DPS 35th Meeting, 1-6 September 2003
Session 2. Io I
Oral, Chair: R. W. Carlson and R. Lopes, Tuesday, September 2, 2003, 10:30am-12:00noon, DeAnza III

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[2.09] Interpreting Observations of Io Plasma Torus Variation

F. Herbert (LPL, University of Arizona)

A useful but hitherto little-used approach for understanding the chemical, energetic, and transport dynamics of the Io plasma torus is time series analysis of torus spectral datasets. Jovian system periodicities (Jovian rotation, Io orbit, ``System IV'', and their beats and harmonics) and randomly varying output from Io's volcanic atmosphere provide fluctuating input to the torus. Each torus component responds to this forcing with its own response time, so that in principle we can observationally constrain torus properties by comparing such responses in repetitive observations of torus spectra. Several such datasets exist, notably those of EUVE, IUE, Voyager UVS, and Cassini UVIS, although their relatively low SNR makes this use challenging. In addition, a Small Explorer mission (JMEX) is being proposed that would greatly expand the dataset applicable to this approach.

To disentangle the random inputs from the deterministic dynamics (the part we're interested in), we can estimate statistics of the data such as the Fourier power spectra and cross-correlations of the ion abundances. Such statistics average out the random part of the fluctuations, leaving the interaction network signatures which can be interpreted by computing the corresponding statistics from models. For example, if neutral cloud S is concentrated near Io, its ionization by torus electrons should produce a ~6.5~hr periodicity (the interval between torus midplane crossings of Io) in the power spectrum of S+, as is observed. Another example: S+++ and O++ abundances should co-vary with fluctuations in the supply of superthermal electrons. The scope for and results of further analysis of this type will be discussed.

This work was made possible by NASA grants NAG5-12944 and 9079 (Geospace Sciences) and NAG5-8952 (Planetary Atmospheres).

The author(s) of this abstract have provided an email address for comments about the abstract: herbert@vega.LPL.arizona.edu

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.