Coupled Fluid-kinetic Global Simulations of Saturn's Magnetopause Dynamics

The giant planets, Jupiter and Saturn, are rapid rotators with strong internal sources of plasma arising from their moons. The role of solar wind-driven transport at these planets has been a topic of intense interest. Both magnetic reconnection and Kelvin-Helmholtz (K-H) instability have been suggested to play an important role in coupling the solar wind with the magnetosphere. To determine the impact of reconnection and K-H instability on the giant planet magnetospheres, we have adapted a coupled fluid-kinetic global model to the giant planets based on the MHD with embedded particle-in-cell (MHD-EPIC) code originally developed for Earth and Ganymede. The MHD code (BATSRUS) is employed over the entire simulation domain, while the fully kinetic code (iPIC3D) covers regions where kinetic physics is important. The two-way coupled MHD-EPIC provides a unique capability of simulating reconnection at kinetic scales while simultaneously capturing large-scale effects of reconnection on a global magnetosphere. Here we report on first results from our MHD-EPIC simulations of the solar wind interaction with Saturn's magnetopause. Our Saturn MHD-EPIC model has been run with various IMF conditions (northward, southward, and spiral configurations) to determine the main process through which the solar wind is coupled to the magnetosphere. We have also performed purely MHD simulations for the same upstream and internal conditions as used in MHD-EPIC to identify how the effects of boundary processes differ in the two models. These comparisons allow us to determine when and where reconnection or K-H like instabilities occur under the external and internal conditions pertinent to the giant planet magnetospheres. As a quantitative measure of the global coupling efficiency, we have analyzed the rate at which open magnetic flux is being added to the polar cap and how it varies with the IMF orientation. It is found that the global reconnection efficiency differs significantly between MHD-EPIC and MHD simulations using the same external and internal conditions.