Thermal structure at the inner core boundary in dynamo simulations with heat equation for the whole core

Abstract:

Resent seismic observations suggests that inner core has a seismic anisotropy. These studies suggest aspherical growth of the inner core, and slow viscous deformation of the inner core and latent heat distribution by flow motion are expected to be the origin of the aspherical growth of the inner core. To explain inner core anisotropy and aspherical growth of the inner core, a number of dynamo simulations have been performed with prescribed boundary conditions at ICB to take into account the inner core heterogeneity. In the present study, geodynamo simulations are performed with considering the heat equation throughout the inner and outer core in order to represent thermal structure of the ICB self-consistently.

To compare simulations with the boundary condition at ICB, we set no heat sources in the outer core and homogeneous heat source in the inner core to conserve the thermal energy. A homogeneous heat flux is set as the thermal boundary condition at CMB. To simplify the model, we assume that the same thermal diffusivity for the inner core and outer core and that latent heat at ICB is not considered. In the present study, We compare the simulations results with the simulations results using fixed heat flux or temperature condition at ICB.

The results show that the time averaged thermal structure at ICB is likely to the simulation results with homogeneous heat flux boundary conditions. The time averaged temperature variation is approximately 26% of the average temperature difference between ICB and CMB, while heat flux variation is only 6% of the average heat flux at the ICB. We also observe small scale temperature and heat flux variations; however, these components vary with time. Furthermore, there is small dependence of the Y₂⁰ component of the temperature variation on the Rayleigh number. Comparing with the non-magnetic cases, the large Y₂⁰ component of temperature variation is observed only in the dynamo cases. Looking at the \phi-component of the force balances averaged over longitudinal direction, the Coriolis force balances with the Lorentz force near the tangent cylinder near the lower boundary. This force balance sustains the poleward flow near the inner core boundary, and generates upwelling flow inside of the tangent cylinder. We conclude that this flow sustains the intense Y₂⁰ component of the temperature.