Comparison between EISCAT UHF and VHF backscattering cross section

Abstract

We present a comparison between the backscattering cross sections at 224 and 933 MHz measured with European Incoherent Scatter (EISCAT) radars during the passage of a discrete arc. It shows a difference of 2 orders of magnitude which cannot simply be explained by normal thermal ionospheric 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 perturbations, caused by low‐energy particle precipitation. Two possible explanations for the observed differences are discussed: spatial localization and k dependence. Using UHF tristatic observations, we show that 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.