's law) and a random component (tilt-angle scatter). We determine the scatter using the observed tilt angle data and study the effects of this scatter on the evolution of the solar surface field using SFT simulations with flux input based upon the recorded sunspot groups. The tilt angle scatter is described in our simulations by a random component according to the observed distributions for different ranges of sunspot group size (total umbral area). By performing simulations with a number of different realizations of the scatter we study the effect of the tilt angle scatter on the global magnetic field, especially on the evolut...
McWilliams, James C
2011-01-01
A quasi-linear theory is presented for how randomly forced, barotropic velocity fluctuations cause an exponentially-growing, large-scale (mean) magnetic dynamo in the presence of a uniform shear flow, $\\vec{U} = S x \\vec{e}_y$. It is a "kinematic" theory for the growth of the mean magnetic energy from a small initial seed, neglecting the saturation effects of the Lorentz force. The quasi-linear approximation is most broadly justifiable by its correspondence with computational solutions of nonlinear magneto-hydrodynamics, and it is rigorously derived in the limit of large resistivity, $\\eta \\rightarrow \\infty$. Dynamo action occurs even without mean helicity in the forcing or flow, but random helicity variance is then essential. In a sufficiently large domain and with small wavenumber $k_z$ in the direction perpendicular to the mean shearing plane, a positive exponential growth rate $\\gamma$ can occur for arbitrary values of $\\eta$, the viscosity $\
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.
Parker's dynamo and geomagnetic reversals
Reshetnyak, M
2011-01-01
Fluctuations of the alpha-effect which break equatorial symmetry of the flow in the kinematic Parker's dynamo are considered. We show, that even small (a few percents) fluctuation can leed to the substantial assymmetry of the magnetic field in the hemispheres as well as the propagation of the dynamo wave through the equator plane. We also consider how change of the dynamo number can be used to explain different regimes of magnetic field generation in geodynamo.
F. H. Busse; Simitev, R.
2009-01-01
Possibilities and difficulties of applying the theory of magnetic field generation by convection flows in rotating spherical fluid shells to the Giant Planets are outlined. Recent progress in the understanding of the distribution of electrical conductivity in the Giant Planets suggests that the dynamo process occurs predominantly in regions of semiconductivity. In contrast to the geodynamo the magnetic field generation in the Giant Planets is thus characterized by strong radial conductivity v...
Cattaneo, F.
2000-05-01
Magnetic activity on the Sun presents us with an interesting dichotomy. On large spatial and temporal scales the solar magnetic field displays a remarkable degree of organization. The 11 years cadence of the solar cycle, Hales' polarity law, and the systematic drift of the regions of emergence of active regions towards the equator throughout the solar cycle are all indicative of a powerful organizing process. On small spatial and temporal scales, the Solar magnetic field appears random and chaotic. It is interesting that recent advances in dynamo theory provide us with a unified approach to solar magnetic activity whereby both large and small scales emerge naturally as dynamo processes associated with rotationally constrained and unconstrained scales of motions in the convection zone (or directly below it). According to this view all coherent scales of motions produce magnetic structures of comparable coherence length. Those that are further endowed with lack of reflectional symmetry by virtue of being rotationally constrained are further associated with inverse cascades that can generate magnetic structures on larger scales still. The picture that emerges is one in which dynamo action proceeds on different time scales all over the convection zone. But only in very special regions, like for instance the solar tachocline, is the magnetic field organized on large scales. This idea provides a natural explanation for the origin of active regions, ephemeral regions, and intra--network fields.
Magnetized Turbulent Dynamo in Protogalaxies
Energy Technology Data Exchange (ETDEWEB)
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
International Nuclear Information System (INIS)
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
Mcwilliams, James C.
2011-01-01
A quasi-linear theory is presented for how randomly forced, barotropic velocity fluctuations cause an exponentially-growing, large-scale (mean) magnetic dynamo in the presence of a uniform shear flow, $\\vec{U} = S x \\vec{e}_y$. It is a "kinematic" theory for the growth of the mean magnetic energy from a small initial seed, neglecting the saturation effects of the Lorentz force. The quasi-linear approximation is most broadly justifiable by its correspondence with computational solutions of non...
The Kinematic Theory of Solar Dynamo
Institute of Scientific and Technical Information of China (English)
无
2003-01-01
Generation of the Sun's magnetic fields by self-inductive processes inthe solar electrically conducting interior, the solar dynamo theory, is a fundamen-tally important subject in astrophysics. The kinematic dynamo theory concernshow the magnetic fields are produced by kinematically possible flows without beingconstrained by the dynamic equation. We review a number of basic aspects of thekinematic dynamo theory, including the magnetohydrodynamic approximation forthe dynamo equation, the impossibility of dynamo action with the solar differentialrotation, the Cowling's anti-dynamo theorem in the solar context, the turbulent al-pha effect and recently constructed three-dimensional interface dynamos controlledby the solar tachocline at the base of the convection zone.
Thermally driven hydromagnetic dynamos
International Nuclear Information System (INIS)
There are still many challenges to be overcome before we can claim to have a full understanding of the generation of the Earth's magnetic field. From a mathematical point of view, the governing equations are nonlinear and must be solved in fully three dimensions, meaning that a numerical method must be employed, although this would probably also be the case for a two-dimensional problem. However, it is only relatively recently that the computer technology has become available to make this possible. Obtaining these solutions remains a highly computationally intensive task, making it difficult to find solutions for a range of parameter values. This is extremely important as a great deal of uncertainty still surrounds the present (and past) geophysical values of the main parameters in the governing equations. Our aim is to try and further understanding of the effect of varying some of these key parameters in simplified, but fully self-consistent hydromagnetic dynamo models. These models will allow us to examine the effect of including the full inertial term to the equations, which has in the past been neglected due to the small geophysical value of the parameter which controls its effect. Further physical insight into the magnetic field generation mechanism will be provided, and we will examine some key issues in numerical dynamo modelling
Effects of anisotropy of turbulent convection in mean-field solar dynamo models
Pipin, V V
2013-01-01
We study how anisotropy of turbulent convection affects diffusion of large-scale magnetic fields and the dynamo process on the Sun. The effect of anisotropy is calculated in a mean-field magneto-hydrodynamics framework using the minimal $\\tau$-approximation. We examine two types of mean-field dynamo models: the well-known benchmark flux-transport model, and a distributed-dynamo model with the subsurface rotational shear layer. For both models we investigate effects of the double-cell meridional circulation, recently suggested by helioseismology. We introduce a parameter of anisotropy as a ratio of the radial and horizontal intensity of turbulent mixing, to characterize the anisotropy effects. It is found that the anisotropy of turbulent convection affects the distribution of magnetic fields inside the convection zone. The concentration of the magnetic flux near the bottom and top boundaries of the convection zone is greater when the anisotropy is stronger. It is shown that the critical dynamo number and the d...
Institute of Scientific and Technical Information of China (English)
无
2002-01-01
A new dynamo model based on the polarization of plasma is presented in this paper.From the Maxwell equations in a moving medium, a magnetization vector can be causedwith Rongon current. The steady solar magnetic field is solved from the equations. Onthe assumption that the meridianal flow is ignored, the distribution of magnetic field isput out. In the model, there is no additional parameter considered. The intensity ofmagnetic field inside the sun ranges from 1-6T. The surface magnetic field around thepole is in the order of 1×10-3T, at low latitude the calculated surface magnetic fieldhas the order of 1×10-2 T. The maximum magnetic field is around 30° in latitude.
Egeland, Ricky; Soon, Willie H.; Baliunas, Sallie L.; Hall, Jeffrey C.; Pevtsov, Alexei A.; Henry, Gregory W.
2016-05-01
We present composite time series of Ca II H & K line core emission indices of up to 50 years in length for a set of 27 solar-analog stars (spectral types G0-G5; within ~10% of the solar mass) and the Sun. These unique data are available thanks to the long-term dedicated efforts of the Mount Wilson Observatory HK project, the Lowell Observatory Solar-Stellar Spectrograph, and the National Solar Observatory/Air Force Research Laboratory/Sacremento Peak K-line monitoring program. The Ca II H & K emission originates in the lower chromosphere and is strongly correlated with the presence of magnetic plage regions in the Sun. These synoptic observations allow us to trace the patterns long-term magnetic variability and explore dynamo behavior over a wide range of rotation regimes and stellar evolution timescales.
Scaling laws of turbulent dynamos
Fauve, Stephan; Petrelis, Francois
2007-01-01
We consider magnetic fields generated by homogeneous isotropic and parity invariant turbulent flows. We show that simple scaling laws for dynamo threshold, magnetic energy and Ohmic dissipation can be obtained depending on the value of the magnetic Prandtl number.
Jones, C. A.
2001-12-01
There has been significant progress in the development of numerical geodynamo models over the last eight years. Advances in computer technology have made it possible to perform three-dimensional simulations, with thermal or compositional convection as the driving mechanism. These numerical simulations give reasonable results for the morphology and strength of the field at the core-mantle boundary, and the models are also capable of giving reversals and excursions which can be compared with paleomagnetic observations. Some useful constraints are obtained by considering the entropy balance and the ohmic dissipation. However, recent studies of plane layer dynamos suggest that the current generation of dynamo models have not yet reached the correct dynamical regime. A rather severe test of how well we understand the geodynamo comes when we try to apply the theory to the magnetic fields of stars and other planets. It becomes clear that not all dynamos are in the same dynamical regime. Some, like the Earth, are in magnetostrophic balance; others like the Sun, are not. Some are in a strong field regime with Elsasser number of order one, others (including some planetary dynamos) are not. Even within late type stars, the rotation rate strongly affects the dynamical regime that the dynamo operates in. The prospects for classifying the various type of convection driven dynamo, by elucidating the possible dynamical regimes, will be reviewed.
Tsunami: ocean dynamo generator.
Sugioka, Hiroko; Hamano, Yozo; Baba, Kiyoshi; Kasaya, Takafumi; Tada, Noriko; Suetsugu, Daisuke
2014-01-01
Secondary magnetic fields are induced by the flow of electrically conducting seawater through the Earth's primary magnetic field ('ocean dynamo effect'), and hence it has long been speculated that tsunami flows should produce measurable magnetic field perturbations, although the signal-to-noise ratio would be small because of the influence of the solar magnetic fields. Here, we report on the detection of deep-seafloor electromagnetic perturbations of 10-micron-order induced by a tsunami, which propagated through a seafloor electromagnetometer array network. The observed data extracted tsunami characteristics, including the direction and velocity of propagation as well as sea-level change, first to verify the induction theory. Presently, offshore observation systems for the early forecasting of tsunami are based on the sea-level measurement by seafloor pressure gauges. In terms of tsunami forecasting accuracy, the integration of vectored electromagnetic measurements into existing scalar observation systems would represent a substantial improvement in the performance of tsunami early-warning systems. PMID:24399356
Ayres, Thomas R.
2016-04-01
The Dynamo Clinical Trial evaluates long-term stellar magnetic health through periodic X-ray examinations (by the Chandra Observatory). So far, there are only three subjects enrolled in the DTC: Alpha Centauri A (a solar-like G dwarf), Alpha Cen B (an early K dwarf, more active than the Sun), and Alpha Canis Majoris A (Procyon, a mid-F subgiant similar in activity to the Sun). Of these, Procyon is a new candidate, so it is too early to judge how it will fare. Of the other two, Alpha Cen B has responded well, with a steady magnetic heartbeat of about 8 years duration. The sickest of the bunch, Alpha Cen A, was in magnetic cardiac arrest during 2005-2010, but has begun responding to treatment in recent years, and seems to be successfully cycling again, perhaps achieving a new peak of magnetic health in the 2016 time frame. If this is the case, it has been 20 years since A's last healthful peak, significantly longer than the middle-aged Sun's 11-year magnetic heartbeat, but perhaps in line with Alpha Cen A's more senescent state (in terms of "relative evolutionary age," apparently an important driver of activity). (By the way, don't miss the exciting movie of the Alpha Cen stars' 20-year X-ray dance.)
Dynamical quenching with non-local alpha and downward pumping
Brandenburg, A; Käpylä, P J
2014-01-01
In light of new results, the one-dimensional mean-field dynamo model of Brandenburg & Kapyla (2007) with dynamical quenching and a nonlocal Babcock-Leighton alpha effect is re-examined for the solar dynamo. We extend the one-dimensional model to include the effects of turbulent downward pumping (Kitchatinov & Olemskoy 2011), and to combine dynamical quenching with shear. We use both the conventional dynamical quenching model of Kleeorin & Ruzmaikin (1982) and the alternate one of Hubbard & Brandenburg (2011), and confirm that with varying levels of non-locality in the alpha effect, and possibly shear as well, the saturation field strength can be independent of the magnetic Reynolds number.
Inverse dynamo problem in a cylinder
Czech Academy of Sciences Publication Activity Database
Šimkanin, Ján; Tilgner, A.
2008-01-01
Roč. 102, č. 2 (2008), s. 205-215. ISSN 0309-1929 R&D Projects: GA AV ČR IAA300120704; GA ČR GP205/04/P182 Grant ostatní: INTAS Foundation(CH) 03-51-5807 Institutional research plan: CEZ:AV0Z30120515 Keywords : inverse dynamo problem * invisible dynamo * kinematic dynamo * helical flows * Ponomarenko dynamo Subject RIV: DE - Earth Magnetism, Geodesy, Geography Impact factor: 1.560, year: 2008
Faraday's first dynamo: A retrospective
Smith, Glenn S.
2013-12-01
In the early 1830s, Michael Faraday performed his seminal experimental research on electromagnetic induction, in which he created the first electric dynamo—a machine for continuously converting rotational mechanical energy into electrical energy. His machine was a conducting disc, rotating between the poles of a permanent magnet, with the voltage/current obtained from brushes contacting the disc. In his first dynamo, the magnetic field was asymmetric with respect to the axis of the disc. This is to be contrasted with some of his later symmetric designs, which are the ones almost invariably discussed in textbooks on electromagnetism. In this paper, a theoretical analysis is developed for Faraday's first dynamo. From this analysis, the eddy currents in the disc and the open-circuit voltage for arbitrary positioning of the brushes are determined. The approximate analysis is verified by comparing theoretical results with measurements made on an experimental recreation of the dynamo. Quantitative results from the analysis are used to elucidate Faraday's qualitative observations, from which he learned so much about electromagnetic induction. For the asymmetric design, the eddy currents in the disc dissipate energy that makes the dynamo inefficient, prohibiting its use as a practical generator of electric power. Faraday's experiments with his first dynamo provided valuable insight into electromagnetic induction, and this insight was quickly used by others to design practical generators.
On the saturation of astrophysical dynamos
DEFF Research Database (Denmark)
Dorch, Bertil; Archontis, Vasilis
2004-01-01
In the context of astrophysical dynamos we illustrate that the no-cosines flow, with zero mean helicity, can drive fast dynamo action and we study the dynamo's mode of operation during both the linear and non-linear saturation regimes. It turns out that in addition to a high growth rate in the...
Cyclic Evolution of Coronal Fields from a Coupled Dynamo Potential-Field Source-Surface Model
Dikpati, Mausumi; Suresh, Akshaya; Burkepile, Joan
2016-02-01
The structure of the Sun's corona varies with the solar-cycle phase, from a near spherical symmetry at solar maximum to an axial dipole at solar minimum. It is widely accepted that the large-scale coronal structure is governed by magnetic fields that are most likely generated by dynamo action in the solar interior. In order to understand the variation in coronal structure, we couple a potential-field source-surface model with a cyclic dynamo model. In this coupled model, the magnetic field inside the convection zone is governed by the dynamo equation; these dynamo-generated fields are extended from the photosphere to the corona using a potential-field source-surface model. Assuming axisymmetry, we take linear combinations of associated Legendre polynomials that match the more complex coronal structures. Choosing images of the global corona from the Mauna Loa Solar Observatory at each Carrington rotation over half a cycle (1986 - 1991), we compute the coefficients of the associated Legendre polynomials up to degree eight and compare with observations. We show that at minimum the dipole term dominates, but it fades as the cycle progresses; higher-order multipolar terms begin to dominate. The amplitudes of these terms are not exactly the same for the two limbs, indicating that there is a longitude dependence. While both the 1986 and the 1996 minimum coronas were dipolar, the minimum in 2008 was unusual, since there was a substantial departure from a dipole. We investigate the physical cause of this departure by including a North-South asymmetry in the surface source of the magnetic fields in our flux-transport dynamo model, and find that this asymmetry could be one of the reasons for departure from the dipole in the 2008 minimum.
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 ...
Magnetic Helicity Conservation and Astrophysical Dynamos
Vishniac, Ethan T.; Cho, Jungyeon
2000-01-01
We construct a magnetic helicity conserving dynamo theory which incorporates a calculated magnetic helicity current. In this model the fluid helicity plays a small role in large scale magnetic field generation. Instead, the dynamo process is dominated by a new quantity, derived from asymmetries in the second derivative of the velocity correlation function, closely related to the `twist and fold' dynamo model. The turbulent damping term is, as expected, almost unchanged. Numerical simulations ...
Numerical dynamo action in cylindrical containers
Nore, Caroline; Castanon Quiroz, Daniel; Guermond, Jean-Luc; Léorat, Jacques; Luddens, Francky
2015-06-01
The purpose of this paper is to present results from numerical simulations of dynamo action in relation with two magnetohydrodynamics (MHD) experiments using liquid sodium in cylindrical containers. The first one is the von Kármán sodium (VKS) experiment from Cadarache (France), the second one is a precession-driven dynamo experiment from the DREsden sodium facility for DYNamo and thermohydraulic studies (DRESDYN). Contribution to the topical issue "Electrical Engineering Symposium (SGE 2014) - Elected submissions", edited by Adel Razek
Formisano, M.; Federico, C.; De Angelis, S.; De Sanctis, M. C.; Magni, G.
2016-05-01
A recent study of Fu et al. analysed the remaining magnetization in the eucrite meteorite Allan Hills A81001, which mostly likely has been produced during the cooling phase of the life of the asteroid Vesta, arguing that an ancient dynamo in the advective liquid metallic core could be set in. Using petrographic and paleomagnetic arguments, Fu et al. estimated a surface magnetic field of at least 2 μT. In this work, we verify the possibility that an early core dynamo took place in Vesta by analysing four different possible fully differentiated configurations of Vesta, characterized by different chondritic compositions, with the constraints on core size and density provided by Ermakov et al. We only incorporate the thermal convection, by neglecting the effects of the compositional convection, so our results in terms of magnetic Reynolds number and duration of the dynamo can be interpreted as a lower bound. The presence of a magnetic field would make Vesta a peculiar object of the Solar system, a `small-Earth', since it has also a differentiated structure like Earth and the magnetic field has preserved Vesta from the space weathering.
Forecasting the solar activity cycle: new insights
Nandy, Dibyendu
2013-01-01
Having advanced knowledge of solar activity is important because the Sun's magnetic output governs space weather and impacts technologies reliant on space. However, the irregular nature of the solar cycle makes solar activity predictions a challenging task. This is best achieved through appropriately constrained solar dynamo simulations and as such the first step towards predictions is to understand the underlying physics of the solar dynamo mechanism. In Babcock-Leighton type dynamo models, the poloidal field is generated near the solar surface whereas the toroidal field is generated in the solar interior. Therefore a finite time is necessary for the coupling of the spatially segregated source layers of the dynamo. This time delay introduces a memory in the dynamo mechanism which allows forecasting of future solar activity. Here we discuss how this forecasting ability of the solar cycle is affected by downward turbulent pumping of magnetic flux. With significant turbulent pumping the memory of the dynamo is ...
Magnetized Turbulent Dynamo in Protogalaxies; TOPICAL
International Nuclear Information System (INIS)
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
Searching invisible helical dynamos in a cylinder
Czech Academy of Sciences Publication Activity Database
Šimkanin, Ján; Tilgner, A.
