0 Assigned ID: 7716 ----- Attempting Submission 1 (Thu Apr 16 22:15:35 GMT 2026) ----- HST Phase I Proposal 7716 successfully submitted. Receipt: # 7716-1 ----- Attempting Submission 2 (Thu Apr 16 23:17:21 GMT 2026) ----- HST Phase I Proposal 7716 successfully submitted. Receipt: # 7716-2 ----- Attempting Submission 3 (Thu Apr 16 23:22:05 GMT 2026) ----- Io is the most volcanically active body in the Solar System and its escaping atmosphere is the dominant source of material for the Jovian magnetosphere. Despite decades of research and major advances in understanding individual processes and features, the connections between Io's volcanos, surface volatiles, atmosphere, magnetospheric plasma-interaction, and beyond to Io's neutral cloud, the Io Plasma Torus, and Jupiter's ionosphere, all remain difficult to quantify and understand. Juno, a polar orbiter originally designed to study Jupiter's deep interior, magnetosphere, and aurora, had two close flybys in HST Cycle 31 which offered an excellent opportunity for combining Juno's intensive in situ measurements with Hubble's remote sensing to advance our understanding of Io's role in driving coupled phenomena within the Jupiter system. The previous HST-Juno Io campaign performed numerous coordinated Io observations to take advantage of this opportunity for combining in situ and remote sensing datasets within a comprehensive modeling framework. We propose an archival study of far-UV data sets obtained to date that targeted Io's escaping atmosphere and neutral cloud spatial distributions. The data also contain ionized sulfur emissions that are useful for constraining Io Plasma Torus models. By comparing the profiles of column densities of neutral O and S species the source rates for the ionization of this material as a replenishing source of the Io Plasma Torus will be characterized for the era of Juno Io encounters. PI K. Retherford has experience from leading one successful HST program as PI (ProgID=13805, Io Lyman-alpha transits Retherford et al., AJ, 2019; large program 17470), several as U.S. administrative PI, and many more as CoI. As PI of the Europa Clipper UV Spectrograph (UVS), Dep. PI of JUICE-UVS, and PI of LRO-LAMP he has extensive far-UV spectroscopic expertise. Fran Bagenal leads the team's model synthesis of numerous datasets from Hubble, Juno, JWST, Keck, and other ground-based observations. Todd Smith, Vincent Dols, Eddie Nerney, Carl Schmidt, and Matthew Burger all have extensive experience working together to constrain models using observations of Io, neutral clouds, and plasma torus. Lorenz Roth has experience from leading ten successful HST programs as PI, including previous programs with similar scope of data-model comparisons and copious publications. Philippa Molyneux has experience with HST data analysis from COS, STIS, and other spectroscopic instruments, with Galilean satellite programs specifically. Michael Velez is a Postdoc and former graduate student with mentor Retherford and has experience in analyzing STIS datasets and planning the observations to be analyzed in this program. Lori Feaga has a deep set of experience observing Io's atmosphere and auroral morphology with all HST STIS modes planned here. Jerry Goldstein has extensive magnetospheric modeling experience including early postdoctoral studies of Jupiter, and additional EUV dataset analysis expertise. Simon Mendenall is a graduate student at UTSA with mentor Retherford and has experience in analyzing STIS COS datasets and planning the observations to be analyzed in this program. Solar System Astronomy 0 false false false false false false false SUPPORTED