Juno-UVS Observations of Io during the PJ58 Flyby

During its first extended mission, NASA’s Juno spacecraft made several close flybys of Jupiter’s Galilean satellites. The final of these was the Io flyby during the perijove (PJ) 58 orbit with closest approach at 17:48:35 UTC on 3 February 2024, about 3h59m prior to PJ58. We used Juno’s UVS, a photon-counting far-ultraviolet (FUV) imaging spectrograph with a bandpass of 68-210 nm, to observe Io’s numerous FUV emissions during the flyby. Due to the high speed and low altitude of the flyby, Io was only observable for a few minutes on either side of Juno’s closest approach. Juno UVS utilizes a high countrate backoff mechanism to protect its MCP detector. Due to the high radiation background of penetrating (>10 Mev) electrons, UVS was only able to sustain its nominal voltage for 3 out the 20 spins of recorded data (± 5 min about closest approach). Out of those only one exhibited low enough radiation levels to allow for clear detections of oxygen and sulfur emissions. With a spatial resolution of ˜5 km at the limb, these observations offer a unique glimpse of the vertical distribution of the O and S emissions. The nadir time of the single low radiation spin was 17:48:09, just prior to closest approach. Juno was at an altitude of 1599 km and the sub spacecraft point was -35.7° latitude and -52.1° East longitude. We will present the observations and a comparison to earlier HST observations and the phenomenological model of Lorenz et al. (2014, DOI 10.1016/j.icarus.2013.10.009).