2005-01-01
Roč. 153, 1/3 (2005), s. 101-107. ISSN 0031-9201 R&D Projects: GA ČR GP205/04/P182 Institutional research plan: CEZ:AV0Z30120515 Keywords : invisible dynamo * inverse dynamo problem * helical flows Subject RIV: DE - Earth Magnetism, Geodesy, Geography Impact factor: 2.420, year: 2005
Magnetic Helicity Conservation and Astrophysical Dynamos
Vishniac, E T; Vishniac, Ethan T.; Cho, Jungyeon
2000-01-01
We construct a magnetic helicity conserving dynamo theory which incorporates a calculated magnetic helicity current. In this model the fluid helicity plays a small role in large scale magnetic field generation. Instead, the dynamo process is dominated by a new quantity, derived from asymmetries in the second derivative of the velocity correlation function, closely related to the `twist and fold' dynamo model. The turbulent damping term is, as expected, almost unchanged. Numerical simulations with a spatially constant fluid helicity and vanishing resistivity are not expected to generate large scale fields in equipartition with the turbulent energy density. In fact, there seems to be little prospect for driving a fast dynamo in a closed box containing homogeneous turbulence. On the other hand, there is an efficient analog to the $\\alpha-\\Omega$ dynamo. Systems whose turbulence is driven by some anisotropic local instability in shearing flow, like real stars and accretion disks, and some computer simulations, ma...
A shell model for turbulent dynamos
Nigro, G.; Perrone, D.; Veltri, P.
2011-06-01
A self-consistent nonlinear dynamo model is presented. The nonlinear behavior of the plasma at small scale is described by using a MHD shell model for fields fluctuations; this allow us to study the dynamo problem in a large parameter regime which characterizes the dynamo phenomenon in many natural systems and which is beyond the power of supercomputers at today. The model is able to reproduce dynamical situations in which the system can undergo transactions to different dynamo regimes. In one of these the large-scale magnetic field jumps between two states reproducing the magnetic polarity reversals. From the analysis of long time series of reversals we infer results about the statistics of persistence times, revealing the presence of hidden long-time correlations in the chaotic dynamo process.
Mechanically-forced dynamos (Invited)
Le Bars, M.
2013-12-01
It is a commonly accepted hypothesis that convection is responsible for planetary dynamos. However, the validity of the convective dynamo model can be questioned in various planets and moons as well as in asteroids, where the constraints from thermal evolution and compositional core models are sometimes difficult to reconcile with available data from paleomagnetism and in situ measurements. Over the last few years, researches have thus been pursued to find alternative mechanisms for sustaining intense three-dimensional motions in liquid cores, a necessary ingredient for planetary dynamo. In particular, mechanical forcings driven by libration, precession, nutation and tides, have received a renewed interest, following the first studies by Malkus in the 60's. A huge reservoir of energy is available in the rotational and orbital motions of all planetary systems. If planetary bodies were completely rigid and rotating at a constant spin rate, their fluid layers in the absence of convection would also behave rigidly and follow the spin of their boundaries. But small periodic perturbations of the shape of the core/mantle boundary (i.e. dynamic tides) and/or small periodic perturbations of the direction of the spin vector (i.e. precession and nutation) and/or small periodic perturbations of the spin rate (i.e. libration) systematically perturb this rigid state. Then, each of these small perturbations is capable of triggering instabilities in fluid layers, conveying energy from the spin and orbital motions to drive intense three-dimensional flows in the liquid cores. With the view to establish a general framework for planetary applications, I will present here the basic physical ingredients of these instabilities, which involve a resonance between the considered mechanical forcing and two inertial waves of the core. I will then review the numerical and experimental validations of this generic principle, and the few magnetohydrodynamic validations of their dynamo capacity
Cross helicity and related dynamo
Yokoi, Nobumitsu
2013-01-01
The turbulent cross helicity is directly related to the coupling coefficients for the mean vorticity in the electromotive force and for the mean magnetic-field strain in the Reynolds stress tensor. This suggests that the cross-helicity effects are important in the cases where global inhomogeneous flow and magnetic-field structures are present. Since such large-scale structures are ubiquitous in geo/astrophysical phenomena, the cross-helicity effect is expected to play an important role in geo/astrophysical flows. In the presence of turbulent cross helicity, the mean vortical motion contributes to the turbulent electromotive force. Magnetic-field generation due to this effect is called the cross-helicity dynamo. Several features of the cross-helicity dynamo are introduced. Unlike the case in the helicity or $\\alpha$ effect, where ${\\bf{J}}$ is aligned with ${\\bf{B}}$ in the turbulent electromotive force, we in general have a finite mean-field Lorentz force ${\\bf{J}} \\times {\\bf{B}}$ in the cross-helicity dynam...
Magnetic Flux Transport and the Long-Term Evolution of Solar Active Regions
Ugarte-Urra, Ignacio; Warren, Harry P; Hathaway, David H
2015-01-01
With multiple vantage points around the Sun, STEREO and SDO imaging observations provide a unique opportunity to view the solar surface continuously. We use He II 304 A data from these observatories to isolate and track ten active regions and study their long-term evolution. We find that active regions typically follow a standard pattern of emergence over several days followed by a slower decay that is proportional in time to the peak intensity in the region. Since STEREO does not make direct observations of the magnetic field, we employ a flux-luminosity relationship to infer the total unsigned magnetic flux evolution. To investigate this magnetic flux decay over several rotations we use a surface flux transport model, the Advective Flux Transport (AFT) model, that simulates convective flows using a time-varying velocity field and find that the model provides realistic predictions when information about the active region's magnetic field strength and distribution at peak flux is available. Finally, we illust...
Magnetic Flux Transport and the Long-term Evolution of Solar Active Regions
Ugarte-Urra, Ignacio; Upton, Lisa; Warren, Harry P.; Hathaway, David H.
2015-12-01
With multiple vantage points around the Sun, Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory imaging observations provide a unique opportunity to view the solar surface continuously. We use He ii 304 Å data from these observatories to isolate and track ten active regions and study their long-term evolution. We find that active regions typically follow a standard pattern of emergence over several days followed by a slower decay that is proportional in time to the peak intensity in the region. Since STEREO does not make direct observations of the magnetic field, we employ a flux-luminosity relationship to infer the total unsigned magnetic flux evolution. To investigate this magnetic flux decay over several rotations we use a surface flux transport model, the Advective Flux Transport model, that simulates convective flows using a time-varying velocity field and find that the model provides realistic predictions when information about the active region's magnetic field strength and distribution at peak flux is available. Finally, we illustrate how 304 Å images can be used as a proxy for magnetic flux measurements when magnetic field data is not accessible.
Nonlinear MHD dynamo operating at equipartition
DEFF Research Database (Denmark)
Archontis, V.; Dorch, Bertil; Nordlund, Åke
2007-01-01
Context.We present results from non linear MHD dynamo experiments with a three-dimensional steady and smooth flow that drives fast dynamo action in the kinematic regime. In the saturation regime, the system yields strong magnetic fields, which undergo transitions between an energy-equipartition a......Context.We present results from non linear MHD dynamo experiments with a three-dimensional steady and smooth flow that drives fast dynamo action in the kinematic regime. In the saturation regime, the system yields strong magnetic fields, which undergo transitions between an energy......-equipartition and a turbulent state. The generation and evolution of such strong magnetic fields is relevant for the understanding of dynamo action that occurs in stars and other astrophysical objects. Aims.We study the mode of operation of this dynamo, in the linear and non-linear saturation regimes. We also consider...... the effect of varying the magnetic and fluid Reymolds number on the non-linear behaviour of the system. Methods.We perform three-dimensional non-linear MHD simulations and visualization using a high resolution numerical scheme. Results.We find that this dynamo has a high growth rate in the linear regime...
Numerical Simulations of Boundary-Driven Dynamos
White, K.; Brummell, N.; Glatzmaier, G. A.
2012-12-01
An important topic of physics research is how magnetic fields are generated and maintained in the many astrophysical bodies where they are ubiquitously observed. Of particular interest, are reversals of magnetic fields of planets and stars, especially those of the Earth and the Sun. In an attempt to provide intuition on this problem, numerous physical dynamo experiments have been performed in different configurations. Recently, a tremendous breakthrough was made in the Von Karman sodium (VKS) experiments in France when the most realistic laboratory fluid dynamo to date was produced by driving an unconstrained flow in a cylinder of liquid sodium (Monchaux et al, 2007, PRL). One of the curiosities of the VKS experiment however is the effect of the composition of the impellers that drive the flow. Steel blades failed to produce a dynamo, but soft iron impellers, which have much higher magnetic permeability, succeeded. The role of the magnetic properties of the boundaries in boundary-driven dynamos is therefore clearly of interest. Kinematic and laminar numerical dynamo simulations (Giesecke et al, 2010, PRL & Gissinger et al, 2008 EPL) have shed some light but turbulent, nonlinear simulations are necessary. Roberts, Glatzmaier & Clune 2010 created a simplified model of the VKS setup by using three-dimensional numerical simulations in a spherical geometry with differential zonal motions of the boundary replacing the driving impellers of the VKS experiment. We have extended these numerical simulations further towards a more complete understanding of such boundary-forced dynamos. In particular, we have examined the effect of the magnetic boundary conditions - changes in the wall thickness, the magnetic permeability, and the electrical conductivity - on the mechanisms responsible for dynamo generation. Enhanced permeability, conductivity and wall thickness all help dynamo action to different degrees. We are further extending our investigations to asymmetric forcing to
Colloquium: Laboratory experiments on hydromagnetic dynamos
International Nuclear Information System (INIS)
Cosmic magnetic fields, including the fields of planets, stars, and galaxies, are believed to be caused by dynamo action in moving electrically conducting fluids. While the theory and numerics of hydromagnetic dynamos have flourished during recent decades, an experimental validation of the effect was missing until recently. We sketch the long history towards a working laboratory dynamo. We report on the first successful experiments at the sodium facilities in Riga and Karlsruhe, and on other experiments which are carried out or planned at various places in the world
Statistical simulation of the magnetorotational dynamo
Energy Technology Data Exchange (ETDEWEB)
Squire, J. [PPPL; Bhattacharjee, A. [PPPL
2014-08-01
We analyze turbulence and dynamo induced by the magnetorotational instability (MRI) using quasi-linear statistical simulation methods. We find that homogenous turbulence is unstable to a large scale dynamo instability, which saturates to an inhomogenous equilibrium with a very strong dependence on the magnetic Prandtl number (Pm). Despite its enormously reduced nonlinearity, the quasi-linear model exhibits the same qualitative scaling of angular momentum transport with Pm as fully nonlinear turbulence. This demonstrates the relationship of recent convergence problems to the large scale dynamo and suggests possible methods for studying astrophysically relevant regimes at very low or high Pm.
Some Recent Developments in Solar Dynamo Theory
Indian Academy of Sciences (India)
Arnab Rai Choudhuri
2006-06-01
We discuss the current status of solar dynamo theory and describe the dynamo model developed by our group. The toroidal magnetic field is generated in the tachocline by the strong differential rotation and rises to the solar surface due to magnetic buoyancy to create active regions. The decay of these active regions at the surface gives rise to the poloidal magnetic field by the Babcock–Leighton mechanism. This poloidal field is advected by the meridional circulation first to high latitudes and then down below to the tachocline. Dynamo models based on these ideas match different aspects of observational data reasonably well.
Dynamo generated by the centrifugal instability
Marcotte, Florence
2016-01-01
We present a new scenario for magnetic field amplification where an electrically conducting fluid is confined in a differentially rotating, spherical shell with thin aspect-ratio. When the angular momentum sufficiently decreases outwards, an hydrodynamic instability develops in the equatorial region, characterised by pairs of counter-rotating toroidal vortices similar to those observed in cylindrical Couette flow. These spherical Taylor-Couette vortices generate a subcritical dynamo magnetic field dominated by non-axisymmetric components. We show that the critical magnetic Reynolds number seems to reach a constant value at large Reynolds number and that the global rotation can strongly decrease the dynamo onset. Our numerical results are understood within the framework of a simple dynamical system, and we propose a low-dimensional model for subcritical dynamo bifurcations. Implications for both laboratory dynamos and astrophysical magnetic fields are finally discussed.
Fast dynamos in weakly ionized gases
Heitsch, Ellen G Zweibel Fabian
2008-01-01
The turnover of interstellar gas on $\\sim 10^9$yr timescales argues for the continuous operation of a galactic dynamo. The conductivity of interstellar gas is so high that the dynamo must be "fast" - i.e. the magnetic field must be amplified at a rate nearly independent of the magnetic diffusivity. Yet, all the fast dynamos so far known - and all direct numerical simulations of interstellar dynamos - yield magnetic power spectra that peak at the resistive scale, while galactic magnetic fields have substantial power on large scales. In this paper we show that in weakly ionized gas the limiting scale may be the ion-neutral decoupling scale, which although still small is many orders of magnitude larger than the resistive scale.
Pallasite paleomagnetism: Quiescence of a core dynamo
Nichols, Claire I. O.; Bryson, James F. J.; Herrero-Albillos, Julia; Kronast, Florian; Nimmo, Francis; Harrison, Richard J.
2016-05-01
Recent paleomagnetic studies of two Main Group pallasites, the Imilac and Esquel, have found evidence for a strong, late-stage magnetic field on the parent body. It has been hypothesized that this magnetic field was generated by a core dynamo, driven by compositional convection during core solidification. Cooling models suggest that the onset of core solidification occurred ∼200 Ma after planetary accretion. Prior to core solidification, a core dynamo may have been generated by thermal convection; however a thermal dynamo is predicted to be short-lived, with a duration of ∼10 Ma to ∼40 Ma after planetary accretion. These models predict, therefore, a period of quiescence between the thermally driven dynamo and the compositionally driven dynamo, when no core dynamo should be active. To test this hypothesis, we have measured the magnetic remanence recorded by the Marjalahti and Brenham pallasites, which based on cooling-rate data locked in any magnetic field signals present ∼95 Ma to ∼135 Ma after planetary accretion, before core solidification began. The cloudy zone, a region of nanoscale tetrataenite islands within a Fe-rich matrix was imaged using X-ray photoemission electron microscopy. The recovered distribution of magnetisation within the cloudy zone suggests that the Marjalahti and Brenham experienced a very weak magnetic field, which may have been induced by a crustal remanence, consistent with the predicted lack of an active core dynamo at this time. We show that the transition from a quiescent period to an active, compositionally driven dynamo has a distinctive paleomagnetic signature, which may be a crucial tool for constraining the time of core solidification on differentiated bodies, including Earth.
Heat flux modulation in domino dynamo model
Reshetnyak, Maxim
2012-01-01
Using domino dynamo model we show how variations of the heat flux at the core-mantle boundary change frequency of geomagnetic field reversals. In fact, we are able to demonstrate effect known from the modern 3D planetary dynamo models using ensemble of the interacting spins, which obey equations of the Langevin-type with a random force. We also consider applications to the giant- planets and offer explanations of some specific episodes of the geomagnetic field in the past.
Two Fluid Dynamo in Reversed Field Pinch
Mirnov, V. V.; Hegna, C. C.; Prager, S. C.; Sovinec, C. R.
2006-10-01
In the Madison Symmetric Torus reversed field pinch experiments, tearing instabilities are observed to generate magnetic field, flow velocity and current density fluctuations that follow a temporally cyclic sawtooth behavior. One of the consequences of these instabilities is the production of dynamos, fluctuation-induced mean electromotive forces in the generalized Ohm's law, that surge during sawtooth crashes. In two-fluid theories, the dynamo is produced from the combination of the MHD (vxB) and Hall (jxB) contributions to Ohm's law. We report new results on the physics of two-fluid dynamos with particular focus on edge-resonant m=0 tearing modes. The two fluid quasilinear theory that was originally derived for a sheared slab [1] is generalized to cylindrical geometry and illuminates the effects of current gradient and field line curvature. The key results are: (1) two fluid effects are important for dynamo through their influence on the phase between the fluctuations; (2) two-fluid theory yields a non-zero flux surface averaged Hall dynamo that is absent in resistive MHD; (3) the two fluid version of the NIMROD code confirms analytic results during the linear stage of the instability but exhibits significant broadening of the Hall dynamo profile on the longer time scales of nonlinear evolution. *Work supported by the USDoE and NSF. [1]V.V.Mirnov, C.C.Hegna, and S.C.Prager, Plasma Physics Report 29, 612 (2003)
Directory of Open Access Journals (Sweden)
R. Nakamura
2009-04-01
Full Text Available We report on the evolution of dipolarization and associated disturbances of the near-Earth current sheet during a substorm on 27 October 2007, based upon Cluster multi-point, multi-scale observations of the night-side plasma sheet at X~−10 R_{E}. Three dipolarization events were observed accompanied by activations on ground magnetograms at 09:07, 09:14, and 09:22 UT. We found that all these events consist of two types of dipolarization signatures: (1 Earthward moving dipolarization pulse, which is accompanied by enhanced rapid Earthward flux transport and is followed by current sheet disturbances with decrease in B_{Z} and enhanced local current density, and subsequent (2 increase in B_{Z} toward a stable level, which is more prominent at Earthward side and evolving tailward. During the 09:07 event, when Cluster was located in a thin current sheet, the dipolarization and fast Earthward flows were also accompanied by further thinning of the current sheet down to a half-thickness of about 1000 km and oscillation in a kink-like mode with a period of ~15 s and propagating duskward. Probable cause of this "flapping current sheet" is shown to be the Earthward high-speed flow. The oscillation ceased as the flow decreased and the field configuration became more dipolar. The later rapid flux transport events at 09:14 and 09:22 UT took place when the field configuration was initially more dipolar and were also associated with B_{Z} disturbance and local current density enhancement, but to a lesser degree. Hence, current sheet disturbances induced by initial dipolarization pulses could differ, depending on the configuration of the current sheet.
Stretch-Twist-Fold and slow filamentary dynamos in liquid sodium Madison Dynamo Experiments
de Andrade, Garcia
2009-01-01
Recently Ricca and Maggione [MHD (2008)] have presented a very simple and interesting model of stretch-twist-fold dynamo in diffusive media based on numerical simulations of Riemannian flux tubes. In this paper we present a yet simpler way of analytically obtaining fast and slow dynamo, generated by by the curvature energy of magnetic filaments in diffusive media. geometrical model for the galactic or accretion disk dynamo in shear flows is presented. In the fast dynamo case it is shown that the absence of stretching leads to the absence of fast dynamos and when torsion of filaments vanishes the dynamo action cannot be support as well. This is the Cowling-Zeldovich theorem for planar flows. Isotropy of the magnetic fields hypothesis is used to compute the fast nature of dynamo. A similar result using non-holonomic Frenet frame has been recently obtained for filamentary dynamos [Garcia de Andrade, AN (2008)]. The stretch-twist-fold (STF) filamented models discussed here may serve to formulate future experiment...
Experimental realization of dynamo action: present status and prospects
Giesecke, Andre; Gundrum, Thomas; Gerbeth, Gunter; Nore, Caroline; Leorat, Jacques
2012-01-01
In the last decades, the experimental study of dynamo action has made great progress. However, after the dynamo experiments in Karlsruhe and Riga, the von-Karman-Sodium (VKS) dynamo is only the third facility that has been able to demonstrate fluid flow driven self-generation of magnetic fields in a laboratory experiment. Further progress in the experimental examination of dynamo action is expected from the planned precession driven dynamo experiment that will be designed in the framework of the liquid sodium facility DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies). In this paper, we briefly present numerical models of the VKS dynamo that demonstrate the close relation between the axisymmetric field observed in that experiment and the soft iron material used for the flow driving impellers. We further show recent results of preparatory water experiments and design studies related to the precession dynamo and delineate the scientific prospects for the final set-up.
Galactic Dynamos and Galactic Winds
Beck, Rainer
2007-01-01
Spiral galaxies host dynamically important magnetic fields which can affect gas flows in the disks and halos. Total magnetic fields in spiral galaxies are strongest (up to 30 \\muG) in the spiral arms where they are mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to 15 \\muG). Faraday rotation measures of radio polarization vectors in the disks of several spiral galaxies reveal large-scale patterns which are signatures of coherent fields generated by a mean-field dynamo. -- Magnetic fields are also observed in radio halos around edge-on galaxies at heights of a few kpc above the disk. Cosmic-ray driven galactic winds transport gas and magnetic fields from the disk into the halo. The magnetic energy density is larger than the thermal energy density, but smaller than the kinetic energy density of the outflow. The orientation of field lines allows to estimate the wind speed and direction. There is no observation ...
