Stable stratification promotes multiple zonal jets in a turbulent jovian dynamo model

Abstract

The ongoing NASA's Juno mission puts new constraints on the internal dynamics of Jupiter. Data gathered by its onboard magnetometer reveal a dipole-dominated surface magnetic field accompanied by strong localised magnetic flux patches. The gravity measurements indicate that the fierce surface zonal jets extend several thousands of kilometers below the cloud level before rapidly decaying below 0.94 − 0.96RJ, RJ being the mean Jovian radius at the one bar level. Jupiter's internal structure can be grossly decomposed in two parts: (i) an outer layer filled with a mixture of molecular hydrogen and helium where the zonal flows are thought to be driven; (ii) an inner region where hydrogen becomes metallic and dynamo action is expected to sustain the magnetic field. Several internal models however suggest a more intricate structure with a thin intermediate region in which helium would segregate from hydrogen, forming a compositionally-stratified layer. Here, we develop the first global Jovian dynamo which incorporates an intermediate stably-stratified layer between 0.82RJ and 0.86RJ. Using much lower diffusivities than previous models enables us to more clearly separate the dynamics of the metallic core and the molecular envelope. Analysing the energy balance reveals that magnetic energy is almost one order of magnitude larger than kinetic energy in the metallic region, while most of the kinetic energy is pumped into zonal motions in the molecular envelope. Those result from the different underlying force hierarchy with a triple balance between Lorentz, Archimedean and ageostrophic Coriolis forces in the metallic core and inertia, buoyancy and ageostrophic Coriolis forces controlling the external layers. The simulation presented here is the first to demonstrate that multiple zonal jets and dipole-dominated dynamo action can be consolidated in a global simulation. The inclusion of a stable layer is a necessary ingredient that allows zonal jets to develop in the outer envelope without contributing to the dynamo action in the deeper metallic region. Stable stratification however also smooths out the small-scale features of the magnetic field by skin effect, yielding a too-dipolar surface field as compared to the observations. These constraints suggest that possible stable layers in Jupiter should be located much closer to the surface (0.9 − 0.95RJ).