Helical dynamo growth at modest versus extreme magnetic Reynolds numbers

TL;DR

This study investigates how large-scale helical magnetic fields grow in astrophysical objects, revealing that under certain conditions, the growth is independent of magnetic Reynolds number and remains significant at very high Rm values.

Contribution

It demonstrates that large-scale dynamo growth can be Rm-independent in the pre-saturation regime and clarifies the impact of simulation parameters on this behavior.

Findings

Large-scale field growth is Rm-independent in the pre-saturation regime.

Predicted Rm-independent and substantial magnetic field magnitude at high Rm.

Modest Rm and hyper-diffusive simulations can mislead about high Rm behavior.

Abstract

Understanding large-scale magnetic field growth in astrophysical objects is a persistent challenge. We tackle the long-standing question of how much helical large-scale dynamo growth occurs independent of the magnetic Reynolds number (Rm) in a closed volume. From modest-Rm numerical simulations, we identify a pre-saturation regime when the large-scale field grows independently of Rm, but to an Rm-dependent magnitude. For plausible magnetic spectra however, the analysis predicts the magnitude to be Rm-independent and substantial as Rm$\to\infty$. This gives renewed optimism for the relevance of closed dynamos and pinpoints how modest Rm and hyper-diffusive simulations can cause misapprehension of the Rm$\to\infty$ behavior.