Energy flowing into Io's hot plasma torus in the vicinity of the satellite causes an azimuthal modulation of the brightness of the torus that is correlated with the position of Io. Voyager ultraviolet spectrometer observations show that the plasma downstream from Io is brighter in S III 685-¿ emission, reflecting a higher electron temperature there. The electron temperature is raised by a mechanism operating within 45¿ of the azimuthal position of Io in its orbit; the interaction region may extend over a much smaller range of azimuth. DIfferences in electron temperature inferred from spectral analysis account for all the observed differences in brightness, implying that no change in the composition or density of the hot plasma occurs. The Io-correlated source provides a time-averaged power of 8¿10-14 erg cm-3 s-1 (4¿1011 W) or about 20% of the power radiated by the torus. The time constant for radiative cooling of the torus is such that most of the remainder of the power must be supplied by the local-time source, which is distinct from the Io source reported here. The specific mechanism by which the electrons are energized is unknown.