Influence of the mass distribution on the magnetic field topology in spherical, anelastic dynamo simulations

Abstract : Numerical modelling of convection driven dynamos in the Boussinesq approximation revealed fundamental characteristics of the dynamo-generated magnetic fields. However, Boussinesq models are not adequate for describing convection in stratified systems, and thus the validity of these previous results remains to be assessed for gas giants and stars. To that end, we carried out a systematic parameter study of spherical dynamo models in the so-called anelastic approximation, which allows for a reference density profile while filtering out sound waves for faster numerical integration. We show that the dichotomy of dipolar and multipolar dynamos identified in Boussinesq simulations is still present in anelastic models, and dipolar dynamos require that the typical length scale of convection is an order of magnitude larger than the Rossby radius. However, the established distinction between dipolar and multipolar dynamos tends to be less clear than it was in Boussinesq studies, since we found a large number of models with a considerable equatorial dipole contribution together with an intermediate overall dipole field strength. This tendency can be found in very weakly stratified models, but it was not reported for previous Boussinesq models assuming a homogeneous mass distribution. In contrast, anelastic models usually assume a central mass distribution, which leads to a gravity profile proportional to 1/r**2. Actually, we show that this choice can result in changes in the magnetic field topology that are mainly due to the concentration of convective cells close to the inner sphere.
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Raphaël Raynaud, Ludovic Petitdemange, Emmanuel Dormy. Influence of the mass distribution on the magnetic field topology in spherical, anelastic dynamo simulations. J. Ballet, F. Bournaud, F. Martins, R. Monier et C. Reylé. Journées de la SF2A, Jun 2014, Paris, France. 2014. ⟨hal-01126612⟩

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