Small-scale magnetic helicity losses from a mean-field dynamo

TL;DR

This paper demonstrates that small-scale magnetic helicity fluxes, driven by various mechanisms, can mitigate catastrophic quenching in mean-field dynamo models, with implications for understanding solar and astrophysical magnetic fields.

Contribution

It introduces a detailed analysis of small-scale magnetic helicity fluxes and their role in alleviating quenching in mean-field dynamo models, considering different flux mechanisms and shear effects.

Findings

Small-scale magnetic helicity fluxes can reduce quenching.

Shear introduces additional fluxes that do not alleviate quenching.

Forcing helicity flux to vanish causes unphysical behavior.

Abstract

Using mean-field models with a dynamical quenching formalism we show that in finite domains magnetic helicity fluxes associated with small-scale magnetic fields are able to alleviate catastrophic quenching. We consider fluxes that result either from advection by a mean flow, the turbulent mixing down the gradient of mean small-scale magnetic helicity concentration, or the explicit removal which may be associated with the effects of coronal mass ejections in the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes are found to be equally strong both for large-scale and small-scale fields. In the presence of shear there is also an additional magnetic helicity flux associated with the mean field, but this flux does not alleviate catastrophic quenching. Outside the dynamo-active region there are neither sources nor sinks of magnetic helicity, so in a steady state this flux must be constant. It is shown that unphysical behavior emerges if the small-scale magnetic helicity flux is forced to vanish within the computational domain.