Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence

TL;DR

This paper calculates the mean electromotive force in magnetohydrodynamic turbulence, revealing how magnetic fluctuations and shear can drive large-scale dynamos, with implications for understanding accretion disk magnetic fields.

Contribution

It introduces a theoretical calculation of the electromotive force considering various effects, highlighting the role of off-diagonal turbulent resistivity in dynamo action.

Findings

Magnetic fluctuations can produce large-scale dynamo action via off-diagonal resistivity.

No definitive sign prediction for the alpha effect due to competing influences.

Results support the shear-current effect as a dynamo mechanism.

Abstract

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 analogue of the "shear-current" effect. In addition, consideration of $\alpha$ effects in the stratified regions of disks gives the puzzling result that there is no strong prediction for a sign of $\alpha$, 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.