PERIODIC INTENSITY VARIATIONS IN SULFUR EMISSIONS FROM THE IO PLASMA TORUS

In November and December 1988, we acquired 157 spectra of [S II]lambda 6731 emissions from the Io plasma torus with a Fabry-Perot interferome ter at the McMath-Pierce Solar Telescope facility on Kitt Peak. A majo r goal of this experiment was to extend our earlier studies of intensi ty variations in the plasma torus. Our earlier analysis (F. L. Roesler , F. Scherb, and R. J. Oliversen, 1984, Geophys. Res. Lett. 11, 128-13 0) of [S III]lambda9531 emission spectra acquired in April 1982 had sh own that the intensity of torus emissions varied periodically with a p eriod of 10.20 +/- 0.06 hr (1-sigma uncertainty level), slightly longe r than Jupiter's System III rotational period of 9.925 hr. We present here a reanalysis of our 1982 data revealing, in addition to the 10.20 -hr period, a clear secondary periodicity at 9.95 +/- 0.06 hr. By cont rast, we show that our new [S II]lambda6731 data have a single period of 10.14 +/- 0.03 hr, which is neither the System III period nor the p roposed ''System IV'' period of 10.224 hr. (C) 1994 Academic Press, In c.