Neutrals Ejected From Io's Plasma/Atmosphere Interaction Region by Physical Chemistry Processes

Abstract Neutrals ejected from Io's atmosphere are the source of many important structures of the jovian magnetosphere: they feed giant neutral clouds, which extend along Io's orbit and nebulae, which extend beyond 500 jovian radii. The neutral loss rate is casually claimed to be ∼1 ton/s, but the processes leading to this loss, their quantitative estimates, and the speed and direction of the ejected neutrals are poorly constrained. In this study, we focus on neutrals ejected by physical chemistry processes resulting from the interaction of the torus plasma interacting with the atmosphere. These processes include electron‐impact dissociative‐ionization and dissociation, symmetrical and asymmetrical charge exchange and ion recombination. Our approach is based on a prescribed atmospheric distribution of SO 2 , SO, S and O. We combine an MHD code to compute the plasma flow into the atmosphere and a Multi‐Species Physical Chemistry code to compute the plasma properties (electrons, SO 2 + , SO + , S + , S ++ , S +++ , O + , O ++ densities and temperatures) and reaction rates along flowlines. In this article, we focus on reactions that specifically produce neutrals and compute their ejection rates, their velocity distribution and ejection direction. Using simplifying assumptions about the atmosphere, the flow and properties of the torus plasma, we provide an upper limit of the neutrals lost by physical chemistry processes ∼1 ton/s, with a velocity distribution specific for each reaction ranging from 0 to 120 km/s. The dominant processes are in the order of importance: molecular ion charge exchange, electron‐impact dissociation and molecular ion dissociative‐recombination, the last of which is prevalent in Io's wake.