Ganymede's Ionosphere Observed by a Dual‐Frequency Radio Occultation With Juno

In June 2021, the Juno spacecraft executed a close flyby of Ganymede. During the encounter, Juno passed behind Ganymede for 15 min as observed from Earth, providing the geometry to conduct a radio occultation experiment to probe Ganymede's tenuous ionosphere. X‐band and Ka‐band radio links were transmitted from Juno to antennas at the Deep Space Network. Electrons encountered along the radio propagation path advance the signal's phase and a linear combination the two frequencies allows for a direct measurement of the electron content along the propagation path. On occultation ingress, an ionosphere peak of 2,000 ± 500 (1‐σ) cm −3 near the surface was observed. On occultation egress, no statistically significant ionosphere was detected. Ingress observation viewed where Ganymede's intrinsic magnetic field lines are open whereas egress observation viewed where the field lines are closed, implying electron impact ionization plays a key role in the generation of the ionosphere.