Detection of SO in Io's Exosphere
C. T. Russell,
M. G. Kivelson
The Galileo orbiter's close pass by Io in 1995 produced
evidence for extensive mass loading of the plasma torus through the ionization of SO2. On 11 October 1999, Galileo passed even
closer to Io, this time across the upstream side relative to the flow of magnetospheric plasma that corotates with Jupiter. On the first flyby, ion cyclotron waves gave direct evidence for the production of
SO2+ ions. On the second flyby, ion cyclotron
waves associated with SO+ were stronger and more
persistent. Moreover, SO+ emissions were seen closer to Io
than SO2+ emissions, suggesting that the
exosphere was spatially inhomogeneous. The location of the waves
suggests a fan-shaped region of ion pickup extending in the
anti-Jupiter direction. Because the wave spectra were different even
where the 1995 and 1999 trajectories crossed, we infer that Io's
exosphere is temporally variable.
Institute of Geophysics and Planetary Physics, 3845 Slichter Hall,
University of California Los Angeles, Los Angeles, CA 90095-1567,
USA.
