Comparison of SuperDARN Irregularity Drift Measurements and F-Region Ion Velocities from the Resolute Bay ISR

Abstract

A number of studies have addressed the principal assumption used by the SuperDARN network of HF radars that the scatter from F region field-aligned irregularities has a Doppler shift given by the cosine component of the EXB plasma drift. However, the slopes of the best-fit line to the measured points have consistently been low, perhaps implying contamination from irregularities with smaller amplitude drifts. This work is motivated by testing the same assumption on a more optimal experimental setting: using the north face of the Resolute Bay incoherent scatter radar (RISR-N), which provides high resolution magnitude and direction measurements of EXB drift in a region where the ionospheric flow is mostly uniform relative to auroral latitudes, thereby reducing echo mixing due to spatio-temporal structuring. We compared the EXB drift measured by RISR-N to the line-of-sight Doppler velocities measured PolarDARN which is composed of Rankin Inlet and Inuvik HF radars, both having a field of view over Resolute Bay. An aggregate scatter plot of all the echoes observed during a 5-day period in early May 2011 contains two distinct groups of echoes. The first group is hypothesized to be from the E region because the echoes appear above a threshold EXB drift and at small flow angles, which are characteristics of primary Farley–Buneman waves. The Doppler velocity of the second group of echoes increase linearly with the EXB drift and is identified here as F region echoes. A special joint fit showed a slope of 0.85 for the F region echoes. During the observation period, the F region electron density has large variations between 1 and 10 1011, in particular due to the polar cap patches. Considering a representative F region electron density of 5 1011 m3, which has a refraction index of 0.86 at 12.5 MHz (for PolarDARN frequencies) and the fact that the measured velocity is the product of the actual velocity and the refractive index, the expected slope is 0.86, in agreement with our measurement. The fit for E region Doppler velocities shows a saturation speed at 170 m/s, while the data were spread between 100 and 300 m/s. Although the saturation speed is somewhat lower than the ion acoustic speed, it is acceptable considering earlier work attributing similarly low velocities to large aspect angles at the scattering altitudes.