Results on relative scattering cross section of 140‐MHz auroral backscatter

Abstract

Observations of power spectra and backscattered signal intensities associated with auroral electron density fluctuations with scale length of nearly 1 m have been made with the Scandinavian Twin Auroral Radar Experiment (STARE). The intensities of the backscattered signals, which are related to the radar scattering cross sections, have been analyzed to determine their angular distribution in the plane perpendicular to the magnetic field with respect to the direction of the electron drift velocity. In the analysis the Doppler spectral characteristics of the data are used in order to identify the type of irregularities. It is found that the primary irregularities, having narrow spectra centered at large Doppler shifts, have a highly anisotropic scattering cross section as compared with the secondary 1‐m waves. The strength of primaries is large and strongly dependent on flow angle θ; typically, the intensity decreases from 0.3 to 0.6 dB/deg with θ increasing. The intensities of the secondaries are relatively weak and rather insensitive to angular variations. We find the ac variation of the backscatter amplitudes associated with primary and secondary irregularities to be closely correlated. This indicates a common modulation mechanism to dominate for the two kinds of scatterers.