Dynamos driven by top-heavy double-diffusive convection in the strong-field regime

TL;DR

This study compares dynamo simulations driven by top-heavy double-diffusive convection with traditional co-density models, finding they produce similar magnetic fields in the strong-field regime, supporting the use of simpler models in planetary studies.

Contribution

It demonstrates that co-density and double-diffusive convection models yield similar magnetic field results, validating the co-density approach in planetary dynamo simulations.

Findings

Both models produce comparable magnetic fields at high magnetic Reynolds numbers.

Dynamo models based on DDC and co-density show similar secular variations.

Magnetic field observations alone may not distinguish buoyancy sources in planetary cores.

Abstract

The magnetic fields of terrestrial planets are generated in their liquid cores through dynamo action driven by thermal and compositional convection. The coexistence of these two buoyancy sources gives rise to double-diffusive convection (DDC) due to the contrast between thermal and compositional diffusivities. However, most dynamo simulations adopt the co-density model, where the two diffusivities are assumed to be equal. In this study, we performed both hydrodynamic and dynamo simulations of top-heavy DDC in a rotating spherical shell with the Lewis number $Le=100$, and compared them with corresponding co-density models. In the hydrodynamic regime, the convective flow morphology is strongly influenced by the nature of the buoyancy sources. However, our dynamo simulations in the strong-field regime demonstrate that the co-density and DDC models yield qualitatively similar magnetic fields at comparable magnetic Reynolds numbers, albeit with some differences in detail. These numerical models further justify the use of the co-density model in planetary dynamo simulations. Finally, we demonstrate that dynamo models based on DDC and co-density produce similar magnetic fields and secular variations at the core-mantle boundary. This suggests that it may not be possible to distinguish the buoyancy sources responsible for planetary dynamos based solely on magnetic field observations.