HF Doppler observations of vertical plasma drifts in the evening F region at the equator

Hf Doppler observations of vertical plasma drifts in the evening F region of the equatorial ionosphere at Trivandrum (8.5 degrees N, 77 degrees E; dip 0.9 degrees S) are presented. The observations reveal a consistent pattern characterized by a post-sun-sent enhancement with peak drift velocity in the range 30-60 m s-1 for quiet conditions. Under moderately disturbed conditions, the enhancement is found to be down by as much as a factor of 2, while no such appreciable change is noted for highly disturbed conditions. The pattern of drift velocity dependence on magnetic activity is seen to be consistent with that of spread F occurrence. A striking feature of the observations is the presence of a significant fluctuating component in the drift velocity with quasiperiods ranging from a few minutes to a few tens of minutes during both quiet and disturbing periods