A brilliant aurora of sulfur has been seen around Io, a moon of Jupiter, by the Voyager spacecraft, now approaching the giant planet.

"We were surprised out of our chairs to see this sepectacular sight," the University of Arizona's Dr. Donald Shemansky said today at news conference at the Jet Propulsion Laboratory where the Voyager flight is being directed. "We expected to see ionized sulfur glowing around Io but nowhere near in the amounts or intensities of what we see."

The brilliance of sulfur aura, which circles Io like a doughnut and remains in place through Io's full orbit of Jupiter, was discovered by an instrument aboard Voyager that measures emissions of ultraviolet light. What this instrument saw is invisible from the earth because ultraviolet light does not peretrate the earth's atmosphere.

The Voyager instrument measured a cloud of ionized sulfur whose individual atoms had been stripped of two electrons, leaving them glowing in space around Io at a temperature of at least 100,000 degrees centigrade. Shemansky said the force needed to electrify sulfur atoms and elevate them to that temperature had to be at least 500 billion watts.

"How this amount of power is injected to maintain this torus (doughnut-shape) is a mystery," She-mansky said. "And what the source of this power is we do not yet know."

Scientists have known of the existence of a cloud of sodium and sulfur around Io for some time, were not aware that it had the brightness measured today by the ultraviolet instrument on Voyager.

The mechanism for creating the clouds of sodium and sulfur has long been felt to be the intense radiation trapped inside Jupiter's magnetic field, which gets so strong close to the planet that it would be enough to tear sulfur and sodium right off the surface of Io. One of the largest of Jupiter's 13 known moons, Io is the closest of the four largest moons to the planet.

As Io orbits Jupiter, it moves through the intense magnetic field in such a way that it generates an electric potential, much the way electricity is produced by a mechanical generator. Electric potentials of as much as 400,000 volts are believed generated at the surface of Io facing Jupiter. So many charged particles are trapped in Jupiter's magnetic field that there is a current of electricity flowing with as much as one million amperes from Io down to the top of the Jovian atmosphere.

Shemansky said the ultraviolet instrument on Voyager measured sulfur concentrations 50 times greater than expected around Io. The instrument also measured temperatures 100 times higher than expected.