Galileo's close-up view of the Io sodium jet

The Galileo spacecraft has imaged a re- markable atmospheric escape process occurring in the atmosphere of Io, Jupiter's volcanic moon. Electrody- namic consequences of Io's motion through Jupiter's magnetosphere drive mega-amp currents through Io's ionosphere; some of the sodium ions carrying this cur- rent are neutralized as they leave the atmosphere. The Galileo images show that the resulting fast sodium jet removes - 5 x 1025 atoms sec - from Io's atmosphere. The spatial profile of the jet shows that the source re- gion is much smaller than Io itself, perhaps confined to volcanically active regions, or to an ionosphere re- stricted to the denser atmosphere near Io's equator. Io. The escape is driven by the electrodynamic poten- tial of 411 kV induced across Io by its motion through Jupiter's magnetic field (Goldreich and Lynden-Bell, 1969; Dessler, 1983). The electric field is directed radially outwards from Jupiter and perpendicular to the magnetic field. Radio occultations by spacecraft have revealed a global ionosphere capable of conduct- ing mega-ampere currents in response to this potential (Hinson et al., 1998). The dominant ionospheric ion is Na + by virtue of sodium's very low ionization potential (Summers and $trobel, 1996). Sodium ions are driven by Pedersen currents radially outward from Jupiter. Ions on the Jupiter-facing side of Io are driven to the sur- face and cannot escape; those on the anti-Jovian side are not blocked. Some of these escaping ions are neu- tralized by charge-exchange reactions and become fast sodium atoms directed away from Io at velocities of tens of km sec -1. The sodium atoms preserve their ion mo- tion at the moment of charge exchange, which includes a substantial gyrovelocity perpendicular to the local mag- netic field. The plane of gyromotion defines the orienta- tion of the jet. Sodium-bearing molecular ions, known to produce fast sodium features farther from Io (Wilson and Schneider, 1994), may contribute sodium atoms to the directional feature. Sodium ejected from Io's iono- sphere adds to the extended disk of sodium atoms ob