Radio Occultation Measurements of Europa's Ionosphere From Juno's Close Flyby

On 29 September 2022 the Juno spacecraft flew within 354 km of Europa's surface while several instruments probed the moon's surroundings. During the close flyby, radio occultations were performed by collecting single-frequency Doppler measurements. These investigations are essential to the study of Europa's ionosphere and represent the first repeat sampling of any set of conditions since the Galileo era. Ingress measurements resulted in a marginal detection with a peak ionospheric density of 4,000 +/- 3,700 cm(-3) (3 sigma) at 22 km altitude. A more significant detection emerged on egress, with a peak density of 6,000 +/- 3,000 cm(-3) (3 sigma) at 320 km altitude. Comparison with Galileo measurements reveals a consistent picture of Europa's ionosphere, and confirms its dependence on illumination conditions and position within Jupiter's magnetosphere. However, the overall lower densities measured by Juno suggest a dependence on time of observation, with implications for the structure of the neutral atmosphere. Plain Language Summary On 29 September 2022, NASA's Juno spacecraft flew very close to Jupiter's moon Europa. During the encounter, a radio occultation experiment was performed, where radio signals are exchanged between the spacecraft and ground stations as the former sets behind or rises from the moon as seen from the Earth. The scope of this experiment was studying the ionosphere of Europa, a layer of electrons and ions surrounding the moon. The Juno measurements confirmed the presence of the layer, with a structure similar to the one observed by the Galileo mission in the late 1990s.