The Juno PJ57 and PJ58 flybys of Io: Multi-species physical chemistry simulations

The Juno spacecraft made close flybys of Io on Dec. 2023 (PJ57) and Feb 2024 (PJ58) above respectively the northern/southern hemisphere with an altitude at closest approach (CA) of ~1,500 km.

On PJ57, Juno went through the Alfven wing and both the Juno/Waves and Radio-occultation measurements showed a surprising large electron density n_el ~ 28,000 near closest approach. On PJ58, Juno flew slightly behind the Alfven wing and the instruments measured a plasma density consistent with the background plasma torus density.

We run numerical simulations of the plasma/atmosphere interaction along teh PJ57 and PJ58 flyby to constrain IO’s polar atmosphere. Our numerical simulations are based on (1) A prescribed atmospheric composition and distribution of S, O, SO₂ and SO; (2) A MHD code to calculate the plasma flow into Io’s atmosphere; (3) A multi-species physical chemistry code to compute the change of the plasma properties (ion densities, composition and temperature) during the plasma/atmosphere interaction (4) a formulation of the ionization by the field-aligned electron beams used for auroral electrons on Earth.

We compute the multi-charged ion composition of the plasma along each flyby and compare to the Juno/JADE measurements to infer the atmosphere composition (O, S, SO2, SO) and density at polar latitudes.