Catastrophic quenching in alpha-Omega dynamos revisited

TL;DR

This paper revisits alpha-Omega dynamo models, identifying issues with the traditional magnetic helicity formalism and proposing an alternative approach that yields larger saturation magnetic fields in oscillatory dynamos.

Contribution

It introduces an improved formalism for magnetic helicity evolution in alpha-Omega dynamos, correcting unphysical predictions of the traditional method.

Findings

Traditional formalism predicts unphysical helicity transfer.

The alternative formalism removes artifacts and predicts larger saturation fields.

The new approach improves the modeling of astrophysical large-scale dynamos.

Abstract

At large magnetic Reynolds numbers, magnetic helicity evolution plays an important role in astrophysical large-scale dynamos. The recognition of this fact led to the development of the dynamical alpha quenching formalism, which predicts catastrophically low mean fields in open systems. Here we show that in oscillatory alpha-Omega dynamos this formalism predicts an unphysical magnetic helicity transfer between scales. An alternative technique is proposed where this artifact is removed by using the evolution equation for the magnetic helicity of the total field in the shearing--advective gauge. In the traditional dynamical alpha quenching formalism, this can be described as an additional magnetic helicity flux of small-scale fields that does not appear in homogeneous alpha-squared dynamos. In alpha-Omega dynamos, the alternative formalism is shown to lead to larger saturation fields than previously obtained with the traditional formalism.