Revised O+ mixing ratio in the Io torus during the Voyager era.

M.H. Taylor (STScI), N.M. Schneider, F. Bagenal (U. Colorado)

The canonical value of the O+ mixing ratio ([O+]/[e-]) in the Io plasma torus during the Voyager encounter is 0.4 based primarily on UVS measurements of the 833 Å emission line (Shemansky, J. Geophys. Res., 92, 6141, 1987). This ratio is important in modeling the relative neutral source rates of oxygen and sulfur to the torus; and hence, the loss rates and abundances at Io. By assuming the in-situ measurement of electron temperature of 5 eV (Sittler and Strobel, J. Geophys. Res., 92, 5741, 1987), we find that the O+ mixing ratio must be reduced by 25% to match the UVS spectra. This brings the value in closer agreement with the nearly simultaneous ground-based observations by Morgan and Pilcher (Astrophys. J., 253, 406, 1982) of the [OII] tex2html_wrap_inline16 3726, 3729 emission. An even further reduction is warranted if the revised in-flight UVS calibration by Holberg et al. (Astrophys. J., 375, 716, 1991) is used. We use the Colorado Io Torus Emissions Package (CITEP; see Taylor et al., J. Geophys. Res., 100, 19541, 1995) to compare the UVS and ground-based observations. We also compare with later observations of O tex2html_wrap_inline18 emission from HST, Galileo, and other satellites, and discuss the new mixing ratio in relation to observed spatial and temporal variations. This work was supported in part by JPL/Galileo grant 958675.