Combining UV Spectra and Physical Chemistry to Constrain the Hot Electron Fraction in the Io Plasma Torus

We have developed a spectral emission model that is a function of the plasma composition, electron temperature, and density in the Io plasma torus. The lines are excited by electron collisions and spontaneously decay resulting in UV emission that is diagnostic of the plasma conditions. In a previous study we used a single Maxwellian distribution to model the UV spectra obtained by the Cassini UVIS instrument in January 2001. We now try to determine the fraction of hot electrons using a double Maxwellian distribution where the core, thermal electron distribution is combined with a hot electron distribution, also assumed to be a Maxwellian. This spectral emission model does not well constrain the fraction of hot electrons, which can be seen by the chi(2) output. By using physical chemistry modeling of equilibrium conditions, we determine the fraction of hot electrons. Our physical chemistry model shows that in order to match the plasma composition and temperatures near the orbit of Io during the Cassini flyby of Jupiter we need the hot electrons to comprise < 0.3% of the total electron density.