When Moons Create Aurora: The Satellite Footprints on Giant Planets

Satellite footprints are localized auroral emissions in the upper atmosphere of Jupiter (Saturn) near the magnetic field lines linking to satellites, Io, Europa, and Ganymede (Enceladus). They are the auroral signatures of the strong electromagnetic interactions taking place between these moons and the intensely magnetized, rapidly rotating planets they orbit. The Io and Europa spots have been shown to be followed by an extended tail. This might also be the case for the Ganymede and Enceladus emissions, although not yet unambiguously observed. Moreover, the main Io spot is accompanied by secondary spots attributed either to reflections of the plasma waves generated at Io on the Io plasma torus boundary or to electrons accelerated in one hemisphere but precipitating in the opposite one. While the horizontal extent of the spots gives a hint of the size of the interaction region in the equatorial plane, the vertical profile of the footprints provides clues to the energy distribution of the precipitating electrons. Moreover, the location of the footprints can be used as constraints for magnetic field models. Finally, the brightness of the footprints is a valuable diagnostic of the interaction mechanism and has been observed to vary at different time scales, each one tentatively associated with a different process.