COMPARISON BETWEEN EISCAT UHF AND VHF BACKSCATTERING CROSS-SECTION

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.