Seminar by Olivier Barrois (Université de Strasbourg) – « Using numerical models and geomagnetism to gain insights into the Earth’s outer core »

In the framework of studying the internal dynamics of the Earth’s outer core using numerical models, we use reduced models under the Quasi-Geostrophic (QG) and Boussinesq hypothesis to solve for the magneto-hydro-dynamics equations (MHD) and improve our knowledge of decadal to centennial core’s processes that could play a role in the so-called Atlantic Gyre or the South Atlantic Gyre.
We thus present numerical hydrodynamic models and dynamos using a hybrid QG-3D approach in a thick spherical shell geometry. Our model is based on a Quasi-Geostrophic convection code extended with a 3D treatment of heat transport and magnetic induction.
We find series of convective models that compare reasonably well with simulations using pure 3D codes, with convective flows that are dominated by zonal jets at mid-depths in the shell and
thermal Rossby waves that are prominent close to the outer boundary. Models that include laterally varying heat flux at the outer boundary are also tested and reproduce regional convection patterns that compare well with those found in similarly forced 3-D models.
We also obtain a collection of self-sustained, multipolar, weak field dynamos with magnetic energy one or two orders of magnitude lower than the kinetic energy. The poloidal magnetic energy is weak and, by construction, there is a lack of equatorially anti-symmetric components in the Buoyancy and Lorentz forces. This leads to configurations where the velocity field is only weakly impacted by the magnetic field, similar to dynamos found in 3D simulations where zonal flows and the Omega-effect dominate. The time- dependence of these dynamos is characterised by quasi-periodic oscillations that we attribute to dynamo waves. The QG-3D dynamos found so far are not Earth-like.
Our models could also be applied to other planetary bodies sharing nearby regions of the parameters’ space.