Coherent HF radar backscatter from small‐scale irregularities in the dusk sector of the subauroral ionosphere

Abstract

The Applied Physics Laboratory high‐frequency (HF) radar located at Goose Bay, Labrador, observes coherent backscatter from decameter‐scale ionospheric plasma density irregularities. Over a period of 5 months centered on winter solstice, the daily activity includes a distinctive feature that appears at dusk and moves in a westerly direction through the radar field of view. The feature typically spans 1 hour in longitude and is observed for 1‐1½ hours. In this paper, we give examples and describe the characteristics of this dusk scatter under geomagnetically quiet conditions. We show that it can be readily distinguished from other types of late afternoon/early evening scatter by the extreme equatorward position of its source region and by the low values of its associated radar Doppler velocities (≤ 200 m/s) and spectral widths (≤ 200 m/s). Comparison of the HF radar observations with nearly simultaneous particle precipitation data obtained with the DMSP F6 satellite demonstrates that the source region lies equatorward of the main auroral zone. We argue that the occurrence of dusk scatter is related to the passage through the radar field of view of the area of intersection of the dusk terminator with the mid‐latitude trough. It is shown that the plasma drifts in the source region are slow (≤ 150 m/s) and sunward in the inertial reference frame, as described for the poleward portion of the trough in the Spiro et al. [1978] model of trough formation. The narrowness of the Doppler spectra is also consistent with the picture of the trough as a region of slow, uniform plasma drift. The small‐scale irregularities responsible for the radar backscatter may be generated by a cascade process from larger‐scale (> 100 m) irregularities that are unstable to the action of the gradient drift instability within the evening trough.