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 S 2+ , 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 C 2+ relative to S 2+ 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.