Periodic Intensity Variations in Sulfur Emissions from the Io Plasma Torus

In November and December 1988, we acquired 157 spectra of [SII]λ6731 emissions from the Io plasma torus with a Fabry-Perot interferometer at the McMath-Pierce Solar Telescope facility on Kitt Peak. A major goal of this experiment was to extend our earlier studies of intensity variations in the plasma torus. Our earlier analysis (F. L. Roesler, F. Scherb, and R. J. Oliversen, 1984, Geophys. Res. Lett. 11, 128-130) of [S III]λ9531 emission spectra acquired in April 1982 had shown that the intensity of torus emissions varied periodically with a period of 10.20 ± 0.06 hr (1.o uncertainty level), slightly longer 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 contrast, we show that our new [S II]λ6731 data have a single period of 10.14 λ 0.03 hr, which is neither the System III period nor the proposed "System IV" period of 10.224 hr.