SEASONAL AND SOLAR-CYCLE ASSOCIATION OF ZONAL DRIFTS OF IONOSPHERIC PLASMA IRREGULARITIES IN THE INDIAN EQUATORIAL REGION

Long series of simultaneous VHF scintillation observations at two stat ions situated in near magnetic east-west direction in the vicinity of the dip equator in the Indian region have been employed to investigate the night-time ionospheric plasma zonal drifts. The drifts are found to be predominantly easterly. On comparing the magnitudes of the drift s with those results derived earlier by HF fading technique, monitorin g signals from two satellites at a station and spaced receiver experim ent, their associations with the season and the degree of solar activi ty are discussed. On a broader scale, the annual mean sunspot number i s shown to have a direct control on the derived drift, the positive re lationship even on day to day basis with the solar flux is established . However, the relationship, as understood by the slope of the best fi t line, in the Indian region (0.27) is found to be weaker when compare d with the similar slope (0.45) in the American sector. There appears to be no geomagnetic activity control on the estimated drifts.