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]
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 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.