Magnetoconvection in a spherical shell: Equatorial symmetry during the transition from the weak- to the strong-field regime

TL;DR

This paper investigates the transition from weak to strong magnetic fields in spherical shell dynamos, revealing that equatorial symmetry breaking is linked to magnetic field growth and is crucial for understanding dynamo regimes.

Contribution

It introduces magnetoconvection simulations that bridge weak and strong-field regimes, highlighting the role of symmetry-breaking in dynamo transitions.

Findings

Symmetry-breaking correlates with magnetic field growth.

Transition to strong-field regime involves equatorial symmetry breaking.

Magnetoconvection simulations help understand dynamo regime changes.

Abstract

At small but supercritical Rayleigh numbers, simulations of dynamos in spherical shells often separate into two broad regimes characterised either by their relative magnetic field strength (weak/strong) or by their dominant force balance (VAC/MAC). These regimes can tend smoothly from one to the other but can also be bistable, a phenomenon which occurs particularly at large $\Pm$. We show that in either case the transition correlates with a breaking of equatorial symmetry. Nonlinear simulations of the geodynamo cannot be performed at accurate parameters and hence it is important to ensure that the correct (strong-field) branch is tracked as a distinguished limit is tracked towards a correct parameterisation from the simulations that we can perform. In order to understand the transition to strong-field dynamos, and better understand the mechanisms that occur in both branches, we report on a series of magnetoconvection simulations (that is, with the magnetic field fixed at the outer boundary) with which we bridge the gap between the strong- and weak-field regimes, and show that symmetry-breaking is triggered by the sudden growth of the magnetic field and in turn supports the dynamo in the strong-field regime.