Helicity and alpha-effect by current-driven instabilities of helical magnetic fields

TL;DR

This study investigates how current-driven instabilities in helical magnetic fields influence helicity and the alpha-effect, revealing small alpha-effects that challenge the feasibility of certain dynamo mechanisms in stellar radiation zones.

Contribution

It provides new insights into the scale and anisotropy of the alpha-effect generated by current-driven instabilities in helical magnetic fields, with implications for stellar dynamo theory.

Findings

Small alpha-effect amplitudes challenge the operation of alpha-Omega dynamos.

Helical background fields produce opposite signs of small-scale and large-scale helicities.

The alpha-tensor is highly anisotropic with opposite signs in different components.

Abstract

Helical magnetic background fields with adjustable pitch angle are imposed on a conducting fluid in a differentially rotating cylindrical container. The small-scale kinetic and current helicities are calculated for various field geometries, and shown to have the opposite sign as the helicity of the large-scale field. These helicities and also the corresponding $\alpha$-effect scale with the current helicity of the background field. The $\alpha$-tensor is highly anisotropic as the components $\alpha_{\phi\phi}$ and $\alpha_{zz}$ have opposite signs. The amplitudes of the azimuthal $\alpha$-effect computed with the cylindrical 3D MHD code are so small that the operation of an $\alpha\Omega$ dynamo on the basis of the current-driven, kink-type instabilities of toroidal fields is highly questionable. In any case the low value of the $\alpha$-effect would lead to very long growth times of a dynamo in the radiation zone of the Sun and early-type stars of the order of mega-years.