Dynamo simulations with a stratified F-layer at the base of Earth's outer core

Seismic observations reveal a slowdown in the P wave velocity at the base of the Earth's outer core compared with PREM, which is attributed to the presence of a stably-stratified layer. This layer is also known as the F-layer and recent studies suggest that this can be explained by a slurry, where solid particles dispersed within the liquid iron alloy snow under gravity towards the inner core. Here we are interested in the search for additional geodynamic evidence of the F-layer. To this end, we numerically model its impact on the geodynamo that generates the Earth's magnetic field. This process is mainly powered by thermal and compositional convection in the liquid bulk of the core, and the dynamics are divided into two distinct regions inside and outside the tangent cylinder that is conventionally tangent to the ICB and aligned with the rotation axis. Using numerical dynamo simulations with core-mantle coupling, we insert a stably stratified layer 360 km thick at the base of the core with varying degrees of stratification that effectively increases the size of the tangent cylinder and alters the dynamics thereof. For a roughly constant level of injected convective power, we find that zonal flows beneath the CMB are significantly enhanced by the F-layer while jets emanating from the tangent cylinder become detached due to rearrangements of the thermal wind balance and of the core-mantle coupling torques. We evaluate the magnetic field at the core surface and determine a possible geomagnetic signature of the F-layer from our simulations independent of the degree of stratification, in which the location of the maximal radial field strength associated with the envelope of a polar magnetic minimum seen in the geomagnetic data is shifted by 10 degrees in latitude. This amounts to 1,000 km at the Earth's surface and may be detectable from observations.