LONGITUDINAL DEPENDENCE OF EQUATORIAL F-REGION VERTICAL PLASMA DRIFTSIN THE DUSK SECTOR

The effect of solar activity on the postsunset peak of F region vertic al plasma drift at Kodaikanal (77 degrees 28'E, dip 3 degrees N) is de termined as a function of season and for two levels of magnetic activi ty using extensive HF phase path observations and compared with that a t Jicamarca (75 degrees W, dip 2 degrees N) reported in the literature . The postsunset peak vertical velocity V-zp at Kodaikanal increases l inearly with 10.7-cm Solar flux (E(10.7)) irrespective of season and l evel of magnetic activity The sensitivity of V-zp to solar flux change s is more at Jicamarca than at Kodaikanal during equinoxes and Decembe r solstice. This feature is virtually absent in June solstice. The sea sonally averaged values of dusktime vertical velocity at Kodaikanal fo r moderate to high solar flux conditions and quiet magnetic conditions (Kp less than or equal to 3) are in reasonable agreement with those r eported very recently for the Indian sector (80 degrees E) from AE-E s atellite data [Fejer et al, 1995]. In June solstice, the evening upwar d vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160 degrees-200 degrees E), while the post reversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230 degr ees-310 degrees E) where there they are higher. The present study supp orts the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June so lstice.