VARIATIONS IN OPTICAL S- EVIDENCE FOR QUASI-PERIODICITY( EMISSION FROM THE IO PLASMA TORUS )

As part of its efforts to monitor and characterize periodic variations in the Io plasma torus, the Wisconsin Space Physics group acquired Fa bry-Perot spectra of [S II] lambda lambda 6716, 6731 for a 5 week peri od in 1988. Previous analysis showed periodicity, but, surprisingly, n ot at 10.2 hr-the persistent 3% subcorotational period, often called S ystem IV, reported in other long-term torus data sets. We show here th at our 1988 data exhibit a quasi-periodic intensity variation at 10.2 hr, characterized by two distinct intensity enhancements that abruptly changed in relative magnitude during the observational run. We also p resent perpendicular ion temperature data from the same period, showin g a periodic variation anticorrelated with intensity after the aforeme ntioned abrupt change, but no significant periodicity beforehand. This non/anticorrelation appears to eliminate the possibility that a slowl y moving magnetic field enhancement is responsible for the subcorotati onal periodicity. Considering these and other long-term data, we sugge st that the various observed subcorotational features are not themselv es responsible for the 3% subcorotational period, but rather are indep endent features modulated or otherwise affected by a permanent, global phenomenon in the torus lagging corotation by 3%.