The Far-Ultraviolet Spectrum of the Io Plasma Torus

The spectrum of the Io plasma torus in the range 905-1187 Å was recorded at 0.26 Å resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2001 January 14. Five orbits of data were obtained with the west ansa of the torus centered and tracked in the 30′′ × 30′′ apertures of the FUSE spectrographs for a total observation time of 9740 s. This region of the spectrum is dominated by transitions of S II, S III and S IV, whose multiplet structure is nearly completely resolved. We confirm our earlier detection of emission from resonance multiplets of Cl III and Cl II and derive an abundance of Cl+2 of 2.1% relative to S2+, leading to an overall chlorine ion abundance in the torus of slightly less than 1%. A number of features near 990 Å remain unidentified, and C III λ977 is detected in two independent channels at the 3 σ level. The inferred relative ion abundance of C2+ relative to S2+ is 3.7 × 10-4. We also present spectra at 0.085 Å resolution taken on 2001 October 19 and 21 with the 4′′ × 20′′ aperture. In these spectra, the observed lines are resolved, and their widths correspond to ion temperatures of 60-70 eV for all three sulfur ions.