Modeling temporal variability of plasma conditions in the Io torus during the Cassini era

Abstract

[1] Observations of ultraviolet (UV) emissions from the major ion species (S + , S ++ , S +++ , O + , O ++ ) of the Io plasma torus made during the Cassini flyby (October 2000 to March 2001) have revealed significant time variability. Using a homogeneous model for mass and energy flow in the torus parameterized by five input variables (transport timescale, neutral source strength, ratio of oxygen to sulfur atoms in the source, fraction of superthermal electrons, and temperature of the hot electrons), we have investigated the time variability of the torus properties (density, composition, and temperature) during the Cassini era. In order to match the changes in emissions, the model suggests that a significant change in the neutral source occurred near the beginning of the observing period, decreasing from >1.8 tons/s to 0.7 tons/s. The changes in the neutral source appear to coincide with the declining phase of a dramatic (i.e., 2-3 order of magnitude) peak in iogenic dust emissions observed by Galileo prior to the Cassini era.

Cassini UVIS observations of the Io plasma torusIII. Observations of temporal and azimuthal variability

“…These compositional changes are consistent with models predicting a factor of 3-4 increase in the amount of neutral material supplied to the torus in early September, 2000. These results are discussed in more detail by Delamere et al (2004).…”

“…Given the phenomenological similarity between the UVIS 10.07 hour periodicity and the System IV periodicity, we propose that the same physical mechanism is responsible for both. It is plausible that the factor of 3-4 increase in the amount of neutrals supplied to the torus in September 2000 (Delamere et al 2004) altered the mechanism responsible for producing the System IV period in such a manner that a 10.07 hour period was produced. Based on measurements of Iogenic dust by the Galileo Dust Detector System (Krüger et al 2003), such events occur relatively infrequently (only (Nozawa et al 2004), which suggests the torus periodicity might have been returning to the "typical" System IV period.…”

“…Since several other brightening events occurred throughout the 45-day observing period, the timing of the two largest events may be coincidental. The next step is to model the UVIS observations by extending the torus chemistry model of Delamere et al (2004). Preliminary results suggest that the interaction of a subcorotating (at 10.07 hours), azimuthally-varying source of hot (∼55 eV) electrons with a corotating (i.e.…”

“…Since the observed variations have a period very close to the rotation period of Jupiter, and since the timescales for chemical processes, such as charge exchange, ionization, recombination, etc., which produce significant changes in the Io torus are generally much longer than this (Shemansky 1988;Barbosa 1994;Schreier et al 1998;Delamere and Bagenal 2003;Delamere et al 2004), the only plausible explanation is azimuthal variability.…”

“…The mixing ratios of O II and S III, the two dominant ion species in the torus, remain relatively constant. The temporal changes in the composition of the torus plasma coupled with observations by the Galileo Dust Detector System of a four-orders-of-magnitude increase in the amount of dust emitted from Io (Krüger et al 2003) led Delamere et al (2004) to propose a factor of 3-4 increase in the amount of neutral material available to the torus on, or around, 4 September 2000. Torus chemistry models including such an increase in the neutral source rate (along with a corresponding increase in the amount of hot electrons in the torus) can closely match the observed changes in plasma composition with time.…”