Double diffusive convection in stable layers within Earth’s core

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The geomagnetic field is generated by thermo-compositional convection in the Earth’s fluid outer core. However, not the entire core is expected to be convectively unstable. The outermost part of the current core is generally believed to be stably stratified, and there are strong arguments for the presence of stable stratification in the Earth's history, especially during the epoch before the formation of the solid inner core. While stable stratification has a tendency to damp vertical motions, stable layers may still support fluid dynamical instabilities of double-diffusive type. These instabilities, which occur on small spatial scales (on the order of several centimetres to meters), can significantly increase the strength of vertical mixing across these core regions. Under favourable conditions, they can even transform the internal structure of the stable region into a stack of vigorously convecting layers, separated by thin, high gradient interfaces. In this project, the properties of double-diffusive convection under conditions characteristic for planetary cores are studied using direct numerical simulations. In particular, the combined effects of Coriolis and Lorentz forces, exerted by the Earth's rotation and by the geomagnetic field, will be taken into consideration. The study explicitly focusses on small, local regions in the core such that the small-scale double diffusive motions can be resolved in a realistic fashion. The simulation results will help to constrain the radial transport through stable layers in the Earth's core, and they will also constrain under which conditions small-scale double diffusive motions would transform a stable region inside Earth's core into a stack of convecting layers. This research contributes to a better understanding of small-scale double diffusive convection in planetary cores and will help to eventually parameterize it in large-scale models of the geodynamo.