Galileo in its December 7, 1995, encounter with Io flew downstream of
Io and through the magnetospheric wake of the satellite with a closest
approach altitude of similar to 900 km. Magnetospheric instruments we
re therefore able to sample the Iogenic plasma source both outside as
well as deeply within the Lagrange sphere of Io (similar to 5.81 satel
lite radii) where the gravity of Io dominates and where the plasma pic
kup processes.are expected to be highly peaked about the satellite. Th
e presence of both long-lived atomic ions (H+, 0(+), O++, S+, S++) and
short-lived molecular ions (SO+, SO2+) was detected along the spacecr
aft trajectory. We have undertaken preliminary calculations for the de
nsity profile of SO2+. These calculations are compared with the SO2+ d
ensity profiles deduced from magnetic field fluctuations with periods
of similar to 2-3 s measured by the Galileo magnetometer and interpret
ed as ion cyclotron waves produced by fresh SO2+ Iogenic pickup ions c
reated near Io. By matching the absolute SO2+ model density with the m
inimum ion density determined by Huddleston et al. [1997] in their ana
lpsis of the ion cyclotron waves, an SO2 source rate of similar to 4 x
10(27) molecules s(-1) (425 kg s(-1)) at Io's exobase and a correspon
ding SO+2 source rate 2.8 X 10(26) ions s(-1) (30 kg s(-1)) in the mag
netosphere are determined. Most of the SO, that undergoes interactions
in the plasma torus is, however, rapidly dissociated primarily by ele
ctron impact, producing O, S, SO, and O-2. These species subsequently
undergo ionization and charge exchange reactions in the plasma torus,
producing much larger mass and energy pickup plasma loading rates, inc
luding an SO+ source rate estimated to be somewhat smaller than the SO
2+ source rate. Since the lifetime of SO2 is highly variable with Io's
position in the plasma torus, it follows that the spatial profile for
the amplitude of these magnetic fluctuations will also be highly spac
e and time variable and will depend upon both Io System III longitude
and Io geocentric phase angle.