On large-scale dynamos with stable stratification and the application to stellar radiative zones

TL;DR

This paper investigates a non-helical large-scale dynamo mechanism in stellar radiative zones, demonstrating its robustness to stable stratification and potential to generate significant magnetic fields without catastrophic quenching.

Contribution

It provides evidence that the magnetic shear-current effect can sustain large-scale magnetic fields in stratified stellar zones, even with stable stratification and high magnetic Reynolds numbers.

Findings

The dynamo mechanism is robust to stable stratification.

Large-scale toroidal fields are generated near equipartition.

The mechanism shows resilience to catastrophic quenching.

Abstract

Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such magnetic fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We explore the effects of stable stratification on mean-field dynamo theory with a particular focus on a non-helical large-scale dynamo (LSD) mechanism known as the magnetic shear-current effect. We find that the mechanism is robust to increasing stable stratification as long as the original requirements for its operation are met: a source of shear and non-helical magnetic fluctuations (e.g. from a small-scale dynamo). Both are plausibly sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a dynamo mechanism resilient to catastrophic quenching, which has been an issue for helical dynamos. These properties -- the absence of catastrophic quenching and robustness to stable stratification -- make the mechanism a plausible candidate for generating in-situ large-scale magnetic fields in stellar radiative zones.