Electrodynamics of the Equatorial F region ionosphere using Post-sunset and Pre-sunrise Vertical Plasma drift Observations

The Multi frequency HF Doppler radar located at the magnetic equatorial station, Trivandrum (8.330 N, 770 E, dip 0.40 N), operating at three fixed frequencies - 2.5 MHz, 3.5 MHz, and 4.5 MHz, is capable of observing nearly simultaneous vertical plasma drift and zonal drift data at three different heights. The radar is used to make observation of the vertical plasma drifts mainly during the post sunset and pre-sunrise periods. The vector plasma drift from multiple altitudes during post sunset hours shows the signature of velocity vortex. While the radar was used to observe vertical plasma drifts at three altitudes during most of the days, the observation at the pre-sunrise period was not possible on all days. The pre-sunrise observation was limited to those days for which the foF2 value falls below 2 MHz around 06:00 LT making it impossible to probe the ionosphere using even the lowest frequency of the radar. To complement the HF Doppler radar observations, the digital ionosonde observation at Trivandrum is used to evaluate the vertical plasma motion from the time variations of h'F during the pre-sunrise period. In the present work, the characteristics of the vertical velocity profiles of post-sunset and pre-sunrise hours are studied and presented. The most striking feature of the plasma drift is the downward excursion of about 20 - 30 minutes duration near sunrise followed by an upward turning beyond 06:30 LT. This feature of plasma drift is similar to the post sunset enhancement followed by the reversal in plasma drift direction. The mechanisms of both the post sunset enhancement (PRE) and pre-sunrise downward excursion (SDE) are to be qualitatively discussed and presented.