The plasma state of the hot torus is studied with a local, homogeneous, steady state model described by a set of coupled quasi-linear equations for the distribution functions of S+, S++, S+++, O+, and O++. The equations contain model Fokker-Planck operators for ion-ion and ion-electron collisions, a species- and energy-independent loss rate &tgr;-1, impact ionization of S, S+, S++, O, and O+, recombination of S+++, and 11 charge exchange reactions. The only free parameters, which are not governed by physical processes contained in the model, are the ion confinement or residence lifetime &tgr; and the neutral densities of sulfur, nS, and oxygen, nO. Equivalently, if nS, nO, and the electron density ne are adopted as parameters, then &tgr; is determined by imposing charge neutrality.

Under the constraint that the pickup mechanism for newly created ions is the dominant energy source and that the torus is in equilibrium between collisional energy loss from ions to electrons and radiative UV loss from ions excited by ion-electron collisions, the densities, average energies, and distribution functions of all ion species and the temperature of the electrons, which are assumed to be Maxwellian, are calculated as functions of the input parameters.

Among the results are the following: (1) The ion velocity distributions are significantly non-Maxwellian for the major species, with high-energy tails extending to the pickup energy. A quasithermal core exists for the O+ and/or S+ ion velocity distribution functions only if the EUV luminosity of the torus is less than 0.2 eV cm-3 s-1. The core ''temperature'' is at most Ti~100 eV; the average energy of the total distribution is generally less than 0.5 of the pickup energy. The ion velocity distributions do not drive the ion loss cone instability for parameters generally typical of the torus. (2) The EUV luminosity of the torus during the Voyager 1 encounter was ~0.15 eV cm-3 s-1, a factor of ~1.8 less than that reported by Shemansky and Smith (1981). This downward revision in the EUV intensities is in agreement with calibration adjustments adopted by Holberg et al. (1982) at longer wavelengths (912-1050 ¿). (3) During the Voyager 1 encounter the approximate average torus ion densities were S+: <350, S++: 420, S+++: 10-20; O+: 660, O++: 40-80 cm-3 with Te~4.8 eV, &tgr;~60 days, and ne~2000 cm-3.

The average neutral torus densities were nS≤6 and n0~30 cm-3 and consistent with an SO2 source. (4) The upper limit obtained by Brown et al. (1983b) on O++ concentrations is only applicable to short duration periods when transient, large eruptions of sulfur-driven volcanoes occur on the surface of Io (McEwen and Soderblom, 1983) and mass loading of the neutral torus is preferentially by sulfur. The resultant plasma torus contains S+ and S++ densities of ~600, O+~120, O++<10, and S+++<40 cm-3. The increased mass loading reduces &tgr; to ~1 week.