At the other end of the field lines: the satellite footprints

The electromagnetic perturbations caused by the interaction between the moons Io, Europa, and Ganymede and
the Jovian magnetosphere propagate along magnetic field lines in the form of Alfvén waves and finally create
auroral footprints in Jupiter’s polar ionosphere. Thanks to the high spatial and temporal resolution of the Hubble
Space Telescope, the analysis of the footprints in the Far UV domain provides extremely valuable information on
these interactions.
In the case of the Io footprint, the analysis of the multiplicity of the auroral spots and of their relative motion
resulted into a new understanding of the far field interaction, which involves trans-hemispheric electron beams.
Consistent with the in-situ energetic electrons observations, this scenario has been later reinforced by theoretical
models simulating the Alfvén wave’s propagation and the ways these waves accelerate electrons. Moreover,
while the altitude and the vertical profile of the auroral emissions provide crucial information on the energy
of the precipitating electrons, the measure of the shape of the different sub-structures of the satellite footprints
provides important clues about the size of the interaction region. The evolution of the footprint brightness is
also an essential parameter for understanding the energies involved in the interaction. Finally, the study of the
satellite footprints can also be used for broader purposes than just the investigation of the satellite-magnetosphere
interaction, such as building more accurate Jovian magnetic field models.