Long-term stability of the Io high-temperature plasma torus

Abstract

The IUE satellite was used to monitor the UV line emission of S⁺, S⁺⁺ and S⁺⁺⁺ ions in the high-temperature portion of the Io plasma torus. Thirteen observations obtained over a 5 yr period from 1979 March to 1984 March show that the plasma parameters of the high-temperature portion of the torus have surprisingly small variations. In particular, the mixing ratio for the dominant constituent, S⁺⁺, and the electron temperature are quite stable. To correct for the geometric effects, a simple three-dimensional model of the plasma torus is used to obtain the ion mixing ratios and the plasma density for each observation. The long-term variations of the mixing ratios and the plasma density are presented and a comparison of the measured ionization balance with that predicted by the effects of ionization, recombination, and the plasma confinement time is given.