Westward plasma drift in the midlatitude ionospheric F region in the midnight‐dawn sector

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Abstract
In this paper we report the observations of a sharp ionospheric plasma velocity reversal boundary at subauroral latitudes during magnetic storms in the midnight‐dawn sector. The zonal plasma drift was eastward poleward of the velocity boundary and became westward equatorward of the boundary. The latitudinal extent of the westward drifts was ∼20°. A local maximum of the drift occurred only 2°–5° below the boundary; the magnitude of the maximum velocity was 200–400 m s−1 between 2100 and 0300 local time (LT) and as high as 780 m s−1 near midnight. The large westward plasma drift immediately equatorward of the velocity boundary first occurred at 2100 LT and continuously extended across midnight to the dawn sector. The location of the maximum velocity coincided with the minimum density within the midlatitude ionospheric trough. The latitude of the velocity boundary decreased from geographic latitude ∼53° to ∼43° (corresponding to geomagnetic latitude ∼65° to ∼55°) between 2100 and 0800 LT. We propose that the large westward drift at subauroral latitudes is driven by the polarization jet electric fields in the ring current and that the lower‐latitude westward drift with moderate magnitude is caused by the disturbance wind dynamo.