Rc. Woodward et al., VARIATIONS IN OPTICAL S- EVIDENCE FOR QUASI-PERIODICITY( EMISSION FROM THE IO PLASMA TORUS ), The Astrophysical journal, 479(2), 1997, pp. 984-991
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%.