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 .