CUTLASS HF radar observations of high-velocity E-region echoes

A short event of high-velocity E-region echo observations by the Pykkvibaer HF radar is analysed to study echo parameters and the echo relation to the Farley-Buneman plasma instability. The echoes were detected in several bea ms aligned closely to the magnetic L-shell direction. Two echo groups were identified: one group corresponded to the classical type 1 echoes with velo cities close to the nominal ion-acoustic speed of 400 ms(-1), while the oth er group had significantly larger velocities, of the order of 700 ms(-1). T he mutual relationship between the echo power, Doppler velocity, spectral w idth and elevation angles for these two groups was studied. Plotting of ech o parameters versus slant range showed that all similar to 700 ms(-1) echoe s originated from larger heights and distances of 500-700 km, while all sim ilar to 400 ms(-1) echoes came from lower heights and from farther distance s; 700-1000 km. We argue that both observed groups of echoes occurred due t o the Farley-Buneman plasma instability excited by strong (similar to 70 mV m(-1)) and uniformly distributed electric fields. We show that the echo vel ocities for the two groups were different because the echoes were received from different heights. Such a separation of echo heights occult-ed due to the differing amounts of ionospheric refraction at short and large ranges. Thus, the ionospheric refraction and related altitude modulation of ionosph eric parameters are the most important factors to consider, when various ch aracteristics of E-region decametre irregularities are derived from HF rada r measurements.