Voyager EUV observations of the Io plasma torus showing a Jupiter local time asymmetry have been interpreted as a 10-hour periodicity in electron temperature in the corotating reference frame. The plasma shows two remarkable morphological characteristics. First, the intensity periodicity shows no tendency to change over the ?0.5-year period of reduced data, indicating it to be a permanent feature of the torus. Second, deviations from the mean behavior, such as short-term magnetic longitude effects, all appear to be caused by electron temperature effects. Thus no local plasma mass variations have been detected during the observational period. These characteristics and the variability of the energy source required to drive the observed asymmetry lead to the conclusion that the dominant mechanism of energy transport to the plasma is electron-electron heating. If the parameters applied in coming to this conclusion are correct, the role played by mass loading from Io and planetary rotation in delivering energy to the plasma becomes much more abscure than the straightforward electron-ion transfer mechanism that has been considered to date.