Measurements of the dispersion characteristics of electrostatic waves reveal the properties of the ambient plasma electrons. In the Io torus, the Ulysses filamental dipole antennae were longer than electrostatic wavelengths; in this case, the antenna's directivity pattern is especially suitable to deduce the electrostatic wave vector modulus k from the observed polarization ([Meyer-Vernet 1994]). This method has been extensively applied to the plasma quasi-thermal noise measured aboard Ulysses between the electron gyroharmonic frequencies, to deduce dispersion characteristics of Bernstein waves in the Io torus ([Moncuquet et al. 1995]).
If the electrons are Maxwellian - with temperature , the normalized dispersion relation is known theoretically ([Bernstein 1958]), and depends only on the magnetic field B and the plasma frequency . Using independent measurements of B from the inboard magnetometer ([Balogh et al. 1992]) or from the wave spectral minima ([Meyer-Vernet et al. 1993]), and of ([Hoang et al. 1993]), one can thus deduce the temperature by fitting the theoretical dispersion curves to the experimental ones ( [Moncuquet et al. 1995]). Since is the sole unknown parameter in the fitting, this determination can be fairly precise. If the electrons are not Maxwellian, this method gives an effective temperature which is defined in Section 2.2.
Figure 1: Electron density and temperature versus Jovian centrifugal latitude, measured in situ by the radio and plasma wave experiment aboard Ulysses. The corresponding Jovicentric distance in Jovian radii is indicated at the top (dotted error bars identify the data for which the measurements of and are not independent of each other).
The temperature thus obtained is plotted in Fig.1 from +15 to Jovian centrifugal latitude (the latitude is referenced to the classical centrifugal surface reference, using an approximate tilted dipole magnetic field model). We have superimposed the density (in S.I. units) determined by [Hoang et al. 1993]. It is important to note that in this region, our measurements of are based on a part of the dispersion relation which is nearly independent of (because is large enough); thus any correlation observed between and cannot be an artefact due to the measuring process.
For negative latitudes beyond in the torus, no independent measurement of could be obtained. In that region, [Moncuquet et al. 1995] deduced both and , by using the full dispersion characteristics. These measurements of and are thus not independent, and are less precise. They are plotted in Fig.1 with dotted error bars, and will not be used in analysing the relation between and .