SHORT-PERIOD (5-33 MIN) VARIATIONS IN VERTICAL DRIFT OF F-REGION PLASMA NEAR THE MAGNETIC EQUATOR

Measurements off region vertical drift, V-z made at Kodaikanal (dip 4 degrees N) on 133 days during March 1991-February 1992 under non-equat orial spread-F(ESF) conditions, are analysed for the characteristics o f short-period (5-33 min) variations in V-z in the evening hours (1630 -0130 IST). The spectral content of the fluctuations in V-z is found t o depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (sigma(2)), increases during the period of po stsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, whi le functuations in V-z in the entire band 5-33 min manifest in the pos tsunset hours, the longer-period (13-33 min) components dominate there after (2200-0000 IST). The amplitude of the fluctuations and its day-t o-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of V-z functuations increases with p eak height in the interval 1756-2004 IST corresponding to the epoch of the evening enhancement in V-z. The results support the view that the short-period variations in F region vertical drift near the dip equat or during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.