Rocket measurements from equatorial regions during equatorial spread F (ESF) consistently reveal the presence of plasma density irregularities in the lower F region (200--300 km) where the initial conditions including the vertical electron density gradient are not favorable for the development of linear instability. An investigation carried out using a nonlinear numerical simulation model and the results obtained from the Ionization Hole campaign revealed that the altitude variation of the recombination coefficient and the vertical polarization velocity due to fringe fields associated with the plasma bubble at the base of the F layer are responsible for changing the polarity of the vertical gradient in the plasma density profile in the lower F region, while the penetration of the fringe fields is mainly responsible for the development of zonal electron density gradient which plays a crucial role in the secondary plasma instabilities resulting in the generation of kilometer scale size irregularities in the vertical direction.¿ 1997 American Geophysical Union