Temporal and spatial variations in the Io torus

Spectrographic data on the Io torus from 15 nights of observations spread over a 4-month period in 1981 are presented here. The SII] λλ6716, 6731; SII] λλ4069, 4076; OII] λλ3726, 3729; and SIII] λ3722 lines were simultaneously measured on each spectrogram. 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 SII] longitudinal asymmetry that were stable over at least 4 days were observed. The magnetic longitudes of ～ 180 and 300° are shown to be of particular interest. Longitudinal structure was not detected in either the OII] intensity or the plasma density as measured by the OII] and SII] doublet ratios. Errors in the line ratios could mask density variations as large as a factor of ～ 1.5. A radial variation in the ratio of OII/SII was observed, with the ratio being largest near Io's orbit. Monthly variabilitywas detected in both the intensity and density of the tours. The SII] line ratios indicated an increase in n_e over the 4-month period that was accompanied by increased intensities. For single measurements, no correlation between the SII] intensities and the SII] λ6716/λ6731 line ratio was detected, but this could be a result of errors in the line ratio determinations. Extremely low values of this same ratio were measured. These appear to indicate errors in the presently accepted SII] transition probabilities. These SII] line ratios indicate that very-high-density regions are present in the torus, and it is shown how these regions could have significantly influenced these measurements.