The Io torus was easily detected on Voyager by the ultraviolet spectrometer, even from a distance of 150 million kilometers. The strongest ultraviolet radiation comes from twice-ionized sulfur (atoms that have lost two electrons) (S III), emitting a wavelength of 69 nanometers or about one-eighth the wavelength of visible light. The spectrometer also detected glows from atoms of triply ionized sulfur (S IV) and twice-ionized oxygen (O III).
At the time of the Voyager 1 encounter, the most abundant heavy ions in the Jovian magnetosphere were sulfur and oxygen. Multiply ionized sulfur and oxygen both emit strongly in the ultraviolet, where they could be observed by the ultraviolet spectrometer. This spectrum of the Io torus registers the tremendous amount of ultraviolet energy (about a million million watts) being radiated. To emit so strongly, the temperatures in the torus must be near 100 000 K.
Direct measurement of the heavy ions associated with the Io torus were made by the Voyager 1 LECP instrument. Here the amounts of various elements are shown for two cases: the Jovian inner magnetosphere (solid line) and a typical solar event. Both the Jovian and the solar particles have been scaled to show similar amounts of oxygen, but the solar particles are also rich in carbon and iron, whereas Jupiter has a great deal of sulfur. Evidence such as this demonstrates that the sulfur does not come from the Sun; rather, the sulfur and most of the oxygen appear to be the product of the Io sulfur dioxide volcanic eruptions.
Scans across the torus showed that it had a thickness of 1.0 RJ and was centered at a distance of 5.9 RJ from Jupiter. The torus is centered on the magnetic, rather than the rotational, equator of the planet. To produce the intense glow observed, the electron temperatures in the torus must be 100 000 K, with an electron density of about 1000 per cubic centimeter. The brightness in the ultraviolet corresponds to a radiated power of more than a million million (10¹²) watts. This enormous amount of energy must be continuously supplied by the magnetosphere.
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 RJ) as had Voyager; interestingly, both agree that the sulfur torus is centered slightly inside the orbit of Io (6.0 RJ).
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.