It is well-known that large wedges of depleted plasma often form in the equatorial ionosphere after sunset. Irregularities in and around these depletions cause scintillation on trans-ionospheric radio paths, as well as strong VHF backscatter. The ultimate stage of depletion growth is controlled by a collisional interchange instability. However, the initiation stages remain the subject of debate. Depletions formed in the first 1–3 hours after sunset are probably seeded by instability processes operating in the valley region from 150–250 km. However, depletions that form later in the evening do not benefit from the rapid pre-reversal enhancement of the zonal electric field nor the sheared flow of the evening vortex. We investigate evidence from airglow images and VHF coherent backscatter that a polarization electric field associated with an ionospheric instability process at middle latitudes may induce the formation of post-midnight depletions at the geomagnetic equator during geomagnetically-quiet periods.