Asymmetry of Io's outer atmosphere: Constraints from five Galileo flybys

The Galileo spacecraft probed the Io atmosphere/torus interaction region along six flybys between 1995 and 2001. The instruments on board provided measurements of the plasma density, average ion temperature, composition, flow, and magnetic perturbations to which we compare models and from which we constrain Io's atmosphere asymmetries. We have developed a model of the local interaction at Io that couples an MHD model of the flow and magnetic perturbations around the moon with a multispecies chemistry model that includes the physical chemistry of the main species: atoms, molecules, and ions derived from Io's volcanic gases ( S, O, SO2, SO). We prescribe several scenarios of the multicomponent neutral atmosphere of Io based on observations and compare our results with the plasma properties inferred from the Galileo measurements. Owing to limited data on the I25 flyby, it is excluded to make a total of five flybys for our comparisons. We show that the ion average temperature profile is a key quantity to constrain the radial extension and longitudinal asymmetries of the atmosphere of Io. We propose that the atmosphere has longitudinal asymmetries: its radial extension is limited upstream and significantly larger on the anti-Jovian downstream side. A very extended corona of SO2 and SO is present mainly downstream of Io. We discuss the possible temporal variability of Io's plasma-atmosphere interaction by comparing two flybys at different times at approximately the same location in the wake. We also discuss the plasma composition in the wake and the O1356A auroral emissions observed on the flanks of Io.