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F. Herbert (U.Arizona), G.R. Gladstone (SWRI)
Observations of the Io plasma torus by the Extreme Ultraviolet Explorer (EUVE) began in 1993 and continue still. EUVE is a spectral imager recording the whole torus, and so can measure the luminosity of the entire torus or selected portions thereof as a function of wavelength within the 300-730Å\ bandpass. Because of the high effective frequency of sampling and the long time baseline of observation, time scales from hours to years are accessible. Certain kinematic time scales, such as the 10 hr Jovian rotational period, the 42 hr Io orbital period, and their 13 hr beat, are expected to appear as quasi-periodic components of the luminosity time series. We also expect to see evidence of dynamical time scales such as the electron cooling and radial transport lifetimes, each of which is believed to lie in the range of several hrs to several tens of hrs.
Therefore, in order to constrain torus dynamics, we have begun computing the power spectra of the observations in the EUVE archive. So far only one several-day set of observations of the dawn and dusk torus halves has been analyzed, yielding significant fluctuation power at periods of 5, 10, 13, and 30-40, and 100-200 hrs. Inasmuch as this data set spans only 127 hrs, the last-named peak has probably been filtered by our observation window so as to eliminate fluctuations of longer period. The 42-hr Io period may have also been modified by dynamical effects with time scales of a few tens of hours.
In order to interpret this data, we have created and similarly analyzed time series from highly simplified zero-dimensional time-dependent torus models. Although the model parameters have not so far been uniquely determinable from comparison of observed and model power spectra, the model results are sensitive to the model parameters and so imply that our on-going analysis of the full archive (plus upcoming EUVE observations) may ultimately place important constraints on torus dynamics.
This work was enabled by support from NASA grant #s NAG5-4158 and NAG5-6362 (FH) and NAGW-2651 (GRG).
The author(s) of this abstract have provided an email address for comments about the abstract: herbert@argus.LPL.arizona.edu