Magnetospheric mapping of the dayside UV auroral oval at Saturn using simultaneous HST images, Cassini IMF data, and a global magnetic field model

Abstract

We determine the field-aligned mapping of Saturn's auroras into the magnetosphere by combining UV images of the southern dayside oval obtained by the Hubble Space Telescope (HST) with a global model of the magnetospheric magnetic field. The model is tailored to simulate prevailing conditions in the interplanetary medium, corresponding to high solar wind dynamic pressure and variable interplanetary magnetic field (IMF) strength and direction determined from suitably lagged field data observed just upstream of Saturn's dayside bow shock by the Cassini spacecraft. Two out of four images obtained in February 2008 when such simultaneous data are available are examined in detail, exemplifying conditions for northward and southward IMF. The model field structure in the outer magnetosphere and tail is found to be very different in these cases. Nevertheless, the dayside UV oval is found to have a consistent location relative to the field structure in each case. The poleward boundary of the oval is located close to the open-closed field boundary and thus maps to the vicinity of the magnetopause, consistent with previous results. The equatorward boundary of the oval then maps typically near the outer boundary of the equatorial ring current appropriate to the compressed conditions prevailing. Similar results are also found for related images from the January 2004 HST data set. These new results thus show that the mapped dayside UV oval typically spans the outer magnetosphere between the outer part of the ring current and the magnetopause. It does not encompass the region of primary corotation flow breakdown within the inner Enceladus torus.