Juno’s high-spatial-resolution ultraviolet observations of Ganymede’s auroral patches

Aims.
We analyzed ultraviolet observations of Ganymede recorded on June 7, 2021, by the Juno spacecraft, which observed the aurora at high spatial resolution in order to detect the presence of auroral sub-structures. The emission mainly comes from two oxygen lines at 130.4 and 135.6 nm, which are excited under electron precipitation.

Methods.
We produced longitude–latitude projections of the oxygen emission lines from Ganymede’s atmosphere to investigate the presence of small-scale auroral features. We adopted a magnetic field model of Ganymede’s magnetosphere to map between the moon surface and the surrounding space, in order to determine the regions magnetically connected to the aurora.

Results.
We find auroral patches on the leading, downstream side of Ganymede. Their typical size is ∼50 km, and they have a brightness of up to ∼200 Rayleigh. These features map approximately to the downstream reconnection region and seemingly resemble the terrestrial and Jovian mesoscale auroras associated with sub-storms and dawn storms, respectively. This could indicate that magnetospheres of celestial bodies host similar physical process(es) despite their different conditions and dynamics.