Shear-driven and diffusive helicity fluxes in alpha-Omega dynamos

TL;DR

This study investigates how magnetic helicity fluxes, especially diffusive and Vishniac-Cho fluxes, influence the saturation and distribution of magnetic fields in solar-like alpha-Omega dynamo models, highlighting their roles at high magnetic Reynolds numbers.

Contribution

The paper introduces a comprehensive model incorporating magnetic helicity fluxes into alpha-Omega dynamo simulations, revealing their effects on quenching and magnetic field distribution in spherical geometry.

Findings

Diffusive fluxes alleviate catastrophic quenching at high Rm.

Vishniac-Cho flux increases magnetic field amplitude for Rm<10^4.

Shear and flux divergence can enable subcritical dynamo action.

Abstract

We present nonlinear mean-field alpha-Omega dynamo simulations in spherical geometry with simplified profiles of kinematic alpha effect and shear. We take magnetic helicity evolution into account by solving a dynamical equation for the magnetic alpha effect. This gives a consistent description of the quenching mechanism in mean-field dynamo models. The main goal of this work is to explore the effects of this quenching mechanism in solar-like geometry, and in particular to investigate the role of magnetic helicity fluxes, specifically diffusive and Vishniac-Cho (VC) fluxes, at large magnetic Reynolds numbers (Rm). For models with negative radial shear or positive latitudinal shear, the magnetic alpha effect has predominantly negative (positive) sign in the northern (southern) hemisphere. In the absence of fluxes, we find that the magnetic energy follows an Rm^-1 dependence, as found in previous works. This catastrophic quenching is alleviated in models with diffusive magnetic helicity fluxes resulting in magnetic fields comparable to the equipartition value even for Rm=10^7. On the other hand, models with a shear-driven Vishniac-Cho flux show an increase of the amplitude of the magnetic field with respect to models without fluxes, but only for Rm<10^4. This is mainly a consequence of assuming a vacuum outside the Sun which cannot support a significant VC flux across the boundary. However, in contrast with the diffusive flux, the VC flux modifies the distribution of the magnetic field. In addition, if an ill-determined scaling factor in the expression for the VC flux is large enough, subcritical dynamo action is possible that is driven by the action of shear and the divergence of current helicity flux.