Temporal and spatial variations in the Io torus

Spectroscopic measurements of lines from both forbidden S II and forbidden O II in the Io torus taken on 15 nights spread over four months in 1981 are presented and discussed. An east-west asymmetry was observed in the optical emissions, showing larger western intensities and a more diffuse and radially extensive nebula to the east. Two configurations of forbidden S II longitudinal asymmetry were stable over at least four days. Longitudinal structure was not detected in either the forbidden O II intensity or the plasma density as measured by the forbidden O II and S II doublet ratios. A radial variation in the ratio of O II/S II was observed, with the ratio being largest near Io's orbit. Monthly variability was detected in both the intensity and density of the torus.