Self-consistent mean field MHD

TL;DR

This paper extends mean field theory to predict the stability of nonlinear MHD states, revealing the importance of coupled equations and transport tensors, and challenges the adequacy of a simple quenched alpha coefficient.

Contribution

It develops a mean field framework for nonlinear MHD stability analysis that accounts for coupling effects and transport tensors, advancing beyond previous kinematic models.

Findings

Mean field theory can predict growth rates of perturbations in nonlinear MHD.

Coupling between momentum and induction equations requires four transport tensors.

A simple quenched alpha coefficient is insufficient for nonlinear regime description.

Abstract

We consider the linear stability of two-dimensional nonlinear magnetohydrodynamic basic states to long-wavelength three-dimensional perturbations. Following Hughes & Proctor (2009a), the 2D basic states are obtained from a specific forcing function in the presence of an initially uniform mean field of strength $\mathcal{B}$. By extending to the nonlinear regime the kinematic analysis of Roberts (1970), we show that it is possible to predict the growth rate of these perturbations by applying mean field theory to \textit{both} the momentum and the induction equations. If $\mathcal{B}=0$, these equations decouple and large-scale magnetic and velocity perturbations may grow via the kinematic $\alpha$-effect and the AKA instability respectively. However, if $\mathcal{B} \neq 0$, the momentum and induction equations are coupled by the Lorentz force; in this case, we show that four transport tensors are now necessary to determine the growth rate of the perturbations. We illustrate these situations by numerical examples; in particular, we show that a mean field description of the nonlinear regime based solely on a quenched $\alpha$ coefficient is incorrect.