Ground scatter (GS) echoes in Super Dual Auroral Radar Network (SuperDARN) observations have been always expected to occur under high-enough electron density in the ionosphere providing sufficient bending of HF radio wave paths toward the ground. In this study we provide direct evidence statistically supporting this notion by comparing the GS occurrence rate for the Rankin Inlet SuperDARN radar and the F region peak electron density NmF2 measured at Resolute Bay by the CADI ionosonde and incoherent scatter radars RISR-N/C. We show that the occurrence rate increases with NmF2 roughly linearly up to about ~4·1011 m-3, and the trend saturates at larger NmF2. One expected consequence of this relationship is correlation in seasonal and solar cycle variations of the GS echo occurrence rate and NmF2. GS occurrence rates for a number of SuperDARN radars at middle latitudes, in the auroral zone and in the polar cap are considered separately for daytime and nighttime. The data indicate that the daytime occurrence rates are maximized in winter and nighttime occurrence rates are maximized in summer for middle latitude and auroral zone radars in the Northern Hemisphere, consistent with the Winter Anomaly (WA) phenomenon. The effect is most evident in the North American and Japanize sectors, and the quality of WA signatures deteriorates in the European and, especially, in the Australian sectors. The effect does not exist in the South American sector and in the polar caps of both hemispheres.

Rankin Inlet (RKN) SuperDARN radar observations simultaneously with the Resolute Bay Incoherent scatter radar in nearly coinciding beams are considered to investigate the relationship between the velocity of HF echoes and ExB plasma drift component along the RKN beam. We focus on a case of observations roughly along the flow direction on 6 March 2016. We show that, depending on HF operating frequency, the RKN radar detects either E or F region echoes. For the E region echoes and fast flows of 700-1000 m/s, HF velocities are of two types: very slow with speeds below 100 m/s and fast with speeds up to 400 m/s. Velocities of slow echoes can be of opposite polarity at 10 and 12 MHz and not coincide with the ExB drift polarity. Velocities of fast echoes are somewhat larger at 12 MHz as compared to 10 MHz and both are less than the expected ion-acoustic speed of plasma at typical electrojet heights. No strong range (presumably, aspect angle) attenuation effect is noticed in the range profiles of such echoes. We relate the first type of echoes to the neutral wind turbulence while the second type – to the Farley-Buneman (FB) plasma instability processes. Periods have been noticed when E region echoes had speeds of ~ 200 m/s which is well below the ion-acoustic speed and ExB drift component. We hypothesize that these echoes are owing to FB irregularities generated at low electrojet heights. The observations show existence of extended periods when the RKN radar detects F region echoes at 10 MHz and E region echoes at 12 MHz at the same ranges implying that the “transition region/ranges” for E and F region detection is very sensitive to the observational conditions.

The study investigates the relationship between SuperDARN HF radar velocities detected at intermediate ranges of 600-100 km from the radar and the plasma drift. Two approaches are implemented. First, a three-hour interval of SuperDARN Rankin Inlet (RKN) radar measurements and Resolute Bay incoherent scatter radar RISR-C measurements in nearly coinciding directions are investigated to show that 1) HF echoes with low velocities (less than 200 m/s) are often detected when drifts are in excess of 1000 m/s, 2) high-velocity HF echoes from the E region have velocities somewhat below the expected values of the ion-acoustic speed of the plasma and the HF velocity does not show a tendency for an increase at the largest drifts, 3) for E region echoes, 12 MHz velocities are slightly larger than those at 10 MHz, and 4) It often occurs that 12 MHz echoes are received from the electrojet heights while 10 MHz echoes are received from the F region heights so that the observed velocities are quite different with the latter reflecting the drift of the plasma. In the second approach, velocities of 10 and 12 MHz RKN echoes are compared for a large data set comprising several months of observations to show that occurrence of 12 MHz low-velocity echoes is fairly common (up to 25% of the time) whenever the flows are fast. Under this condition, the SuperDARN cross polar cap potential is underestimated by ~4 kilovolts.