Rotational effects on the small-scale dynamo

TL;DR

This study investigates how rotation influences the small-scale dynamo in turbulent flows, revealing that slow rotation suppresses dynamo action, but large-scale vortices at higher rotation rates can enhance it, with implications for astrophysical and geophysical magnetic fields.

Contribution

It provides new insights into the rotational effects on small-scale dynamo growth and saturation, highlighting the transition from suppression to enhancement due to vortex formation.

Findings

Slow rotation reduces dynamo growth rate and saturation level.

Large-scale vortices form beyond a rotation threshold, boosting dynamo activity.

Rotational suppression persists at high Reynolds numbers, with magnetic energy ratios reduced by up to 35%.

Abstract

Using direct numerical simulations of forced rotating turbulence, we study the effect of rotation on the growth rate and the saturation level of the small-scale dynamo. For slow rotation rates, increasing the rotation rate reduces both the growth rate and the saturation level. Once the rotation rate crosses a threshold, large-scale vortices are formed which enhance the growth rate and the saturation level. Below this threshold, the suppression of the small-scale dynamo with increasing rotation is explained by the fact that at scales close to, but smaller than, the forcing scale, rotating turbulence is one-dimensionalized, with the velocity component along the rotation axis being larger than the other two components. This is due to the rotational destabilization of vortices produced by the forcing function. While the rotational effect on the growth rate becomes small at high Re, the ratio of the steady-state magnetic to kinetic energies remains suppressed by up to 35% as compared to the non-rotating case.