Ion Precipitation Into Io's Poles Driven by a Strong Sub‐Alfvénic Interaction

Juno performed two close flybys of Io and found enhanced field-aligned proton fluxes are absorbed by Io. These protons are absorbed at mass input rates comparable to previous estimates for hydrogen losses from Io, hence Jupiter is likely the source of hydrogen at Io. The conditions necessary for this to occur are: (a) formation of Alfvén waves at Io, (b) wave-particle coupling to energize protons, (c) anti-planetward transport of ions due to the magnetic mirror force and/or parallel acceleration, and (d) strong sub-Alfvénic interaction slowing the flow connected to Io's fluxtube allowing for sufficient travel time for energized ions to transit to Io. The derived slowdown of ≤12% the upstream value is linked to filamentation within the Alfvén wing. This mechanism is likely operating at all strongly interacting satellites and provides an avenue to transfer material from a planetary body to its satellites, including exoplanets and brown dwarfs.