Vertical and Temporal H3+ Structure at the Auroral Footprint of Io

Abstract

We report the first observation of the vertical and temporal structure of the H _3 ^+ emission at the auroral footprint of Io, as observed by Juno/JIRAM. The brightness vertical profile shows a maximum at 600 km above 1 bar, with no apparent difference between the main Alfvén wing (MAW) spot emission and the tail of the footprint. This observation better aligns with a broadband energy distribution of the precipitating electrons, instead of a monoenergetic one. The temporal profile of H _3 ^+ column density has been observed after the passage of the MAW and shows a hyperbolic decrease. A model of H _3 ^+ decay is proposed, which takes into account the second-order kinetics of dissociative recombination of H _3 ^+ ions with electrons. The model is found to be in very good agreement with Juno observations. The conversion factor from radiance to column density has been derived, as well as the half-life for H _3 ^+ , which is not constant but inversely proportional to the H _3 ^+ column density. This explains the wide range of H _3 ^+ lifetimes proposed before.

Keywords

• Near infrared astronomy
• Jupiter
• Aurorae