Ten years of Hubble Space Telescope observations of the variation of the Jovian satellites' auroral footprint brightness

During the past decade, FUV imaging of Jupiter's auroral region by the Hubble Space Telescope (HST) using two instruments, the Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS), has provided detailed information on the electrodynamic interaction between Io's, Ganymede's, and Europa's atmospheres and plasma in Jupiter's magnetosphere. This interaction is responsible for the satellites' auroral footprints in Jupiter's atmosphere connected via magnetic flux tubes to the satellites' interaction regions. The observed brightness of each auroral footprint is considered to be one main observable quantity to characterize the interaction environment at the satellites. Previous observations of Io's magnetic footprints using HST STIS images showed that the footprint emission appears brightest when Io is centered in the plasma torus. With the much larger data set obtained from the 2007 HST campaigns, we find the same variation observed by Serio and Clarke (2008), but with significantly better statistics over a time period of 10 years. These results confirm that Io's footprint brightness varies mainly with the satellite's location in Jupiter's plasma torus over a long time scale. Additional observations of the downstream emissions and their variations were presented by Bonfond et al. (2007). In Ganymede's case, the relation between the footprint brightness and the satellite's position in Jupiter's magnetosphere shows some evidence for the same general trend, although the data are noisier than the data for Io. Ganymede's footprint brightness appears to be less consistent over time than Io's. The variation of Ganymede's footprints over short time periods was studied by Grodent et al. (2009). Europa's fainter footprint brightness makes it difficult to see any systematic trend.