Both of the above properties are systematically observed in our data (see
Figures 2a and 2b (top)).
This allows us to locate the largest 
resonance in each observed harmonic band: a check of self-consistency can be
made by comparing the different 
resonances, using the theoretical
Doppler-shifted dispersion curves, which are plotted, in the same frequency
range, under each spectrum of Figures 2a and 2b.
The abrupt drop of the spectral density is actually observed in about 80%
of the spectra acquired by Ulysses between 1800 and 2300 UT.
Because (1)
contains 
, this detection of a high noise level band (where multiple
resonances occur), abruptly followed by the forbidden
band where 
is low, will allow us to determine the plasma density
in a number of spectra.
The problems to be solved are first to clearly identify the detected and
then to derive from that detection the plasma frequency
or more exactly the cold plasma frequency as discussed below.
Figure 2a: Two power spectra at 1912 and 1931 UT showing abrupt drops together
with the corresponding
dispersion curves and the derived 
. (top) The solid
line is the average of individual measurements (dots) over a frequency step
(i.e., 4 measurements acquired in 2 s). The abrupt drop of the signal level at
is indicated by an arrow.
The variation of 
is plotted at the top with its minima
indicated as vertical dotted lines. (bottom) Dispersion curves
computed by fitting the values of to the observed abrupt drop
frequencies are shown; they are plotted with fixed 
and various
(10, 15, 25, 35, and 50). Note that which defines the low-frequency
side of the forbidden band (grey strip), is only a function of 
. The
gyrofrequency 
is deduced from the magnetometer data at the time of the
signal drop.
Figure 2b: Two power spectra at 1944 and 2008 UT,
showing the abrupt drops (top)
and the dispersion curves (bottom) with the forbidden bands (grey strips),
plotted as in Figure 2a with the same notations. Since was
lower than for the spectra shown in Figure 2a, we observe
an additional intraharmonic band 
including the resonance
which defines the low-frequency side of a second forbidden band.