In order to understand observations of modulated irregularities in a localized region on the bottomside of the equatorial ionosphere without the presence of vertically rising large-scale plume events, an investigation is carried out by means of a nonlinear numerical simulation model using different background ionospheric conditions. The investigation revealed that the development of long wavelength (200 km) perturbations can only be confined to the bottomside of the ionosphere in a localized region with the combined effects of shear in the zonal plasma drift and a nighttime westward electric field. The variabilities in the occurrence characteristic are shown to depend on the variabilities in the prereversal enhancement in the zonal electric field. Further, the resurgence of a plume event at a later time, as seen by the Jicamarca radar, is explained on the basis of confined (preseeded) structures subject to a fresh instability source. First evidence is presented that a source of fresh instability may actually be a wind and/or a temperature disturbance launched hours earlier by a small substorm. ¿ 1998 American Geophysical Union