A unified theory has been developed to explain the formation of a quasi-dc electric pulse (&ohgr;≤&OHgr;i) induced by an ionizing neutral barium beam across an ionospheric plasma. We obtained a generalized form for the dc electric field in the plane perpendicular to the magnetic field within a simplified slab barium cloud moving perpendicular to the geomagnetic fields. A current system associated with the quasi-dc electric field was also proposed to provide a way to transfer the momentum between the streaming barium cloud and the ambient plasma. The characteristic time derived by using this model for momentum coupling between the streaming barium cloud and the ambient plasma is found to be in agreement with the previous result. The quasi-dc electric field predicted by this model is reasonably consistent with the CRIT II critical ionization velocity measurements. On the basis of the constrains of the conservation of energy and momentum, we found that Alfv¿n's critical ionization velocity (CIV) effect is a self-limiting ionization process in a finite extent neutral cloud. It may be the reason why the CIV effect took place in CRIT II but lasted only for a very short period, and it may have resulted in low ionization yields in most of space CIV experiments.¿ 1996 American Geophysical Union