When the moons create aurora: the satellite footprints on giant planets

Aurora on giant planets present many differences with
the terrestrial aurora and the satellite footprints are among
the most striking examples. Created by the interaction
between the moons and the magnetospheric plasma, these
auroral features are observed as spots or curtains located
close to the field lines connecting the satellites to the planet.
On Jupiter, the footprints of Io, Europa, and Ganymede have
been discovered from ground observations in the infrared
domain, Hubble Space Telescope observations in the
ultraviolet domain, and Galileo spacecraft images in the
visible domain. On Saturn, recent observations from the
Cassini spacecraft tentatively identified the UV footprint of
Enceladus. First we will briefly review the mechanisms
driving these electro-magnetic interactions and their related
observational evidences. Then we will discuss the
characteristics of these various footprints, focusing on the
better-studied cases of the Io and Ganymede footprints. We
will see that the analysis of the footprint morphology,
location, spatial extent, and brightness provides extremely
valuable information both on the planetary magnetic field
topology and on the processes at play in the interaction. And
these processes are not so different from those creating some
components of the Earth aurora.