HF radar observations of high-velocity E region echoes from the eastward auroral electrojet

We present a statistical analysis of short-range E region echoes observed by the Super Dual Auroral Radar Network (SuperDARN) HF radars in the evening sector (16-22 MLT) over 3 years. Significant populations of the high-velocity (350-450 m/s) E region echoes similar to the classical Type 1 echoes are observed by 4 zonally-looking SuperDARN radars at small magnetic L-shell angles. The spatial occurrence pattern of Type 1 echoes is investigated. It is shown that the latitudinal (slant range) extent of the region where Type 1 echoes occur increases as the L-shell angle decreases, which is interpreted as widening of the aspect angle instability cone with the flow angle decrease. The echoes with unusually high velocities (500-600 m/s) observed by the Syowa East HF radar are also investigated. These echoes are seen at all L-shell angles (15 degrees-75 degrees) and their Doppler velocity increases with range and exhibits little variation with L-shell angle. The echoes occur at ranges 360-495 km when the strong low-velocity echoes (P > 30 dB, V < 200 m/s) are observed at ranges 225-360 km. We analyze the echo occurrence and velocity dependencies on the simultaneously observed F region echo velocity (plasma drift velocity) and E region echo power. It is suggested that the auroral E region echoes with unusually high velocities observed at large flow angles are similar to the vertically propagating Type 1 echoes reported previously in observations at the magnetic equator and are likely to be secondary waves generated through nonlinear mode coupling processes.