Io Source of Heavy Ions in Jupiter’s Equatorial Ionosphere

The JADE instrument on Juno has measured cold (<1 eV) heavy ions (sulfur and oxygen) in the equatorial ionosphere of Jupiter as the spacecraft made its closest approach to the planet (~3,500 km above 1 bar level), as reported by Valek et al. (2020) for 17 passes (between perijoves 6 and 23). The source of these heavy ions in Jupiter’s proton-dominated ionosphere remains unclear. The presence of sulfur and oxygen suggests they ultimately came from Io. Inward transport of plasma from the Io torus is slow and the total flux very small. Moreover, adiabatic heating would produce energetic particles reaching the equatorial ionosphere rather than cold populations. An alternative possible source could be neutral atoms that are ejected by the plasma interaction with Io’s atmosphere. We take estimates of the neutral fluxes from models of the Io plasma-atmosphere interaction and explore how the flux of neutrals reaching Jupiter depends on the ejection speed and direction. We then consider typical equatorial atmospheric conditions at Jupiter and evaluate how the incoming neutrals become cold heavy ions mixed into the ionosphere.