What can we learn from the Io footprint on Jupiter?

The Io footprint (IFP) is one of the most spectacular signatures of the Io-Jupiter electromagnetic interaction. The IFP consists of several auroral spots and an extended tail which are located close to the feet of the Io flux tube. A careful analysis of the IFP based on observations acquired with the STIS and ACS UV cameras on board the Hubble Space Telescope can provide us with essential information on the ongoing physical processes. We address5 basic questions: “What is the Io footprint?”, “Where is the Io footprint?”, “How high is the Io footprint?”, “How big is the Io footprint?” and finally: “How bright is the Io footprint?” The answers to each of these questions have profound implications for the understanding of the phenomenon.