In situ and remote observations of the ultraviolet footprint of the moon Callisto by the Juno spacecraft

Jupiter exhibits peculiar multiwavelength auroral emissions resulting from the electromagnetic interactions of Io, Europa, and Ganymede with the magnetospheric plasma flow. Characterizing the faint auroral footprint of the fourth Galilean moon, Callisto, has always been challenging because of its expected weakness and its proximity to Jupiter’s bright main aurora. Here, we report on unusual magnetospheric conditions that led to an equatorward shift of Jupiter’s main auroral oval unveiling the auroral footprints of the four Galilean moons in a single observation. Remote observations by the Juno spacecraft reveal a double-spot structure, characteristic of the footprints of the other three moons, with a maximum ultraviolet brightness of 137±15 kR. Concurrent observations within Callisto’s flux tube reveal field-aligned electrons with a characteristic energy of 10 keV, depositing an energy flux of 55 mW.m^-2 in Jupiter’s atmosphere. The electron properties are consistent with the triggering of radio emissions with intensities lower than 5 × 10^-18 W.m^-2.Hz^-1.