Multicolored Dynamos on Toroidal Meshes
Brunetti, Sara; Quattrociocchi, Walter
2010-01-01
Detecting on a graph the presence of the minimum number of nodes (target set) that will be able to "activate" a prescribed number of vertices in the graph is called the target set selection problem (TSS) proposed by Kempe, Kleinberg, and Tardos. In TSS's settings, nodes have two possible states (active or non-active) and the threshold triggering the activation of a node is given by the number of its active neighbors. Dealing with fault tolerance in a majority based system the two possible states are used to denote faulty or non-faulty nodes, and the threshold is given by the state of the majority of neighbors. Here, the major effort was in determining the distribution of initial faults leading the entire system to a faulty behavior. Such an activation pattern, also known as dynamic monopoly (or shortly dynamo), was introduced by Peleg in 1996. In this paper we extend the TSS problem's settings by representing nodes' states with a "multicolored" set. The extended version of the problem can be described as foll...
Convective Dynamo Simulation with a Grand Minimum
Augustson, Kyle C.; Brun, A. S.; Miesch, Mark; Toomre, Juri
2015-01-01
The global-scale dynamo action achieved in a simulation of a Sun-like star rotating at thrice the solar rate is assessed. The 3-D MHD Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity minimization scheme, is employed to capture convection and dynamo processes in this G-type star. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. It is found that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals occurring roughly every 6.2 years. The magnetic energy also rises and falls with a regular period. The magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the processes leading to polarity reversals are more complex, appearing to arise from the interaction of convection with the mean toroidal fields. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation, and potentially to a nonlinear dynamo wave. This simulation also enters a grand minimum lasting roughly 20 years, after which the dynamo recovers its regular polarity cycles.
Turbulent dynamo in a collisionless plasma
Rincon, François; Califano, Francesco; Schekochihin, Alexander A.; Valentini, Francesco
2016-04-01
Magnetic fields pervade the entire universe and affect the formation and evolution of astrophysical systems from cosmological to planetary scales. The generation and dynamical amplification of extragalactic magnetic fields through cosmic times (up to microgauss levels reported in nearby galaxy clusters, near equipartition with kinetic energy of plasma motions, and on scales of at least tens of kiloparsecs) are major puzzles largely unconstrained by observations. A dynamo effect converting kinetic flow energy into magnetic energy is often invoked in that context; however, extragalactic plasmas are weakly collisional (as opposed to magnetohydrodynamic fluids), and whether magnetic field growth and sustainment through an efficient turbulent dynamo instability are possible in such plasmas is not established. Fully kinetic numerical simulations of the Vlasov equation in a 6D-phase space necessary to answer this question have, until recently, remained beyond computational capabilities. Here, we show by means of such simulations that magnetic field amplification by dynamo instability does occur in a stochastically driven, nonrelativistic subsonic flow of initially unmagnetized collisionless plasma. We also find that the dynamo self-accelerates and becomes entangled with kinetic instabilities as magnetization increases. The results suggest that such a plasma dynamo may be realizable in laboratory experiments, support the idea that intracluster medium turbulence may have significantly contributed to the amplification of cluster magnetic fields up to near-equipartition levels on a timescale shorter than the Hubble time, and emphasize the crucial role of multiscale kinetic physics in high-energy astrophysical plasmas.
Current helicity constraints in solar dynamo models
Sokoloff, D; Moss, D; Kleeorin, N; Kuzanyan, K; Rogachevski, I; Gao, Yu; Xu, H
2012-01-01
We investigate to what extent the current helicity distribution observed in solar active regions is compatible with solar dynamo models. We use an advanced 2D mean-field dynamo model with dynamo action largely concentrated near the bottom of the convective zone, and dynamo saturation based on the evolution of the magnetic helicity and algebraic quenching. For comparison, we also studied a more basic 2D mean-field dynamo model with simple algebraic alpha quenching only. Using these numerical models we obtain butterfly diagrams for both the small-scale current helicity and the large-scale magnetic helicity, and compare them with the butterfly diagram for the current helicity in active regions obtained from observations. This comparison shows that the current helicity of active regions, as estimated by $-A \\cdot B$ evaluated at the depth from which the active region arises, resembles the observational data much better than the small-scale current helicity calculated directly from the helicity evolution equation....
Seasonal characteristics of the large-scale moisture flux transport over the Arabian Peninsula
Athar, H.; Ammar, K.
2016-05-01
The relationship between the lower tropospheric (1000 to 850 hPa) large-scale moisture flux transport and the precipitation over the Arabian Peninsula (AP), on a seasonal basis, using the NCEP-NCAR gridded dataset for the 53-year period (1958-2010), is investigated. The lower tropospheric moisture flux divergence occurs due to the Hadley cell-based descending air over the AP, as well as due to the presence of Somali jet in dry season (June to September) for the southern (≤22° N) AP domain, leading to significantly reduced precipitation in the AP. The AP thus acts more as a net transporter of moisture flux from adjacent Sea areas to nearby regions. The North Atlantic Oscillation (NAO) and the Artic Oscillation (AO) climatic indices are found to modulate significantly the net seasonal moisture flux into the AP region animating from the Mediterranean Sea, and the Arabian Sea, both for the northern (≥22° N) and southern AP domains.
Lagrangian coherent structures in a nonlinear dynamo
Rempel, Erico L; Brandenburg, Axel
2010-01-01
Turbulence and chaos play a fundamental role in stellar convective zones through the transport of particles, energy and momentum, and in fast dynamos, through the stretching, twisting and folding of magnetic flux tubes. A particularly revealing way to describe turbulent motions is through the analysis of Lagrangian coherent structures (LCS), which are material lines or surfaces that act as transport barriers in the fluid. We report the detection of Lagrangian coherent structures in helical MHD dynamo simulations with scale separation. Two dynamo regimes, a propagating coherent mean--field regime and an intermittent regime, are identified as the magnetic diffusivity is varied. The sharp contrast between the chaotic tangle of attracting and repelling LCS in both regimes permits a unique analysis of the impact of the magnetic field on the velocity field. An interpretation for the origin of intermittency in the magnetic field evolution is presented.
Dynamos and MHD theory of turbulence suppression
International Nuclear Information System (INIS)
Characteristics of electrically-conducting media are reviewed from the macroscopic viewpoint based on the mean-field magnetohydrodynamics, while being compared with the methodology and knowledge in fluid mechanics. The themes covered in this review range from the generation mechanism of stellar magnetic fields (dynamo) to transport properties in fusion. The primary concern here is to see the characteristics common to these apparently different phenomena, within the framework of the mean-field theory. Owing to the intrinsic limitation of the approach, the present discussions are limited more or less to specific aspects of phenomena. They are supplemented with the reference to theoretical, numerical, and observational approaches intrinsic to each theme. In the description of dynamo phenomena, an emphasis is put on the cross-helicity dynamo. Features common to the stellar magnetic-field generation and the rotational-motion drive in toroidal plasmas are illustrated on this basis. (author)
Numerical simulation of Martian historical dynamo: Impact of the Rayleigh number on the dynamo state
Institute of Scientific and Technical Information of China (English)
WANG TianYuan; KUANG WeiJia; MA ShiZhuang
2009-01-01
The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core.We use the MoSST core dynamics model to simulate the Martian historical dynamo,focusing on the variation of the dynamo states with the Rayleigh number Ra (a non-dimensional parameter describing the buoyancy force in the core).Our numerical results show that the mean field length scale does not vary monotonically with the Rayleigh number,and the field morphology at the core mantle boundary changes with Rayleigh number.In particular,it drifts westward with a speed decreasing with Rayleigh number.
Spectral gaps, inertial manifolds and kinematic dynamos
Energy Technology Data Exchange (ETDEWEB)
Nunez, Manuel [Departamento de Analisis Matematico, Universidad de Valladolid, 47005 Valladolid (Spain)]. E-mail: mnjmhd@am.uva.es
2005-10-17
Inertial manifolds are desirable objects when ones wishes a dynamical process to behave asymptotically as a finite-dimensional ones. Recently [Physica D 194 (2004) 297] these manifolds are constructed for the kinematic dynamo problem with time-periodic velocity. It turns out, however, that the conditions imposed on the fluid velocity to guarantee the existence of inertial manifolds are too demanding, in the sense that they imply that all the solutions tend exponentially to zero. The inertial manifolds are meaningful because they represent different decay rates, but the classical dynamos where the magnetic field is maintained or grows are not covered by this approach, at least until more refined estimates are found.
Mean-field magnetohydrodynamics and dynamo theory
Krause, F
2013-01-01
Mean-Field Magnetohydrodynamics and Dynamo Theory provides a systematic introduction to mean-field magnetohydrodynamics and the dynamo theory, along with the results achieved. Topics covered include turbulence and large-scale structures; general properties of the turbulent electromotive force; homogeneity, isotropy, and mirror symmetry of turbulent fields; and turbulent electromotive force in the case of non-vanishing mean flow. The turbulent electromotive force in the case of rotational mean motion is also considered. This book is comprised of 17 chapters and opens with an overview of the gen
Dynamo theory in the Sun and stars
International Nuclear Information System (INIS)
Dynamo theory, a plausible and successful theory in capturing the essential processes of stellar activity, is discussed, as well as several uncertainties and intrinsic limits. Some alternative or complementary suggestions are also considered. It is stressed that the importance of magnetoconvection and flux tube studies improves our understanding of both large-scale and small-scale interaction of rotation, turbulent convection and magnetic field. Finally, recent models of stellar activity are critically reviewed. It is pointed out that the confrontation with the new stellar data should extend our comprehension of the dynamo operation modes, which probably depend on stellar structure, rotation and age. (Auth.)
Towards a precession driven dynamo experiment
Stefani, F; Gerbeth, G; Giesecke, A; Gundrum, T; Herault, J; Nore, C; Steglich, C
2014-01-01
The most ambitious project within the DREsden Sodium facility for DYNamo and thermohydraulic studies (DRESDYN) at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is the set-up of a precession-driven dynamo experiment. After discussing the scientific background and some results of water pre-experiments and numerical predictions, we focus on the numerous structural and design problems of the machine. We also outline the progress of the building's construction, and the status of some other experiments that are planned in the framework of DRESDYN.
The dynamo basis of solar cycle precursor schemes
Charbonneau, Paul; Barlet, Guillaume
2011-02-01
We investigate the dynamo underpinning of solar cycle precursor schemes based on direct or indirect measures of the solar surface magnetic field. We do so for various types of mean-field-like kinematic axisymmetric dynamo models, where amplitude fluctuations are driven by zero-mean stochastic forcing of the dynamo number controlling the strength of the poloidal source term. In all stochastically forced models considered, the surface poloidal magnetic field is found to have precursor value only if it feeds back into the dynamo loop, which suggests that accurate determination of the magnetic flux budget of the solar polar fields may hold the key to dynamo model-based cycle forecasting.
Modeling astrophysical outflows via the unified Dynamo-Reverse Dynamo mechanism
Lingam, Manasvi
2015-01-01
The unified Dynamo-Reverse Dynamo (Dy-RDy) mechanism, capable of simultaneously generating large scale outflows and magnetic fields from an ambient microscopic reservoir, is explored in a broad astrophysical context. The Dy-RDy mechanism is derived via Hall magnetohydrodynamics, which unifies the evolution of magnetic field and fluid vorticity. It also introduces an intrinsic length scale, the ion skin depth, allowing for the proper normalization and categorization of microscopic and macroscopic scales. The large scale Alfv\\'en Mach number $\\mathcal{M}_{A}$, defining the relative "abundance" of the flow field to the magnetic field is shown to be tied to a microscopic scale length that reflects the characteristics of the ambient short scale reservoir. The dynamo (Dy), preferentially producing the large scale magnetic field, is the dominant mode when the ambient turbulence is mostly kinetic, while the outflow producing reverse dynamo (RDy) is the principal manifestation of a magnetically dominated turbulent res...
Configuration Design of Novel Manually Operated Dynamo Flashlights
Yan, Hong-Sen; Wang, Hsin-Te
This paper synthesizes novel configurations of manually operated dynamo flashlights. Topology and motion characteristics of existing gear dynamos are modified and concluded. The structural sketches and corresponding graph representations for gear trains and dynamos with the defined induced magnetic circuits are defined. Through the concepts of generalization and specialization, the atlas of the structural sketches and graphs of the embedded gear dynamos is obtained subject to the defined design requirements and constraints. And, a systematic approach is proposed to synthesize the novel mechanisms of the embedded gear dynamos. As a result, the embedded three-link and four-link gear dynamos have 12 and 24 novel design configurations, respectively. One prototype of the embedded three-link and another of the embedded four-link gear dynamo are built.
Predicting the Amplitude and Hemispheric Asymmetry of Solar Cycle 25 with Surface Flux Transport
Hathaway, David H.; Upton, Lisa
2016-05-01
Evidence from 40 years of magnetic field measurements, 110 years of polar faculae counts, and 150 years of geomagnetic field measurements, strongly indicates that the strength of the magnetic field at the Sun's poles near the time of a sunspot cycle minimum determines the strength of the following solar activity cycle. The processes that produce these polar fields are well observed and accurately modeled as the transport of magnetic flux (which emerges in active regions) by the horizontal flows in the Sun's near-surface shear layer, i.e. differential rotation, poleward meridional flow, and cellular convective motions. We use our Advective Flux Transport (AFT) code, with flows fully constrained by observations, to simulate the evolution of the Sun's polar magnetic fields from early 2016 to the end of 2019 – near the expected time of Cycle 24/25 minimum. We assimilate active regions from Cycle 14 (107 years earlier) to represent the continued development of Cycle 24. Cycle 14 was similar to Cycle 24 in size, shape, and hemispheric asymmetry. We run a series of simulations in which the uncertain conditions (convective motion details, active region tilt, and meridional flow profile) are varied within expected ranges. We find that the ensemble average of the strength of the polar fields near the end of Cycle 24 is about the same as that measured near the end of Cycle 23, indicating that Cycle 25 will be similar in strength to the current cycle with an expected maximum sunspot number (Version 2.0) of 100±15. In all cases within our ensemble the polar fields are asymmetric with fields in the south stronger than those in the north. After just four years of simulation the variability across our ensemble indicates an uncertainty of about 15%. This stochastic variability, intrinsic to the Sun itself, suggests that we may never be able to reliably predict solar cycles more than one cycle into the future.
Evidence in Magnetic Clouds for Systematic Open Flux Transport on the Sun
Crooker, N. U.; Kahler, S. W.; Gosling, J. T.; Lepping, R. P.
2008-01-01
Most magnetic clouds encountered by spacecraft at 1 AU display a mix of unidirectional suprathermal electrons signaling open field lines and counterstreaming electrons signaling loops connected to the Sun at both ends. Assuming the open fields were originally loops that underwent interchange reconnection with open fields at the Sun, we determine the sense of connectedness of the open fields found in 72 of 97 magnetic clouds identified by the Wind spacecraft in order to obtain information on the location and sense of the reconnection and resulting flux transport at the Sun. The true polarity of the open fields in each magnetic cloud was determined from the direction of the suprathermal electron flow relative to the magnetic field direction. Results indicate that the polarity of all open fields within a given magnetic cloud is the same 89% of the time, implying that interchange reconnection at the Sun most often occurs in only one leg of a flux rope loop, thus transporting open flux in a single direction, from a coronal hole near that leg to the foot point of the opposite leg. This pattern is consistent with the view that interchange reconnection in coronal mass ejections systematically transports an amount of open flux sufficient to reverse the polarity of the heliospheric field through the course of the solar cycle. Using the same electron data, we also find that the fields encountered in magnetic clouds are only a third as likely to be locally inverted as not. While one might expect inversions to be equally as common as not in flux rope coils, consideration of the geometry of spacecraft trajectories relative to the modeled magnetic cloud axes leads us to conclude that the result is reasonable.
Converting DYNAMO simulations to Powersim Studio simulations
Energy Technology Data Exchange (ETDEWEB)
Walker, La Tonya Nicole; Malczynski, Leonard A.
2014-02-01
DYNAMO is a computer program for building and running 'continuous' simulation models. It was developed by the Industrial Dynamics Group at the Massachusetts Institute of Technology for simulating dynamic feedback models of business, economic, and social systems. The history of the system dynamics method since 1957 includes many classic models built in DYANMO. It was not until the late 1980s that software was built to take advantage of the rise of personal computers and graphical user interfaces that DYNAMO was supplanted. There is much learning and insight to be gained from examining the DYANMO models and their accompanying research papers. We believe that it is a worthwhile exercise to convert DYNAMO models to more recent software packages. We have made an attempt to make it easier to turn these models into a more current system dynamics software language, Powersim © Studio produced by Powersim AS^{2} of Bergen, Norway. This guide shows how to convert DYNAMO syntax into Studio syntax.
Magnetic Helicity in a Cyclic Convective Dynamo
Miesch, Mark S.; Zhang, Mei; Augustson, Kyle C.
2016-05-01
Magnetic helicity is a fundamental agent for magnetic self-organization in magnetohydrodynamic (MHD) dynamos. As a conserved quantity in ideal MHD, it establishes a strict topological coupling between large and small-scale magnetic fields. The generation of magnetic fields on scales larger than the velocity field is linked to an upscale transfer of magnetic helicity, either locally in spectral space as in the inverse cascade of magnetic helicity in MHD turbulence or non-locally, as in the turbulent alpha-effect of mean-field dynamo theory. Thus, understanding the generation, transport, and dissipation of magnetic helicity is an essential prerequisite to understanding manifestations of magnetic self-organization in the solar dynamo, including sunspots, the prominent dipole and quadrupole moments, and the 22-year magnetic activity cycle. We investigate the role of magnetic helicity in a convective dynamo model that exhibits regular magnetic cycles. The cycle is marked by coherent bands of toroidal field that persist within the convection zone and that are antisymmetric about the equator. When these toriodal bands interact across the equator, it initiates a global restructuring of the magnetic topology that contributes to the reversal of the dipole moment. Thus, the polar field reversals are preceeded by a brief reversal of the subsurface magnetic helicity. There is some evidence that the Sun may exhibit a similar magnetic helicity reversal prior to its polar field reversals.
Azimuthal dynamo wave in spherical shell convection
Cole, Elizabeth; Mantere, Maarit J; Brandenburg, Axel
2013-01-01
We report the finding of an azimuthal dynamo wave of a low-order (m=1) mode in direct numerical simulations (DNS) of turbulent convection in spherical shells. Such waves are predicted by mean field dynamo theory and have been obtained previously in mean-field models. Observational results both from photometry and Doppler imaging have revealed persistent drifts of spots for several rapidly rotating stars, but, although an azimuthal dynamo wave has been proposed as a possible mechanism responsible for this behavior, it has been judged as unlikely, as practical evidence for such waves from DNS has been lacking. The large-scale magnetic field in our DNS, which is due to self-consistent dynamo action, is dominated by a retrograde m=1 mode. Its pattern speed is nearly independent of latitude and does not reflect the speed of the differential rotation at any depth. The extrema of magnetic m=1 structures coincide reasonably with the maxima of m=2 structures of the temperature. These results provide direct support for...
The RFP dynamo: MHD to kinetic regimes
Sarff, J. S.; Almagri, A. F.; den Hartog, D. J.; McCollam, K. J.; Nornberg, M. D.; Sauppe, J. P.; Sovinec, C. R.; Terry, P. W.; Triana, J. C.; Brower, D. L.; Ding, W. X.; Parke, E.
2015-11-01
The hallmark of magnetic relaxation in an RFP plasma is profile flattening of J0 .B0 /B2 effected by a dynamo-like emf in Ohm's law. This is well-studied in single-fluid MHD, but recent MST results and extended MHD modeling show that both and the Hall emf, - /ene , are important, revealing decoupled electron and ion motion. Since dynamo is current-related, the electron fluid emf, , captures both effects. In MST, the electron flow is dominantly Ve , 1 ~E1 ×B0 /B2 , implying ~ / B . This and the Hall emf are measured in MST for comparison in Ohm's law. A finite-pressure response is also possible, e.g., ``diamagnetic dynamo'', ∇ . /ene , associated with diamagnetic drift, and ``kinetic dynamo'' associated with collisionless streaming of electrons in a stochastic magnetic field. Correlation measurements and using FIR interferometry and Thomson scattering reveal these as small but finite in MST. A kinetic emf might be expected for any high-beta plasma with inhomogeneous pressure. Support by DOE/NSF.
