Effects of Shell Thickness on Cross-Helicity Generation in Convection-Driven Spherical Dynamos

• Main Authors: Luis Silva, Parag Gupta, David MacTaggart, Radostin D. Simitev
• Format: Article
• Language: English
• Published: MDPI AG 2020-12-01
• Series: Fluids
• Subjects: rotating thermal convection; convection-driven dynamos; numerical simulations; bistability; mean-field magnetohydrodynamics; spherical shells
• Online Access: https://www.mdpi.com/2311-5521/5/4/245

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 behaviour and spatial structures of both magnetic fields and rotating thermal convection. The helicity <inline-formula><math display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-effect and the cross-helicity <inline-formula><math display="inline"><semantics><mi>γ</mi></semantics></math></inline-formula>-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 <inline-formula><math display="inline"><semantics><mi>α</mi></semantics></math></inline-formula>-effect dominates by approximately two orders of magnitude and becomes stronger with decreasing shell thickness.