We present a comparison between the backscattering cross sections at 2
24 and 933 MHz measured with European Incoherent Scatter (EISCAT) rada
rs during the passage of a discrete are. It shows a difference of 2 or
ders of magnitude which cannot simply be explained by normal thermal i
onospheric density fluctuations. We claim that the observed difference
in the scattering cross sections is due to so-called anomalous echoes
which typically occur in the vicinity of high-altitude ionospheric pe
rturbations, caused by low-energy particle precipitation. Two possible
explanations for the observed differences are discussed: spatial loca
lization and k dependence. Using UHF tristatic observations, we show t
hat there, indeed, is a possibility that the observed echoes are very
localized in space. Analyzing the possible k dependence, we show that
the experimental geometry used at least in principle will allow us to
rule out one of the earlier proposed production mechanisms.