A multispecies chemistry model of Io's local interaction with the Plasma Torus

Abstract

We model the local interaction between the plasma in the torus and Io's neutral corona (inside ∼6 RIo), focusing on the multispecies chemistry outside the collision‐dominated ionosphere. We include a detailed chemistry of S, O, SO2, SO under ionization, charge exchange, and recombination processes in a parcel of plasma that follows a prescribed flow field around Io's ionosphere and interacts with the neutral density profiles in the corona, as constrained by available observations. We compare the model results to the Galileo plasma observations in Io's wake (GLL/J0). We conclude the following: (1) The plasma characteristics along GLL/J0 require a dense SO2 corona confined close to Io (<2 RIo). (2) Molecular SO2 chemistry dominates the interaction, with atomic chemistry playing a negligible role. (3) SO2+ is the main output ion of the local interaction, but it recombines and dissociates rapidly so that the contribution of the local interaction to the global mass and energy supply of the torus is very small. (4) The high density of plasma observed in the wake requires a supplemental ionization source beyond the thermal electrons of the torus, and we estimate the ionization due to the (∼350 eV) field‐aligned electron beams observed around Io. (5) Assuming a SO2 atmosphere/corona with a vertical column of 6 × 1016 cm−2 and a prescribed deceleration of the flow around Io, we estimate a local ion mass production rate of ∼200 kg/s. We also compute a neutral loss rate of ∼2400 kg/s and a pick‐up current of ∼5.2 MAmp, both dominated by the SO2 resonant charge exchange reaction.