The dependence of the relative backscatter cross section of 1‐m density fluctuations in the auroral electrojet on the angle between electron drift and radar wave vector

Abstract

With the STARE radar system it is possible to measure, with high spatial and temporal resolution, the electron drift velocity VD and the relative amplitude of electron density fluctuations of 1‐m wavelength in the auroral electrojet. These density fluctuations are generated by the combined effects of the two‐stream and the gradient drift instabilities. We have determined the angular dependence of the backscatter intensity (which is proportional to the square of the amplitude of the density fluctuations) on the angle θ between the electron drift direction and the direction from the scattering volume to the radar in the plane perpendicular to the magnetic field. We find a fluctuation minimum for θ = 90° and an increase towards θ = 0° over the whole velocity range up to 1000 m/s. This increase is small for velocities below the ion acoustic velocity Cs but reaches over 20 dB gain in the backscatter intensity (corresponding to a density fluctuation more than 10 times as great) for higher velocities. We explain that the backscatter is caused mainly by two‐stream instability in the range cos θ > Cs/Vd and by secondary gradient drift instability elsewhere.