Multifrequency measurements of HF Doppler velocity in the auroral E region

Abstract

[1] HF measurements in Prince George, British Columbia (Canada), at five radar frequencies between 9.3 and 15.7 MHz are considered to study the Doppler velocity of E region coherent echoes. One event showing a regular variation of velocity with radar frequency, slant range, and azimuth of observations is analyzed in detail. For this event, plasma drifts were in access of 700 m s À1 , but the observed velocities were below 250 m s À1since measurements were performed at large flow angles (L shell angles 45°< f< 100°). We show that measured Doppler velocity depends on irregularity scale but only within the Farley-Buneman (F-B) instability cone (45°< f< 75°). We demonstrate that maximum velocities measured at the highest radar frequency are $1.3 times larger than those at the lowest frequency. We also show that for observations inside the instability cone, the velocity magnitude strongly decreases with aspect angle and the rate of the decrease is scale sensitive. The effect can be described by the fluid theory formula if the nominal electron collision frequencies are replaced by anomalous collision frequencies that are $5 times larger. However, for observations outside the F-B instability cone (75°< f< 100°), the Doppler velocity does not show significant variation with aspect angle. For these directions, velocity change with flow angle was insignificant, very similar at all radar frequencies, and not consistent with the expected ''cosine'' law. The implications of the measurements on the theory of electrojet instabilities and the processes of coherent echo formation are discussed.

Simultaneous HF measurements of E- and F-region Doppler velocities at large flow angles

“…1. To some extent, this result reminds us of observations of Makarevitch et al (2002a), who reported the absence of the aspect angle attenuation for the Doppler velocity at large L-shell angles φ ∼ =90…”

A first comparison of irregularity and ion drift velocity measurements in the E-region

“…(1) is small the phase velocity is determined by the second term and hence would be representative of the ion drift velocity component alongk and independent of the aspect angle. This argument has been employed by Makarevitch et al (2002) in order to explain an absence of velocity variation with slant range (and hence aspect angle) for certain directions as seen by the Prince George Super-DARN HF radar. The above argument would be valid, however, only for the relatively small range of flow angle near perpendicularity to V d if perfect aspect angle conditions (α=0) are assumed.…”

“…More observations casting doubt on the validity of the cosine law have been presented in a series of recent studies by and Uspensky et al (2003) at VHF and by Makarevitch et al (2002Makarevitch et al ( , 2004a and Milan et al (2004) at HF. It has been suggested that the observations could be explained if the ion motion contribution to the phase velocity of E-region irregularities is taken into account as prescribed by generalized formulas of the linear theory in the frame of reference of the neutrals.…”

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

“…These have recently been identified as backscatter from high-aspect angles (the high aspect angle irregularity region (HAIR) echoes), that is from density structures not aligned along the magnetic field, whose Doppler velocity is determined by the ion drift rather than the electron drift Drexler and St.-Maurice 2005). Aspect angle effects have also been found to be important in modifying the Doppler shift characteristics of other echo types (Makarevitch et al 2002.…”

“…More recently, SuperDARN observations have challenged this view and it has become clear that the slow background echoes do not represent the background E Â B drift but have u los values that are only *10-40% of v los (Makarevitch et al 2002;Milan et al 2003a;Koustov et al 2005). Understanding that the most ubiquitous echoes are not representative of the electric field prompted a reclassification of the echo-types seen at HF frequencies into five populations i to v, drawing a clear distinction from VHF nomenclature Lester 1999, 2001):…”