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N. M. Schneider (LASP, U Colorado), M. E. Kueppers (LASP, U. Colorado), M. E. Brown (CalTech), J. T. Trauger (JPL)
We attempt to reconcile apparent discrepancies in reports of periodicities in the Io plasma torus. Groundbased imaging studies (reviewed by Thomas, JGR 98, 18737, 1993) consistently report lamIII peaks intensities around lamIII ~ 200 degrees on most nights of observing, but lack sufficient duration to detect lamIV intensity peaks. The peak is observed to drift in longitude, but never appears more than 90 degrees from lamIII ~ 200 degrees. By contrast, the most extensive groundbased spectroscopic study (Brown, JGR 100, 21683, 1995) detects both lamIII and lamIV periodicities via periodogram analysis, sometimes finding stronger variation in lamIV. The two periodicities appear to interact with each other, and simple separable sinusoidal variations with lamIII and lamIV clearly cannot match both datasets.
We investigate the complex connection between lamIII and lamIV by (1) including longer-baseline imaging studies; (2) examining the short-term behavior in spectroscopic studies, and (3) focusing on simultaneous spectroscopic and imaging campaigns. We will present empirical descriptions of the two kinds of variability and their interaction, and speculate on the physical mechanisms responsible for them.
This work has been supported by NASA's Planetary Astronomy and Planetary Atmospheres programs.