NEUTRAL SULFUR EMISSION FROM THE IO TORUS MEASURED WITH THE HOPKINS-ULTRAVIOLET-TELESCOPE

During the Astro 1 mission in 1990 December, ultraviolet spectra of th e Io plasma torus were obtained using the Hopkins Ultraviolet Telescop e. These spectra cover a wavelength range of 830-1860 Angstrom in firs t order and 415-930 Angstrom in second order. Two separate observation s were made. Observation 1, which nominally contained Io within the ap erture, centered a 17 '' x 116 '' aperture on the west ansa of the tor us, while observation 2, which excluded Io, centered a 9 ''.4 x 116 '' aperture on the east ansa. Emission from the S I lambda 1479 multiple t is detected in both observations. The spectral width of this emissio n in observation 1 is too broad to be completely associated with Io. T he best-fit is obtained with a narrow (3.0 Angstrom) emission associat ed with Io and a broad (6.1 Angstrom) emission associated with the tor us. The derived emission rate from Io is consistent with IUE observati ons of Io and shows that only similar to 33% of the S I emission measu red by HUT can be accounted for as emission from Io. The rest of this emission is due to a diffuse neutral sulfur component of the torus. Th e detection of S I emission in observation 2 sets a lower limit to the angular extent of the neutral sulfur component of the torus to at lea st 20 degrees in front of Io. Emission from the S I lambda 1429 multip let is also detected in observation 1 and marginally detected in obser vation 2. Emission from the S I lambda 1814 multiplet was not detected in either observation but the derived upper limits are consistent wit h the expected flux based on the IUE measurements of Io. Observation 1 also shows emission from the S I lambda 1389 multiplet and a strong n arrow line at 1398.7 Angstrom, which is tentatively identified with th e S I lambda 1405 multiplet. The lambda 1389 and 1405 emissions are no t present in observation 2. A comparison of S I multiplet emission rat ios from Io with laboratory results does not support either e + S or e + SO2 as the excitation mechanism for neutral sulfur emission from Io .