A potential thermal dynamo and its astrophysical applications
TL;DR: In this article, it is shown that thermal turbulence, not unlike the standard kinetic and magnetic turbulence, can be an effective driver of a mean-field dynamo, and the thermal drives are likely to be dominant in systems that are endowed with subsonic, low-β turbulence.
Abstract: 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.
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TL;DR: In this paper, the authors construct a magnetic helicity conserving dynamo theory which incorporates a calculated magnetic heli-ity current, which plays a small role in large scale magnetic field generation.
Abstract: 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. The prospects for driving a fast dynamo under these circumstances are uncertain, but if it is possible, then the field must be largely force-free. 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 a shearing flow, like real stars and accretion disks, and some computer simulations, may successfully drive the generation of strong large scale magnetic fields, provided that $\partial_r\Omega >0$. We show that this criterion is usually satisfied. Such dynamos will include a persistent, spatially coherent vertical magnetic helicity current with the same sign as $-\partial_r\Omega$, that is, positive for an accretion disk and negative for the Sun. We comment on the role of random magnetic helicity currents in storing turbulent energy in a disordered magnetic field, which will generate an equipartition, disordered field in a turbulent medium, and also a declining long wavelength tail to the power spectrum. As a result, calculations of the galactic `seed' field are largely irrelevant.
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TL;DR: In this article , the authors investigated the dynamo effect of multiple tearing modes in a force-free plasma using resistive magnetohydrodynamics equations and found that the profile of a finite plasma region can be flattened if there are enough tearing modes.
Abstract: The dynamo effect of multiple tearing modes in a force-free plasma is investigated using resistive magnetohydrodynamics equations. In a steady state, two tearing modes are considered. It is found that the dynamo effect is related to the distance between the two rational surfaces and the magnetic island width. The [Formula: see text] profile is flatter for closer rational surfaces and wider magnetic islands. The case of an arbitrary number of tearing modes is also considered, and it is found that the λ profile in a finite plasma region can be flattened by the dynamo effect if there are enough tearing modes. This indicates that λ can be flattened in the entire plasma region, which makes it clear that the dynamo effect actually flattens λ rather than the current density. In the growth stage, the case of a growing tearing mode and two saturated modes is considered. The calculation shows that the middle tearing mode makes connections between the two modes on each side, playing the role of a mediator. Our results provide a more clear explanation for the dynamo effect of multiple tearing modes as a possible mechanism behind the Taylor relaxation process.
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TL;DR: The current understanding of astrophysical magnetic fields is reviewed in this paper, focusing on their generation and maintenance by turbulence, where analytical and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamo, where some form of parity breaking is crucial.
1,548 citations
TL;DR: In this paper, a wealth of information obtained from quasistationary laboratory experiments for plasma confinement is reviewed for drift waves driven unstable by density gradients, temperature gradients and trapped particle effects.
Abstract: Drift waves occur universally in magnetized plasmas producing the dominant mechanism for the transport of particles, energy and momentum across magnetic field lines. A wealth of information obtained from quasistationary laboratory experiments for plasma confinement is reviewed for drift waves driven unstable by density gradients, temperature gradients and trapped particle effects. The modern understanding of Bohm transport and the role of sheared flows and magnetic shear in reducing the transport to the gyro-Bohm rate are explained and illustrated with large scale computer simulations. The types of mixed wave and vortex turbulence spontaneously generated in nonuniform plasmas are derived with reduced magnetized fluid descriptions. The types of theoretical descriptions reviewed include weak turbulence theory, Kolmogorov anisotropic spectral indices, and the mixing length. A number of standard turbulent diffusivity formulas are given for the various space-time scales of the drift-wave turbulent mixing.
1,076 citations