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

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.