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

Abstract

Abstract.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 beams aligned closely to the magnetic L-shell direction. Two echo groups were identified: one group corresponded to the classical type 1 echoes with velocities close to the nominal ionacoustic speed of 400 ms −1 , while the other group had significantly larger velocities, of the order of 700 ms −1 . The mutual relationship between the echo power, Doppler velocity, spectral width and elevation angles for these two groups was studied. Plotting of echo parameters versus slant range showed that all ∼700 ms −1 echoes originated from larger heights and distances of 500-700 km, while all ∼400 ms −1 echoes came from lower heights and from farther distances; 700-1000 km. We argue that both observed groups of echoes occurred due to the Farley-Buneman plasma instability excited by strong (∼70 mVm −1 ) and uniformly distributed electric fields. We show that the echo velocities for the two groups were different because the echoes were received from different heights. Such a separation of echo heights occurred due to the differing amounts of ionospheric refraction at short and large ranges. Thus, the ionospheric refraction and related altitude modulation of ionospheric parameters are the most important factors to consider, when various characteristics of E-region decametre irregularities are derived from HF radar measurements.

A decade of the Super Dual Auroral Radar Network (SuperDARN): scientific achievements, new techniques and future directions

“…There is also interferometer evidence that different echo types arise from slightly differing altitudes within the E region (e.g. Milan et al , 2003aUspensky et al 2001). This suggests that different irregularity types can occur at the same position, though horizontally stratified, giving rise to co-located echoes with different Doppler characteristics, despite the driving electric field being common to both.…”

“…These echoes are characterized by particularly high backscatter power but appear to be sensitive to the radar operating frequency, being seen predominantly near 10-12 MHz (Milan et al 2003a) • Like population ii, population iv echoes have u los values which appear proportional to v los , but with a constant of proportionality close to *0.8 (Uspensky et al 2001;Milan et al 2003a). …”

On the relationship between the velocity of E-region HF echoes and <i>E</i>x<i>B</i> plasma drift

“…At small flow angles (presumably within the FB instability cone), from one side, a clear statistical clustering of HF velocities near 400 m/s (typical C s value) was reported (Hanuise et al, 1991;Milan and Lester, 2001). On the other hand, large velocities of more than 700 m/s are not rare (Milan and Lester, 1998; and strong velocity change with the flow angle within the FB instability cone has been reported (Milan and Lester, 2001;Uspensky et al, 2001;Makarevitch et al, 2002b). We should note that the velocity variation with flow angle inside the FB instability cone was recently reported for the STARE radars (Nielsen et al, 2002).…”

HF echo types revisited: aspect angle attenuation effects

“…Both the HF power and velocity behaviour were typical of those observed in other studies that employed oblique-sounding auroral HF radars Makarevitch et al, 2002b) with the power and velocity reaching their respective maxima at ranges r P and r V (with r P typically being close to r V ) and decreasing with range increasing/decreasing from r P or r V . Such a behavior is believed to be related to the power and velocity attenuation with the aspect angle as demonstrated by numerous previous studies at HF (Koustov et al, , 2002Uspensky et al, 2001;Makarevitch et al, 2001Makarevitch et al, , 2002bMakarevitch et al, , 2004 and as described below.…”

“…Over the last decade, the HF coherent radar studies of the plasma waves in the auroral E region have been extremely successful in identifying fundamental differences between irregularity properties at decameter scales as compared to meter-scale irregularities observed with VHF radars (Milan and Lester, 1998, 1999Jayachandran et al, 2000;Uspensky et al, 2001;Koustov et al, 2001Koustov et al, , 2002Koustov et al, , 2005Makarevitch et al, 2001Makarevitch et al, , 2002bMakarevitch et al, ,a, 2004Milan et al, 2003Milan et al, , 2004Makarevich, 2008;Carter and Makarevich, 2009). In particular, the HF echoes have been found to exhibit a wider Correspondence to: R. A.…”