[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.

Abstract. Data collected by the CUTLASS Finland HF radar are used to illustrate the significant difference between the cosine component of the plasma convection in the F-region and the Doppler velocity of the E-region coherent echoes observed at large flow angles. We show that the E-region velocity is ∼5 times smaller in magnitude and rotated by ∼30 • clockwise with respect to convection in the F-region. Also, measurements at flow angles larger than 90 • exhibit a completely new feature: Doppler velocity increase with the expected aspect angle and spatial anticorrelation with the backscatter power. By considering DMSP drift-meter measurements we argue that the difference between F-and Eregion velocities cannot be interpreted in terms of the convection change with latitude. The observed features in the velocity of the E-region echoes can be explained by taking into account the ion drift contribution to the irregularity phase velocity as predicted by the linear fluid theory.

Abstract. In this study, velocities of E-region HF echoes observed by the Stokkseyri HF radar are compared with E×B plasma drifts in the F-region measured by the DMSP satellites. Events were selected for which the DMSP track projected to the height of 110 km was almost perpendicular to the central beams of the radar, resulting in a direct comparison of the cross-track component of the E×B drift and the line-of-sight HF velocity. We found that the typical ratio of HF velocity to the DMSP drift is ∼0.35 in a range of DMSP drifts of 0-1700 m/s. It is suggested that E-region HF velocities, observed both along the electrojet and at large flow angles, are significantly affected by scatter from the bottom of the electrojet layer where the irregularity phase velocity is expected to be strongly depressed with respect to the E×B flow.

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[1] Cooperative UK Twin Located Auroral Sounding System Finland HF radar observations of E region coherent spectra are presented for several events. The Burg spectrum method is applied to study microstructure of the spectra. It is discovered that many spectra are two-peaked, and quite often there is a systematic pattern in their occurrence; the double-peak echoes are typically observed at intermediate and far ranges of the E region echo band and at ranges farther than the power maximum in the range profile. The typical separation between the peaks is about 150 m/s and hardly changes with the azimuth of observations. Velocities of two components in a double-peak spectrum are typically larger and smaller than the velocity of the unresolved spectrum and singlepeaked velocity of echoes at shorter ranges. It is hypothesized that the two components of the echoes occur because of signal reception from the top and bottom of the electrojet layer.