Propagation angle dependence of radar auroral E-region irregularities

Abstract

The Sweden and Britain Radar aurora Experiment (SABRE) measures the phase velocity and backscatter amplitude of 1 m wavelength irregularities in the auroral zone E-region. These irregularities are produced by two types of plasma instabilities and theory predicts that the variation in backscatter amplitude with the angle between the electron drift velocity V d and the radar vector k, the so called propagation angle, is different for the two cases. An isotropic angular dependence is expected for gradient drift instabilities, but for the alternative two stream instability process a minimum is produced in the backscatter amplitude for directions orthogonal to the flow. The SABRE data have been averaged to remove small scale fluctuations and for drift velocities less than ~200 ms −1 the backscatter dependence on the propagation angle is isotropic, strongly suggesting that gradient drift processes are dominant. At higher drift velocities anisotropies develop in the angular dependence with deep minima at approximately 90° to the flow and a subsidiary minimum at 0° to this direction. Although these characteristics suggest a two stream instability mechanism, there are features in the experimental data that are not predicted by this simple theory. It is suggested that ion drifts may in part be responsible for these additional features.