Reconstructing Europa’s atmosphere from H+ 2 pickup ions detected during the Juno PJ45 flyby

We use a suite of single-species (H2) and multispecies (H2+O2+H+O) Direct Simulation Monte Carlo models of Europa’s atmosphere to interpret the H+ 2 pickup ions detected by Juno at distances beyond 2 Europa radii (RE) during the Perijove 45 flyby. The observations require contributions from both Europa’s H2 atmosphere and a background H+ 2 torus; the latter has local densities ranging from ∼1.1 cm−3 on the inbound leg (anti-Jovian hemisphere) to ∼0.9 cm−3 on the outbound leg (sub-Jovian hemisphere)—an asymmetry in the H+ 2 torus consistent with recent plasma-advection modeling. We show that the magnitude and spatial structure of the observed H+ 2 signatures are reproduced to within ±5% over ∼4RE along both legs of the flyby by a purely thermal H2 atmosphere embedded within the H+ 2 torus.