On Large-Scale Dynamo Action at High Magnetic Reynolds Number

TL;DR

This paper investigates how large-scale magnetic dynamo waves can persist at high magnetic Reynolds numbers, emphasizing the role of shear in suppressing small-scale dynamo action to enable large-scale magnetic field generation.

Contribution

It demonstrates that shear suppresses small-scale dynamo action at high Rm, allowing large-scale dynamo waves to persist, supporting a suppression principle for large-scale magnetic field generation.

Findings

Large-scale dynamo waves persist at high Rm with narrow-band helical flow and sufficient shear.

Wide-band helical flows lead to small-scale dynamo dominance unless shear is increased.

Shear acts to suppress small-scale dynamo action, facilitating large-scale dynamo emergence.

Abstract

We consider the generation of magnetic activity --- dynamo waves --- in the astrophysical limit of very large magnetic Reynolds number. We consider kinematic dynamo action for a system consisting of helical flow and large-scale shear. We demonstrate that large-scale dynamo waves persist at high $Rm$ if the helical flow is characterised by a narrow band of spatial scales and the shear is large enough. However for a wide band of scales the dynamo becomes small-scale with a further increase of $Rm$, with dynamo waves re-emerging only if the shear is then increased. We show that at high $Rm$ the key effect of the shear is to suppress small-scale dynamo action, allowing large-scale dynamo action to be observed. We conjecture that this supports a general "suppression principle" --- large-scale dynamo action can only be observed if there is a mechanism that suppresses the small-scale fluctuations.