Effects of shell thickness on cross-helicity generation in convection-driven spherical dynamos

TL;DR

This study investigates how shell thickness influences the roles of helicity and cross-helicity in magnetic field generation within convection-driven spherical dynamos, revealing distinct dynamo regimes and the dominance of effects depending on shell parameters.

Contribution

It demonstrates the coexistence of two dynamo regimes and analyzes how shell thickness affects the relative importance of helicity effects in magnetic field generation.

Findings

Two dynamo regimes coexist in simulations.

Cross-helicity and helicity effects are comparable in fluctuating regime.

Helicity effect dominates in mean-field regime, especially with thinner shells.

Abstract

The relative importance of the helicity and cross-helicity electromotive dynamo effects for self-sustained magnetic field generation by chaotic thermal convection in rotating spherical shells is investigated as a function of shell thickness. Two distinct branches of dynamo solutions are found to coexist in direct numerical simulations for shell aspect ratios between 0.25 and 0.6 - a mean-field dipolar regime and a fluctuating dipolar regime. The properties characterising the coexisting dynamo attractors are compared and contrasted, including differences in temporal behavior and spatial structures of both the magnetic field and rotating thermal convection. The helicity $\alpha$-effect and the cross-helicity $\gamma$-effect are found to be comparable in intensity within the fluctuating dipolar dynamo regime, where their ratio does not vary significantly with the shell thickness. In contrast, within the mean-field dipolar dynamo regime the helicity $\alpha$-effect dominates by approximately two orders of magnitude and becomes stronger with decreasing shell thickness.