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Ion-molecule charge exchange in Io’s extended atmosphere: Velocity dependence

Presentation #111.04 in the session Io (Poster)

Published onOct 23, 2023
Ion-molecule charge exchange in Io’s extended atmosphere: Velocity dependence

The Io plasma torus is mostly composed of multiply-charged O and S ions. These ions interact with the neutral species of Io’s atmosphere (S, O, SO2 and SO) through symmetrical (e.g., O+ + O ⇒ O + O+) and asymmetrical (e.g., S+ + O ⇒ S + O+) charge exchange reactions. Moreover, molecular ions (SO2+, SO+) produced in Io’s atmosphere also charge exchange with the atmospheric neutrals (e.g., SO2+ + SO2 ⇒ SO2 + SO2+).

Cross sections for many charge exchange reactions were estimated in Johnson & Strobel (1982) and McGrath & Johnson (1989) for ion speeds of 60 km/s appropriate for torus/neutral cloud interactions where the plasma is approximately at corotation. But close to Io, the plasma flow velocity and ion temperature are drastically reduced (Vflow < 10 km/s and Tion < 50 eV) and greatly increased on its flanks (Vflow ~ 80 km/s and Tion ~ 400 eV). Thus, velocity-dependent charge exchange cross sections are critical for numerical simulations of the Io plasma/atmosphere interaction.

We provide estimates of velocity-dependent cross sections for ion/neutral charge exchanges in the range of velocity [5–120] km/s representative of ion velocities at Io. We combine several methods valid at the low and higher velocities in this range: Two-state impact parameter approximation, quasi-thermal rate constant extrapolation, Langevin-orbiting cross sections and scaling from experimental data.

Our estimates at 60 km/s are 2–3 times smaller or larger than previously published values, depending on the reaction. Most are generally large (~60–90 Å2) at small velocities.

We conclude that velocity-dependent cross sections are critical to study the ion composition close to Io and also stress the need for experimental determination of cross sections for sulfur-bearing components.

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