Abstract:
The effect of solar activity on the postsunset peak of F region vertical plasma drift at Kodaikanal (77°28'E, dip 3°N) is determined as a function of season and for two levels of magnetic activity using extensive HF phase path observations and compared with that at Jicamarca (75°W, dip 2°N) reported in the literature. The postsunset peak vertical velocity Vzp at Kodaikanal increases linearly with 10.7-cm solar flux (F10.7) irrespective of season and level of magnetic activity. The sensitivity of Vzp to solar flux changes is more at Jicamarca than at Kodaikanal during equinoxes and December solstice. This feature is virtually absent in June solstice. The seasonally averaged values of dusktime vertical velocity at Kodaikanal for moderate to high solar flux conditions and quiet magnetic conditions (Kp<=3) are in reasonable agreement with those reported very recently for the Indian sector (80°E) from AE-E satellite data [Fejer et al., 1995]. In June solstice, the evening upward vertical velocities at Kodaikanal are, on the average, much smaller than in the Pacific sector (160°-200°E), while the postreversal downward velocities are more or less of the same magnitude in all longitude sectors except in the western American sector (230°-310°E) where there they are higher. The present study supports the emerging view that the dusktime equatorial F region vertical plasma drifts exhibit large longitudinal variations during the June solstice.
