Detection of Chlorine Ions in the [ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Spectrum of the Io Plasma Torus

Abstract

The spectrum of the Io plasma torus in the range of 995-1187 A ˚ was recorded at 0.26 A ˚ resolution by the Far Ultraviolet Spectroscopic Explorer (FUSE) on 2000 January 20. Five orbits of data were obtained in point-and-shoot mode (no tracking of the moving target), with the east ansa of the torus initially centered in the aperture of the FUSE LiF spectrographs yielding a total observation time of 3405 s. The spectral 30 # 30 resolution exceeds by a factor of 10 that of the data obtained by the Hopkins Ultraviolet Telescope (HUT) during the Astro-1 mission. This region of the spectrum is dominated by resonance multiplets of S iii and S iv, whose multiplet structures are nearly completely resolved, as well as numerous S ii multiplets originating on the 2 o D state of the ground configuration. Weak emission from a few lines of the resonance multiplets of Cl iii and Cl ii is detected at or above the 3 j level, Cl iii being stronger with two components roughly one-tenth the brightness of the main components of S iii l1018. We derive an abundance of Cl of 3% relative to S 2 , leading 2 to an overall chlorine ion abundance in the torus of ≈1%. The ratio of S iv to S iii brightness is about twice that observed by HUT, which, when the different slit geometries are accounted for, supports the earlier analysis that S iv emissions originate from a region more extended out of the centrifugal plane than the S iii emissions. Subject headings: planets and satellites: individual (Io, Jupiter)-ultraviolet: solar system On-line material: color figure