An implication for the origin of stratification below the core–mantle boundary region in numerical dynamo simulations in a rotating spherical shell

• We investigated dynamo solutions with the stratified layer at the top of core.
• Three dynamo solutions are found in the space of Pm and the layer thickness.
• Bi-modal solution is found in the weak field regime.
• The chemical origin is preferred for the stratified layer at the top of core.

We investigate numerical dynamo simulations in a rotating spherical shell including a stably stratified region below the outer boundary to discuss dynamo solution behaviors and some implications for the dynamics of Earth’s core. As a consequence of over 50 numerical dynamo simulations, we found strong field, weak field and failed dynamos as a function of magnetic Prandtl number and thickness of stratified region. The weak field regime is not a very stable solution of dynamo action with a stably stratified region because a bistable solution is found depending on the initial amplitude of magnetic field. When the initial amplitude of magnetic field is given as a large value, the weak field regime changes into the strong field regime. One outcome of the study is that when the Earth’s core evolution has high conductivity, the dynamo action with purely thermal stratification may not be suitable for explaining the generation of geomagnetic field. This is because the dynamo solution is likely to fail with a thick stratified region. This implies that the origin of stratification is likely to be compositional rather thermal in origin. This is consistent with the 1-D seismic velocity anomalies found from seismic waveform analyses.