DSMC simulations of the plasma bombardment on Io's sublimated and sputtered atmosphere

The DSMC method is used to model the interaction of the jovian plasma torus with Io's SO₂ sublimation and sputtered atmosphere just prior to eclipse. The SO₂ frost sublimes on the warm dayside and photo and neutral chemistry, the dominant source of the daughter species (SO, O ₂, O, and S) are included. To model the plasma interaction with the sublimation atmosphere, a two-timestep method is utilized in which the neutrals are assumed to be stationary while electrons and ions are moved and collided over a much smaller timestep. The dominant ion-neutral interactions (non-reactive and resonant charge exchange) are included. Sputtering of SO ₂ molecules from the frost-covered surface is dependent on the incident ion energy and the surface frost temperature. Io's surface is assumed to be uniformly covered by SO₂ surface frosts with the temperature computed based on radiative equilibrium with insolation. We investigate the effect that the plasma interaction with Io's atmosphere has on atmospheric composition and structure, circumplanetary winds, and the escape rate of material from Io to the plasma torus. The dense sublimation atmosphere reduces sputtering from SO₂ surface frosts over much of the dayside; however, sputtering was found to be a significant contributor to the nightside atmosphere. The plasma pressure on the sublimation atmosphere has a substantial effect on the day-to-night winds. Not only does the plasma pressure induce an overall retrograde wind in Io's atmosphere just prior to entry into eclipse, but the atmospheric scale height is reduced by the plasma pressure on the trailing hemisphere. Molecular oxygen is a minor species on the dayside but is found to be the dominant nightside species because it is non-condensable and the loss rates due to atmospheric escape or dissociation are slow.