The UV footprint emission of Io: morphology, brightness and control by Io

Emissions from the magnetic footprint of Io on Jupiter's upper atmosphere has been observed with the Space Telescope Imaging Spectrograph (STIS) since 1997 and more recently with the Advanced Camera for Surveys (ACS) on board HST. The observations cover a wide range of central meridian Jovian longitudes and Io orbital positions. Most images also exhibit a long trailing tail associated with Io's wake plasma. The brightness of the footprint shows variations by over an order of magnitude and appears to be controlled by the distance from Jupiter's central meridian and the longitude of Io. Multiple footprints are also occasionally observed with varying relative brightness and mutual distance. These features apparently correspond to precipitation generated by the reflection of Alfvèn waves between Jupiter's ionosphere and the plasma torus. Observations collected in 2005 with ACS have provided high quality contrasted images that provide a direct indication of the altitude of the trailing tail and its orientation.