A comprehensive model of ion diffusion and charge exchange in the cold io torus

An analytic model of lossy radial diffusion of ions in the cold Io torus is developed. The model includes ion‐neutral charge exchange reactions with extended atomic and molecular clouds of S, O, SO, and SO2 which act as combined sources/sinks of ions that couple the species and their various charge states. The ion‐molecule reactions are crucial to the analysis; in particular, the exothermic reaction S+ + SO → S + SO+ followed by rapid dissociative recombination for the resultant SO+ ion is the only viable way of removing S+ from the cold torus in order that its density conform with the many independent measurements of it. Model calculations indicate optimum values for the neutral densities are [S] = 7 cm−3, [O] = 30 cm−3, [SO] = 10 cm−3, and [SO2] = 20 cm−3. Such a neutral cloud environment yields an excellent correspondence of hot torus ion partitioning models with that of the cold torus through lossy radial diffusion. The preferred model of the analysis for the hot torus composition is [S+] = 220 cm−3, [S2+] = 340 cm−3, [S3+] = 40 cm−3, [O+] = 800 cm−3, and [O2+] = 90 cm−3 at Voyager 1 epoch. These numbers imply not only that the plasma torus is SO2‐based but also that dissociation of the molecular SO2 cloud is the primary source of all other neutrals that are distributed throughout the Io torus region. A unique feature of the model is the production of a boundary layer between the cold and hot tori where cold S+ ions mix with hot plasma torus ions in a manner that reproduces the ribbonlike plasma structure described by Trauger (1984).