Satellite and rocket measurements of ion and electron density indicate that near the magnetic equator, in the nighttime (dusk) ionospheric F1 region, NO+ is often the dominant ion up to altitudes of 250 to 300 km. To study this region we theoretically calculate NO+, O2+, and O+ ion densities by solving the coupled, time-dependent, nonlinear ion continuity equations numerically and include the effects of production, loss and transport by diffusion and ?¿? drift. Incorporation of a realistic vertical ?¿? drift model which includes a prereversal enhancement in upward drift, produces NO+, O2+, and O+ density profiles which are consistent with the observations. The chemical reaction O2++N(4S)→NO++O is an important one and acts as a sink for O2+ ions and a source of NO+ as the F layer moves upward under the action of ?¿? drift. Both the observations and calculations show that where NO+ is the dominant ion, the electron and ion densities are nearly constant in altitude, i.e., the plasma scale height becomes very large. The implications of this on the growth of large scale irregularities are briefly discussed.