Magnetospheric configuration and dynamics of Saturn's magnetosphere: A global MHD simulation

We investigate the solar wind interaction with Saturn's magnetosphere by using a global magnetohydrodynamic simulation driven by an idealized time-varying solar wind input that includes features of Corotating Interaction Regions typically seen at Saturn. Our model results indicate that the compressibility of Saturn's magnetosphere is intermediate between the Earth's and Jupiter's, and the magnetopause location appears insensitive to the orientation of the interplanetary magnetic field. The modeled dependences of both the magnetopause and bow shock locations on the solar wind dynamic pressure agree reasonably well with those of data-based empirical models. Our model shows that the centrifugal acceleration of mass-loaded flux tubes leads to reconnection on closed field lines forming plasmoids, an intrinsic process ("Vasyliunas-cycle") in Saturn's magnetosphere taking place independent of the external conditions. In addition, another type of reconnection process involving open flux tubes ("Dungey-cycle") is also seen in our simulation when the external condition is favorable for dayside reconnection. Under such circumstances, plasmoid formation in the tail involves reconnection between open field lines in the lobes, producing stronger global impacts on the magnetosphere and ionosphere compared to that imposed by the Vasyliunas-cycle directly. Our model also shows that large-scale tail reconnection may be induced by compressions driven by interplanetary shocks. In our simulation, large-scale tail reconnection and plasmoid formation take place in a quasi-periodic manner but the recurrence rate tends to be higher as the dynamic pressure becomes higher. While large-scale plasmoid release clearly is an important process in controlling the magnetospheric dynamics, it appears insufficient to account for all the losses of plasma added by the magnetospheric sources. We find that a large fraction of the planetary plasma is lost through the magnetotail near the flanks probably through relatively small-scale plasmoids, a situation that may also exist at Jupiter.