Astronomy and Astrophysics – Astrophysics – Solar and Stellar Astrophysics
Scientific paper
2009-09-03
Proc. R. Soc. A 466, 583 (2010)
Astronomy and Astrophysics
Astrophysics
Solar and Stellar Astrophysics
Submitted to Proc R. Soc., 22/07/09 Accepted subject to minor revisions, 11/08/09. Revised version resubmitted, 25/09/09
Scientific paper
10.1098/rspa.2009.0384
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.
Courvoisier Alice
Hughes David W. W.
Proctor Michael R. E.
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