The High Plasma Density Observed Along the PJ57 Juno Flyby of Io Implies the Presence of a Dense Atomic Corona

Abstract On December 2023, the Juno spacecraft made a flyby of Io above the northern hemisphere at a closest approach (CA) altitude of ∼1,500 km (PJ57). The Juno/Waves and Radio‐occultation measurements showed a surprising large electron density ∼28,000 cm −3 near closest approach. We run 2D numerical simulations of the plasma/atmosphere interaction to explore the causes of this high plasma density. Our numerical simulations are based on (a) A prescribed atmospheric composition and distribution of S, O, SO 2 , and SO; (b) A MHD code to calculate the plasma flow into Io's atmosphere; (c) A multi‐species physical chemistry code to compute the change in plasma properties (electron and ion densities, composition and temperature) during the plasma/atmosphere interaction; and (d) Ionization by field‐aligned electron beams. We show that during the PJ57 flyby, the increased plasma density is strictly localized inside the Alfven Wing and is caused by ionization of the polar atmosphere by both the thermal electrons of the torus penetrating the Alfven Wing and field‐aligned electron beams detected inside the wing. We explore several hypotheses leading to the very large electron density observed along PJ57: a dense upstream plasma impinging on the atmosphere; a dense polar atmosphere; and a change in its composition. We also assess the sensitivity of our results to the plasma flow speed inside the Alfven Wing. We show that the high electron density observed by Juno requires a significant atmospheric density above the pole, the composition of which is still undetermined.