Juno/JIRAM Observations of Jupiter’s Main Aurorae and Satellite Footprints.

JIRAM (Jovian Infrared Auroral Mapper) on board the NASA/Juno spacecraft is an imaging /spectrometer in the 2-5 um range. One of the imager channels (L band 3.3-3.6 um) is designed to study the Jovian H3+ auroral emission. The very good angular resolution of the camera, combined with the unique vantage point provided by Juno, allows to JIRAM to observe the aurorae with unprecedented details. Here we present the results of ~2 years of auroral observations, with particular emphasis  on the auroral footprints of the Galilean moons. These are bright spots (with associated tail) that appear in Jupiter’s ionosphere at the base of the magnetic field lines that 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, resembling a repeating pattern of swirling vortices (von Kármán vortex street) shed by a cylinder in the path of a flowing fluid. Contrary to the larger spots seen in the UV, the small scale of these multiple features (~100 km) is incompatible with the simple paradigm of multiple Alfvén wave reflections. The small scale of these multiple features (~100 km) shows that this particular multiplicity is not generated by multiple Alfven wave reflections. Observations of Io’s trailing tail well downstream of the leading feature reveal a pair of closely spaced parallel arcs that were previously unresolved. Both of Ganymede’s footprint spots (main and secondary) appear as a pair of emission features that evidently provides a remote measure of Ganymede’s magnetosphere, mapped from its distant orbit onto Jupiter’s ionosphere.