RESPONSE OF EQUATORIAL F-LAYER VERTICAL DRIFT IN THE DUSK SECTOR TO THE CHANGE IN THE RADIATIVE OUTPUT OF THE SUN IN 1992

A study is made of the effect of the rapid and large decline in the so lar radiative output that occurred from March through May 1992 on the postsunset maximum in F layer vertical drift V-zp over Kodaikanal (dip 3 degrees N). Between the periods July 1991 through February 1992 and June 1992 through February 1993, there is a 35% To decrease in the me an amplitude of the postsunset peak in F layer vertical drift (V-zp) o ver 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 lo cal winter months preceding and following the rapid decline in the 10. 7-cm flux. The results illustrate the sensitivity of equatorial F laye r vertical drift in the dusk sector to short-term variability the sola r radiative output Solar flux related changes in the longitudinal (loc al 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.