HST Observations of Aurora from the Magnetic Footprints of Io, Ganymede, and Europa during the Millennium Campaign

UV images of Jupiter's aurora obtained with the HST STIS instrument reveal much detail in the distribution of emissions from Io's magnetic footprint on Jupiter, including extended trails of emission in the downstream direction. Emissions are also seen from the magnetic footprints of Ganymede and Europa, relatively much fainter and point-like in spatial extent. Knowledge of the statistical properties of these emissions, in terms of their locations and brightnesses, was greatly advanced in a comprehensive series of images obtained during the Millennium campaign in Dec. 2000 - Jan. 2001. These images provide sufficient spatial coverage to give a good indication of the auroral oval locations mapping to Io and Ganymede, and a few points mapping to Europa. The observed variations in footprint properties provide indications of the nature of the electrodynamic interactions of the satellites with Jupiter's magnetic field. These and other indicators also show the direction that magnetic field models should take to better describe Jupiter's internal magnetic field. Contributing evidence comes from the locus of footprint latitudes, and their deviations from the VIP4 model. The latitudinal distance between the Io and Ganymede footprint loci indicates variations in the local field strength, with larger separations corresponding to a weaker field. These separations, and the distance from and distortion of the main oval, indicate an anomalously weak field region in the north near 90[SUP]o[/SUP] longitude. This work has been supported by NASA in STScI grants GO-08171-97A and GO-08657-01A to the University of Michigan.