Efficient quasi-kinematic large-scale dynamo as the small-scale dynamo saturates

TL;DR

This paper demonstrates a new quasi-kinematic large-scale dynamo mechanism that operates efficiently after small-scale dynamo saturation, contributing to the understanding of magnetic field generation in astrophysical systems.

Contribution

It introduces a novel quasi-kinematic dynamo process that functions post small-scale dynamo saturation, advancing the comprehension of magnetic field evolution in turbulent astrophysical environments.

Findings

Both small-scale and large-scale dynamos can operate sequentially.

The quasi-kinematic dynamo sustains large-scale magnetic fields after small-scale saturation.

Simulations confirm the coexistence and efficiency of both dynamo processes.

Abstract

Large-scale magnetic fields in stars and galaxies are thought to arise by mean-field dynamo action due to the combined influence of both helical turbulence and shear. Those systems are also highly conducting and the turbulence therein leads to a fluctuation (or small-scale) dynamo which more rapidly amplifies magnetic field fluctuations on the eddy scales and smaller. Will this then interfere with and suppress the mean (or large-scale) field growth? Using direct numerical simulations of helical turbulence (with and without shear), we identify a novel quasi-kinematic large-scale dynamo which operates as the small-scale dynamo saturates. Thus both dynamos operate efficiently, one after the other, and lead to the generation of significant large-scale fields.