The plasma/atmosphere interaction at Io: revisiting the ion/neutral charge-exchange cross-sections and their ion-velocity dependency

<p>The Io Torus plasma is mostly composed of singly and multi-charged S and O ions. These ions interact with the neutrals of Io’s atmosphere (S, O, SO<sub>2</sub> and SO) through symmetrical (i.e. O<sup>+</sup> + O => O + O<sup>+</sup>) and asymmetrical (i.e. S<sup>++</sup> + O => S + O<sup>++</sup>) charge-exchanges. Charge-exchange cross-sections were estimated in Johnson & Strobel, 1982 and McGrath & Johnson, 1989 at 60 km/s (the plasma corotation velocity in Io’s frame), and are used in numerical simulations of the torus/neutral cloud interaction (i.e. Delamere and Bagenal, 2003).</p> <p>Dols et al., 2008 proposed numerical simulations of the multi-species chemistry interaction at Io using these cross-sections at 60 km/s. The plasma/atmosphere interaction at Io is strong and the flow velocity and ion temperature are drastically reduced close to Io (v < 10 km/s). Thus, velocity-dependent charge-exchange cross-sections are critical for such numerical simulations and their effect on the local plasma and neutral supply at Io should be explored.</p> <p>We propose to revisit the calculation of ion/neutral charge-exchange cross-sections following Johnson & Strobel, 1982’s approach for plasma velocities relevant to the local interaction at Io (V=10-120 km/s). More sophisticated calculations were proposed in McGrath & Johnson, 1989 but both publications offered very few details about their procedure.</p> <p>We will illustrate the effect of using velocity-depend charge-exchange cross-sections in numerical simulations of the multi-species plasma/atmosphere interaction at Io.</p> <p>More generally, this presentation aimed at providing an incentive for the community to expand the work of McGrath & Johnson, 1989.</p> <p> </p> <p><em>Johnson & Strobel, Charge-exchange in the Io torus and exosphere, JGR, 87,1982</em></p> <p><em>McGrath & Johnson, Charge exchange cross sections for the Io plasma torus, JGR, 94, 1989</em></p> <p><em>Delamere & Bagenal, Modeling variability of plasma conditions in the Io torus, JGR, 108, 2003</em></p> <p><em>Dols, Delamere, Bagenal, Kurth, Paterson, A multi-species chemistry model of Io’s local interaction with the plasma torus, JGR, 113, 2008</em></p>