Long-term Brightness Variations of the Io UV Footprint

Since the finding of the UV Io footprint in 1996, the successive UV instruments on board the Hubble Space Telescope (HST) allowed us to considerably improve the understanding of the Io-Jupiter electro-magnetic interaction and its auroral counterpart. It has been shown that the Io footprint is generally formed by one bright spot preceded or followed by secondary spots whose relative positions are linked to the location of Io in the plasma torus. We also know that these spots experience brightness variations from minutes to hours. The Io footprint brightness varies over hours with the longitude of Io in the Jovian magnetic field (System III longitude) but until recently, huge gaps existed in the longitude coverage. Part of these gaps has now been filled during the latest HST imaging campaign and a more complete spot brightness versus Io System III longitude diagram emerges. Additionally, we compare spot brightness between images obtained a few minutes apart but from opposite hemispheres. Based on images gathered from 1997 to 2007 with the STIS and the ACS cameras, we also show that the footprint morphology and the spots brightness, including their relative brightness, can vary significantly from one year to another. Finally, we discuss the brightness variations from hours to years in terms of plasma torus density and position of Io in the plasma torus as well as in Jovian magnetic field.