Stochastic flux freezing and magnetic dynamo
International Nuclear Information System (INIS)
Magnetic flux conservation in turbulent plasmas at high magnetic Reynolds numbers is argued neither to hold in the conventional sense nor to be entirely broken, but instead to be valid in a statistical sense associated to the ''spontaneous stochasticity'' of Lagrangian particle trajectories. The latter phenomenon is due to the explosive separation of particles undergoing turbulent Richardson diffusion, which leads to a breakdown of Laplacian determinism for classical dynamics. Empirical evidence is presented for spontaneous stochasticity, including numerical results. A Lagrangian path-integral approach is then exploited to establish stochastic flux freezing for resistive hydromagnetic equations and to argue, based on the properties of Richardson diffusion, that flux conservation must remain stochastic at infinite magnetic Reynolds number. An important application of these results is the kinematic, fluctuation dynamo in nonhelical, incompressible turbulence at magnetic Prandtl number (Prm) equal to unity. Numerical results on the Lagrangian dynamo mechanisms by a stochastic particle method demonstrate a strong similarity between the Prm=1 and 0 dynamos. Stochasticity of field-line motion is an essential ingredient of both. Finally, some consequences for nonlinear magnetohydrodynamic turbulence, dynamo, and reconnection are briefly considered.
New results on an equipartition dynamo
DEFF Research Database (Denmark)
Dorch, S. B. F.; Archontis, V.
2006-01-01
linear and non-linear saturation regimes. The means were 3-d non-linear MHD simulations and visualization using the high resolution numerical scheme by Nordlund, Galsgaard and others. We have found that the dynamo has a high growth rate in the linear regime, and that it can saturate at a level...
HELIOSEISMIC DATA INCLUSION IN SOLAR DYNAMO MODELS
International Nuclear Information System (INIS)
An essential ingredient in kinematic dynamo models of the solar cycle is the internal velocity field within the simulation domain-the solar convection zone (SCZ). In the last decade or so, the field of helioseismology has revolutionized our understanding of this velocity field. In particular, the internal differential rotation of the Sun is now fairly well constrained by helioseismic observations almost throughout the SCZ. Helioseismology also gives us some information about the depth dependence of the meridional circulation in the near-surface layers of the Sun. The typical velocity inputs used in solar dynamo models, however, continue to be an analytic fit to the observed differential rotation profile and a theoretically constructed meridional circulation profile that is made to match the flow speed only at the solar surface. Here, we take the first steps toward the use of more accurate velocity fields in solar dynamo models by presenting methodologies for constructing differential rotation and meridional circulation profiles that more closely conform to the best observational constraints currently available. We also present kinematic dynamo simulations driven by direct helioseismic measurements for the rotation and four plausible profiles for the internal meridional circulation-all of which are made to match the helioseismically inferred near-surface depth dependence, but whose magnitudes are made to vary. We discuss how the results from these dynamo simulations compare with those that are driven by purely analytic fits to the velocity field. Our results and analysis indicate that the latitudinal shear in the rotation in the bulk of the SCZ plays a more important role, than either the tachocline or surface radial shear, in the induction of the toroidal field. We also find that it is the speed of the equatorward counterflow in the meridional circulation right at the base of the SCZ, and not how far into the radiative interior it penetrates, that primarily
Simoniello, R; Jain, K; Hill, F
2016-01-01
The solar cycle onset at mid-latitudes, the slow down of the sunspot drift toward the equator, the tail-like attachment and the overlap of successive cycles at the time of activity minimum are delicate issues in $\\alpha\\Omega$ dynamo wave and flux transport dynamo models. Very different parameter values produce similar results, making it difficult to understand the origin of these solar cycle properties. We use GONG helioseismic data to investigate the progression of the solar cycle as observed in intermediate-degree global $p$-mode frequency shifts at different latitudes and subsurface layers, from the beginning of solar cycle 23 up to the maximum of the current solar cycle. We also analyze those for high-degree modes in each hemisphere obtained through the ring-diagram technique of local helioseismology. The analysis highlighted differences in the progression of the cycle below 15\\degr\\ compared to higher latitudes. While the cycle starts at mid-latitudes and then migrates equatorward/poleward, the sunspot ...
Tidally Driven Dynamos in a Rotating Sphere
Cébron, D.; Hollerbach, R.
2014-07-01
Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star τ-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker & Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.
The precession dynamo experiment at HZDR
Giesecke, A.; Gundrum, T.; Herault, J.; Stefani, F.; Gerbeth, G.
2015-12-01
In a next generation dynamo experiment currently under development atthe Helmholtz-Zentrum Dresden-Rossendorf (HZDR) a fluid flow of liquidsodium, solely driven by precession, will be considered as a possiblesource for magnetic field generation. The experiment is mainlymotivated by alternative concepts for astrophysical dynamos that arebased on mechanical flow driving. For example, it has long beendiscussed whether precession may be a complementary power source forthe geodynamo (Malkus, Science 1968) or for the ancient lunar dynamodue to the Earth-driven precession of the lunar spin axis (Dwyer, Nature 2011).We will present the current state of development of the dynamoexperiment together with results from non-linear hydrodynamicsimulations with moderate precessional forcing. Our simulations reveala non-axisymmetric forced mode with an amplitude of up to one fourthof the rotation velocity of the cylindrical container confirming thatprecession provides a rather efficient flow driving mechanism even atmoderate precession rates.More relevant for dynamo action might be free Kelvin modes (thenatural flow eigenmodes in a rotating cylinder) with higher azimuthalwave number. These modes may become relevant when constituting atriadic resonance with the fundamental forced mode, i.e., when theheight of the container matches their axial wave lengths. We findtriadic resonances at aspect ratios close to those predicted by thelinear theory except around the primary resonance of the forcedmode. In that regime we still identify free Kelvin modes propagatingin retrograde direction but none of them can be assigned to a triade.Our results will enter into the development of flow models that willbe used in kinematic simulations of the electromagnetic inductionequation in order to determine whether a precession driven flow willbe capable to drive a dynamo at all and to limit the parameter spacewithin which the occurrence of dynamo action is most promising.
TIDALLY DRIVEN DYNAMOS IN A ROTATING SPHERE
Energy Technology Data Exchange (ETDEWEB)
Cébron, D.; Hollerbach, R., E-mail: david.cebron@ujf-grenoble.fr, E-mail: r.hollerbach@leeds.ac.uk [Institut für Geophysik, Sonneggstrasse 5, ETH Zürich, Zürich CH-8092 (Switzerland)
2014-07-01
Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star τ-boo, Mars, or the early Moon. By simulating a small local patch of a rotating fluid, Barker and Lithwick have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested in the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation in the bulk of the fluid. This allows us to use an efficient spectral code to solve the magnetohydrodynamic problem. We first compare the hydrodynamic results with theoretical asymptotic results and numerical results obtained in a truly deformed ellipsoid, which confirms the presence of elliptical instability. We then perform magnetohydrodynamic simulations and investigate the dynamo capability of the flow. Kinematic and self-consistent dynamos are finally simulated, showing that the elliptical instability is capable of generating a dipole-dominated large-scale magnetic field in global simulations of a fluid rotating sphere.
Saturation of Zeldovich Stretch-Twist-Fold Map Dynamos
Seta, Amit; Subramanian, Kandaswamy
2014-01-01
Zeldovich's stretch-twist fold (STF) dynamo provided a breakthrough in conceptual understanding of fast dynamos, including fluctuation or small scale dynamos. We study the evolution and saturation behaviour of two types of Baker's map dynamos, which have been used to model Zeldovich's STF dynamo process. Using such maps allows one to analyze dynamos at much higher magnetic Reynolds numbers $R_M$ as compared to direct numerical simulations. In the 2-strip map dynamo there is constant constructive folding while the 4-strip map dynamo also allows the possibility of field reversal. Incorporating a diffusive step parameterised by $R_M$, we find that the magnetic field $B(x)$ is amplified only above a critical $R_M=R_{crit} \\sim 4$ for both types of dynamos. We explore the saturation of these dynamos in 3 ways; by a renormalized decrease of the effective $R_M$ (Case I) or due to a decrease in the efficiency of field amplification by stretching (Case II), or a combination of both effects (Case III). For Case I, we s...
Statistical Mechanics of Turbulent Dynamos
Shebalin, John V.
2014-01-01
Incompressible magnetohydrodynamic (MHD) turbulence and magnetic dynamos, which occur in magnetofluids with large fluid and magnetic Reynolds numbers, will be discussed. When Reynolds numbers are large and energy decays slowly, the distribution of energy with respect to length scale becomes quasi-stationary and MHD turbulence can be described statistically. In the limit of infinite Reynolds numbers, viscosity and resistivity become zero and if these values are used in the MHD equations ab initio, a model system called ideal MHD turbulence results. This model system is typically confined in simple geometries with some form of homogeneous boundary conditions, allowing for velocity and magnetic field to be represented by orthogonal function expansions. One advantage to this is that the coefficients of the expansions form a set of nonlinearly interacting variables whose behavior can be described by equilibrium statistical mechanics, i.e., by a canonical ensemble theory based on the global invariants (energy, cross helicity and magnetic helicity) of ideal MHD turbulence. Another advantage is that truncated expansions provide a finite dynamical system whose time evolution can be numerically simulated to test the predictions of the associated statistical mechanics. If ensemble predictions are the same as time averages, then the system is said to be ergodic; if not, the system is nonergodic. Although it had been implicitly assumed in the early days of ideal MHD statistical theory development that these finite dynamical systems were ergodic, numerical simulations provided sufficient evidence that they were, in fact, nonergodic. Specifically, while canonical ensemble theory predicted that expansion coefficients would be (i) zero-mean random variables with (ii) energy that decreased with length scale, it was found that although (ii) was correct, (i) was not and the expected ergodicity was broken. The exact cause of this broken ergodicity was explained, after much
Double Dynamo Signatures in a Global MHD Simulation and Mean-field Dynamos
Beaudoin, Patrice; Simard, Corinne; Cossette, Jean-François; Charbonneau, Paul
2016-08-01
The 11 year solar activity cycle is the most prominent periodic manifestation of the magnetohydrodynamical (MHD) large-scale dynamo operating in the solar interior, yet longer and shorter (quasi-) periodicities are also present. The so-called “quasi-biennial” signal appearing in many proxies of solar activity has been gaining increasing attention since its detection in p-mode frequency shifts, which suggests a subphotospheric origin. A number of candidate mechanisms have been proposed, including beating between co-existing global dynamo modes, dual dynamos operating in spatially separated regions of the solar interior, and Rossby waves driving short-period oscillations in the large-scale solar magnetic field produced by the 11 year activity cycle. In this article, we analyze a global MHD simulation of solar convection producing regular large-scale magnetic cycles, and detect and characterize shorter periodicities developing therein. By constructing kinematic mean-field α 2Ω dynamo models incorporating the turbulent electromotive force (emf) extracted from that same simulation, we find that dual-dynamo behavior materializes in fairly wide regions of the model’s parameters space. This suggests that the origin of the similar behavior detected in the MHD simulation lies with the joint complexity of the turbulent emf and differential rotation profile, rather that with dynamical interactions such as those mediated by Rossby waves. Analysis of the simulation also reveals that the dual dynamo operating therein leaves a double-period signature in the temperature field, consistent with a dual-period helioseismic signature. Order-of-magnitude estimates for the magnitude of the expected frequency shifts are commensurate with helioseismic measurements. Taken together, our results support the hypothesis that the solar quasi-biennial oscillations are associated with a secondary dynamo process operating in the outer reaches of the solar convection zone.
The New Mexico alpha-omega Dynamo Experiment Modeling Astrophysical Dynamos
Colgate, S A; Beckley, H F; Ferrel, R; Romero, V D; Weatherall, J C
2001-01-01
A magnetic dynamo experiment is under construction at the New Mexico Institute of Mining and Technology. The experiment is designed to demonstrate in the laboratory the alpha-omega magnetic dynamo, which is believed to operate in many rotating and conducting astrophysical objects. The experiment uses the Couette flow of liquid sodium between two cylinders rotating with different angular velocities to model the omega-effect. The alpha-effect is created by the rising and expanding jets of liquid sodium driven through a pair of orifices in the end plates of the cylindrical vessel, presumably simulating plumes driven by buoyancy in astrophysical objects. The water analog of the dynamo device has been constructed and the flow necessary for the dynamo has been demonstrated. Results of the numerical simulations of the kinematic dynamo are presented. The toroidal field produced by the omega-effect is predicted to be B_{\\phi} \\simeq (R_m/2\\pi) B_{poloidal}\\simeq 20 \\times B_{poloidal} for the expected magnetic Reynold...
Dynamos and anti-dynamos as thin magnetic flux ropes in Riemannian spaces
de Andrade, L Garcia
2007-01-01
Two examples of magnetic anti-dynamos in magnetohydrodynamics (MHD) are given. The first is a 3D metric conformally related to Arnold cat fast dynamo metric: ${ds_{A}}^{2}=e^{-{\\lambda}z}dp^{2}+e^{{\\lambda}z}dq^{2}+dz^{2}$ is shown to present a behaviour of non-dynamos where the magnetic field exponentially decay in time. The curvature decay as z-coordinates increases without bounds. Some of the Riemann curvature components such as $R_{pzpz}$ also undergoes dissipation while component $R_{qzqz}$ increases without bounds. The remaining curvature component $R_{pqpq}$ is constant on the torus surface. The other anti-dynamo which may be useful in plasma astrophysics is the thin magnetic flux rope or twisted magnetic thin flux tube which also behaves as anti-dynamo since it also decays with time. This model is based on the Riemannian metric of the magnetic twisted flux tube where the axis possesses Frenet curvature and torsion. Since in this last example the Frenet torsion of the axis of the rope is almost zero, o...
A potential thermal dynamo and its astrophysical applications
Lingam, Manasvi; Mahajan, Swadesh M.
2016-05-01
It is shown that thermal turbulence, not unlike the standard kinetic and magnetic turbulence, can be an effective driver of a mean-field dynamo. In simple models, such as hydrodynamics and magnetohydrodynamics, both vorticity and induction equations can have strong thermal drives that resemble the α and γ effects in conventional dynamo theories; the thermal drives are likely to be dominant in systems that are endowed with subsonic, low-β turbulence. A pure thermal dynamo is quite different from the conventional dynamo in which the same kinetic/magnetic mix in the ambient turbulence can yield a different ratio of macroscopic magnetic/vortical fields. The possible implications of the similarities and differences between the thermal and non-thermal dynamos are discussed. The thermal dynamo is shown to be highly important in the stellar and planetary context, and yields results broadly consistent with other theoretical and experimental approaches.
Magnetic Wreaths and Cycles in Convective Dynamos
Nelson, Nicholas J; Brun, A Sacha; Miesch, Mark S; Toomre, Juri
2012-01-01
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to explore the convective origins of this activity, we have carried out a series of global 3D magnetohydrodynamic (MHD) simulations with the anelastic spherical harmonic (ASH) code. Here we report on the dynamo mechanisms achieved as the effects of artificial diffusion are systematically decreased. The simulations are carried out at a nominal rotation rate of three times the solar value (3$\\Omega_\\odot$), but similar dynamics may also apply to the Sun. Our previous simulations demonstrated that convective dynamos can build persistent toroidal flux structures (magnetic wreaths) in the midst of a turbulent convection zone and that high rotation rates promote the cyclic reversal of these wreaths. Here we demonstrate that magnetic cycles can also be achieved by reducing the diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these more turbulent models, diffusive processes no longer play a significant role in the key dynam...
Invisible dynamo in mean-field models
Reshetnyak, M. Yu.
2016-07-01
The inverse problem in a spherical shell to find the two-dimensional spatial distributions of the α-effect and differential rotation in a mean-field dynamo model has been solved. The derived distributions lead to the generation of a magnetic field concentrated inside the convection zone. The magnetic field is shown to have no time to rise from the region of maximum generation located in the lower layers to the surface in the polarity reversal time due to magnetic diffusion. The ratio of the maximum magnetic energy in the convection zone to its value at the outer boundary reaches two orders of magnitude or more. This result is important in interpreting the observed stellar and planetary magnetic fields. The proposed method of solving the inverse nonlinear dynamo problem is easily adapted for a wide class of mathematical-physics problems.
Differential Rotation in Solar Convective Dynamo Simulations
Fan, Yuhong
2015-01-01
We carry out a magneto-hydrodynamic (MHD) simulation of convective dynamo in the rotating solar convective envelope driven by the solar radiative diffusive heat flux. The simulation is similar to that reported in Fan & Fang (2014) but with further reduced viscosity and magnetic diffusion. The resulting convective dynamo produces a large scale mean field that exhibits similar irregular cyclic behavior and polarity reversals, and self-consistently maintains a solar-like differential rotation. The main driver for the solar-like differential rotation (with faster rotating equator) is a net outward transport of angular momentum away from the rotation axis by the Reynolds stress, and we found that this transport is enhanced with reduced viscosity and magnetic diffusion.
Subcritical dynamo bifurcation in the Taylor Green flow
Ponty, Yannick; Dubrulle, Berengere; Daviaud, François; Pinton, Jean-François
2007-01-01
We report direct numerical simulations of dynamo generation for flow generated using a Taylor-Green forcing. We find that the bifurcation is subcritical, and show its bifurcation diagram. We connect the associated hysteretic behavior with hydrodynamics changes induced by the action of the Lorentz force. We show the geometry of the dynamo magnetic field and discuss how the dynamo transition can be induced when an external field is applied to the flow.
History and results of the Riga dynamo experiments
Gailitis, Agris; Gundrum, Thomas; Lielausis, Olgerts; Platacis, Ernests; Stefani, Frank
2008-01-01
On 11 November 1999, a self-exciting magnetic eigenfield was detected for the first time in the Riga liquid sodium dynamo experiment. We report on the long history leading to this event, and on the subsequent experimental campaigns which provided a wealth of data on the kinematic and the saturated regime of this dynamo. The present state of the theoretical understanding of both regimes is delineated, and some comparisons with other laboratory dynamo experiments are made.
Subcritical dynamo bifurcation in the Taylor Green flow
Ponty, Yannick; Laval, Jean-Phillipe; Dubrulle, Berengere; Daviaud, François; Pinton, Jean-François
2007-01-01
4 pages We report direct numerical simulations of dynamo generation for flow generated using a Taylor-Green forcing. We find that the bifurcation is subcritical, and show its bifurcation diagram. We connect the associated hysteretic behavior with hydrodynamics changes induced by the action of the Lorentz force. We show the geometry of the dynamo magnetic field and discuss how the dynamo transition can be induced when an external field is applied to the flow.
Period doubling route to chaos in Taylor-Green dynamo
Yadav, R; Verma, M K; Paul, S; Wahi, P
2010-01-01
We perform spectral simulations of dynamo for magnetic Prandtl number of one with Taylor-Green forcing. We observe dynamo transition through a supercritical pitchfork bifurcation. Beyond the transition, the numerical simulations reveal complex dynamo states with windows of constant, periodic, quasiperiodic, and chaotic magnetic field configurations. For some forcing amplitudes, multiple attractors were obtained for different initial conditions. We show that one of the chaotic windows follows the period-doubling route to chaos.
Dynamo quenching due to shear flow
Leprovost, Nicolas; Kim, Eun-Jin
2008-01-01
We provide a theory of dynamo ($\\alpha$ effect) and momentum transport in three-dimensional magnetohydrodynamics. For the first time, we show that the $\\alpha$ effect is severely reduced by the shear even in the absence of magnetic field. The $\\alpha$ effect is further suppressed by magnetic fields well below equipartition (with the large-scale flow) with different scalings depending on the relative strength of shear and magnetic field. The turbulent viscosity is also found to be significantl...
Nowada, M.; Fu, S.-Y.; Parks, G. K.; Pu, Z.-Y.; Angelopoulos, V.; Carlson, C. W.; Auster, H.-U.
