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G. Lichtenberg, N. Thomas (Max-Planck-Institut fuer Aeronomie)
The physical processes maintaining the Io plasma torus (IPT) are still not fully understood. One gap in the knowledge of the IPT is the origin of highly ionized species such as OIII and SIV. The electron temperatures in the IPT are too low to maintain these species if the electrons are in thermal equilibrium. One way to explain the presence of these species is a polytropic electron distribution leading to a higher percentage of hot electrons if compared to a Maxwellian distribution (Meyer-Vernet et al 1995).
But while the presence of OIII is well established from measurements in various wavelength regions, observations of SIV, which could set further constraints on the physical properties of the IPT are somewhat contradictory. For example, the densities calculated from IUE observations are 4 times lower than those derived from Voyager UVS observations. Since SIV has no optical emissions and the UV line is rather weak, up to now there was no way to resolve this contradiction. The launch of ISO made it possible for us to observe the SIV emission at 10.51 um, which is considerably stronger than the UV emission.
The observations took place on 25 May 1997. SIV was detected with the Short Wavelength Spectrometer at both ansae at an intensity of approximately 0.5 Jy. Observations were obtained at 4 different magnetic longitudes. Furthermore, serendipitous observations of the SIII 33.48 um emission were obtained simultaneously. To analyse the data we are using a semi-empirical model first described by Thomas in 1992. The model gives us brightness predictions and allows us to separate line-of-sight-effects from physical effects. In this talk we want to present first results.
REFERENCES: Thomas, N.: Optical observations of Io's neutral clouds and plasma torus (Surv.Geophys. 13, pp. 91-164, 1992)
Meyer-Vernet, N., Moncuquet, M., Hoang, S.: Temperature inversion in the Io plasma torus (Icarus 116, pp. 202-213, 1995)
The author(s) of this abstract have provided an email address for comments about the abstract: lichten@linmpi.mpg.de