Height gradient of F region vertical drift in the evening equatorial ionosphere

Simultaneous measurements of F‐region vertical drift are made in the evening hours (1700–2100 IST) at Trivandrum (dip 0.6°N) and Kodaikanal (dip 4°N) on fifteen days during December 1993–January 1994 using the HF phase path technique on two different probing frequencies. The data are used to study the height dependence of vertical plasma drift in the bottomside F‐region in the dusk sector after correcting the drifts (at Kodaikanal) for meridional wind effects and chemical loss. It is found that growth and decay of a positive height gradient in vertical drift occurs fairly regularly in the dusk period. On the average the vertical velocity gradient is positive in in the interval 1815–1925 IST and is preceded by negative values. The positive height gradient of vertical plasma drift below the F layer peak is interpreted in terms of altitude dependence of the relative contributions of E and F region dynamos to the electric fields responsible for plasma drifts (vertical and zonal) of the dusktime equatorial F‐region. These results are for winter solstice solar minimum conditions.