HEIGHT GRADIENT OF F-REGION VERTICAL DRIFT IN THE EVENING EQUATORIAL IONOSPHERE

Simultaneous measurements of F-region vertical drift are made in the e vening hours (1700-2100 IST) at Trivandrum (dip 0.6 degrees N) and Kod aikanal (dip 4 degrees N) on fifteen days during December 1993-January 1994 using the HF phase path technique on two different probing frequ encies. The data are used to study the height dependence of vertical p lasma drift in the bottomside F-region in the dusk sector after correc ting the drifts (at Kodaikanal) for meridional wind effects and chemic al loss. It is found that growth and decay of a positive height gradie nt in vertical drift occurs fairly regularly in the dusk period. On th e average the vertical velocity gradient is positive in in the interva l 1815-1925 IST and is preceded by negative values. The positive heigh t gradient of vertical plasma drift below the F layer peak is interpre ted in terms of altitude dependence of the relative contributions of E and F region dynamos to the electric fields responsible for plasma dr ifts (vertical and zonal) of the dusktime equatorial F-region. These r esults are for winter solstice solar minimum conditions.