Effect of width, amplitude, and position of a core mantle boundary hot spot on core convection and dynamo action

TL;DR

This study uses numerical simulations to explore how hot spots at the core-mantle boundary influence core convection and magnetic field generation, revealing that the size and position of these anomalies significantly affect flow symmetry and magnetic field characteristics.

Contribution

It provides a detailed analysis of how parametrized CMB heat flux anomalies impact core convection and dynamo processes, highlighting the importance of anomaly size and location.

Findings

Anomaly size comparable to outer core radius significantly influences flow and magnetic symmetry.

Self-consistent dynamo models show either suppression or enhancement of equatorial antisymmetric flows.

CMB heat flux anomalies cannot explain the observed crustal magnetization asymmetry on Mars.

Abstract

Within the fluid iron cores of terrestrial planets, convection and the resulting generation of global magnetic fields are controlled by the overlying rocky mantle. The thermal structure of the lower mantle determines how much heat is allowed to escape the core. Hot lower mantle features, such as the thermal footprint of a giant impact or hot mantle plumes, will locally reduce the heat flux through the core mantle boundary (CMB), thereby weakening core convection and affecting the magnetic field generation process. In this study, we numerically investigate how parametrised hot spots at the CMB with arbitrary sizes, amplitudes, and positions affect core convection and hence the dynamo. The effect of the heat flux anomaly is quantified by changes in global flow symmetry properties, such as the emergence of equatorial antisymmetric, axisymmetric (EAA) zonal flows. For purely hydrodynamic models, the EAA symmetry scales almost linearly with the CMB amplitude and size, whereas self-consistent dynamo simulations typically reveal either suppressed or drastically enhanced EAA symmetry depending mainly on the horizontal extent of the heat flux anomaly. Our results suggest that the length scale of the anomaly should be on the same order as the outer core radius to significantly affect flow and field symmetries. As an implication to Mars and in the range of our model, the study concludes that an ancient core field modified by a CMB heat flux anomaly is not able to heterogeneously magnetise the crust to the present-day level of north--south asymmetry on Mars. The resulting magnetic fields obtained using our model either are not asymmetric enough or, when they are asymmetric enough, show rapid polarity inversions, which are incompatible with thick unidirectional magnetisation.