On the occurrence of high-velocity E-region echoes in SuperDARN observations

Abstract

A comprehensive data set comprising 3 years of E-region backscatter observations by the entire Super Dual Auroral Radar Network (SuperDARN) of coherent high-frequency (HF) radars is analyzed statistically. The average spectral characteristics are examined for both morning and evening short-range echoes to develop a simple criterion for separating E- and F-region HF backscatter. Different E-region echo populations are considered for all 16 auroral radars with half of the radars detecting substantial numbers of echoes with Doppler velocities above the nominal value of the ion-acoustic speed Cs of 350-450 ms-1. The high-velocity echoes with Doppler velocity distributions that are asymmetric about zero velocity are examined separately for velocities near and well above the nominal Cs. Indicators of the total numbers of echoes near the Cs are shown to decrease from radar to radar as the radar's boresite direction approaches an L-shell angle of 90°. The very high-velocity echoes, on the other hand, are predominantly seen by the two most zonally looking radars. These echoes are typically observed at greater slant ranges than echoes both below and near the Cs and both their occurrence and velocity are independent of the direction of observation. Moreover, their velocity polarity is inconsistent with that of the plasma convection component. The observations provide further support for the idea that the auroral decameter-scale irregularities with unusually high phase velocities independent of the flow angle are generated through nonlinear mode coupling processes.