Abstract. The Direct Simulation Monte Carlo (DSMC) method is used to simulate the interaction of Io’s atmosphere with the Jovian plasma torus and the results are compared to observations. These comparisons help constrain the relative contributions of atmospheric support as well as highlight the most important physics in Io’s atmosphere.  These rarefied gas dynamics simulations improve upon earlier models by using a three-dimensional domain encompassing the entire planet computed in parallel. A series of three large-scale multi-physics global DSMC atmospheric simulations are performed. The effects of plasma heating, planetary rotation, inhomogeneous surface frost, molecular residence time of SO₂ on the exposed non-frost surface, and surface temperature distribution are investigated in the first set of atmospheric simulations.

The results of the first set of global DSMC atmospheric simulations of Io’s atmosphere show that the most important and sensitive parameter is the SO₂ surface frost temperature. To improve upon the original surface temperature model, we constrain Io’s surface thermal distribution by a parametric study of its thermophysical properties. Our thermal model solves the one-dimensional heat conduction equation in depth into Io’s surface and includes the effects of eclipse by Jupiter, radiation from Jupiter, and latent heat of sublimation and condensation. The second set of global DSMC atmospheric simulations show that the sub-Jovian hemisphere is significantly affected by the daily solar eclipse. The simulations are compared to Lyman-a observations in an attempt to explain the asymmetry between the dayside atmospheres of the anti-Jovian and sub-Jovian hemispheres. The comparison indicates that the sub-Jovian hemisphere should have lower average column densities than the anti-Jovian hemisphere (with the strongest effect at the sub-Jovian point) due entirely to the diurnally averaged effect of eclipse.

Lastly, a particle description of the plasma is coupled with the sophisticated thermal model and a final set of global DSMC atmospheric simulations are performed. The particle description of energetic ions from the Jovian plasma torus allows for momentum transfer from the ions to the neutral atmosphere. Also, the energetic ions can dissociate the neutral atmosphere and cause sputtering of SO₂ surface frosts. SO₂ remains the dominant dayside species (>90%) despite being dissociated by ions and photons to form O, O₂, S, and SO. O₂ becomes the dominant atmospheric species above coldest areas of the surface because it is non-condensable at Io’s surface temperatures and other species are sticking to the surface. The momentum transfer from the plasma is found to have substantial effect on the global wind patterns. O₂ is pushed to the nightside by the circumplanetary winds where it builds-up until it reaches an equilibrium column density where it is destroyed by ion dissociation as quickly as it is transported. Large cyclones develop in the northern and southern hemispheres and are most apparent in the O₂ wind patterns because other species condense out on the nightside.