VHF coherent radar signals from the E region ionosphere and the relationship to electron drift velocity and ion acoustic velocity

The Scandinavian Twin Auroral Radar Experiment (STARE) coherent radar system measures the Doppler shifts caused by ∼1 m plasma waves in the high-latitude E region ionosphere. These Doppler velocities are here related to the electron drift velocity and ion acoustic velocity derived from measurements with the incoherent radar system European Incoherent Scatter (EISCAT). The Doppler velocity is limited in magnitude to near the ion acoustic velocity in the plasma. For large flow angles θ, i.e., the angle between the radar line of sight and the electron drift velocity, the Doppler shifts are equal to the component of the electron drift velocity on the line of sight. For θ ∼ 40° the Doppler velocity is equal to the ion acoustic velocity at 105-km altitude, and for decreasing flow angle the Doppler velocity increases. For 0° < θ < 60° the variation with flow angle can be described as cosαθ, where the α decreases from 0.8 to 0.2 with an increase in drift speed from ∼400 to 1600 ms−1. The ratio of the line-of-sight velocity for θ ∼ 0° to the ion acoustic velocity decreases from 1.2 at low velocities to 1.05 at large velocities. The systematic variations of the Doppler shifts with drift speed and flow angle make it possible, in principle, to recover the electron velocity from the coherent radar measurements. The observations are used to illustrate how well the recovery is possible in practice