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Chemical convection and stratification at the top of the Earth's outer core

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The outer core of the Earth is a liquid layer made of iron, plus a small fraction of nickel and lighter elements (O/S/Si…). In this layer, the combination of thermal and compositional effects drives convective currents which generate the magnetic field of the Earth by dynamo action. However, vigorous convection may not take place in the entire outer core since seismic and magnetic observations suggest that its top 60-300km is stably stratified. This stratified layer at the top of the core could have important consequences for the terrestrial dynamo, the dynamics and evolution of the core, but its origin remains enigmatic. Several scenarios have been proposed to explain the formation of such a layer. One possibility is that light chemical plumes and “blobs” emitted at the bottom of the outer core by the crystallization of the inner core could accumulate below the top of the core, gradually forming a chemically stratified layer. The plausibility of this scenario can be assessed using a new code based on a particle-in-cell method I developed during my PhD, which allows to deal with the low value of the compositional molecular diffusivity (the chemical Prandtl number is large); an ingredient neglected in previous simulations. In this seminar, I will present the results of recent numerical simulations of rotating chemical convection in a spherical shell. I will describe the general characteristics of rotating convection at large chemical Prandtl number and will show that a chemically stratified layer forms at the top of the shell in all simulations. I will then discuss the extrapolation to the Earth’s outer core.

This talk is part of the Geophysical and Environmental Processes (DAMTP/BPI) series.

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