Evidence of a velocity shear in bulk plasma motion associated with the post‐sunset rise of the equatorial F‐layer

Abstract

Using the east‐west drift of equatorial spread‐F (ESF) irregularities as tracers for horizontal bulk plasma motion, we show that a velocity shear (with altitude) in east‐west plasma drift exists in the nighttime equatorial F‐layer. When combined with incoherent‐scatter measurements of the vertical motion of the F‐layer, we find evidence that the two‐dimensional plasma flow pattern in the F‐region around E‐region sunset resembles a vortex. The velocity vortex is attributed to (1) a velocity shear that is set up by the F‐region dynamo that begins to function around E‐region sunset, and (2) the postsunset rise of the F‐layer. The existence of a velocity shear verifies the importance of the eastward neutral wind and F‐region dynamo in nighttime equatorial plasma electrodynamics, and allows new interpretation for ESF generation processes.