Galileo Plasma Wave Observations in the Io Plasma Torus and Near Io

The Galileo plasma wave instrument detected jovian radio emissions, narrowband upper hybrid waves, and whistler-mode emissions during the inbound pass through the Io torus. The upper hybrid waves provided an accurate profile of electron density through the Io torus and in the vicinity of Io. These measurements show that the torus density has increased by about a factor of 2 since the Voyager 1 flyby in 1979. A well-defined peak in the electron density was observed in the wake of Io, with densities as high as about 4 × 10 4 per cubic centimeter.