The Unique Observation Geometries of Juno’s Radio Occultations of the Galilean Satellites

During Juno’s extended mission, the spacecraft performed four close flybys of the Galilean satellites. Each encounter with a Galilean satellite occurred during the ascending node crossing of the orbit and reduced the orbital period of the spacecraft, phasing the orbit for the subsequent encounter. During each encounter, a radio occultation experiment could be performed using Juno’s radio science instrumentation. During the Ganymede encounter in June 2021 and Europa encounter in September 2022, occultations of the moon’s ionospheres were performed. Both yielded detections of the moon’s ionospheres, with the Ganymede occultation revealing the importance of electron impact ionization. During the close encounters with Io in December 2023 and February 2024, although the spacecraft was not occulted by the limb of the moon, it was occulted by the Alfven wing connecting Io to Jupiter. Increased electron density was detected in the Alfven wing with the occultation method for the first time, providing independent verification of in-situ measurements of the wing. The unique observation geometries of each of these four flybys – a consequence of the complex interaction between Jupiter’s magnetosphere and the moons – required adapting traditional radio occultation techniques to invert the radio frequency measurements into electron density estimates.