Massively parallel MHD simulation of convection and auroral emissions in Saturn’s magnetosphere driven by the observed solar wind

In a series of simulation studies we have reported that vortices formed at Saturn's dawn magnetopause in simulations when IMF was northward. We interpreted these vortices as resulting from the Kelvin Helmholtz (K-H) instability. Thanks to recent advances in computer performance, we have been able to perform high resolution global MHD simulations of the Kronian magnetosphere with 0.1 Rs grid spacing. In these simulations we obtained the signature of the field-aligned currents from the K-H vortices in Saturn's auroral ionosphere and found small patchy regions of upward field-aligned current which may be related to auroral emissions. These patchy aurorae resembling our results have been reported from Cassini observations.

In our previous simulations we have used the constant and simple solar wind conditions to understand the basic behavior of Kronian magnetosphere. In this study we have used Cassini observations of the solar wind upstream of Saturn to drive a massively parallel simulation. Using these solar wind data we simulated the Kronian magnetosphere from 2008-02-12/14:00:31 to 2008-02-13/01:59:31 when the Hubble Space Telescope (HST) observed Kronian UV auroral emissions. For these solar wind conditions there are several enhancements of the solar wind dynamic pressure (shocks) and a polarity reversal in the IMF components.

From these simulations we obtained the dynamically changing shape and convection pattern of the Kronian magnetosphere in response to the variation of solar wind dynamic pressure and IMF direction. In particular, a layered convection pattern formed between the corotation region and magnetopause. The layers in this convection system interacted with each other, forming large vortices. We calculated the configuration of field aligned currents from the simulation and found layered and patchy distributions in the ionosphere. The pattern of these upward field aligned currents in the dawn side ionosphere resembles the configuration of auroral emission observed by HST well.