F. Bagenal

In 1995, the Galileo spacecraft traversed the wake of Io at ∼900 km altitude. The instruments onboard detected intense bi‐directional field‐aligned electron beams (∼140 eV–150 keV), embedded in a dense, cold and slow plasma wake ( N el ∼ 35,000 cm −3 , T i < 10 eV, V < 3 km/s). Similar electron beams were also detected along subsequent Galileo flybys. Using numerical simulations, we show that these electron beams are responsible for the formation of Io's dense plasma wake. We prescribe the composition of Io's atmosphere in S, O, SO and SO 2 , compute the atmospheric ionization by the beams with a parameterization adapted from study of auroral electrons at Earth, the plasma flow into Io's atmosphere with a Magneto‐Hydro‐Dynamic code, and the ion composition and temperature with a multi‐species physical chemistry code. Results reveal contrasting chemistries between the upstream and wake regions, leading to different ion compositions. The upstream chemistry is driven by the torus thermal electrons at 5 eV with SO 2 + becoming the dominant ion because of electron‐impact ionization of the SO 2 atmosphere. The wake chemistry is driven by the high‐energy electrons in the beams with S + and SO + becoming the dominant ions produced by dissociative‐ionization of SO 2 . We show that the wake ion composition is highly sensitive to the atmospheric composition. Juno, in its extended mission, will traverse Io's wake and determine its ion composition, which, compared with our numerical simulations will enable us to infer the detailed composition of the atmosphere.