HF Doppler radar observations of vertical and zonal plasma drifts-Signature of a plasma velocity vortex in evening F-region

The simultaneous vertical and zonal plasma drift measurements using an HF Doppler radar system around March equinox of 1995 at the magnetic equatorial station, Trivandrum, India (8.33 deg N, 77 deg E, dip 0.4 deg N) are analyzed to evaluate the vector plasma drift at the F-region. It is found that the pre-reversal enhancement in vertical drift and the direction change of zonal drift from westward to eastward occur almost simultaneously. The velocity vector in the vertical-zonal plane, the resultant of vertical and zonal velocities, exhibits a gradual rotation in the evening time. The characteristics of velocity vector clearly promulgate the existence of a plasma velocity vortex in the equatorial F-region during post-sunset period.