Large-scale-vortex dynamos in planar rotating convection

TL;DR

This paper investigates the magnetic field generation (dynamo action) in rotating convection flows, revealing a large-scale-vortex dynamo at moderate magnetic Reynolds numbers and its suppression at higher values, with implications for planetary magnetic fields.

Contribution

It introduces the concept of a large-scale-vortex dynamo in rotating convection and analyzes its behavior across different magnetic Reynolds numbers, highlighting its relevance to planetary magnetic fields.

Findings

Large-scale magnetic fields are generated at moderate Rm.

Oscillations in magnetic energy are linked to large-scale vortices.

High Rm leads to small-scale dynamo suppressing large vortices.

Abstract

Several recent studies have demonstrated how large-scale vortices may arise spontaneously in rotating planar convection. Here we examine the dynamo properties of such flows in rotating Boussinesq convection. For moderate values of the magnetic Reynolds number ($100 \lesssim Rm \lesssim 550$, with $Rm$ based on the box depth and the convective velocity), a large-scale (i.e. system-size) magnetic field is generated. The amplitude of the magnetic energy oscillates in time, nearly out of phase with the oscillating amplitude of the large-scale vortex. The large-scale vortex is disrupted once the magnetic field reaches a critical strength, showing that these oscillations are of magnetic origin. The dynamo mechanism relies on those components of the flow that have length scales lying between that of the large-scale vortex and the typical convective cell size; smaller-scale flows are not required. The large-scale vortex plays a crucial role in the magnetic induction despite being essentially two-dimensional; we thus refer to this dynamo as a large-scale-vortex dynamo. For larger magnetic Reynolds numbers, the dynamo is small scale, with a magnetic energy spectrum that peaks at the scale of the convective cells. In this case, the small-scale magnetic field continuously suppresses the large-scale vortex by disrupting the correlations between the convective velocities that allow it to form. The suppression of the large-scale vortex at high $Rm$ therefore probably limits the relevance of the large-scale-vortex dynamo to astrophysical objects with moderate values of $Rm$, such as planets. In this context, the ability of the large-scale-vortex dynamo to operate at low magnetic Prandtl numbers is of great interest.