Solar cycle and seasonal variations in F-region vertical drifts over Kodaikanal, India

Abstract

Measurements of the changes in phase path of F-region reflections at normal incidence at Kodaikanal (77°28'E, 10°14'N, dip 3°N) from February 1991 to February 1993 are used to determine the variation of the equatorial evening F-region vertical drifts (Vz) with season, solar and magnetic activity. It is found that on average, at Kodaikanal, the post-sunset peak in Vz (Vzp) is higher in equinox and local winter months than in local summer. The day-to-day variability in Vzp is highest in summer and lowest in winter. This seasonal trend persists even on magnetically quiet days (Ap≤14). Vzp is found to increase with 10.7 cm solar flux in all three seasons but tends to saturate for large flux values (>230 units) during local summer and winter months. Magnetic activity [represented by Ap as well as the time-weighted accumulations of ap and ap(τ)] does not seem to have any statistically significant effect on Vzp, except during equinoctial months of moderate solar activity, when Vzp decreases as magnetic activity increases.