Abstract
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.
Highlights
[1] 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
Jupiter’s bright and complex auroral region has traditionally been divided into three regions: the main auroral emission, polar emission, and footprint emission [Clarke et al, 1998; Grodent et al, 2003; Hill, 2004]
The Alfvénic travel time was found to be longer than the time predicted by the steady state unipolar inductor model [Bagenal, 1994], implying that the direct current loop cannot close between Io and Jupiter’s ionosphere
Summary
[2] A number of direct observations and theoretical studies of planetary aurora have provided fruitful information about their magnetospheres [McPherron, 1995; Carlson and Egeland, 1995; Cowley, 1998; Kivelson, 2005; Stallard et al, 2008]. The various ranges of temporal variations in the plasma torus brightness were presented Their results suggest a different plasma environment to that observed during the Voyager era [Steffl et al, 2008]. The plasma densities near Io measured by different spacecraft differ by less than one order of magnitude, in general, the density is suggested to be most concentrated near Io’s orbit Supporting those direct observations, UV imaging by HST (SC08) has revealed a strong relation between Io’s magnetic footprint brightness and its location in Jupiter’s plasma torus. According to previous observations [Clarke et al, 1998; Gérard et al, 2006], Io’s footprint could appear as multiple spots, presumably as a result of the Io‐ generated waves reflected at plasma density gradients in the plasma torus. Io’s footprint brightness is averaged over a 0.25 × 0.25 arc sec area, which is smaller than areas used in previous studies [Bonfond et al, 2007; SC08], to focus on the brightest spots
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