This is the published abstract of a `poster talk' presented at the 1995 annual meeting of the Division for Planetary Sciences of the American Astronomical Association.

Periodic and Quasiperiodic Variations in Optical S+ and S++ Emissions from the Io Plasma Torus

R. C. Woodward [unaffiliated], F. Scherb, F. L. Roesler [U. Wisconsin], W. H. Smyth [AER], R. J. Oliversen [GSFC]

The Wisconsin Space Physics group acquired Fabry-Perot spectra of the bright optical lines of S++ for a three week period in 1982, and of S+ and S++ for a six week period in 1988. In earlier work we discovered a subcorotational periodicity in the 1982 S++ 9531Å emission intensity of 10.2 hours (Roesler et al. 1984), and a distinct subcorotational periodicity of 10.1 hours in the 1988 S+ 6731Å emission (Woodward et al. 1994). Here we will extend our earlier work. We will show that a) the 1988 S+ intensity data can also be well described by two separate periodic variations occurring over two different time intervals of the six week run, both now consistent with the System IV period of 10.2 hours, but approximately 180° out of phase with each other, and b) the corresponding temperature data also show quasiperiodic variations. We will compare the 1982 and 1988 S++ intensity and temperature data. With the aid of torus modelling code developed at AER, we will show how the same torus conditions can give rise to different periodicity results from different instruments, and will explore possible torus conditions that these data could imply.