2012-04-01
The fast plasma flows in the geomagnetotail are observed during both geomagnetically active and quiet times. However, it has been unclear about the fundamental difference in the plasma fast flows between at two different geomagnetic conditions, that is, the generation mechanism of, and pictures of the energy transport and balance at the fast plasma flows. Magnetic reconnection in the magnetotail has been believed as one of the most possible mechanisms to generate the fast plasma flows regardless of the geomagnetic conditions. Recently, Nowada et al. [2012], however, demonstrated that the magnetotail magnetic reconnection does not always contribute to the generation of the fast plasma flows at geomagnetically quiet times based on the THEMIS measurements. It is very important to reveal how the energy transport and balance in the magnetotail in association with these plasma fast flows are on obtaining a clue to elucidate an essential difference in the plasma fast flows between during active and quiet geomagnetic conditions. Based on three events of the magnetotail plasma flow bursts, which are transient fast plasma flows with the durations between 1 and 2 minutes, during geomagnetically quiet times, observed by THEMIS, we examined detailed variations of the electric field as a proxy of the flux transport aspect, and associated pressure. The main characteristics of these events are shown as follows; 1) the GSM-X component of the plasma velocity (Vx) was higher than 300 km/s 2) associated parallel (V//) and perpendicular (V⊥) velocities to the local magnetic field line were higher than 200 km/s 3) the flow bursts were observed during which AL and AU indices were lower than 40 nT, and simultaneous Kp index range was between -1 and 1. For almost events, the parallel (E//) and perpendicular (E⊥) components of the electric field to the local magnetic field line were much stronger than the dawn-dusk electric field component (Ey). This result implies that a larger amount
Convective Dynamo Simulation with a Grand Minimum
Augustson, Kyle; Miesch, Mark; Toomre, Juri
2015-01-01
The global-scale dynamo action achieved in a simulation of a Sun-like star rotating at thrice the solar rate is assessed. The 3-D MHD Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity minimization scheme, is employed to capture convection and dynamo processes in this G-type star. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. It is found that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals occurring roughly every 6.2 years. The magnetic energy also rises and falls with a regular period. The magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the processes leading to polarity reversals are more complex, appearing to arise from the interaction of convection with the mean toroidal fields. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation, and potentially to a no...
Turbulent 2.5 dimensional dynamos
Seshasayanan, K
2015-01-01
We study the dynamo instability driven by a turbulent two dimensional flow with three components of the form (u(x, y, t), v(x, y, t), w(x, y, t)) sometimes referred to as a 2.5 dimensional flow. This type of flows provides an approximation to very fast rotating flows often observed in nature. The low dimensionality of the system allows the investigation of a wide range of fluid Reynolds number Re, magnetic Reynolds number Rm and forcing length scales relative to the domain size that is still prohibited for full three dimensional numerical simulations. We were thus able to determine the properties of the dynamo onset as a function of Re and and the asymptotic behavior of the most unstable mode in the large Rm limit. In particular it has been shown that: In a non-helical flow in an infinite domain the critical magnetic Reynolds number Rmc becomes a constant in the large Re limit. A helical flow always results in dynamo in agreement with mean field predictions. For thin layers for both helical and nonhelical flo...
Tidally driven dynamos in a rotating sphere
Cébron, David
2014-01-01
Large-scale planetary or stellar magnetic fields generated by a dynamo effect are mostly attributed to flows forced by buoyancy forces in electrically conducting fluid layers. However, these large-scale fields may also be controlled by tides, as previously suggested for the star $\\tau$-boo, Mars or the Early Moon. By simulating a small local patch of a rotating fluid, \\cite{Barker2014} have recently shown that tides can drive small-scale dynamos by exciting a hydrodynamic instability, the so-called elliptical (or tidal) instability. By performing global magnetohydrodynamic simulations of a rotating spherical fluid body, we investigate if this instability can also drive the observed large-scale magnetic fields. We are thus interested by the dynamo threshold and the generated magnetic field in order to test if such a mechanism is relevant for planets and stars. Rather than solving the problem in a geometry deformed by tides, we consider a spherical fluid body and add a body force to mimic the tidal deformation ...
The α Dynamo Effects in Laboratory Plasmas
International Nuclear Information System (INIS)
A concise review of observations of the alpha dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This alpha-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-and-relax cycle, or the so-called self-organization process, happens in magnetized plasmas in a timescale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed alpha-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed
Kinematic dynamo, supersymmetry breaking, and chaos
Ovchinnikov, Igor V.; Enßlin, Torsten A.
2016-04-01
The kinematic dynamo (KD) describes the growth of magnetic fields generated by the flow of a conducting medium in the limit of vanishing backaction of the fields onto the flow. The KD is therefore an important model system for understanding astrophysical magnetism. Here, the mathematical correspondence between the KD and a specific stochastic differential equation (SDE) viewed from the perspective of the supersymmetric theory of stochastics (STS) is discussed. The STS is a novel, approximation-free framework to investigate SDEs. The correspondence reported here permits insights from the STS to be applied to the theory of KD and vice versa. It was previously known that the fast KD in the idealistic limit of no magnetic diffusion requires chaotic flows. The KD-STS correspondence shows that this is also true for the diffusive KD. From the STS perspective, the KD possesses a topological supersymmetry, and the dynamo effect can be viewed as its spontaneous breakdown. This supersymmetry breaking can be regarded as the stochastic generalization of the concept of dynamical chaos. As this supersymmetry breaking happens in both the diffusive and the nondiffusive cases, the necessity of the underlying SDE being chaotic is given in either case. The observed exponentially growing and oscillating KD modes prove physically that dynamical spectra of the STS evolution operator that break the topological supersymmetry exist with both real and complex ground state eigenvalues. Finally, we comment on the nonexistence of dynamos for scalar quantities.
Study on the large scale dynamo transition
Nigro, Giuseppina
2010-01-01
Using the magnetohydrodynamic (MHD) description, we develop a nonlinear dynamo model that couples the evolution of the large scale magnetic field with turbulent dynamics of the plasma at small scale by electromotive force (e.m.f.) in the induction equation at large scale. The nonlinear behavior of the plasma at small scale is described by using a MHD shell model for velocity field and magnetic field fluctuations.The shell model allow to study this problem in a large parameter regime which characterizes the dynamo phenomenon in many natural systems and which is beyond the power of supercomputers at today. Under specific conditions of the plasma turbulent state, the field fluctuations at small scales are able to trigger the dynamo instability. We study this transition considering the stability curve which shows a strong decrease in the critical magnetic Reynolds number for increasing inverse magnetic Prandlt number $\\textrm{Pm}^{-1}$ in the range $[10^{-6},1]$ and slows an increase in the range $[1,10^{8}]$. We...
The Alpha Dynamo Effects in Laboratory Plasmas
Energy Technology Data Exchange (ETDEWEB)
Hantao Ji; Stewart C. Prager
2001-10-16
A concise review of observations of the alpha dynamo effect in laboratory plasmas is given. Unlike many astrophysical systems, the laboratory pinch plasmas are driven magnetically. When the system is overdriven, the resultant instabilities cause magnetic and flow fields to fluctuate, and their correlation induces electromotive forces along the mean magnetic field. This alpha-effect drives mean parallel electric current, which, in turn, modifies the initial background mean magnetic structure towards the stable regime. This drive-and-relax cycle, or the so-called self-organization process, happens in magnetized plasmas in a timescale much shorter than resistive diffusion time, thus it is a fast and unquenched dynamo process. The observed alpha-effect redistributes magnetic helicity (a measure of twistedness and knottedness of magnetic field lines) but conserves its total value. It can be shown that fast and unquenched dynamos are natural consequences of a driven system where fluctuations are statistically either not stationary in time or not homogeneous in space, or both. Implications to astrophysical phenomena will be discussed.
Hall-magnetohydrodynamic small-scale dynamos.
Gómez, Daniel O; Mininni, Pablo D; Dmitruk, Pablo
2010-09-01
Magnetic field generation by dynamo action is often studied within the theoretical framework of magnetohydrodynamics (MHD). However, for sufficiently diffuse media, the Hall effect may become non-negligible. We present results from three-dimensional simulations of the Hall-MHD equations subjected to random nonhelical forcing. We study the role of the Hall effect in the dynamo efficiency for different values of the Hall parameter. For small values of the Hall parameter, the small-scale dynamo is more efficient, displaying faster growth and saturating at larger amplitudes of the magnetic field. For larger values of the Hall parameter, saturation of the magnetic field is reached at smaller amplitudes than in the MHD case. We also study energy transfer rates among spatial scales and show that the Hall effect produces a reduction of the direct energy cascade at scales larger than the Hall scale, therefore leading to smaller energy dissipation rates. Finally, we present results stemming from simulations at large magnetic Prandtl numbers, which is the relevant regime in the hot and diffuse interstellar medium. In the range of magnetic Prandtl numbers considered, the Hall effect moves the peak of the magnetic energy spectrum as well as other relevant magnetic length scales toward the Hall scale. PMID:21230195
Conserved quantities in kinematic dynamo theory
International Nuclear Information System (INIS)
Using a Lagrangian approach to the magnetic induction equation in an infinite medium, it is demonstrated that there exist seven conserved quantities which, by analogy with classical mechanics, labeled as ''energy,'' ''momentum,'' and ''angular momentum.'' For prescribed fluid motions the detailed conservation equations are spelled out. For a fluid motion which is turbulent one also gives the average conserved quantities. In a pragmatic sense it is expected that these conservation laws will be of use in attempts to obtain numerically accurate solutions to the turbulent kinematic dynamo equations. Since the magnetic induction equation is not self-adjoint, numerical attempts to date have to impose some extraneous ad hoc ''criteria of goodness'' at any given level of numerical truncation. The conserved quantities given provide an internal check of the accuracy of any numerical calculation without the necessity for arbitrarily imposed external criteria of accuracy. As such they should be a powerful tool in rapidly increasing the accuracy of numerical solutions to the kinematic dynamo equations. It is also pointed out that the conserved quantities can be used to indicate the possibility of kinematic dynamo activity ahead of any detailed calculations
Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
Riols, A; Cossu, C; Lesur, G; Ogilvie, G I; Longaretti, P-Y
2016-01-01
In Keplerian accretion disks, turbulence and magnetic fields may be jointly excited through a subcritical dynamo process involving the magnetorotational instability (MRI). High-resolution simulations exhibit a tendency towards statistical self-organization of MRI dynamo turbulence into large-scale cyclic dynamics. Understanding the physical origin of these structures, and whether they can be sustained and transport angular momentum efficiently in astrophysical conditions, represents a significant theoretical challenge. The discovery of simple periodic nonlinear MRI dynamo solutions has recently proven useful in this respect, and has notably served to highlight the role of turbulent magnetic diffusion in the seeming decay of the dynamics at low magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a common regime in accretion disks. The connection between these simple structures and the statistical organization reported in turbulent simulations remained elusive, though. Here, we report the n...
Iida, Y.
2016-01-01
The solar dynamo problem is the question of how the cyclic variation in the solar magnetic field is maintained. One of the important processes is the transport of magnetic flux by surface convection. To reveal this process, the dependence of the squared displacement of magnetic flux concentrations upon the elapsed time is investigated in this paper via a feature-recognition technique and a continual five-day magnetogram. This represents the longest time scale over which a satellite observatio...
Shear dynamo, turbulence, and the magnetorotational instability
Squire, Jonathan
The formation, evolution, and detailed structure of accretion disks remain poorly understood, with wide implications across a variety of astrophysical disciplines. While the most pressing question --- what causes the high angular momentum fluxes that are necessary to explain observations? --- is nicely answered by the idea that the disk is turbulent, a more complete grasp of the fundamental processes is necessary to capture the wide variety of behaviors observed in the night sky. This thesis studies the turbulence in ionized accretion disks from a theoretical standpoint, in particular focusing on the generation of magnetic fields in these processes, known as dynamo. Such fields are expected to be enormously important, both by enabling the magnetorotational instability (which evolves into virulent turbulence), and through large-scale structure formation, which may transport angular momentum in different ways and be fundamental for the formation of jets. The central result of this thesis is the suggestion of a new large-scale dynamo mechanism in shear flows --- the "magnetic shear-current effect" --- which relies on a positive feedback from small-scale magnetic fields. As well as being a very promising candidate for driving field generation in the central regions of accretion disks, this effect is interesting because small-scale magnetic fields have historically been considered to have a negative effect on the large-scale dynamo, damping growth and leading to dire predictions for final saturation amplitudes. Given that small-scale fields are ubiquitous in plasma turbulence above moderate Reynolds numbers, the finding that they could instead have a positive effect in some situations is interesting from a theoretical and practical standpoint. The effect is studied using direct numerical simulation, analytic techniques, and novel statistical simulation methods. In addition to the dynamo, much attention is given to the linear physics of disks and its relevance to
Helioseismic Constraints and Paradigm Shift in Solar Dynamo
Kosovichev, Alexander G; Zhao, Junwei
2014-01-01
Helioseismology provides important constraints for the solar dynamo problem. However, the basic properties and even the depth of the dynamo process, which operates also in other stars, are unknown. Most of the dynamo models suggest that the toroidal magnetic field that emerges on the surface and forms sunspots is generated near the bottom of the convection zone, in the tachocline. However, there is a number of theoretical and observational problems with justifying the deep-seated dynamo models. This leads to the idea that the subsurface angular velocity shear may play an important role in the solar dynamo. Using helioseismology measurements of the internal rotation and meridional circulation, we investigate a mean-field MHD model of dynamo distributed in the bulk of the convection zone but shaped in a near-surface layer. We show that if the boundary conditions at the top of the dynamo region allow the large-scale toroidal magnetic fields to penetrate into the surface, then the dynamo wave propagates along the...
Dikpati, Mausumi; Anderson, Jeffrey L.; Mitra, Dhrubaditya
2016-09-01
We implement an Ensemble Kalman Filter procedure using the Data Assimilation Research Testbed for assimilating “synthetic” meridional flow-speed data in a Babcock–Leighton-type flux-transport solar dynamo model. By performing several “observing system simulation experiments,” we reconstruct time variation in meridional flow speed and analyze sensitivity and robustness of reconstruction. Using 192 ensemble members including 10 observations, each with 4% error, we find that flow speed is reconstructed best if observations of near-surface poloidal fields from low latitudes and tachocline toroidal fields from midlatitudes are assimilated. If observations include a mixture of poloidal and toroidal fields from different latitude locations, reconstruction is reasonably good for ≤slant 40 % error in low-latitude data, even if observational error in polar region data becomes 200%, but deteriorates when observational error increases in low- and midlatitude data. Solar polar region observations are known to contain larger errors than those in low latitudes; our forward operator (a flux-transport dynamo model here) can sustain larger errors in polar region data, but is more sensitive to errors in low-latitude data. An optimal reconstruction is obtained if an assimilation interval of 15 days is used; 10- and 20-day assimilation intervals also give reasonably good results. Assimilation intervals \\lt 5 days do not produce faithful reconstructions of flow speed, because the system requires a minimum time to develop dynamics to respond to flow variations. Reconstruction also deteriorates if an assimilation interval \\gt 45 days is used, because the system’s inherent memory interferes with its short-term dynamics during a substantially long run without updating.
Stable Alfven wave dynamo action in the reversed field pinch
International Nuclear Information System (INIS)
Recent advances in linear resistive MHD stability analysis are used to calculate the quasi-linear dynamo mean electromotive force of Alfven waves. This emf is incorporated into a one-dimensional transport and mean-field evolution code. The changing equilibrium is then fed back to the stability code to complete a computational framework that self-consistently evaluates a dynamic plasma dynamo. Static quasi-linear Alfven wave calculations have shown that dynamo emfs on the order of eta vector J are possible. This suggested a possible explanation of RFP behavior and a new (externally driven) mechanism for extending operation and controlling field profiles (possibly reducing plasma transport). This thesis demonstrates that the dynamo emf can quickly induce plasma currents whose emf cancels the dynamo effect. This thesis also contains extensive studies of resistive Alfven wave properties. This includes behavior versus spectral location, magnetic Reynolds number and wave number
Could giant basin-forming impacts have killed Martian dynamo?
Kuang, W.; Jiang, W.; Roberts, J.; Frey, H. V.
2014-11-01
The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30° of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16° polar reorientation is needed if Utopia is the dynamo killer.
Could Giant Basin-Forming Impacts Have Killed Martian Dynamo?
Kuang, W.; Jiang, W.; Roberts, J.; Frey, H. V.
2014-01-01
The observed strong remanent crustal magnetization at the surface of Mars suggests an active dynamo in the past and ceased to exist around early to middle Noachian era, estimated by examining remagnetization strengths in extant and buried impact basins. We investigate whether the Martian dynamo could have been killed by these large basin-forming impacts, via numerical simulation of subcritical dynamos with impact-induced thermal heterogeneity across the core-mantle boundary. We find that subcritical dynamos are prone to the impacts centered on locations within 30 deg of the equator but can easily survive those at higher latitudes. Our results further suggest that magnetic timing places a strong constraint on postimpact polar reorientation, e.g., a minimum 16 deg polar reorientation is needed if Utopia is the dynamo killer.
Evaluation of Model Operational Analyses during DYNAMO
Ciesielski, Paul; Johnson, Richard
2013-04-01
A primary component of the observing system in the DYNAMO-CINDY2011-AMIE field campaign was an atmospheric sounding network comprised of two sounding quadrilaterals, one north and one south of the equator over the central Indian Ocean. During the experiment a major effort was undertaken to ensure the real-time transmission of these data onto the GTS (Global Telecommunication System) for dissemination to the operational centers (ECMWF, NCEP, JMA, etc.). Preliminary estimates indicate that ~95% of the soundings from the enhanced sounding network were successfully transmitted and potentially used in their data assimilation systems. Because of the wide use of operational and reanalysis products (e.g., in process studies, initializing numerical simulations, construction of large-scale forcing datasets for CRMs, etc.), their validity will be examined by comparing a variety of basic and diagnosed fields from two operational analyses (ECMWF and NCEP) to similar analyses based solely on sounding observations. Particular attention will be given to the vertical structures of apparent heating (Q1) and drying (Q2) from the operational analyses (OA), which are strongly influenced by cumulus parameterizations, a source of model infidelity. Preliminary results indicate that the OA products did a reasonable job at capturing the mean and temporal characteristics of convection during the DYNAMO enhanced observing period, which included the passage of two significant MJO events during the October-November 2011 period. For example, temporal correlations between Q2-budget derived rainfall from the OA products and that estimated from the TRMM satellite (i.e., the 3B42V7 product) were greater than 0.9 over the Northern Sounding Array of DYNAMO. However closer inspection of the budget profiles show notable differences between the OA products and the sounding-derived results in low-level (surface to 700 hPa) heating and drying structures. This presentation will examine these differences and
MHD dynamo action in space plasmas
International Nuclear Information System (INIS)
Electric currents are now recognized to play a major role in the physical process of the Earths magnetosphere as well as in distant astrophysical plasmas. In driving these currents MHD dynamos as well as generators of a thermoelectric nature are important. The primary source of power for the Earths magnetospheric process is the solar wind, which supplies a voltage of the order of 200 kV across the magnetosphere. The direction of the large-scale solar wind electric field varies of many different time scales. The power input to the magnetosphere is closely correlated with the direction of the large-scale solar wind electric field in such a fashion as to mimick the response of a half-wave rectifier with a down-to-dusk conduction direction. Behind this apparently simple response there are complex plasma physical processes that are still very incompletely understood. They are intimately related to auroras, magnetic storms, radiation belts and changes in magnetospheric plasma populations. Similar dynamo actions should occur at other planets having magnetospheres. Recent observations seem to indicate that part of the power input to the Earths magnetosphere comes through MHD dynamo action of a forced plasma flow inside the flanks of the magnetopause and may play a role in other parts of the magnetosphere, too. An example of a cosmical MHD connected to a solid load is the corotating plasma of Jupiters inner magnetosphere, sweeping past the plants inner satelites. In particular the electric currents thereby driven to and from the satellite Io have attracted considerable interest.(author)
Czech Academy of Sciences Publication Activity Database
Šimkanin, Ján; Hejda, Pavel
2009-01-01
Roč. 53, č. 1 (2009), s. 99-110. ISSN 0039-3169 R&D Projects: GA AV ČR IAA300120704 Institutional research plan: CEZ:AV0Z30120515 Keywords : hydromagnetic dynamos * control volume method * numerical dynamo benchmark * efficiency of parallelization Subject RIV: DE - Earth Magnetism, Geodesy, Geography Impact factor: 1.000, year: 2009
Solar and stellar dynamos -- latest developments
Brandenburg, A; Brandenburg, Axel; Dobler, Wolfgang
2002-01-01
Recent progress in the theory of solar and stellar dynamos is reviewed. Particular emphasis is placed on the mean-field theory which tries to describe the collective behavior of the magnetic field. In order to understand solar and stellar activity, a quantitatively reliable theory is necessary. Much of the new developments center around magnetic helicity conservation which is seen to be important in numerical simulations. Only a dynamical, explicitly time dependent theory of alpha-quenching is able to describe this behavior correctly.
