From several years of Atmosphere Explorer data we show that large-scale plasma density depletions (horizontal scale greater than 1000 km) in the nocturnal F region occur mainly during the first half of the night. The geomagnetic latitude of these depletions depends on season: at solstice periods the occurrence is mainly in the summer hemisphere and at equinoxes mainly on the geomagnetic equator. Large-scale depletions occur most often at longitudes characterized by a large magnetic declination. We interpret the large-scale plasma depletions to be a consequence of imposed upward drift which raises the F region above the satellite trajectory. We have tested this interpretation by calculating the vertical ion drift velocity imposed by zonal and meridional winds and electrostatic fields, with allowance for the effects of diffusion. The calculations reveal a correspondence between the locations of maxima in the calculated vertical ion drift velocity and the large-scale plasma depletions.