DISENTANGLING ELECTRON-TEMPERATURE AND DENSITY IN THE IO PLASMA TORUS

One of the torus characteristics of most interest for understanding to rus energization is its electron temperature T-e. Yet deriving T-e has always been difficult because the measured quantity (emission brightn ess) is controlled jointly by T-e and other unknowns, such as ion and electron density. In order to solve this problem, we have used a new t echnique to estimate T-e from spectral images of the Io plasma torus i n the 350 to 700 Angstrom region obtained by the Extreme Ultraviolet E xplorer (EUVE), Because of the lack of information available on the co llision strengths of important lines between 350 and 600 Angstrom, we have simultaneously attempted to constrain the unknown collision stren gths and also to deduce the time-varying torus characteristics by fitt ing analytic models which exploit the both the commonalities and the v ariations among the observations. However, because of present limitati ons of the data set, we can only deduce relative variations in torus T -e, total electron number N-e (a proxy for total torus mass), and ioni c composition. In the 1993 - 1995 data set, T-e and N-e were anticorre lated, while total torus luminosity remained steadier than either T-e or N-e. One interpretation of the anticorrelation of N-e and T-e is th at torus luminosity may be primarily determined by a relatively consta nt power-limited energy supply, so that as N-e increases (decreases), T-e sags (surges) in response, This anticorrelation is a constraint on theories of torus energization and transport. There also seems to hav e been an abrupt 20% decrease in N-e at about the time of the comet Sh oemaker-Levy 9 impacts on Jupiter, as though a magnetospheric disturba nce had increased the convective loss rate of the torus, but this may well be a coincidence.