Effects of Penetrative Convection on Solar Dynamo
Masada, Youhei; Kageyama, Akira
2013-01-01
Spherical solar dynamo simulations are performed. Self-consistent, fully compressible magnetohydrodynamic system with a stably stratified layer below the convection zone are numerically solved with a newly developed simulation code based on the Yin-Yang grid. The effects of penetrative convection are studied by comparing two models with and without the stable layer. A solar-like differential rotation profile is established when the penetrative convection is taken into account without assuming any forcing. A large-scale magnetic field is also spontaneously organized in the underlying stable layer. The embedded field has a dipole symmetry about the equator and it shows polarity reversals in time.
Synthesis of Small and Large scale Dynamos
Subramanian, K
2000-01-01
Using a closure model for the evolution of magnetic correlations, we uncover an interesting plausible saturated state of the small-scale fluctuation dynamo (SSD) and a novel anology between quantum mechanical tunneling and the generation of large-scale fields. Large scale fields develop via the $\\alpha$-effect, but as magnetic helicity can only change on a resistive timescale, the time it takes to organize the field into large scales increases with magnetic Reynolds number. This is very similar to the results which obtain from simulations using full MHD.
Constraints on dynamo action in plasmas
Helander, P; Schekochihin, A A
2016-01-01
Upper bounds are derived on the amount of magnetic energy that can be generated by dynamo action in collisional and collisionless plasmas with and without external forcing. A hierarchy of mathematical descriptions is considered for the plasma dynamics: ideal MHD, visco-resistive MHD, the double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD, and other kinetic models. It is found that dynamo action is greatly constrained in models where the magnetic moment of any particle species is conserved. In the absence of external forcing, the magnetic energy then remains small at all times if it is small in the initial state. In other words, a small "seed" magnetic field cannot be amplified significantly, regardless of the nature of flow, as long as the collision frequency and gyroradius are small enough to be negligible. A similar conclusion also holds if the system is subject to external forcing as long as this forcing conserves the magnetic moment of at least one plasma species and does not greatly i...
Why dynamos are prone to reversals
Stefani, F; Günther, U; Xu, M; Stefani, Frank; Gerbeth, Gunter; Guenther, Uwe; Xu, Mingtian
2005-01-01
In a recent paper (Phys. Rev. Lett. 94 (2005), 184506; physics/0411050) it was shown that a simple mean-field dynamo model with a spherically symmetric helical turbulence parameter alpha can exhibit a number of features which are typical for Earth's magnetic field reversals. In particular, the model produces asymmetric reversals, a positive correlation of field strength and interval length, and a bimodal field distribution. All these features are attributable to the magnetic field dynamics in the vicinity of an exceptional point of the spectrum of the non-selfadjoint dynamo operator. The negative slope of the growth rate curve between the nearby local maximum and the exceptional point makes the system unstable and drives it to the exceptional point and beyond into the oscillatory branch where the sign change happens. A weakness of this reversal model is the apparent necessity to fine-tune the magnetic Reynolds number and/or the radial profile of alpha. In the present paper, it is shown that this fine-tuning i...
MAGNETIC WREATHS AND CYCLES IN CONVECTIVE DYNAMOS
International Nuclear Information System (INIS)
Solar-type stars exhibit a rich variety of magnetic activity. Seeking to explore the convective origins of this activity, we have carried out a series of global three-dimensional magnetohydrodynamic simulations with the anelastic spherical harmonic code. Here we report on the dynamo mechanisms achieved as the effects of artificial diffusion are systematically decreased. The simulations are carried out at a nominal rotation rate of three times the solar value (3 Ω☉), but similar dynamics may also apply to the Sun. Our previous simulations demonstrated that convective dynamos can build persistent toroidal flux structures (magnetic wreaths) in the midst of a turbulent convection zone and that high rotation rates promote the cyclic reversal of these wreaths. Here we demonstrate that magnetic cycles can also be achieved by reducing the diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these more turbulent models, diffusive processes no longer play a significant role in the key dynamical balances that establish and maintain the differential rotation and magnetic wreaths. Magnetic reversals are attributed to an imbalance in the poloidal magnetic induction by convective motions that is stabilized at higher diffusion levels. Additionally, the enhanced levels of turbulence lead to greater intermittency in the toroidal magnetic wreaths, promoting the generation of buoyant magnetic loops that rise from the deep interior to the upper regions of our simulated domain. The implications of such turbulence-induced magnetic buoyancy for solar and stellar flux emergence are also discussed.
Two spinning ways for precession dynamo
Cappanera, L.; Guermond, J.-L.; Léorat, J.; Nore, C.
2016-04-01
It is numerically demonstrated by means of a magnetohydrodynamic code that precession can trigger dynamo action in a cylindrical container. Fixing the angle between the spin and the precession axis to be 1/2 π , two limit configurations of the spinning axis are explored: either the symmetry axis of the cylinder is parallel to the spin axis (this configuration is henceforth referred to as the axial spin case), or it is perpendicular to the spin axis (this configuration is referred to as the equatorial spin case). In both cases, the centro-symmetry of the flow breaks when the kinetic Reynolds number increases. Equatorial spinning is found to be more efficient in breaking the centro-symmetry of the flow. In both cases, the average flow in the reference frame of the mantle converges to a counter-rotation with respect to the spin axis as the Reynolds number grows. We find a scaling law for the average kinetic energy in term of the Reynolds number in the axial spin case. In the equatorial spin case, the unsteady asymmetric flow is shown to be capable of sustaining dynamo action in the linear and nonlinear regimes. The magnetic field is mainly dipolar in the equatorial spin case, while it is is mainly quadrupolar in the axial spin case.
Stellar Magnetic Dynamos and Activity Cycles
Wright, Nicholas J
2013-01-01
Using a new uniform sample of 824 solar and late-type stars with measured X-ray luminosities and rotation periods we have studied the relationship between rotation and stellar activity that is believed to be a probe of the underlying stellar dynamo. Using an unbiased subset of the sample we calculate the power law slope of the unsaturated regime of the activity -- rotation relationship as $L_X/L_{bol}\\propto Ro^\\beta$, where $\\beta=-2.70\\pm0.13$. This is inconsistent with the canonical $\\beta = -2$ slope to a confidence of 5$\\sigma$ and argues for an interface-type dynamo. We map out three regimes of coronal emission as a function of stellar mass and age, using the empirical saturation threshold and theoretical super-saturation thresholds. We find that the empirical saturation timescale is well correlated with the time at which stars transition from the rapidly rotating convective sequence to the slowly rotating interface sequence in stellar spin-down models. This may be hinting at fundamental changes in the ...
On the mean-field theory of the Karlsruhe Dynamo Experiment
Directory of Open Access Journals (Sweden)
K.-H. Rädler
2002-01-01
Full Text Available In the Forschungszentrum Karlsruhe an experiment has been constructed which demonstrates a homogeneous dynamo as is expected to exist in the Earth's interior. This experiment is discussed within the framework of mean-field dynamo theory. The main predictions of this theory are explained and compared with the experimental results. Key words. Dynamo, geodynamo, dynamo experiment, mean-field dynamo theory, a-effect
The initial temporal evolution of a feedback dynamo for Mercury
Heyner, D; Wicht, J; Glassmeier, K -H; Korth, H; Motschmann, U
2010-01-01
Various possibilities are currently under discussion to explain the observed weakness of the intrinsic magnetic field of planet Mercury. One of the possible dynamo scenarios is a dynamo with feedback from the magnetosphere. Due to its weak magnetic field Mercury exhibits a small magnetosphere whose subsolar magnetopause distance is only about 1.7 Hermean radii. We consider the magnetic field due to magnetopause currents in the dynamo region. Since the external field of magnetospheric origin is antiparallel to the dipole component of the dynamo field, a negative feedback results. For an alpha-omega-dynamo two stationary solutions of such a feedback dynamo emerge, one with a weak and the other with a strong magnetic field. The question, however, is how these solutions can be realized. To address this problem, we discuss various scenarios for a simple dynamo model and the conditions under which a steady weak magnetic field can be reached. We find that the feedback mechanism quenches the overall field to a low va...
The manifold zoology of anelastic dynamos with variable conductivity
Dietrich, Wieland; Jones, Chris
2015-04-01
Whereas the dynamo processes in terrestrial planets is strongly influenced by the overlying rocky mantle, the induction of global magnetic fields in gas giants is mainly affected by internal properties, such as the rapid outward decay of static density, pressure and temperature throughout the gaseous shell. Further for Jupiter and Saturn it is well known that the transition from metallic to molecular hydrogen leads to a steep decrease in the electrical conductivity. This drop-off radius is closer to the surface for heavy Jupiter (at 90% of its respective radius), but much deeper for the less massive Saturn (65%). From the modelling perspective this leads to an inner conducting shell where the magnetic fields dominate the dynamics, and outer hydro dynamic shell where the strong Coriolis force reigns. Within this study we parametrise the conductivity drop-off radius and investigate the interaction between these shells, such as the emergence of differential rotation and induction of magnetic fields. Remarkably, we could identify numerous rather different self-consistent dynamo solutions. E.g., hemispherical dynamos, quadrupolar dynamos, octupolar dynamos, dipolar dynamo waves or many mixed modes, such as solutions where the quadrupole is stable in time and the dipole periodically reverses. In summary, our results suggest anelastic dynamo models with variable conductivity yield manifold different solutions in close poriximity in the parameter space. Unfortunately for Saturn-like models with deep conductivity drop-off, Saturn-like magnetic field (stable, strongly dipolar) seemed rather unlikely.
Mean-field theory and self-consistent dynamo modeling
International Nuclear Information System (INIS)
Mean-field theory of dynamo is discussed with emphasis on the statistical formulation of turbulence effects on the magnetohydrodynamic equations and the construction of a self-consistent dynamo model. The dynamo mechanism is sought in the combination of the turbulent residual-helicity and cross-helicity effects. On the basis of this mechanism, discussions are made on the generation of planetary magnetic fields such as geomagnetic field and sunspots and on the occurrence of flow by magnetic fields in planetary and fusion phenomena. (author)
The effects of vertical outflows on disk dynamos
Bardou, A.; Rekowski, B. v.; Dobler, W.; Brandenburg, A.; Shukurov, A.
2000-01-01
We consider the effect of vertical outflows on the mean-field dynamo in a thin disk. These outflows could be due to winds or magnetic buoyancy. We analyse both two-dimensional finite-difference numerical solutions of the axisymmetric dynamo equations and a free-decay mode expansion using the thin-disk approximation. Contrary to expectations, a vertical velocity can enhance dynamo action, provided it is not too strong. In the nonlinear regime this can lead to super-exponential growth of the ma...
3-dimensional simulation of dynamo effect of reversed field pinch
International Nuclear Information System (INIS)
A non-linear numerical simulation of the dynamo effect of a reversed field pinch (RFP) with finite beta is presented. It is shown that the m=-1, n=(9,10,11,....,19) modes cause the dynamo effect and sustain the field reversed configuration. The role of the m=0 modes on the dynamo effect is carefully examined. Our simulation shows that the magnetic field fluctuation level scales as S-0.2 or S-0.3 in the range of 103 5, while Nebel, Caramana and Schnack obtained the fluctuation level is independent of S for a pressureless RFP plasma. (author)
Mean-field theory and self-consistent dynamo modeling
Energy Technology Data Exchange (ETDEWEB)
Yoshizawa, Akira; Yokoi, Nobumitsu [Tokyo Univ. (Japan). Inst. of Industrial Science; Itoh, Sanae-I [Kyushu Univ., Fukuoka (Japan). Research Inst. for Applied Mechanics; Itoh, Kimitaka [National Inst. for Fusion Science, Toki, Gifu (Japan)
2001-12-01
Mean-field theory of dynamo is discussed with emphasis on the statistical formulation of turbulence effects on the magnetohydrodynamic equations and the construction of a self-consistent dynamo model. The dynamo mechanism is sought in the combination of the turbulent residual-helicity and cross-helicity effects. On the basis of this mechanism, discussions are made on the generation of planetary magnetic fields such as geomagnetic field and sunspots and on the occurrence of flow by magnetic fields in planetary and fusion phenomena. (author)
The Current Status of Kinematic Solar Dynamo Models
Indian Academy of Sciences (India)
Arnab Rai Choudhuri
2000-09-01
This review provides a historical overview of how research in kinematic solar dynamo modeling evolved during the last few decades and assesses the present state of research. The early pioneering papers assumed the dynamo to operate in the convection zone. It was suggested in the 1980s that the dynamo operates in a thin layer at the bottom of the convection zone. Some researchers in recent years are arguing that the poloidal field is produced near the surface—an idea that goes back to Babcock (1961) and Leighton (1969).
Magnetic dipole moment estimates for an ancient lunar dynamo
Anderson, K. A.
1983-01-01
The four measured planetary magnetic moments combined with a recent theoretical prediction for dynamo magnetic fields suggests that no dynamo exists in the moon's interior today. For the moon to have had a magnetic moment in the past of sufficient strength to account for at least some of the lunar rock magnetism, the rotation would have been about twenty times faster than it is today and the radius of the fluid, conducting core must have been about 750 km. The argument depends on the validity of the Busse solution to the validity of the MHD problem of planetary dynamos.
Introduction to Plasma Dynamo, Reconnection and Shocks
Energy Technology Data Exchange (ETDEWEB)
Intrator, Thomas P. [Los Alamos National Laboratory
2012-08-30
In our plasma universe, most of what we can observe is composed of ionized gas, or plasma. This plasma is a conducting fluid, which advects magnetic fields when it flows. Magnetic structure occurs from the smallest planetary to the largest cosmic scales. We introduce at a basic level some interesting features of non linear magnetohydrodynamics (MHD). For example, in our plasma universe, dynamo creates magnetic fields from gravitationally driven flow energy in an electrically conducting medium, and conversely magnetic reconnection annihilates magnetic field and accelerates particles. Shocks occur when flows move faster than the local velocity (sonic or Alfven speed) for the propagation of information. Both reconnection and shocks can accelerate particles, perhaps to gigantic energies, for example as observed with 10{sup 20} eV cosmic rays.
Computer simulation of a magnetohydrodynamic dynamo II
International Nuclear Information System (INIS)
We performed a computer simulation of a magnetohydrodynamic dynamo in a rapidly rotating spherical shell. Extensive parameter runs are carried out changing the electrical resistivity. It is found that the total magnetic energy can grow more than ten times larger than the total kinetic energy of the convection motion when the resistivity is sufficiently small. When the resistivity is relatively large and the magnetic energy is comparable or smaller than the kinetic energy, the convection motion maintains its well-organized structure. However, when the resistivity is small and the magnetic energy becomes larger than the kinetic energy, the well-organized convection motion is highly disturbed. The generated magnetic field is organized as a set of flux tubes which can be divided into two categories. The magnetic field component parallel to the rotation axis tends to be confined inside the anticyclonic columnar convection cells. On the other hand, the component perpendicular to the rotation axis is confined outside the convection cells. (author)
Nonhelical mean-field dynamos in a sheared turbulence
Rogachevskii, I
2008-01-01
Mechanisms of nonhelical large-scale dynamos (shear-current dynamo and effect of homogeneous kinetic helicity fluctuations with zero mean) in a homogeneous turbulence with large-scale shear are discussed. We have found that the shear-current dynamo can act even in random flows with small Reynolds numbers. However, in this case mean-field dynamo requires small magnetic Prandtl numbers (i.e., ${\\rm Pm} < {\\rm Pm}^{\\rm cr}<1$). The threshold in the magnetic Prandtl number, ${\\rm Pm}^{\\rm cr} = 0.24$, is determined using second order correlation approximation (or first-order smoothing approximation) for a background random flow with a scale-dependent viscous correlation time $\\tau_c=(\
Wave-driven dynamo action in spherical MHD systems
Reuter, K; Tilgner, A; Forest, C B
2009-01-01
Hydrodynamic and magnetohydrodynamic numerical studies of a mechanically forced two-vortex flow inside a sphere are reported. The simulations are performed in the intermediate regime between the laminar flow and developed turbulence where a hydrodynamic instability is found to generate internal waves with a characteristic m=2 zonal wave number. It is shown that this time-periodic flow acts as a dynamo although snapshots of the flow as well as the mean flow are not dynamos. The magnetic fields' growth rate exhibits resonance effects depending on the wave frequency. Furthermore, a cyclic self-killing and self-recovering dynamo based on the relative alignment of the velocity and magnetic fields is presented. The phenomena are explained in terms of a mixing of non-orthogonal eigenstates of the time dependent linear operator of the magnetic induction equation. The potential relevance of this mechanism to dynamo experiments is discussed.
Magnetic flux concentrations from dynamo-generated fields
Jabbari, Sarah; Losada, Illa R; Kleeorin, Nathan; Rogachevskii, Igor
2014-01-01
The mean-field theory of magnetized stellar convection gives rise to the two possibility of distinct instabilities: the large-scale dynamo instability, operating in the bulk of the convection zone, and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation, but the growth rate of NEMPI is suppressed with increasing rotation rates, although recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an alpha squared mean-field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. We use both DNS and mean-field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal field. We use the test-field method to compute relevant dynamo coefficients. DNS show that magnetic flux ...
Energy transfers in dynamos with small magnetic Prandtl numbers
Kumar, Rohit
2015-06-25
We perform numerical simulation of dynamo with magnetic Prandtl number Pm = 0.2 on 10243 grid, and compute the energy fluxes and the shell-to-shell energy transfers. These computations indicate that the magnetic energy growth takes place mainly due to the energy transfers from large-scale velocity field to large-scale magnetic field and that the magnetic energy flux is forward. The steady-state magnetic energy is much smaller than the kinetic energy, rather than equipartition; this is because the magnetic Reynolds number is near the dynamo transition regime. We also contrast our results with those for dynamo with Pm = 20 and decaying dynamo. © 2015 Taylor & Francis.
Solar Magnetic Field Reversals and the Role of Dynamo Families
DeRosa, M L; Hoeksema, J T
2012-01-01
The variable magnetic field of the solar photosphere exhibits periodic reversals as a result of dynamo activity occurring within the solar interior. We decompose the surface field as observed by both the Wilcox Solar Observatory and the Michelson Doppler Imager into its harmonic constituents, and present the time evolution of the mode coefficients for the past three sunspot cycles. The interplay between the various modes is then interpreted from the perspective of general dynamo theory, where the coupling between the primary and secondary families of modes is found to correlate with large-scale polarity reversals for many examples of cyclic dynamos. Mean-field dynamos based on the solar parameter regime are then used to explore how such couplings may result in the various long-term trends in the surface magnetic field observed to occur in the solar case.
Measurements of the Hall Dynamo in MST Plasmas
Triana, J. C.; Almagri, A. F.; McCollam, K. J.; Sarff, J. S.; Sauppe, J. P.; Sovinec, C. R.
2015-11-01
Fluctuation-induced emfs correlated with tearing mode activity govern the relaxation process in RFP plasmas. Previous radial profile measurements in the edge of MST plasmas (ra/> 0 . 8) revealed a competition of the Hall, 1 ne || , and MHD, || , terms in Ohm's law. A robust magnetic probe allows measurements of the Hall-dynamo profile much deeper in the plasma (ra > 0 . 4) for low current conditions. The mode composition of the dynamo emf is computed using pseudospectral (cross-correlation) analysis with the spectrum measured from a toroidal magnetic array at the plasma surface. Extended MHD simulations with parameters comparable to the experiment have been performed using NIMROD. They predict complex variation of the Hall and MHD dynamo profiles across the plasma radius. Measurements of the Hall-dynamo profile can inform future computational work in addition to directing future experimental measurements of the MHD term. Work supported by U.S. DOE and NSF.
Early Mars Chronology: When did the Dynamo Really Die?
Lillis, R. J.; Stewart, S. T.; Roberts, J.; Bottke, W. F.; Manga, M.; Frey, H. V.; Kuang, W.; Robbins, S.
2014-07-01
Mars' dynamo is a key aspect of early Mars evolution. It likely started some time after primordial crust formation and ceased before the Utopia impact. Its total duration depends on impactor flux following accretion and the timing of the LHB.
