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 ( tex2html_wrap_inline45 ). Yet deriving tex2html_wrap_inline45 has always been difficult because the measured quantity (emission brightness) is controlled jointly by tex2html_wrap_inline45 and a second unknown, electron density. In order to solve this problem, we have used a new technique to estimate tex2html_wrap_inline45 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 tex2html_wrap_inline45 , total electron number ( tex2html_wrap_inline55 - a proxy for total torus mass), and ionic composition. In the 1993 - 1995 data set, tex2html_wrap_inline45 and tex2html_wrap_inline55 were strongly anti-correlated, while total torus luminosity remained steadier than either tex2html_wrap_inline45 or tex2html_wrap_inline55 . The anti-correlation of tex2html_wrap_inline55 and tex2html_wrap_inline45 suggests that torus luminosity may be primarily determined by a relatively constant power-limited energy supply, so that as tex2html_wrap_inline55 increases (decreases), tex2html_wrap_inline45 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 tex2html_wrap_inline55 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 tex2html_wrap_inline75 line do not constrain tex2html_wrap_inline55 decreases of this size, despite their low sensitivity to tex2html_wrap_inline45 , because they measure total torus mass only approximately.