Dynamics and magnetic field generation in Jupiter and Saturn

The interior of a gas giant planet is known to be mostly composed of hydrogen. The commonly accepted three-layer model consists of a small inner rocky core, an intermediate fluid hydrogen layer and an outer molecular hydrogen envelope. The high pressures and temperatures reached between 80 and 90% of the planetary radius result in a phase transition of hydrogen to a metallic state. The electrical conductivity, however, increases rapidly with depth due to the growing degree of ionisation. The density increase across the interior of the giant gas planets is estimated to be a few orders of magnitude. Many numerical models for the interior dynamics have nevertheless neglected the stratification for simplicity. This thesis and the related publications represent the first systematic explanation of the effects of density stratification in a planetary context. The goal of this work is to systematically explore the effects of density stratification and electrical conductivity variations in numerical models. The anelastic version of the MHD code MagIC is used, which solves for convection and magnetic field generation in a rotating spherical shell. A big advantage of this approximation is that sound waves can be neglected, which could severely slow down numerical calculations due to their high speed relatively to the significantly slower convective motions. It is found that even a mild density contrast affects the dynamics. The scale of convection decreases considerably with increasing density stratification across the shell radius. The entropy gradient in the outer part of the shell grows more rapidly, which also alters the location of the axial convective motions by moving them toward the outer boundary. For the majority of the simulations a free-slip mechanical outer boundary condition is used which is more appropriate for gas planets as it allows strong zonal winds to develop. In a second step an electrical conductivity profile was introduced. The profile starts to matter when the thickness of the weakly conducting outer layer is at east 80% of the total shell. Increasing also the density contrast revealed a more relevant setup for a gas giant where both strong zonal winds and dipolar field coexist. In such dipole-dominated models, the equatorial jet remains confined to the outer weakly conducting layer.