Following Jupiter's Satellite Footprints with JWST

A striking feature of Jupiter’s aurora are the emissions induced by the Galilean satellites. Their auroral footprints represent the ionospheric signature of the electromagnetic interaction between the Galilean moons and Jupiter's co-rotating magnetospheric plasma. We present a new high-resolution view of the auroral footprints of Io and Europa in the near-infrared, as observed by the Near-Infrared Spectrograph (NIRSpec) onboard the James Webb Space Telescope (JWST). We report measurements of ionospheric H₃⁺ spectral radiance, total emission, column-averaged temperature and ion density, as well as the spectral radiance from lower altitude CH₄.

The H₃⁺ spectral radiance for both footprints is found to be primarily driven by the ion densities, as opposed to the local thermospheric temperature, with significant variability on the timescales of ∼30 minutes observed in both the H₃⁺ temperature (∼50%) and density (∼500%) at the core of the Io spot. A spatially confined cold structure was uniquely seen, localised to the centre of the Io spot, with extremely high H₃⁺ densities and surrounded by a “halo” of relatively hot and less dense H₃⁺ extending into the footprint tail. The variability in the ionization of H₃⁺ at the Io spot is likely driven by changes in the precipitating electron flux and energy creating H₃⁺ across different altitudes, therefore sampling various regions of the ionosphere’s altitudinal temperature profile, and/or NIRSpec’s changing observing geometry is revealing different vertical extents of the footprint. There are also suggestions of a similar, yet less extreme, cold and dense population of H₃⁺ associated with the Europa footprint.   

Our findings highlight the unique insights gained from analysing the near-infrared emissions from the auroral footprints of the Galilean moons. Such observations can supply valuable context for the in-situ measurements acquired by Juno as it traversed within the moons’ orbits during its prime and extended missions, as well as to support future investigations made by JUICE and Europa Clipper.