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 gathered measurements about the moon’s surroundings. During the close flyby, a radio occultation experiment was performed by collecting single-frequency Doppler measurements on ingress and egress. These investigations are essential to the study of Europa’s ionosphere and represent the first repeated measurements of charged particle densities around the moon since the Galileo era. Ingress measurements resulted in a marginal detection with a peak ionospheric density of 4,000 ± 3,700 cm−3 (3σ) at 22 km altitude. A more significant detection emerged on egress, with a peak density of 6,000 ± 3,000 cm−3 (3σ) 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, although the over all lower densities measured by Juno suggest a dependence on time of observation.