The occurrence of equatorial density depletions in the nightside F region ionosphere has been investigated by using observations gathered by the polar-orbiting Dynamics Explorer 2 satellite from August 1981 to February 1983. A variety of electric field/plasma drift patterns were observed within these depletions, including updrafting, downdrafting, bifurcating, converging, subsonic, and supersonic flows. The depletions, 116 events in total, are distributed over two groups: group I (eveningside depletions) consists of the events in the 1900--2300 MLT sector, and group II (morningside depletions) are the events in the 2300--0600 MLT sector. A statistical analysis reveals clear differences in the density depletion occurrence rates between the two groups. Magnetic activity appears to suppress the generation of eveningside depletions with a delay of 2--3 hours. In the morningside the probability of observing depletions increases instantly with increasing magnetic activity; yet the best correlation is found with a 2-hour delay. This indicates that in the premidnight sector the substorm-induced dynamo and prompt penetration electric fields induce westward electric fields in the equatorial ionosphere 2 hours after the onset in the auroral region. In the postmidnight sector, high-latitude ionospheric disturbances induce instantly equatorial eastward electric fields that move the F layer to higher altitudes, where it can become unstable to the Rayleigh-Taylor instability. Eveningside depletions were observed at all longitudes except those over the Pacific Ocean, while the morningside depletions occurred mostly over the Pacific and Atlantic-African sectors. ¿ 2000 American Geophysical Union