EXTREME-ULTRAVIOLET EXPLORER SATELLITE OBSERVATION OF JUPITER IO PLASMA TORUS

We present the first Extreme Ultraviolet Explorer satellite observatio n of the Jupiter system, obtained during the 2 day period 1993 March 3 0 through April 1, which shows a rich emission-line spectrum from the lo plasma torus spanning wavelengths 370 to 735 angstrom. The emission features correspond primarily to known multiplets of oxygen and sulfu r ions, but a blended feature near 372 angstrom is a plausible Na II t ransition. The summed detected energy flux of (7.2 +/- 0.2) x 10(-11) ergs cm-2 s-1 corresponds to a radiated power of almost-equal-to 4 x 1 0(11) W in this spectral range. All ansa emissions show a distinct daw n-dusk brightness asymmetry and the measured dusk/dawn ratio of the br ight S III lambda680 feature is 2.3 +/-0.3, significantly larger than the ratio measured by the Voyager spacecraft UV instruments. A prelimi nary estimate of ion partitioning indicates that the oxygen/sulfur ion ratio is almost-equal-to 2, compared to the value almost-equal-to 1.3 measured by Voyager, and that [Na+]/[e] > 0.01.