Multifrequency measurements of HF Doppler velocity in the auroral E region

Abstract

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−1, but the observed velocities were below 250 m s−1 since measurements were performed at large flow angles (L shell angles 45°< ϕ< 100°). We show that measured Doppler velocity depends on irregularity scale but only within the Farley‐Buneman (F‐B) instability cone (45°< ϕ< 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°< ϕ< 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.