  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 S II is strongly correlated with
the mixing ratio of S III and the equatorial electron density and strongly
anti-correlated with the mixing ratios of both S IV and O II and the equatorial
electron temperature. Surprisingly, the azimuthal variation in ion composition
is observed to have a period of 10.07 hours--1.5% longer than the System III
rotation period of Jupiter, yet 1.3% shorter than the System IV period defined
by Brown (1995). Although the amplitude of the azimuthal variation of S III and
O II remained in the range of 2-5%, the amplitude of the S II and S IV
compositional variation ranged between 5-25% during the UVIS observations.
Furthermore, the amplitude of the azimuthal variations of S II and S IV appears
to be modulated by its location in System III longitude, such that when the
region of maximum S II mixing ratio (minimum S IV mixing ratio) is aligned with
a System III longitude of ~200 +/- 15 degrees, the amplitude is a factor of ~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.
