HF echo types revisited: aspect angle attenuation effects

The high-spatial-resolution observations of the auroral E-region echoes by the Super Dual Auroral Radar Network (SuperDARN) HF radar at Pykkvibaer, Iceland considered in a previous study are re-examined. Both the spectral power and Doppler velocity of the E-region HF echoes exhibit strong dependence on the slant range and expected off-perpendicular magnetic aspect angle (aspect angle attenuation). We consider the aspect angle attenuation effects in identification of spectral HF echo types. It is argued that echoes with Doppler velocities close to the nominal value of the ion-acoustic speed Cs (~350 m/s) and echoes with velocities greatly exceeding the Cs (up to 600 m/s) are more related in their generation mechanisms than previously thought. It is proposed to treat the echoes near the Cs as aspect-angle-attenuated counterparts of the high-velocity echoes and that both types are generated directly by the modified two-stream instability with aspect angle attenuation resulting in the preferential phase velocity saturation at the Cs