Neutral Sulfur Emission from the Io Torus Measured with the Hopkins Ultraviolet Telescope

Abstract

During the Astro 1 mission in 1990 December, ultraviolet spectra of the Io plasma torus were obtained using the Hopkins Ultraviolet Telescope. These spectra cover a wavelength range of 830-1860 Å in first order and 415-930 Å in second order. Two separate observations were made. Observation 1, which nominally contained Io within the aperture, centered a 17" x 116" aperture on the west ansa of the torus, while observation 2, which excluded Io, centered a 9"4 x 116" aperture on the east ansa. Emission from the S I λ1479 multiplet is detected in both observations. The spectral width of this emission in observation 1 is too broad to be completely associated with Io. The best-fit is obtained with a narrow (3.0 Å) emission associated with Io and a broad (6.1 Å) emission associated with the torus. The derived emission rate from Io is consistent with IUE observations of Io and shows that only ∼33% of the S I emission measured 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. The 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 least 20° in front of Io. Emission from the S I λ1429 multiplet is also detected in observation 1 and marginally detected in observation 2. Emission from the S I λ1814 multiplet was not detected in either observation but the derived upper limits are consistent with the expected flux based on the IUE measurements of Io. Observation 1 also shows emission from the S I λ1389 multiplet and a strong narrow line at 1398.7 Å, which is tentatively identified with the S I λ1405 multiplet. The λ1389 and λ1405 emissions are not present in observation 2. A comparison of S I multiplet emission ratios from Io with laboratory results does not support either e + S or e + SO₂ as the excitation mechanism for neutral sulfur emission from Io.