Non-linear Galactic Dynamos and The Magnetic Pitch Angle
Chamandy, Luke
2015-01-01
Pitch angles $p$ of the large-scale magnetic fields $\\overline{\\bf{\\it{B}}}$ of spiral galaxies have previously been inferred from observations to be systematically larger in magnitude than predicted by standard mean-field dynamo theory. This discrepancy is more pronounced if dynamo growth has saturated, which is reasonable to assume given that such fields are generally inferred to be close to energy equipartition with the interstellar turbulence. This 'pitch angle problem' is explored using local numerical mean-field dynamo solutions as well as asymptotic analytical solutions. It is first shown that solutions in the saturated or kinematic regimes depend on only 5 dynamo parameters, two of which are tightly constrained by observations of galaxy rotation curves. The remaining 3-dimensional (dimensionless) parameter space can be constrained to some extent using theoretical arguments. Predicted values of $|p|$ can be as large as $\\sim40^\\circ$, which is similar to the largest values inferred from observations, b...
Resistive and ferritic-wall plasma dynamos in a sphere
International Nuclear Information System (INIS)
We numerically study the effects of varying electric conductivity and magnetic permeability of the bounding wall on a kinematic dynamo in a sphere for parameters relevant to Madison plasma dynamo experiment. The dynamo is excited by a laminar, axisymmetric flow of von Kármán type. The flow is obtained as a solution to the Navier-Stokes equation for an isothermal fluid with a velocity profile specified at the sphere's boundary. The properties of the wall are taken into account as thin-wall boundary conditions imposed on the magnetic field. It is found that an increase in the permeability of the wall reduces the critical magnetic Reynolds number Rmcr. An increase in the conductivity of the wall leaves Rmcr unaffected but reduces the dynamo growth rate.
Sharp magnetic structures from dynamos with density stratification
Jabbari, Sarah; Kleeorin, Nathan; Rogachevskii, Igor
2016-01-01
Recent direct numerical simulations (DNS) of large-scale turbulent dynamos in strongly stratified layers have resulted in surprisingly sharp bipolar structures at the surface. Here we present new DNS of helically and non-helically forced turbulence with and without rotation and compare with corresponding mean-field simulations (MFS) to show that these structures are a generic outcome of a broader class of dynamos in density-stratified layers. The MFS agree qualitatively with the DNS, but the period of oscillations tends to be longer in the DNS. In both DNS and MFS, the sharp structures are produced by converging flows at the surface and are driven by the Lorentz force associated with the large-scale dynamo-driven magnetic field if the dynamo number is at least 5 times supercritical.
Mean-Field Theory of the Solar Dynamo
Schmitt, D.
The generation of the solar magnetic field is generally ascribed to dynamo processes in the convection zone. The dynamo effects, differential rotation (Omega-effect) and helical turbulence (alpha-effect) are explained, and the basic properties of the mean-field dynamo equations are discussed in view of the observed properties of the solar cycle. Problems of the classical picture of a dynamo in the convection zone (fibril state of magnetic flux, field strength, magnetic buoyancy, polarity rules, differential rotation and butterfly diagram) are addressed and some alternatives to overcome these problems are presented. A possibility to make up for the missing radial gradient of rotation in the convection zone is an alpha^2-Omega-dynamo with an anisotropic alpha-tensor. Dynamo solutions then might have the characteristics of the butterfly diagram. Another approach involves meridional circulation as the cause of the migration of a dynamo wave. Another suggestion is that the solar dynamo operates in the overshoot region at the base of the convection zone where strong fields, necessary to explain the polarity rules, can be stored and radial gradients in the angular velocity occur. As an alternative to the turbulent alpha-effect a dynamic alpha-effect based on magnetostrophic waves driven by a magnetic buoyancy instability of a magnetic flux layer is introduced. Model calculations which use the internal rotation of the Sun as deduced from helioseismology only show solar cycle behaviour if the turbulent diffusivity is reduced in the layer and the alpha-effect is concentrated near the equator. Another possibility is a combined model. The non-uniform rotation and most of the azimuthal magnetic flux are confined to a thin layer at the bottom of the convection zone where turbulent diffusion is greatly reduced, with the convective region above containing only weak fields for which the alpha-effect and turbulent diffusion operate in the conventional manner. The dynamo takes on the
Magnetic Field Saturation in the Riga Dynamo Experiment
Gailitis, A; Platacis, E; Dementev, S; Cifersons, A; Gerbeth, G; Gundrum, T; Stefani, F; Christen, M; Will, G; Gailitis, Agris; Lielausis, Olgerts; Platacis, Ernests; Dement'ev, Sergej; Cifersons, Arnis; Gerbeth, Gunter; Gundrum, Thomas; Stefani, Frank; Christen, Michael; Will, Gotthard
2001-01-01
After the dynamo experiment in November 1999 had shown magnetic field self-excitation in a spiraling liquid metal flow, in a second series of experiments emphasis was placed on the magnetic field saturation regime as the next principal step in the dynamo process. The dependence of the strength of the magnetic field on the rotation rate is studied. Various features of the saturated magnetic field are outlined and possible saturation mechanisms are discussed.
The Role of Magnetic Helicity Transport in Nonlinear Galactic Dynamos
Kleeorin, N.; Moss, D.; Rogachevskii, I.; Sokoloff, D.
2002-01-01
We consider the magnetic helicity balance for the galactic dynamo in the framework of the local dynamo problem, as well as in the no-z model (which includes explicitly the radial distribution of the magnetic fields). When calculating the magnetic helicity balance we take into account the redistribution of the small-scale and large-scale magnetic fields between the magnetic helicities, as well as magnetic helicity transport and diffusion due to small-scale turbulence. We demonstrate that the m...
Nonlinear regimes in mean-field full-sphere dynamo
Pipin, V V
2016-01-01
The mean-field dynamo model is employed to study the non-linear dynamo regimes in a fully convective star of mass 0.3$M_{\\odot}$ rotating with period of 10 days. The differential rotation law was estimated using the mean-field hydrodynamic and heat transport equations. For the intermediate parameter of the turbulent magnetic Reynolds number, $Pm_{T}=3$ we found the oscillating dynamo regimes with period about 40Yr. The higher $Pm_{T}$ results to longer dynamo periods. The meridional circulation has one cell per hemisphere. It is counter-clockwise in the Northen hemisphere. The amplitude of the flow at the surface around 1 m/s. Tne models with regards for meridional circulation show the anti-symmetric relative to equator magnetic field. If the large-scale flows is fixed we find that the dynamo transits from axisymmetric to non-axisymmetric regimes for the overcritical parameter of the $\\alpha$effect. The change of dynamo regime occurs because of the non-axisymmetric non-linear $\\alpha$-effect. The situation pe...
Bekheit, A. H.
2010-08-01
The effect of toroidal rotation on heat flux transport in the edge plasma of small size divertor was simulated by B2SOLP0.5.2D transport code. The main results of simulation shows that, the following: (1) the radial heat flux is strongly influenced by toroidal rotation. (2) The amplification of conduction part of radial heat flux imposes nonresilient profile of ion temperature, under which the effect of toroidal rotation on ion temperature profile is strong. (3) The ion distribution and its gradients are lower for counter-injection neutral beam than for co-injection neutral beam. (4) Reversal of toroidal rotation during using neutral beam injection result in reverses of radial electric field and E × B drift velocity. (5) The toroidal rotation strong influence on the ion temperature scale length of the ion temperature gradient (ITG). (6) Switch on and off all drifts leads to higher change in the ion density distribution in edge plasma of small size divertor tokamak when the unbalance neutral beam injection are considered (7) the comparison between radial heat flux at different momentum input shows that, the radial ion heat flux with larger ion temperature scale length in the case of co-injection neutral beam is larger than the ion heat flux with smaller ion temperature scale length in the case of counter-injection neutral beam.
Metric-torsion preheating: cosmic dynamo mechanism?
de Andrade, L C Garcia
2014-01-01
Earlier Bassett et al [Phys Rev D 63 (2001) 023506] investigated the amplification of large scale magnetic fields during preheating and inflation in several different models. They argued that in the presence of conductivity resonance effect is weakened. From a dynamo equation in spacetimes endowed with torsion recently derived by Garcia de Andrade [Phys Lett B 711: 143 (2012)] it is shown that a in a universe with pure torsion in Minkowski spacetime the cosmological magnetic field is enhanced by ohmic or non-conductivity effect, which shows that the metric-torsion effects is worth while of being studied. In this paper we investigated the metric-torsion preheating perturbation, which leads to the seed cosmological magnetic field in the universe with torsion is of the order of $B_{seed}\\sim{10^{-37}Gauss}$ which is several orders of magnitude weaker than the decoupling value obtained from pure metric preheating of $10^{-15}Gauss$. Despite of the weakness of the magnetic field this seed field may seed the galact...
Measurement of the dynamo effect in a plasma
International Nuclear Information System (INIS)
A series of the detailed experiments has been conducted in three laboratory plasma devices to measure the dynamo electric field along the equilibrium field line (the α effect) arising from the correlation between the fluctuating flow velocity and magnetic field. The fluctuating flow velocity is obtained from probe measurement of the fluctuating E x B drift and electron diamagnetic drift. The three major findings are (1) the α effect accounts for the dynamo current generation, even in the time dependence through a ''sawtooth'' cycle; (2) at low collisionality the dynamo is explained primarily by the widely studied pressureless Magnetohydrodynamic (MHD) model, i.e., the fluctuating velocity is dominated by the E x B drift; (3) at high collisionality, a new ''electron diamagnetic dynamo'' is observed, in which the fluctuating velocity is dominated by the diamagnetic drift. In addition, direct measurements of the helicity flux indicate that the dynamo activity transports magnetic helicity from one part of the plasma to another, but the total helicity is roughly conserved, verifying J.B. Taylor's conjecture
Numerical Simulations of Dynamos Generated in Spherical Couette Flows
Guervilly, Céline; 10.1080/03091920903550955
2010-01-01
We numerically investigate the efficiency of a spherical Couette flow at generating a self-sustained magnetic field. No dynamo action occurs for axisymmetric flow while we always found a dynamo when non-axisymmetric hydrodynamical instabilities are excited. Without rotation of the outer sphere, typical critical magnetic Reynolds numbers $Rm_c$ are of the order of a few thousands. They increase as the mechanical forcing imposed by the inner core on the flow increases (Reynolds number $Re$). Namely, no dynamo is found if the magnetic Prandtl number $Pm=Rm/Re$ is less than a critical value $Pm_c\\sim 1$. Oscillating quadrupolar dynamos are present in the vicinity of the dynamo onset. Saturated magnetic fields obtained in supercritical regimes (either $Re>2 Re_c$ or $Pm>2Pm_c$) correspond to the equipartition between magnetic and kinetic energies. A global rotation of the system (Ekman numbers $E=10^{-3}, 10^{-4}$) yields to a slight decrease (factor 2) of the critical magnetic Prandtl number, but we find a peculi...
THE TURBULENT DYNAMO IN HIGHLY COMPRESSIBLE SUPERSONIC PLASMAS
International Nuclear Information System (INIS)
The turbulent dynamo may explain the origin of cosmic magnetism. While the exponential amplification of magnetic fields has been studied for incompressible gases, little is known about dynamo action in highly compressible, supersonic plasmas, such as the interstellar medium of galaxies and the early universe. Here we perform the first quantitative comparison of theoretical models of the dynamo growth rate and saturation level with three-dimensional magnetohydrodynamical simulations of supersonic turbulence with grid resolutions of up to 10243 cells. We obtain numerical convergence and find that dynamo action occurs for both low and high magnetic Prandtl numbers Pm = ν/η = 0.1-10 (the ratio of viscous to magnetic dissipation), which had so far only been seen for Pm ≥ 1 in supersonic turbulence. We measure the critical magnetic Reynolds number, Rmcrit=129−31+43, showing that the compressible dynamo is almost as efficient as in incompressible gas. Considering the physical conditions of the present and early universe, we conclude that magnetic fields need to be taken into account during structure formation from the early to the present cosmic ages, because they suppress gas fragmentation and drive powerful jets and outflows, both greatly affecting the initial mass function of stars
Spatial Nonlocality of the Small-Scale Solar Dynamo
Lamb, Derek A; DeForest, Craig E
2014-01-01
We explore the nature of the small-scale solar dynamo by tracking magnetic features. We investigate two previously-explored categories of the small-scale solar dynamo: shallow and deep. Recent modeling work on the shallow dynamo has produced a number of scenarios for how a strong network concentration can influence the formation and polarity of nearby small-scale magnetic features. These scenarios have measurable signatures, which we test for here using magnetograms from the Narrowband Filter Imager (NFI) on Hinode. We find no statistical tendency for newly-formed magnetic features to cluster around or away from network concentrations, nor do we find any statistical relationship between their polarities. We conclude that there is no shallow or "surface" dynamo on the spatial scales observable by Hinode/NFI. In light of these results, we offer a scenario in which the sub-surface field in a deep solar dynamo is stretched and distorted via turbulence, allowing the field to emerge at random locations on the photo...
The fate of alpha dynamos at large $Rm$
Cameron, Alexandre
2016-01-01
At the heart of today's solar magnetic field evolution models lies the alpha dynamo description. In this work, we investigate the fate of alpha-dynamos as the magnetic Reynolds number $Rm$ is increased. Using Floquet theory, we are able to precisely quantify mean field effects like the alpha and beta effect (i) by rigorously distinguishing dynamo modes that involve large scale components from the ones that only involve small scales, and by (ii) providing a way to investigate arbitrary large scale separations with minimal computational cost. We apply this framework to helical and non-helical flows as well as to random flows with short correlation time. Our results determine that the alpha-description is valid for $Rm$ smaller than a critical value $Rm_c$ at which small scale dynamo instability starts. When $Rm$ is above $Rm_c$ the dynamo ceases to follow the mean field description and the growth rate of the large scale modes becomes independent of the scale separation while the energy in the large scale modes ...
Spatial nonlocality of the small-scale solar dynamo
International Nuclear Information System (INIS)
We explore the nature of the small-scale solar dynamo by tracking magnetic features. We investigate two previously explored categories of the small-scale solar dynamo: shallow and deep. Recent modeling work on the shallow dynamo has produced a number of scenarios for how a strong network concentration can influence the formation and polarity of nearby small-scale magnetic features. These scenarios have measurable signatures, for which we test using magnetograms from the Narrowband Filter Imager (NFI) on board Hinode. We find no statistical tendency for newly formed magnetic features to cluster around or away from network concentrations, nor do we find any statistical relationship between their polarities. We conclude that there is no shallow or 'surface' dynamo on the spatial scales observable by Hinode/NFI. In light of these results, we offer a scenario in which the subsurface field in a deep solar dynamo is stretched and distorted via turbulence, allowing the small-scale field to emerge at random locations on the photosphere.
THE TURBULENT DYNAMO IN HIGHLY COMPRESSIBLE SUPERSONIC PLASMAS
Energy Technology Data Exchange (ETDEWEB)
Federrath, Christoph [Research School of Astronomy and Astrophysics, The Australian National University, Canberra, ACT 2611 (Australia); Schober, Jennifer [Universität Heidelberg, Zentrum für Astronomie, Institut für Theoretische Astrophysik, Albert-Ueberle-Strasse 2, D-69120 Heidelberg (Germany); Bovino, Stefano; Schleicher, Dominik R. G., E-mail: christoph.federrath@anu.edu.au [Institut für Astrophysik, Georg-August-Universität Göttingen, Friedrich-Hund-Platz 1, D-37077 Göttingen (Germany)
2014-12-20
The turbulent dynamo may explain the origin of cosmic magnetism. While the exponential amplification of magnetic fields has been studied for incompressible gases, little is known about dynamo action in highly compressible, supersonic plasmas, such as the interstellar medium of galaxies and the early universe. Here we perform the first quantitative comparison of theoretical models of the dynamo growth rate and saturation level with three-dimensional magnetohydrodynamical simulations of supersonic turbulence with grid resolutions of up to 1024{sup 3} cells. We obtain numerical convergence and find that dynamo action occurs for both low and high magnetic Prandtl numbers Pm = ν/η = 0.1-10 (the ratio of viscous to magnetic dissipation), which had so far only been seen for Pm ≥ 1 in supersonic turbulence. We measure the critical magnetic Reynolds number, Rm{sub crit}=129{sub −31}{sup +43}, showing that the compressible dynamo is almost as efficient as in incompressible gas. Considering the physical conditions of the present and early universe, we conclude that magnetic fields need to be taken into account during structure formation from the early to the present cosmic ages, because they suppress gas fragmentation and drive powerful jets and outflows, both greatly affecting the initial mass function of stars.
Iida, Y
2016-01-01
The solar dynamo problem is the question of how the cyclic variation in the solar magnetic field is maintained. One of the important processes is the transport of magnetic flux by surface convection. To reveal this process, the dependence of the squared displacement of magnetic flux concentrations upon the elapsed time is investigated in this paper via a feature-recognition technique and a continual five-day magnetogram. This represents the longest time scale over which a satellite observation has ever been performed for this problem. The dependence is found to follow a power-law and differ significantly from that of diffusion transport. Furthermore there is a change in the behavior at a spatial scale of 10^{3.8} km. A super-diffusion behavior with an index of 1.4 is found on smaller scales, while changing to a sub-diffusion behavior with an index of 0.6 on larger ones. I interpret this difference in the transport regime as coming from the network-flow pattern.
A hemispherical dynamo model : Implications for the Martian crustal magnetization
Dietrich, Wieland
2014-01-01
Mars Global Surveyor measurements revealed that the Martian crust is strongly magnetized in the southern hemisphere while the northern hemisphere is virtually void of magnetization. Two possible reasons have been suggested for this dichotomy: A once more or less homogeneously magnetization may have been destroyed in the northern hemisphere by, for example, resurfacing or impacts. The alternative theory we further explore here assumes that the dynamo itself produced a hemispherical field. We use numerical dynamo simulations to study under which conditions a spatial variation of the heat flux through the core-mantle boundary (CMB) may yield a strongly hemispherical surface field. We assume that the early Martian dynamo was exclusively driven by secular cooling and we mostly concentrate on a cosine CMB heat flux pattern with a minimum at the north pole, possibly caused by the impacts responsible for the northern lowlands. This pattern consistently triggers a convective mode which is dominated by equatorially ant...
Dynamo regimes and transitions in the VKS experiment
Berhanu, M; Boisson, J; Gallet, B; Gissinger, C; Fauve, S; Mordant, N; Pétrélis, F; Bourgoin, M; Odier, P; Pinton, J -F; Plihon, N; Aumaître, S; Chiffaudel, A; Daviaud, F; Dubrulle, B; Pirat, C
2015-01-01
The Von K{\\'a}rm{\\'a}n Sodium experiment yields a variety of dynamo regimes, when asymmetry is imparted to the flow by rotating impellers at different speed F1 and F2. We show that as the intensity of forcing, measured as F1 + F2, is increased, the transition to a self-sustained magnetic field is always observed via a supercritical bifurcation to a stationary state. For some values of the asymmetry parameter $\\\\theta$ = (F1--F2)/(F1+F2), time dependent dynamo regimes develop. They are observed either when the forcing is increased for a given value of asymmetry, or when the amount of asymmetry is varied at sufficiently high forcing. Two qualitatively different transitions between oscillatory and stationary regimes are reported, involving or not a strong divergence of the period of oscillations. These transitions can be interpreted using a low dimensional model based on the interactions of two dynamo modes.
Hall current effects in mean-field dynamo theory
Lingam, Manasvi
2016-01-01
The role of the Hall term on large scale dynamo action is investigated by means of the First Order Smoothing Approximation. It is shown that the standard $\\alpha$ coefficient is altered, and is zero when a specific double Beltrami state is attained, in contrast to the Alfv\\'enic state for MHD dynamos. The $\\beta$ coefficient is no longer positive definite, and thereby enables dynamo action even if $\\alpha$-quenching were to operate. The similarities and differences with the (magnetic) shear-current effect are pointed out, and a mechanism that may be potentially responsible for $\\beta < 0$ is advanced. The results are compared against previous studies, and their astrophysical relevance is also highlighted.
