The ultraviolet emissions from the Io torus seen by Voyager were dramatically different from those seen in 1973 and 1974 with the simpler ultraviolet instrument on board Pioneers 10 and 11. These changes correspond to more than a factor of 10 in brightness. As noted by the Voyager Team, “Because of the remarkable differences we conclude that the Jupiter-Io environment has changed significantly since December 1973. The observed differences are so spectacularly large that this conclusion does not depend on a detailed comparison of the two instruments, their calibrations, or the observing geometry.” The reason for this change, or the degree to which it reflects a large-scale variation in the volcanic activity of Io, is one of the major questions arising from the Voyager mission. The Io torus can also be observed from the ground. In 1976, spectra showed the glow of singly ionized sulfur (S II), and in 1979 singly ionized oxygen (O II) was detected. The observations of S II and O II are particularly interesting because they provide a measure of the density and temperature of the plasma. In April 1979, between the two Voyager flybys, Carl Pilcher of the University of Hawaii succeeded in obtaining a telescopic image of the torus in the light of S II. He measured nearly the same ring diameter (5.3-5.7 R_J) as had Voyager; interestingly, both agree that the sulfur torus is centered slightly inside the orbit of Io (6.0 R_J). [Illustration: The emission of light from sulfur ions in the Io torus is so strong it can be measured from the Earth. In these pictures, University of Hawaii astronomer Carl Pilcher photographed the torus in the light of ionized sulfur on the night of April 9, 1979. As the planet rotates, the torus is seen first partly opened, then edge-on, and again opened in the opposite direction. The dark band on the right of each image is due to light from Jupiter scattered in the telescope, as shown in the bottom picture, which contains the scattered light only.] Direct measurements of the torus were made from Voyager 1 as the spacecraft passed twice through this region, once inbound and once outbound. The low energy charged particle instrument and the cosmic ray instrument both determined that the composition of the ions in the Io torus was primarily sulfur and oxygen. Ionized sodium was also observed. For several years, ground-based telescope observations had revealed a cloud of neutral sodium around Io; the Voyager instruments picked up these atoms after they had each lost an electron and become trapped in the magnetosphere. These instruments also derived the electron and ion density (about 1000 per cubic centimeter) and confirmed that the ions were co-rotating with the inner magnetosphere.