Rotationally induced coherence in turbulent kinematic dynamos

TL;DR

This paper investigates how rotation enhances the coherence and efficiency of turbulent kinematic dynamos at low magnetic Prandtl numbers, highlighting the role of large-scale eddies in dynamo growth.

Contribution

It demonstrates that rotation induces long-lived large-scale eddies that significantly increase the dynamo growth rate, reducing the energy threshold compared to non-rotating flows.

Findings

Rotation increases spatio-temporal coherence in turbulent flows.

Rotationally induced eddies enhance dynamo growth rates.

Rotation reduces the critical magnetic Reynolds number needed for dynamo action.

Abstract

We consider rotating, kinematic dynamos at low magnetic Prandtl number $Pm$. We show that the inclusion of rotation leads to an increase in spatio-temporal coherence and a modification of the turbulent spectrum. These effects make the flow more efficient in driving the dynamo, in the sense that the energy injection rate required to reach the critical value of the magnetic Reynolds number $Rm_c$ is reduced in comparison with a non-rotating dynamo (Seshasayanan et al. 2017). For random dynamos it is known that the growth-rate would largely be determined by the spectral index of the flow at the resistive scale. Here, however, we demonstrate that the dynamo growth-rate in rotating flows is increased by the rotationally induced long-lived large scale eddies with a coherence time greater than the local turnover time. These eddies play the major role in determining the dynamo growth-rate.