Suppression of a kinematic dynamo by large shear
Sood, Aditi; Kim, Eun-jin
2016-01-01
We numerically solve the magnetic induction equation in a spherical shell geometry, with a kinematically prescribed axisymmetric flow that consists of a superposition of a small-scale helical flow and a large-scale shear flow. The small-scale flow is chosen to be a local analog of the classical Roberts cells, consisting of strongly helical vortex rolls. The large-scale flow is a shearing motion in either the radial or the latitudinal directions. In the absence of large-scale shear, the small-scale flow is an efficient dynamo, in agreement with previous results. Adding increasingly large shear flows strongly suppresses the dynamo efficiency, indicating that shear is not always a favourable ingredient in dynamo action.
The solar dynamo: inferences from observations and modeling
Kitchatinov, L L
2014-01-01
It can be shown on observational grounds that two basic effects of dynamo theory for solar activity - production of the toroidal field from the poloidal one by differential rotation and reverse conversion of the toroidal field to the poloidal configuration by helical motions - are operating in the Sun. These two effects, however, do not suffice for constructing a realistic model for the solar dynamo. Only when a non-local version of the alpha-effect is applied, is downward diamagnetic pumping included and field advection by the equatorward meridional flow near the base of the convection zone allowed for, can the observed activity cycles be closely reproduced. Fluctuations in the alpha-effect can be estimated from sunspot data. Dynamo models with fluctuating parameters reproduce irregularities of solar cycles including the grand activity minima. The physics of parametric excitation of irregularities remains, however, to be understood.
The Magnetic Furnace: Intense Core Dynamos in B-stars
Augustson, Kyle C; Toomre, Juri
2016-01-01
The dynamo action achieved in the convective cores of main-sequence massive stars is explored here through 3-D global simulations of convective core dynamos operating within a young 10$M_{\\mathrm{sun}}$ B-type star, using the anelastic spherical harmonic (ASH) code. These simulations capture the inner 65% of this star by radius, encompassing the convective nuclear-burning core (about 23% by radius) and a portion of the overlying radiative envelope. Eight rotation rates are considered, ranging from 0.05% to 16% of the surface breakup velocity, thereby capturing both convection barely sensing the effects of rotation to others in which the Coriolis forces are prominent. The vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (ME) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (KE). The core convection typically involves turbulent colum...
Mechanism of local dynamo action on the Sun
Kitiashvili, I N; Mansour, N N; Wray, A A
2013-01-01
In the quiet Sun, magnetic fields are usually observed as small-scale magnetic elements, `salt and pepper', covering the entire solar surface. By using 3D radiative MHD numerical simulations we demonstrate that these fields are a result of local dynamo action in the top layers of the convection zone, where extremely weak `seed' magnetic fields can locally grow above the mean equipartition field (e.g., from a $10^{-6}$ G `seed' field to more than 1000 G magnetic structures). We find that the local dynamo action takes place only in a shallow, about 500 km deep, subsurface layer, from which the generated field is transported into deeper layers by convection downdrafts. We demonstrate that the observed dominance of vertical magnetic fields at the photosphere and the horizontal fields above the photosphere can be explained by multi-scale magnetic loops produced by the dynamo.
Spherical single-roll dynamos at large magnetic Reynolds numbers
Latter, Henrik
2010-01-01
This paper concerns kinematic helical dynamos in a spherical fluid body surrounded by an insulator. In particular, we examine their behaviour in the regime of large magnetic Reynolds number $\\Rm$, for which dynamo action is usually concentrated upon a simple resonant stream-surface. The dynamo eigensolutions are computed numerically for two representative single-roll flows using a compact spherical harmonic decomposition and fourth-order finite-differences in radius. These solutions are then compared with the growth rates and eigenfunctions of the Gilbert and Ponty (2000) large $\\Rm$ asymptotic theory. We find good agreement between the growth rates when $\\Rm>10^4$, and between the eigenfunctions when $\\Rm>10^5$.
Consistent scaling laws in anelastic spherical shell dynamos
Yadav, Rakesh K; Christensen, Ulrich R; Duarte, Lúcia D V
2013-01-01
Numerical dynamo models always employ parameter values that differ by orders of magnitude from the values expected in natural objects. However, such models have been successful in qualitatively reproducing properties of planetary and stellar dynamos. This qualitative agreement fuels the idea that both numerical models and astrophysical objects may operate in the same asymptotic regime of dynamics. This can be tested by exploring the scaling behavior of the models. For convection-driven incompressible spherical shell dynamos with constant material properties, scaling laws had been established previously that relate flow velocity and magnetic field strength to the available power. Here we analyze 272 direct numerical simulations using the anelastic approximation, involving also cases with radius-dependent magnetic, thermal and viscous diffusivities. These better represent conditions in gas giant planets and low-mass stars compared to Boussinesq models. Our study provides strong support for the hypothesis that b...
Numerical demonstration of fluctuation dynamo at low magnetic Prandtl numbers
Iskakov, A B; Cowley, S C; McWilliams, J C; Proctor, M R E
2007-01-01
Direct numerical simulations of incompressible nonhelical randomly forced MHD turbulence are used to demonstrate for the first time that the fluctuation dynamo exists in the limit of large magnetic Reynolds number Rm>>1 and small magnetic Prandtl number Pm6000 compared to Rm_c~60 for Pm>1. Is is not as yet possible to determine numerically whether the growth rate of the magnetic energy is ~Rm^{1/2} in the limit Rm->infinity, as should be the case if the dynamo is driven by the inertial-range motions at the resistive scale.
Cause of equatorward migration in global convective dynamo simulations
Warnecke, Jörn; Käpylä, Maarit J; Brandenburg, Axel
2014-01-01
We present results from four convectively-driven stellar dynamo simulations in spherical wedge geometry. All of these simulations produce cyclic and migrating mean magnetic fields. Through detailed comparisons we show that the migration direction can be explained by an $\\alpha\\Omega$ dynamo wave following the Parker--Yoshimura rule. We conclude that the equatorward migration in this and previous work is due to a positive (negative) $\\alpha$ effect in the northern (southern) hemisphere and a negative radial gradient of $\\Omega$ outside the inner tangent cylinder of these models. This idea is supported by a strong correlation between negative radial shear and toroidal field strength in the region of equatorward propagation.
Outflows from dynamo-active protostellar accretion discs
Von Rekowski, B; Dobler, W; Shukurov, A M; Brandenburg, Axel; Dobler, Wolfgang; Rekowski, Brigitta von; Shukurov, Anvar
2003-01-01
An axisymmetric model of a cool, dynamo-active accretion disc is applied to protostellar discs. Thermally and magnetically driven outflows develop that are not collimated within 0.1 AU. In the presence of a central magnetic field from the protostar, accretion onto the protostar is highly episodic, which is in agreement with earlier work.
Optimized boundary driven flows for dynamos in a sphere
Khalzov, I V; Cooper, C M; Weisberg, D B; Forest, C B; 10.1063/1.4764048
2012-01-01
We perform numerical optimization of the axisymmetric flows in a sphere to minimize the critical magnetic Reynolds number Rm_cr required for dynamo onset. The optimization is done for the class of laminar incompressible flows of von Karman type satisfying the steady-state Navier-Stokes equation. Such flows are determined by equatorially antisymmetric profiles of driving azimuthal (toroidal) velocity specified at the spherical boundary. The model is relevant to the Madison plasma dynamo experiment (MPDX), whose spherical boundary is capable of differential driving of plasma in the azimuthal direction. We show that the dynamo onset in this system depends strongly on details of the driving velocity profile and the fluid Reynolds number Re. It is found that the overall lowest Rm_cr~200 is achieved at Re~240 for the flow, which is hydrodynamically marginally stable. We also show that the optimized flows can sustain dynamos only in the range Rm_cr
Turbulent dynamo in a conducting fluid and partially ionized gas
Xu, Siyao
2016-01-01
By following the Kazantsev theory and taking into account both microscopic and turbulent diffusion of magnetic fields, we develop a unified treatment of the kinematic and nonlinear stages of turbulent dynamo, and study the dynamo process for a full range of magnetic Prandtl number Pm and ionization fractions. We find a striking similarity between the dependence of dynamo behavior on Pm in a conducting fluid and R (a function of ionization fraction) in partially ionized gas. In a weakly ionized medium, the kinematic stage is largely extended, including not only exponential growth but a new regime of dynamo characterized by linear-in-time growth of magnetic field strength, and the resulting magnetic energy is much higher than the kinetic energy carried by viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear stage is unaffected by microscopic diffusion processes and has a universal linear-in-time growth of magnetic energy with the growth rate as a constant fraction $3/38$ of the turbulent e...
Toward an asymptotic behaviour of the ABC dynamo
Bouya, Ismaël
2016-01-01
The ABC flow was originally introduced by Arnol'd to investigate Lagrangian chaos. It soon became the prototype example to illustrate magnetic-field amplification via fast dynamo action, i.e. dynamo action exhibiting magnetic-field amplification on a typical timescale independent of the electrical resistivity of the medium. Even though this flow is the most classical example for this important class of dynamos (with application to large-scale astrophysical objects), it was recently pointed out (Bouya Isma\\"el and Dormy Emmanuel, Phys. Fluids, 25 (2013) 037103) that the fast dynamo nature of this flow was unclear, as the growth rate still depended on the magnetic Reynolds number at the largest values available so far $(\\text{Rm} = 25000)$ . Using state-of-the-art high-performance computing, we present high-resolution simulations (up to 40963) and extend the value of $\\text{Rm}$ up to $ 5\\cdot10^5$ . Interestingly, even at these huge values, the growth rate of the leading eigenmode still depends on the controll...
Instability-driven interfacial dynamo in protoneutron stars
Mastrano, Alpha
2011-01-01
The existence of a tachocline in the Sun has been proven by helioseismology. It is unknown whether a similar shear layer, widely regarded as the seat of magnetic dynamo action, also exists in a protoneutron star. Sudden jumps in magnetic diffusivity $\\eta$ and turbulent vorticity $\\alpha$, for example at the interface between the neutron-finger and convective zones, are known to be capable of enhancing mean-field dynamo effects in a protoneutron star. Here we apply the well-known, plane-parallel, MacGregor-Charbonneau analysis of the Solar interfacial dynamo to the protoneutron star problem and calculate the growth rate analytically under a range of conditions. It is shown that, like the Solar dynamo, it is impossible to achieve self-sustained growth if the discontinuities in $\\alpha$, $\\eta$, and shear are coincident and the magnetic diffusivity is isotropic. In contrast, when the jumps in $\\eta$ and $\\alpha$ are situated away from the shear layer, self-sustained growth is possible for $P\\lesssim 49.8$ ms (i...
On the role of tachoclines in solar and stellar dynamos
Guerrero, G; Pino, E M de Gouveia Dal; Kosovichev, A G; Mansour, N N
2015-01-01
Rotational shear layers at the boundary between radiative and convective zones, tachoclines, play a key role in the dynamo process of magnetic field generation in the Sun and solar-like stars. We present two sets of global simulations of rotating turbulent convection and dynamo. The first set considers a stellar convective envelope only; the second one, aiming at the formation of a tachocline, considers also the upper part of the radiative zone. Our results indicate that dynamo properties like the growth rate, the saturation energy and mode depend on the Rossby (Ro) number. The models with slow rotation in the first set of simulations reproduce remarkably well the solar differential rotation in the convection zone. Depending on the value of Ro either oscillatory (with ~2 yr period) or steady dynamo solutions are obtained. The models in the second set naturally develop a tachocline which, in turn, leads to the generation of strong mean magnetic field (~1 Tesla). Since the field is also deposited into the stabl...
Solar small-scale dynamo and polarity of sunspot groups
Sokoloff, D; Abramenko, V
2015-01-01
In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, we suggest an observational test for small-scale dynamo action based on statistics of anti-Hale sunspot groups. As we have shown, according to theoretical expectations the small-scale dynamo action has to provide a population of sunspot groups which do not follow the Hale polarity law, and the density of such groups on the time-latitude diagram is expected to be independent on the phase of the solar cycle. Correspondingly, a percentage of the anti-Hale groups is expected to reach its maximum values during solar minima. For several solar cycles, we considered statistics of anti-Hale groups obtained by several scientific teams, including ours, to find that the percentage of anti-Hale groups becomes indeed maximal during a solar minimum. Our interpretation is that this fact may be explained by the small-scale dynamo action inside the solar convective zone.
On the effects of turbulence on a screw dynamo
Rädler, K H
2006-01-01
In an experiment in the Institute of Continuous Media Mechanics in Perm (Russia) an non--stationary screw dynamo is intended to be realized with a helical flow of liquid sodium in a torus. The flow is necessarily turbulent, that is, may be considered as a mean flow and a superimposed turbulence. In this paper the induction processes of the turbulence are investigated within the framework of mean--field electrodynamics. They imply of course a part which leads to an enhanced dissipation of the mean magnetic field. As a consequence of the helical mean flow there are also helical structures in the turbulence. They lead to some kind of $\\alpha$--effect, which might basically support the screw dynamo. The peculiarity of this $\\alpha$--effect explains measurements made at a smaller version of the device envisaged for the dynamo experiment. The helical structures of the turbulence lead also to other effects, which in combination with a rotational shear are potentially capable of dynamo action. A part of them can basi...
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
Numerical experiments on dynamo action in sheared and rotating turbulence
Yousef, T A; Rincon, F; Schekochihin, A A; Kleeorin, N; Rogachevskii, I; Cowley, S C; McWilliams, J C
2008-01-01
Numerical simulations of forced turbulence in elongated shearing boxes are carried out to demonstrate that a nonhelical turbulence in conjunction with a linear shear can give rise to a mean-field dynamo. Exponential growth of magnetic field at scales larger than the outer (forcing) scale of the turbulence is found. Over a range of values of the shearing rate S spanning approximately two orders of magnitude, the growth rate of the magnetic field is proportional to the imposed shear, gamma ~ S, while the characteristic spatial scale of the field is l_b ~ S^(-1/2). The effect is quite general: earlier results for the nonrotating case by Yousef et al. 2008 (PRL 100, 184501) are extended to shearing boxes with Keplerian rotation; it is also shown that the shear dynamo mechanism operates both below and above the threshold for the fluctuation dynamo. The apparently generic nature of the shear dynamo effect makes it an attractive object of study for the purpose of understanding thegeneration of magnetic fields in ast...
Helicity of Solar Active Regions from a Dynamo Model
Indian Academy of Sciences (India)
Piyali Chatterjee
2006-06-01
We calculate helicities of solar active regions based on the idea that poloidal flux lines get wrapped around a toroidal flux tube rising through the convection zone, thereby giving rise to the helicity. We use our solar dynamo model based on the Babcock–Leighton -effect to study how helicity varies with latitude and time.
Transitions in Dynamo Modes Controlled by the Domain Aspect Ratio
Goudard, L.; Dormy, E.
2007-12-01
Magnetic fields of internal origin are observed on many planets in the solar system. The Sun itself acts as a dynamo. While these natural objects are very different in their composition, when it comes to dynamo modeling the governing equations are remarkably similar. One of the controlling parameters to distinguish between these objects is the aspect ratio of the convecting domain. Comparing the Sun to the Earth raises the issue of the nature of reversals. A challenging issue is to determine why the geomagnetic field reverses polarity on an irregular basis, whereas the Sun --which is a much larger object, governed by stronger nonlinearities-- reverses its magnetic polarity on a quasi-periodic timescale of 11 yrs. We use a three-dimensional Boussinesq model (the Parody code) to investigate the transition between these two types of behavior. We show that the aspect ratio of the convecting domain controls the nature of the dynamo field. We report a butterfly-like diagram at large aspect ratio, with magnetic activity near 30° of latitudes, which migrates with time toward the equator. We trace the existence of the dynamo wave solution at various aspect ratio and suggest possible consequences for the geomagnetic secular variation.
Dipole collapse and reversal precursors in a numerical dynamo
Olson, Peter; Driscoll, Peter; Amit, Hagay
2009-03-01
Precursors to extreme geomagnetic field changes are examined in a numerical dynamo with a reversing dipolar magnetic field. A dynamo model with compositional convection in a rotating spherical shell produces a strongly dipolar external magnetic field over 6 Myr of simulated paleomagnetic time, with stable polarity epochs and occasional dipole collapses, some of which result in polarity reversals or dipole axis excursions. We analyze the model behavior during two dipole collapses, one that leads to a polarity reversal and one that does not, focusing on observable precursors. Reversed magnetic field induced in the dynamo interior by intermittent convective variability is the primary cause of dipole collapse. Spots of reversed magnetic flux emerge on the outer boundary at an early stage, then re-emerge with greater intensity at the height of the collapse. The energy in the external field cascades to higher harmonics as these reversed patches appear. Butterfly diagrams showing the reversed and normal flux contributions to the axial dipole reveal poleward migration of the patches during dipole collapse. Axial dipole reduction by precursory reversed flux is several times larger in the reversing case, compared to the non-reversing case. A butterfly diagram of the geomagnetic field since 1840 shows high latitude reversed flux emerging on the core-mantle boundary. Although the reversed geomagnetic flux is presently too weak to be labeled a reversal precursor, it is consistent with early stage dipole collapse in the dynamo model.
Can a Sinking Metallic Diapir Generate a Dynamo?
Schaeffer, N.; Monteux, J.; Amit, H.; Cardin, P.
2012-12-01
Metallic diapirs may have strongly contributed to core formations during the first million years of planetary evolutions. The length-scales of these diapirs can range from several centimetres to several hundred kilometres. The aim of this study is to determine whether the dynamics enhanced by the diapir sinking can drive a dynamo and to characterize the required conditions on the size of the diapir, the mantle viscosity and the planetary latitude at which the diapir sinks. We impose a classical Hadamard flow solution for the motion at the interface between a sinking diapir and a viscous mantle on dynamical simulations that account for rotational and inertial effects in order to model the flow within the diapir. The flows are confined to a velocity layer with a thickness that decreases with increasing rotation rate. This decrease depends on the initial latitude of the diapir. This 3D flow is then used as input for kinematic dynamo simulations to determine the critical magnetic Reynolds number for dynamo onset. Our results demonstrate that the flow pattern occurring inside a diapir sinking through a partially molten mantle within a rotating planet can generate a magnetic field. This dynamo generation is more favourable for a large diapir sinking from the equator than from the planet's rotational pole.
Gravitational dynamos and the low-frequency geomagnetic secular variation.
Olson, P
2007-12-18
Self-sustaining numerical dynamos are used to infer the sources of low-frequency secular variation of the geomagnetic field. Gravitational dynamo models powered by compositional convection in an electrically conducting, rotating fluid shell exhibit several regimes of magnetic field behavior with an increasing Rayleigh number of the convection, including nearly steady dipoles, chaotic nonreversing dipoles, and chaotic reversing dipoles. The time average dipole strength and dipolarity of the magnetic field decrease, whereas the dipole variability, average dipole tilt angle, and frequency of polarity reversals increase with Rayleigh number. Chaotic gravitational dynamos have large-amplitude dipole secular variation with maximum power at frequencies corresponding to a few cycles per million years on Earth. Their external magnetic field structure, dipole statistics, low-frequency power spectra, and polarity reversal frequency are comparable to the geomagnetic field. The magnetic variability is driven by the Lorentz force and is characterized by an inverse correlation between dynamo magnetic and kinetic energy fluctuations. A constant energy dissipation theory accounts for this inverse energy correlation, which is shown to produce conditions favorable for dipole drift, polarity reversals, and excursions. PMID:18048345
Solar small-scale dynamo and polarity of sunspot groups
Sokoloff, D.; Khlystova, A.; Abramenko, V.
2015-08-01
In order to clarify a possible role of small-scale dynamo in formation of solar magnetic field, we suggest an observational test for small-scale dynamo action based on statistics of anti-Hale sunspot groups. As we have shown, according to theoretical expectations the small-scale dynamo action has to provide a population of sunspot groups which do not follow the Hale polarity law, and the density of such groups on the time-latitude diagram is expected to be independent on the phase of the solar cycle. Correspondingly, a percentage of the anti-Hale groups is expected to reach its maximum values during solar minima. For several solar cycles, we considered statistics of anti-Hale groups obtained by several scientific teams, including ours, to find that the percentage of anti-Hale groups becomes indeed maximal during a solar minimum. Our interpretation is that this fact may be explained by the small-scale dynamo action inside the solar convective zone.
Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
Energy Technology Data Exchange (ETDEWEB)
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