Semi-convection in rotating spherical shells: flows, layers and dynamos

Large regions of giant planets are thought to possess unstable thermal gradients stabilised by gradients of heavy-element composition. Such a fluid can develop semi-convection, a double-diffusive instability driven by the unequal molecular diffusivities of heat and composition. While previous studies have focused mainly on local Cartesian models, we investigate semi-convection in rotating spherical shells, the geometry relevant to planetary interiors, using direct numerical simulations. In a first nonlinear phase, the flow spontaneously forms concentric density staircases composed of well-mixed layers separated by thin, strongly stratified interfaces. We propose scalings for both the thickness of these layers and their survival time in terms of the rotation rate and stratification. Over longer time scales, layers merge to produce statistically steady states consisting either of a fully convective shell, or of a convective interior overlain by a persistent stably stratified layer (SSL), depending on the balance between stratification and rotation. Dynamo simulations show that flow within the convective region can generate a self-sustained magnetic field, which is filtered by zonal flows in the overlying SSL. This results in a strongly dipolar and axisymmetric external field, in encouraging agreement with Saturn’s magnetic field. Across the explored parameter range, both the Rossby number and the thickness of the SSL are governed by a single combination of control parameters. This enables identification of a regime favourable to planetary-like dynamos.