VARIABILITY OF [O-I] 6300-ANGSTROM EMISSION NEAR IO

Observations of [O I] 6300-angstrom emission near Io have been obtaine d in 1990, 1991, and 1992 by the National Solar Observatory staff usin g the solar-stellar spectrograph on the McMath-Pierce telescope at Kit t Peak. High-resolution spectra with a resolving power of about 1.2 x 10(5) were obtained with an integration time of 10-15 min each. The vi ewing aperture for the observations was 5.2 arc sec x 5 arc sec center ed on Io, with spatial resolution limited within this area by seeing c onditions. Observations thus far have been reduced to obtain average b rightness values over the aperture which range from approximately 200 to 1000 R for a number of different Io phase angles and Io system III longitudes. The [O I] 6300-angstrom emission brightness exhibits an ea st-west asymmetry, where the average intensity for Io phase angles in the west (receding ansa) is 1.5 times brighter than in the east (appro aching ansa). Similar east-west intensity ratios have also been observ ed for neutrals near Io and ions in the plasma torus for a number of o ther optical and ultraviolet emission lines which are excited by elect ron impact. In addition to the east-west asymmetry, the [O I] 6300-ang strom emission brightness exhibits a strong dependence on the Io syste m III longitude angle, with a maximum value occurring in the range 200 -degrees +/- 50-degrees. Earlier IUE observations of ultraviolet emiss ion lines of O and S near Io obtained over a number of years have meas ured the east-west asymmetry, but the long IUE integration times of ap proximately 7-14 hours masked any detection of system III variability. For the [O I] 6300-angstrom emission, the O(1D) state may be excited by electron impact of atomic oxygen and by electron impact dissociatio n of SO. The molecule SO may be present at the exobase or may be produ ced above the exobase as the dominant product of SO2 dissociation by e lectron impact. Preliminary assessment indicates that production of O( 1D) by molecular dissociation may be more important. The [O I] 6300-an gstrom emission may therefore provide a remote signature for monitorin g (1) the upward transport rates of molecular species in Io's atmosphe re, (2) the relative abundance and time-variable dissociation of SO2 a nd/or SO at the exobase and in the corona of Io, and (3) the spatial d istribution of these escaping molecular and atomic species and their i on production rates in the planetary magnetosphere.