AN INITIAL LOOK AT THE IOGENIC SO2+ SOURCE DURING THE GALILEO FLYBY OF IO

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.