Observations of satellite footprints in Jupiter's Aurorae

JIRAM (Jovian Infrared Auroral Mapper) on board Juno is an imager/spectrometer in the 2-5 um range. One imaging channel is designed to study the Jovian H3+ auroral emissions. Its high angular resolution, combined with the unique vantage point provided by Juno, allows JIRAM to observe the aurorae in unprecedented detail. Here we present the results from ~2 years of observations of the auroral footprints of the Galilean moons. These are bright spots and
associated tail that appear in Jupiter’s ionosphere at the base of the magnetic field lines which sweep past Io, Europa, and Ganymede. The moons are obstacles in the path of Jupiter’s rapidly rotating magnetospheric plasma, and the resulting electromagnetic interaction launches Alfvén waves along the magnetic field towards Jupiter, where intense electron bombardment of the hydrogen atmosphere causes it to glow. Recent observations reveal for the first time that the footprint of Io is comprised of a regularly spaced array of emission features, extending downstream of the leading footprint. Contrary to the larger spots seen in lower resolution images, the small scale of these multiple features (~100 km) is incompatible with the simple paradigm of multiple Alfvén wave reflections. Additionally, observations of Io’s trailing tail well downstream of the main footprint reveal a pair of closely spaced parallel arcs, previously unresolved.
The temperatures of the main spot and tail, retrieved with the JIRAM spectrometer, are lower than the main auroral oval. This could indicate that the emission is located at a deeper level, possibly
caused by higher energy electrons. Ganymede’s footprint spots (main and secondary) appear as a
pair of emission features that provide a remote measure of the size Ganymede’s magnetosphere, mapped from its distant orbit onto Jupiter’s magnetosphere.