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>

How much Io material reaches Europa?

<p>The plasma interaction with Io’s atmosphere results in at least a ton per second of escaping neutrals. Most of these neutrals supply extended neutral clouds along Io's orbit  and eventually become ionized and accelerated to corotation with Jupiter, populating the Io plasma torus as well as spreading out to fill Jupiter’s vast magnetosphere. About half to two-thirds of the plasma torus ions charge-exchange with the extended neutral clouds  and leave the torus as energetic neutral atoms, passing Europa’s orbit. Energetic neutrals are also produced directly in the plasma-atmosphere interaction, escaping with sufficient speed to reach Europa’s orbit before being ionized. The iogenic ions that are accelerated to high energies in the middle magnetosphere ultimately move back inward, again crossing Europa’s orbit. We present estimates of the fluxes of these various iogenic populations and how much oxygen, sulfur and sodium might be hitting Europa.</p>

Modelling the electron density distribution in the Io Plasma Torus using Juno radio occultations

<p>The innermost galileian moon Io hosts an intense volcanic activity, which ejects about 10<sup>3</sup> kg/s of gas into Jupiter's magnetosphere. Here these neutrals are ionized by interaction with the background plasma and they are accelerated from keplerian velocity to corotation velocity thanks to Alfvén's theorem. This plasma cloud around the planet (the so-called Io Plasma Torus or IPT) slowly diffuses across Jupiter's magnetic field, but high electron densities (>1000-2000 cm<sup>-3</sup>) are found between 5-8 R<sub>J</sub>.</p><p>Juno is travelling along highly eccentric, polar orbits around the planet and flies very close to Jupiter's surface during each perijove. Thus, the radio links used for ground communication and radio science cross the IPT both in the uplink and the downlink leg. Being a dispersive medium, the torus introduces a different path delay on the X/X and Ka/Ka links established between the Ground Station and the spacecraft. Thus, the path delay can be extracted through a linear combination of the two links, and then quantitatively analyzed and fitted to different parametric models of the IPT.</p><p>In this work we have used almost all the available Juno radio occultations of the IPT in order to improve an already existing model by introducing both longitudinal and temporal variations of the electron density. To this end, we looked for the 2D Fourier expansion in longitude and time of the parameters of this model with the goal of minimizing the residuals of the fit and pointing out periodicities in the morphology of the torus.</p>