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

In this third paper in a series presenting observations by the Cassini Ultraviolet Imaging Spectrometer (UVIS) of the Io plasma torus, we show remarkable, though subtle, spatio-temporal variations in torus properties. The Io torus is found to exhibit significant, near-sinusoidal variations in ion composition as a function of azimuthal position. The azimuthal variation in composition is such that the mixing ratio of SII is strongly correlated with the mixing ratio of SIII and the equatorial electron density and strongly anti-correlated with the mixing ratios of both SIV and OII and the equatorial electron temperature. Surprisingly, the azimuthal variation in ion composition is observed to have a period of 10.07 h-1.5% longer than the System III rotation period of Jupiter, yet 1.3% shorter than the System IV period defined by [Brown, M.E., 1995. J. Geophys. Res. 100, 21683-21696]. Although the amplitude of the azimuthal variation of SIII and OII remained in the range of 2-5%, the amplitude of the SII and SIV compositional variation ranged between 5 and 25% during the UVIS observations. Furthermore, the amplitude of the azimuthal variations of SII and SIV appears to be modulated by its location in System III longitude, such that when the region of maximum S 11 mixing ratio (minimum S IV mixing ratio) is aligned with a System III longitude of similar to 200 degrees +/- 15 degrees, the amplitude is a factor of similar to 4 greater than when the variation is anti-aligned. This behavior can explain numerous, often apparently contradictory, observations of variations in the properties of the Io plasma torus with the System III and System IV coordinate systems. (c) 2005 Elsevier Inc. All rights reserved.