Short-Period (5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator

Measurements of F region vertical drift, Vz made at Kodaikanal (dip 4°N) on 133 days during March 1991-February 1992 under non-equatorial spread-F(ESF) conditions, are analysed for the characteristics of short-period (5-33 min) variations in Vz in the evening hours (1630-0130 IST). The spectral content of the fluctuations in Vz is found to depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (σ2), increases during the period of postsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, while functuations in Vz in the entire band 5-33 min manifest in the postsunset hours, the longer-period (13-33 min) components dominate thereafter (2200-0000 IST). The amplitude of the fluctuations and its day-to-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of Vz functuations increases with peak height in the interval 1756-2004 IST corresponding to the epoch of the evening enhancement in Vz. The results support the view that the short-period variations in F region vertical drift near the dip equator during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.