SOLAR-CYCLE AND SEASONAL-VARIATIONS IN F-REGION VERTICAL DRIFTS OVER KODAIKANAL, INDIA

Measurements of the changes in phase path of F-region reflections at n ormal incidence at Kodaikanal (77 degrees 28'E, 10 degrees 14'N, dip 3 degrees N) from February 1991 to February 1993 are used to determine the variation of the equatorial evening F-region vertical drifts (V-z) with season, solar and magnetic activity. It is found that on average , at Kodaikanal, the post-sunset peak in V-z (V-zp) is higher in equin ox and local winter months than in local summer. The day-to-day variab ility in V-zp is highest in summer and lowest in winter. This seasonal trend persists even on magnetically quiet days (A(p) less than or equ al to 14). V-zp is found to increase with 10.7 cm solar flux in all th ree seasons but tends to saturate for large flux values (> 230 units) during local summer and winter months. Magnetic activity [represented by A(p) as well as the time-weighted accumulations of a(p) and a(p)(ta u)] does not seem to have any statistically significant effect on V-zp , except during equinoctial months of moderate solar activity, when V- zp decreases as magnetic activity increases.