The morphology of Saturn's ultraviolet auroral oval and its time variations

Global images of the FUV auroral emission surrounding Saturn's south pole have been obtained with Space Telescope Imaging Spectrograph (STIS) since 1998. During this period, the planet's tilt offered a nearly complete view of the south auroral oval. Several tens of images were obtained in January 2004, concurrent with in situ measurements of the solar wind parameters made with instruments on board the Cassini probe. This unique set of auroral images includes time-tagged exposures providing information on fast time variations. A subset of collected images is used to define a quiet reference oval. This oval serves a framework to investigate local time and longitudinal variations of brightness and latitudinal motions of the oval, including variation of its radius. Occasionally, the oval is not closed, or reduces to a bright spot apparently as a consequence of enhanced solar wind pressure pulses reaching the front of the magnetosphere. Comparisons with terrestrial counterpart indicate that auroral substorms are not observed on the nightside oval. Instead, some features are seen to rotate with the planet as in the Jovian aurora. These characteristics will be compared with predictions of recent models describing the global current system coupling the magnetosphere and the ionosphere. In particular, the latitude of the oval, departure from corotation and the longitudinal brightness distribution can be used to test the paradigm that the main auroral oval maps to a region of enhanced field-aligned current and to improve current models