Observation and theoretical modeling of a region of downward field-aligned flow of O(+) in the winter dayside polar cap

Optical and radar measurements of ion drift at high latitudes near the terminator, which show that there is a region of large downward field-aligned ion flow below the F-peak in the dayside of the polar cap, are presented and examined. The region starts around noon LMT, extending towards 1800 LMT, following the 72-deg invariant latitude circle. When the field perpendicular component in the plane of the observation is about 100 m/s poleward, the maximum field-aligned velocities are found to be in the region of 400 m/s. The Sheffield ionospheric (Quegan et al., 1982) model is consistent with the variation of field-aligned ion flow in latitude, time, and sign, but does not match the high velocities.