A new interpretation of the Io UV footprint morphology

The electromagnetic interaction between Io and the jovian magnetic field leads to an
auroral UV footprint consisting of single or multiple UV spots in both jovian hemi-
spheres. According to current theories, the perturbation induced by the motion of Io
in the plasma torus propagates along the field lines in the form of Alfvén waves and
finally causes electron precipitation in the jovian ionosphere. The occurrence and mul-
tiplicity of the secondary spots appear to be related to the position of Io in the plasma
torus and have been attributed to partial reflections of the Alfvén waves on the torus
boundaries. Nevertheless, the discrepancies between the predicted inter-spot distances
and the measurements were found difficult to explain. Additionally, some crucial con-
figurations of Io in the torus had never been observed.
Our recent HST/ACS observations of the footprint in so far unexplored Io-plasma
torus configurations lead to the finding of a new feature in the footprint: a faint spot
upstream of the main spot. The observations of this precursor emission, together with
the inconsistencies related to the inter-spot distances, suggest a new interpretation of
the footprint morphology. We propose that the precursor and the first secondary spot
stem from the same mechanism and we interpret them as the counterparts of the main
spot occurring in the opposite hemisphere.