UC Santa Cruz

This doctoral work is motivated by the Sun and solar-type stars, which consist of an unstable convection zone (CZ) that lies on top of a stably stratified radiative zone (RZ). The dynamics occurring at the CZ-RZ interface are not well understood, and yet they are known to play a significant role in processes such as transport of chemical species, angular momentum and magnetic fields. To shed some new light on this complicated problem, we have compartmentalized this work into three main chapters. In the first part, in order to mimic stellar-like conditions, we study convection in a weakly non-Boussinesq gaseous spherical shell in the low-Prandtl number regime assuming a constant adiabatic temperature gradient and employing fixed flux at the inner boundary. We find the remarkable emergence of a subadiabatic layer within the domain for sufficiently turbulent flows enhanced by large variations in the superadiabaticity across the shell. However, convection remains vigorous everywhere across the shell thus indicating that it is a highly non-local process. In the second part, we further extend our study to include a stable region below the convective zone and we investigate the dynamics of overshooting/penetrative convection. We observe that the overshooting of the turbulent motions into the RZ depends on three different parameters: the relative stability of the stable zone, the transition width between the two, and the intensity of the turbulence. We find that, in the parameter regime studied, these overshooting motions manage to partially alter the thermal stratification, but not so efficiently as to create a fully mixed adiabatic region. We have built a model of these processes that could be useful for stellar evolution codes. In the third and final part, we also add a poloidal dipole magnetic field initially contained in the stable zone and study its interaction with the turbulent motions. Our numerical results are categorized into non-dynamo and dynamo cases. In the non-dynamo cases, the field diffuses outward, and its field lines open up and penetrate in the CZ. At the same time, a large fraction of its energy is removed due to the turbulent diffusion by the convective motions. In the dynamo cases, the field starts diffusing outward but its interaction with the turbulent motions leads to a small-scale essentially kinematic dynamo within the CZ and the overshoot region. In both of these cases, we find that the dipole field cannot remain confined in the RZ by the turbulent motions.