A study is made of the effect of the rapid and large decline in the solar radiative output that occurred from March through May 1992 on the postsunset maximum in F layer vertical drift Vzp over Kodaikanal (dip 3¿N). Between the periods July 1991 through February 1992 and June 1992 through February 1993, there is a 35% decrease in the mean amplitude of the postsunset peak in F layer vertical drift (Vzp) over Kodaikanal, corresponding to the 40% decrease in the mean level of the 10.7-cm solar radio flux.

Changes of the same magnitude are also seen when the comparison is restricted to a shorter interval of two local winter months preceding and following the rapid decline in the 10.7-cm flux. The results illustrate the sensitivity of equatorial F layer vertical drift in the dusk sector to short-term variability in the solar radiative output. Solar flux related changes in the longitudinal (local time) gradient in E region conductivity as well as in the F region zonal winds are suggested as the cause of the positive response of the dusktime peak in F layer vertical drift to the rapid decline in solar radiative output that occurred in early 1992. ¿American Geophysical Union 1995