Geomagnetic signatures of the slurry F-layer inferred from dynamo simulations

Abstract: Seismic observations indicate that the lowermost portion of Earth's liquid core is density stratified. The existence of this so-called F-layer challenges classical theories of core dynamics, where the geodynamo process that generates Earth's main magnetic field is assumed to be powered by heat and light element release at the inner core boundary. The seismically-inferred thickness, density, and velocity anomaly can be reproduced by a dynamical model that represents the F-layer as a two-phase two-component slurry on the liquidus, with a ``snow'' of solid iron particles falling through a quasi-static iron-oxygen liquid. Here, we present the first fluid dynamical simulations of thermochemically driven rotating convection and dynamo action that include a simple representation of the stratified slurry F-layer at the base of the spherical shell geometry. We show that the F-layer can create a barrier to columnar quasi-geostrophic flow, which is expressed near the core surface as a migration of peak radial and azimuthal flow speeds to lower latitudes as the thickness and stratification strength increase. In dynamo simulations, this effect induces polar minima in the radial magnetic field at the outer boundary ($B_r$) that strengthen and deepen with increasing stratification, and peaks in latitudinal profiles of $B_r$ moving to lower latitudes with reduced temporal variability. The geomagnetic signature of the F-layer is most prominent in time-averaged $B_r$, when resolved to at least spherical harmonic degree 5, and a trend of increasingly negative zonal degree 3 and 5 Gauss coefficients as the F-layer thickness and stratification strength increase. Our results suggest that an F-layer thickness of 600~km is incompatible with geomagnetic observations and favour weak stratification (normalised Brunt-Väisälä frequency $<1$) and a layer $<400$~km thick.