ION TEMPERATURES IN THE IO PLASMA TORUS

Observations of the radial dependence of the perpendicular ion temperatures of the four major ion species in the Io plasma torus are reported. Doppler-resolved forbidden line emissions of S+, S2+, O+, and O2+ were observed within a period of 72 hours on March 3-5 1993, with the 3.6-m telescope of the European Southern Observatory. The long slit spectra of S+ show a sharp (factor of 4) increase in the perpendicular ion temperature at the position of the ''ribbon'' where enhanced electron densities and [S II] emission were observed. The observations are broadly consistent with models of line profiles based on average ion temperatures derived from Voyager 1 plasma science data, The signal to noise for the S+ observations was sufficient to reveal two velocity components in the distribution, at 33 and 188 eV, inside Io's orbit. Modeling shows that the two components arise from the superposition of hot outer ions upon a core of cool inner torus ions as a result of line of sight effects. Outside Io's orbit, the temperature of S+ ions was observed to decrease from 120 eV at 6.0 R(J) to 80 eV at 6.8 R(J).