On radiation-zone dynamos

TL;DR

This paper investigates how magnetic current-driven instabilities in radiation zones generate flow and magnetic field patterns with helicity and alpha-effects, depending on the background magnetic field's current helicity, with implications for stellar magnetic dynamics.

Contribution

It demonstrates that the kink-type instability produces helicity and alpha-effects only if the background magnetic field has non-zero current helicity, providing strict sign rules for cylindric and spherical setups.

Findings

Positive large-scale current helicity leads to negative small-scale kinetic helicity.

The alpha-effect is positive when the background field has positive current helicity.

Results are applicable to cylindric and spherical magnetic configurations.

Abstract

It is shown that the magnetic current-driven (`kink-type') instability produces flow and field patterns with helicity and even with \alpha-effect but only if the magnetic background field possesses non-vanishing current helicity \bar{\vec{B}}\cdot curl \bar{\vec{B}} by itself. Fields with positive large-scale current helicity lead to negative small-scale kinetic helicity. The resulting \alpha-effect is positive. These results are very strict for cylindric setups without <i>z/I>-dependence of the background fields. The sign rules also hold for the more complicated cases in spheres where the toroidal fields are the result of the action of differential rotation (induced from fossil poloidal fields) at least for the case that the global rotation is switched off after the onset of the instability.