What does ``abrupt drop'' mean? This needs to be defined because the frequency
and time resolution of the receiver are limited and because we do not know the
signal level enhancement at the resonance frequencies. We shall consider that
the signal plummets if its decrease is much larger (in spectral density) and
much sharper (in frequency) than the attenuation due to the antenna response
and to the damping of Bernstein modes
observed at low-order gyroharmonics where no forbidden bands exist. During the
period under study, the instrument operating mode was such that the antenna
response was poorly known and differed from that calculated by [Meyer-Vernet, Hoang and Moncuquet, 1993].
However, one can see in Figures 2a and 2b that the
variation in 
at the abrupt drop is about a factor 100, that is,
at least 10
times larger than any periodic effect due to the antenna spin.
Each spectrum can include several gyroharmonic bands containing
forbidden bands and can thus exhibit multiple signal drops (see Figure 2b).
Since we
detect the abrupt drop by comparison with the signal variations in the
low-order gyroharmonic bands, we will interpret the lowest detected abrupt drop
frequency as the Doppler shifted 
in the lowest gyroharmonic band where
these resonances can theoretically occur (that is the band of 
).