Effect of a stably stratified layer near the outer boundary in numerical simulations of a magnetohydrodynamic dynamo in a rotating spherical shell and its implications for Earth’s core

We investigate the effects of stable thermal stratification near the outer boundary in numerical simulations of a magnetohydrodynamic (MHD) dynamo in a rotating spherical shell. The stratified layer acts as an interface for the flow between the convective and stratified regions and filters the magnetic field. Moreover, the stratified boundary also filters the energy generated in the convective region. The strongly multipolar magnetic field at high Rayleigh numbers without a stratified region could filter a dipolar-likemagnetic field when the stratified region is imposed. The main findings are described as follows: Zonal flow would be expected in high latitude region in a stratified layer. The penetrative flow in the stratified region is caused by a thermal wind at high latitudes but is not found at mid to low latitudes. The horizontal scale of convective flow with and without stratification does not differ greatly. The implications for the dynamics in Earth’s core are as follows: (1) A stratified region near the top of the core would be expected at low latitudes but not at high latitudes because thermal winds can penetrate into the stratified region if a dynamo exists. (2) For the existence of small-scale features in the geomagnetic field, laterally heterogeneous heat flux across the core–mantle boundary would be necessary or helpful if a stratified region near the top of the Earth’s core is plausible.

► We investigated effects of a stably stratified layer in numerical dynamo simulations. ► A stratified layer works as a filter of both magnetic field and convective flow. ► The filtering effects are likely to be the dipole field even in chaotic solutions. ► The rigid boundary enhances the filtering by the shear flow in a stratified layer. ► Earth’s core could have a stably stratified layer but further studies are required.