Turbulent Dynamos and Magnetic Helicity

Ji, Hantao

doi:10.1103/physrevlett.83.3198

Cited by 72 publications

(113 citation statements)

References 31 publications

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“…Even when the Hall effect is marginal and acts only close to the dissipation scales ( ¼ 0:066), the generation of mean magnetic helicity decreases by a factor of 2. This strong difference in the evolution of the magnetic helicity between the MHD and the Hall-MHD dynamos is in good agreement with previous theoretical results (Ji 1999), which predict that MHD turbulent dynamo action converts magnetic helicity from the turbulent field to the mean field, but in the presence of the Hall effect, the magnetic helicity is only transported across space.…”

Section: No 1 2003 Dynamo Action In Mhd and Hall-mhdsupporting

confidence: 80%

“…Regardless of suppression or enhancement of the magnetic energy, the Hall effect in our simulations always and strongly suppresses the production of net magnetic helicity. This result, extremely relevant to helicitygeneration schemes, is in good agreement with previous theoretical results (Ji 1999).…”

supporting

confidence: 81%

“…A few exceptions are studies by Heintzmann (1983) and Galanti, Kleeorin, & Rogachevskii (1994), focused on particular geometries and showing either suppression or enhancement of dynamo action depending on external parameters, an analysis of the effect of turbulent dynamo action on magnetic helicity generation (Ji 1999), and a recent general closure proposed to compute the contribution of the Hall term to the -effect (Mininni, Gó mez, & Mahajan 2002). Preliminary results from the latter study show that both large suppression or enhancement of dynamo action (as compared to standard MHD) are possible, depending on parameters that describe the state of the system.…”

Section: Introductionmentioning

confidence: 99%

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Dynamo Action in Magnetohydrodynamics and Hall‐Magnetohydrodynamics

Mininni

Gomez

Mahajan

2003

ApJ

128

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The first direct numerical simulations of turbulent Hall dynamos are presented. The evolution of an initially weak and small-scale magnetic field in a system maintained in a stationary regime of hydrodynamic turbulence (by a stirring force at a macroscopic scale) is studied to explore the conditions for exponential growth of the magnetic energy. The Hall current is shown to have a profound effect on turbulent dynamo action; it can strongly enhance or suppress the generation of the large-scale magnetic energy depending on the relative values of the length scales of the system.

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Section: No 1 2003 Dynamo Action In Mhd and Hall-mhdsupporting

confidence: 80%

supporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

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Dynamo Action in Magnetohydrodynamics and Hall‐Magnetohydrodynamics

Mininni

Gomez

Mahajan

2003

ApJ

128

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“…However, since magnetic helicity is conserved, and was zero initially, it should remain zero, at least on a dynamical time scale (Berger 1984). This condition can be obeyed if the magnetic field is bi-helical, i.e., with opposite signs of magnetic helicity at large and small wavenumbers (Seehafer 1996;Ji 1999). We now ask whether signatures of this could in principle be detected at the solar surface.…”

Section: Surface Diagnosticsmentioning

confidence: 99%

Competition of rotation and stratification in flux concentrations

Losada

Brandenburg

Kleeorin

et al. 2013

A&A

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Context. In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable and spontaneously form local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism, which is called negative effective magnetic pressure instability (NEMPI), is of interest in connection with dynamo scenarios in which most of the magnetic field resides in the bulk of the convection zone and not at the bottom, as is often assumed. Recent work using mean-field hydromagnetic equations has shown that NEMPI becomes suppressed at rather low rotation rates with Coriolis numbers as low as 0.1. Aims. Here we extend these earlier investigations by studying the effects of rotation both on the development of NEMPI and on the effective magnetic pressure. We also quantify the kinetic helicity resulting from direct numerical simulations (DNS) with Coriolis numbers and strengths of stratification comparable to values near the solar surface and compare it with earlier work at smaller scale separation ratios. Further, we estimate the expected observable signals of magnetic helicity at the solar surface. Methods. To calculate the rotational effect on the effective magnetic pressure we consider both DNS and analytical studies using the τ approach. To study the effects of rotation on the development of NEMPI we use both DNS and mean-field calculations of the three-dimensional hydromagnetic equations in a Cartesian domain. Results. We find that the growth rates of NEMPI from earlier mean-field calculations are well reproduced with DNS, provided the Coriolis number is below 0.06. In that case, kinetic and magnetic helicities are found to be weak and the rotational effect on the effective magnetic pressure is negligible as long as the production of flux concentrations is not inhibited by rotation. For faster rotation, dynamo action becomes possible. However, there is an intermediate range of rotation rates where dynamo action on its own is not yet possible, but the rotational suppression of NEMPI is being alleviated. Conclusions. Production of magnetic flux concentrations through the suppression of turbulent pressure appears to be possible only in the uppermost layers of the Sun, where the convective turnover time is less than two hours.

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“…This argument was advanced by Gruzinov and Diamond [34,35] who pointed out that magnetic helicity conservation combined with the assumption of stationary statistics for small scale structure implied almost complete suppression of the kinematic dynamo. We note also that the form of the parallel component of the electromotive force given in equation (22) has been suggested before [6], although the interpretation that the relevant current is a magnetic helicity current appeared somewhat later [42,46]. Here we will follow the treatment in [83], where the magnetic helicity current was derived for the first time for homogeneous turbulence.…”

Section: Magnetic Helicity Conservation Constraintmentioning

confidence: 99%

Problems and Progress in Astrophysical Dynamos

Vishniac

Lazarían²,

Cho³

Turbulence and Magnetic Fields in Astrophysics

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Abstract. Astrophysical objects with negligible resistivity are often threaded by large scale magnetic fields. The generation of these fields is somewhat mysterious, since a magnetic field in a perfectly conducting fluid cannot change the flux threading a fluid element, or the field topology. Classical dynamo theory evades this limit by assuming that magnetic reconnection is fast, even for vanishing resistivity, and that the large scale field can be generated by the action of kinetic helicity. Both these claims have been severely criticized, and the latter appears to conflict with strong theoretical arguments based on magnetic helicity conservation and a series of numerical simulations. Here we discuss recent efforts to explain fast magnetic reconnection through the topological effects of a weak stochastic magnetic field component. We also show how mean-field dynamo theory can be recast in a form which respects magnetic helicity conservation, and how this changes our understanding of astrophysical dynamos. Finally, we comment briefly on why an asymmetry between small scale magnetic and velocity fields is necessary for dynamo action, and how it can arise naturally.

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Turbulent Dynamos and Magnetic Helicity

Cited by 72 publications

References 31 publications

Dynamo Action in Magnetohydrodynamics and Hall‐Magnetohydrodynamics

Dynamo Action in Magnetohydrodynamics and Hall‐Magnetohydrodynamics

Competition of rotation and stratification in flux concentrations

Problems and Progress in Astrophysical Dynamos

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