Cross helicity and related dynamo

Yokoi, Nobumitsu

doi:10.1080/03091929.2012.754022

Cited by 96 publications

(143 citation statements)

References 71 publications

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“…2 We assume that the EMF due to the background turbulence vanishes, u × b 0 = 0 (note that, due to the statistical average, this does not necessarily restrict the turbulent cross helicity u · b 0 ). Such a B independent contribution could be important in some situations (see, for example, Yoshizawa and Yokoi [32]) and the method applied here can be used to calculate well-known effects of this type if desired, for instance the cross-helicity effect [28]. In addition, we do not calculate the components of the Reynolds stress, which would force a mean-field velocity U.…”

Section: Fundamentals Of Mean-field Electrodynamicsmentioning

confidence: 99%

“…(12)]. However, there is no particular reason to expect the saturation of the dynamo to be so simple; because the mean field itself strongly influences the turbulence, a wide variety of nonlinear effects are possible beyond a simple dependence of transport coefficients on the mean field, for instance those related to magnetic helicity transport [27] or generation of cross helicity [28] (see Sec. V for further discussion).…”

Section: Introductionmentioning

confidence: 99%

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Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Squire¹,

Bhattacharjee²

2015

Phys. Rev. E

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This article presents a calculation of the mean electromotive force arising from general small-scale magnetohydrodynamical turbulence, within the framework of the second-order correlation approximation. With the goal of improving understanding of the accretion disk dynamo, effects arising through small-scale magnetic fluctuations, velocity gradients, density and turbulence stratification, and rotation, are included. The primary result, which supplements numerical findings, is that an off-diagonal turbulent resistivity due to magnetic fluctuations can produce large-scale dynamo action-the magnetic analog of the "shear-current" effect. In addition, consideration of α effects in the stratified regions of disks gives the puzzling result that there is no strong prediction for a sign of α, since the effects due to kinetic and magnetic fluctuations, as well as those due to shear and rotation, are each of opposing signs and tend to cancel each other.

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Section: Fundamentals Of Mean-field Electrodynamicsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Squire¹,

Bhattacharjee²

2015

Phys. Rev. E

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“…The eddy-viscosity (EV) model has the longest tradition with roots going back even further than its formulation for LES by Smagorinsky [25]. While originally developed for the kinetic SGS stress tensor in hydrodynamics, the general idea has been transferred to MHD [18,21], where the EMF closure is usually referred to as anomalous or eddy-resistivity. The names of these functional models stem from their primary feature: purely dissipative behavior analogous to e.g.…”

Section: Methodsmentioning

confidence: 99%

“…However, work in the realm of MHD and in particular compressible MHD is scarce, see [16] and [17] for recent reviews. Directly linked to this work are the MHD simulations of (decaying) turbulent boxes in 3D [18], in 2D [19] and in the incompressible case [20,21]. However, all these groups use different numerical schemes, e.g.…”

Section: Introductionmentioning

confidence: 99%

Comparative statistics of selected subgrid-scale models in large-eddy simulations of decaying, supersonic magnetohydrodynamic turbulence

et al. 2017

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Large eddy simulations (LES) are a powerful tool in understanding processes that are inaccessible by direct simulations due to their complexity, for example, in the highly turbulent regime. However, their accuracy and success depends on a proper subgrid-scale (SGS) model that accounts for the unresolved scales in the simulation. We evaluate the applicability of two traditional SGS models, namely the eddy-viscosity (EV) and the scale-similarity (SS) model, and one recently proposed nonlinear (NL) SGS model in the realm of compressible MHD turbulence. Using 209 simulations of decaying, supersonic (initial sonic Mach number Ms ≈ 3) MHD turbulence with a shock-capturing scheme and varying resolution, SGS model and filter, we analyze the ensemble statistics of kinetic and magnetic energy spectra and structure functions. Furthermore, we compare the temporal evolution of lower and higher order statistical moments of the spatial distributions of kinetic and magnetic energy, vorticity, current density, and dilatation magnitudes. We find no statistical influence on the evolution of the flow by any model if grid-scale quantities are used to calculate SGS contributions. In addition, the SS models, which employ an explicit filter, have no impact in general. On the contrary, both EV and NL models change the statistics if an explicit filter is used. For example, they slightly increase the dissipation on the smallest scales. We demonstrate that the nonlinear model improves higher order statistics already with a small explicit filter, i.e. a three-point stencil. The results of e.g. the structure functions or the skewness and kurtosis of the current density distribution are closer to the ones obtained from simulations at higher resolution. In addition, no additional regularization to stabilize the model is required. We conclude that the nonlinear model with a small explicit filter is suitable for application in more complex scenarios when higher order statistics are important.

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“…A more comprehensive form for the electromotive force using up to the first-order derivatives of the mean fields is expressed as (Yokoi, 2013) …”

Section: Introductionmentioning

confidence: 99%

Evaluation of electromotive force in interplanetary space

Narita¹,

Vörös²

2018

Ann. Geophys.

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Abstract. Electromotive force plays a central role in the turbulent dynamo mechanism and carries important information on the nature of the turbulent fields. In this study, an analysis method is developed for the electromotive force and the transport coefficients such as those for the α effect (coefficient α) and the turbulent diffusivity (coefficient β). The method is applied to a magnetic cloud event observed by the Helios 2 spacecraft in the inner heliosphere. The electromotive force is enhanced together with the magnetic cloud event by 1 or 2 orders of magnitude, suggesting that the magnetic field can locally be amplified in the heliosphere, presumably for a short time.

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Cross helicity and related dynamo

Cited by 96 publications

References 71 publications

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

Comparative statistics of selected subgrid-scale models in large-eddy simulations of decaying, supersonic magnetohydrodynamic turbulence

Evaluation of electromotive force in interplanetary space

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