Return to Io by the Galileo spacecraft: Plasma observations

On October 11, 1999, a series of high-resolution plasma measurements were obtained during the second close flyby of Io of the Galileo mission. The closest approach to lo occurred at an altitude of 617 km along the flank of Jupiter's torus plasma flows past this moon. Energy/charge (E/Q) and mass/charge (M/Q) measurements with the plasma analyzer were used to identify the primary ions. At closest approach the ion number densities increased to their maximum values of 1200 cm(-3) as calculated from the plasma moments. These ions were dominated by two thermal populations of torus ions with M/Q = 16, one with density and temperature of 800 cm(-3) and kT = 40 eV and the other with 200 cm(-3) and 160 eV. On the basis of the previous Voyager 1 remote measurements with a spectrometer, these two thermal ion plasmas are identified as O+ and S++, respectively. A substantial population of pickup ions was also observed, with densities of about 100 cm-3 each for S+ and SO+. The plasma bulk flows were strongly deflected around the body of Io, and the speed of the bulk flow increased to a factor of 1.3 greater than that for rigid corotational flow. The torus ion densities just outside of Io's orbit were about 800 cm(-3). These densities at the immediate position of Io are smaller by a factor of about 4 relative to those during the first flyby on December 7, 1995, but should not be taken as indicative of a similar decrease in the bulk of the plasma torus. Two major plasma regimes were encountered before the closest approach to Io. The first was a region of hot ions located inside of Io's orbit which was previously identified as the "ribbon" of enhanced plasma densities with Voyager 1 plasma measurements and with ground-based imagery. The ion composition in the ribbon included O+ and S++ The second region, closer to Io, exhibited a hot torus plasma which was mixed with pickup hydrogen ions. These hydrogen ions were detected at distances beginning at 7.8 R-Io from this moon, which were generally inside the Hill (Lagrange) "sphere" for Io's atmosphere.