Models of the Io Torus

Three-dimensional models of the Io torus are employed to analyze the spectroscopic data reported by J.S. Morgan (1985, Icarus62, 389–414). These models are used to compare Morgan's ground-based spectroscopic data with R.J. Oliversen's (1983, The Io Plasma Torus: Its Structure and Sulfur Emission Spectra. Ph.D. thesis, University of Wisconsin-Madison) nearly simultaneous SII] images and with the in situ measurements made by Voyager 1. The models are also used to investigate whether the observed SII] longitudinal intensity variations were caused by intrinsic or geometric effects, and to test the hypothesis that the observed optical east-west variations are consistent with the convective motions suggested by D.D. Barbosa and M.G. Kivelson (1983, Geophys. Res. Lett.10, 210–213) and W.-H. Ip and C. K. Goertz (1983, Nature302, 232–233). Oliversen's images are found to be in good agreement with Morgan's spectroscopic measurements. Three significant differences exist between these data and the torus described in the Voyager 1 experiments: (1) the torus beyond ～5.7R_J was found to be at least 1.5 to 2 times denser in 1981 than at the time of the Voyager 1 measurements in 1979, (2) the outer torus SII ion temperatures were approximately two times cooler than those measured by Voyager 1, and (3) in 1981, the outer torus OII mixing ratios were lower than were suggested by the Voyager 1 experiments. The 1981 ground-based OII/SII intensity ratios are found to be consistent with a radial peak near 6.0R_J in the ratio of oxygen to sulfur. At its maximum this ratio is ～2, and it falls to ～1 within ～0.5R_J inside and outside of this radius. Viewing geometry variations were found to be inadequate to account for the longitudinal variations observed by Morgan (1984). Intrinsic longitudinal intensity changes of about a factor of 2 are required to match the 1981 observations. Convective motions were found to adequately explain the observed optical east-west intensity asymmetry, but problems in interpreting the OII] doublet line ratios still remain. It is suggested that systematic errors are present in the measurements of the OII] line ratios.