Galileo measurements of plasma density in the Io torus

The measurements of electron density made by the Plasma Wave Subsystem instruments on Galileo during its pass through the torus on December 7th, 1995 are compared with a model based on Voyager 1 measurements made in March 1979. Outside Io's orbit, the plasma densities observed by Galileo are approximately a factor of two higher than the Voyager values. Shortly after crossing Io's orbit, the Galileo density profile dropped sharply and remained at low values for the rest of the inbound leg, suggesting that the 'ribbon' region was either absent or much farther from Jupiter than usual. The peak density on the outbound leg is consistent with Voyager-based predictions for the cold torus in both location (5.1 RJ) and magnitude (950 cm(-3)). Inside 5 RJ the density dropped sharply to less than 3 cm(-3).