The URAP experiment includes a relaxation sounder, which can detect the plasma
resonance frequencies [Stone et al., 1992a]. This instrument emits a short
quasi-monochromatic pulse and records the reflected signal a few milliseconds after the
excitation has stopped.
If the antenna geometry and orientation are adequate, a
``ringing'' of the plasma is expected to be observed at the frequencies where
the wave group velocity in the antenna frame vanishes [see, e.g.,
[Fejer and Yu, 1970].
This instrument was operated at the rate of one frequency sweep every
40 min near the Io torus. Before 19 hours (at 
), its data could not be
processed, so that only few spectra were available for comparison with the
radio data. The theory of the sounder has not yet been developed to give the
amplitude of the resonances with the URAP antenna, which cannot be modeled as
an infinitesimal dipole.
Hence we shall only use the sounder data to confirm our identification of the
frequencies and our interpretation in terms of Bernstein waves. From
the quasi-thermal noise drop (Figure 4, top),
we get the Doppler-shifted
frequency in the 
harmonic band at 
kHz, from which we deduce the cold plasma frequency 
kHz.
Figure 4 (middle) shows the corresponding (Doppler-shifted)
dispersion curves for several values of the hot electron parameters in the
range expected at this location. We focus on the resonances at the
(Doppler-shifted) frequencies 
and the gyroharmonics 
which are a direct consequence of the radio spectrum since they are roughly
independent of the hot population. The peaks of the sounder spectrum (Figure
4, bottom)
coincide with these resonance frequencies within 1%, except
, which barely emerges from the background level. This
comparison uses the instantaneous values of 
deduced from the
magnetometer, which are 1% larger for the sounder spectrum than for the radio
spectrum since the latter was acquired 2 min after.
Figure 4: Comparison between the radio and the sounder data. (top)
The solid line is the average of radio measurements (dots) over a
frequency step; the abrupt drop yields the maximum Doppler-shifted ,
from which we deduce 
. (middle)
Dispersion curves deduced from
this value of (for 
or 0.25 and 
, 25, or
50)
showing the resonances at the Doppler-shifted gyroharmonics and
frequencies, which are roughly independent of the hot electron parameters.
(bottom) Sounder spectrum and identification of these resonances
(arrows),
using the instantaneous values of deduced from the magnetometer.