- In this fourth paper in a series, we present a model of the remarkable; temporal and azimuthal variability of the Io plasma torus observed during the; Cassini encounter with Jupiter. Over a period of three months, the Cassini; Ultraviolet Imaging Spectrograph (UVIS) observed a dramatic variation in the; average torus composition. Superimposed on this long-term variation, is a; 10.07-hour periodicity caused by an azimuthal variation in plasma composition; subcorotating relative to System III longitude. Quite surprisingly, the; amplitude of the azimuthal variation appears to be modulated at the beat; frequency between the System III period and the observed 10.07-hour period.; Previously, we have successfully modeled the months-long compositional change; by supposing a factor of three increase in the amount of material supplied to; Io's extended neutral clouds. Here, we extend our torus chemistry model to; include an azimuthal dimension. We postulate the existence of two azimuthal; variations in the number of super-thermal electrons in the torus: a primary; variation that subcorotates with a period of 10.07 hours and a secondary; variation that remains fixed in System III longitude. Using these two hot; electron variations, our model can reproduce the observed temporal and; azimuthal variations observed by Cassini UVIS.