Disentangling Electron Temperature and Density in the Io Plasma Torus
Floyd Herbert (U. Arizona), Doyle T. Hall (U. Colorado)
One of the torus characteristics of most interest for understanding torus energization is its electron temperature ( ). Yet deriving has always been difficult because the measured quantity (emission brightness) is controlled jointly by and a second unknown, electron density. In order to solve this problem, we have used a new technique to estimate from spectral images of the Io plasma torus in the 350 to 700Å region obtained by the Extreme Ultraviolet Explorer (EUVE). Because of the lack of information available on the collision strengths of important lines between 350 and 600Å, we have attempted to simultaneously deduce both the unknown collision strengths and also the time-varying torus characteristics by fitting analytic models which exploit the both the commonalities and the variations among the observations. However, because of present limitations of the method, we can only deduce relative variations in torus , total electron number ( - a proxy for total torus mass), and ionic composition. In the 1993 - 1995 data set, and were strongly anti-correlated, while total torus luminosity remained steadier than either or . The anti-correlation of and suggests that torus luminosity may be primarily determined by a relatively constant power-limited energy supply, so that as increases (decreases), sags (surges) in response. A corollary to this hypothesis would be that the mass loading rate is only weakly coupled to torus energization, contradicting the class of plasma torus models called ``neutral cloud theory.'' There also seems to have been an abrupt 20% decrease in at about the time of the comet Shoemaker-Levy 9 impacts on Jupiter, as though a magnetospheric disturbance had increased the convective loss rate of the torus, but this may well be a coincidence. Simultaneous ground-based observations of the 6731Å S line do not constrain decreases of this size, despite their low sensitivity to , because they measure total torus mass only approximately.