Ryoichi Koga

The Jovian system is very intriguing with extremely different particle sources. As the dominant source of particles to Jupiter's magnetosphere, Io is of particular importance. The JAXA Hisaki (SPRINT‐A) mission is providing insightful observations with regard to Io. In particular, its UV neutral oxygen line of sight observations provide the best glimpse so far of Jovian neutral particle populations. This is exciting in that for the first time, the neutral tori can be directly observed on time scales that constrain satellite sources. This data also sheds unprecedented insight into neutral torus distributions, which could subsequently provide essential information about the sources and mechanisms from Io. However, 3‐D modeling is also required to interpret the complex and dynamic observational geometries. Here, we present the first 3‐D distribution of the Io‐generated neutral oxygen torus using JAXA Hisaki neutral oxygen observations as modeling constraints to provide characterization of the Io magnetospheric source and the resulting neutral tori. Major findings of our research include: (a) the effective total original SO 2 source rate from Io during a low activity period is ∼800 kg/s; (b) Io has two separate exospheric sources: O at ∼200 kg/s and SO 2 at ∼400 kg/s; (c) particles escape from Io preferentially on the Jupiter upstream facing hemisphere; (d) the oxygen neutral torus populates the entire orbit and significantly dominates over the SO 2 , SO, and S neutral toroidal clouds; and (e) large ionization and dissociation rates limit the significant expansion of Io‐genic neutral particles in Jupiter's magnetosphere beyond ∼2 Rj from Io's orbit.