Jonathan Mound, Chris Davies , Sebastian Rost, Jonathan M. Aurnou

Earths magnetic field is generated by turbulent motion in its fluid outer core. Although the bulk of the outer core is vigorously convecting and well-mixed, some seismic, geomagnetic, and geodynamic evidence suggests that a global stably stratified layer exists at the top of Earths core. Such a layer would strongly influence thermal, chemical, and momentum exchange across the core-mantle boundary (CMB) and thus have significant implications for the dynamics and evolution of the core. Here we argue that the relevant scenario is not global but regional stratification arising solely from the lateral variations in CMB heat flux. Based on our extensive suite of numerical simulations we predict that regional inversion layers extend 100s of kilometres into the core under anomalously hot regions of the lowermost mantle. Although the majority of the outermost core remains actively convecting, sufficiently large and strong regional inversion layers produce a 1D temperature profile that mimics a globally stratified layer below the CMB, an apparent thermal stratification despite the average heat flux across the CMB being strongly superadiabatic.