Galileo plasma spectrometer measurements of composition and temperature in the Io plasma torus

On December 7, 1995, the Galileo spacecraft passed through the Io plasma torus, and made observations of this dense, heavy ion plasma. We report an analysis of the data from the spacecraft's plasma spectrometer which measures the angular distribution of the ions. This enables us to determine the composition of the plasma. We compare our results to the models based on the Voyager 1 encounter of 1979, Galileo ultraviolet observations, and Earth-based observations. At the time and longitude of the December 7 Galileo encounter the abundance of oxygen was higher than observed by Voyager, and the O++:O+ ratio was greater. The ion temperatures were not in thermal equilibrium and were generally cooler than Voyager values.