By using a numerical model we have studied electric field effects on the generation of equatorial spread F (ESF) in the postmidnight hours. Equatorial electric fields are zonally westward during late evening and postmidnight hours on normal days, while they sometimes reverse to eastward in magnetically disturbed days. The quiet time westward configuration and typical low altitude of the layer results in stable bottomside gradient or at most one with a very low generalized Rayleigh-Taylor growth rate. The model calculation shows that the growth rate becomes significant when the electric field reverses to eastward and may remain so even when the electric field disturbance weakens due to the upward altitude excursion of the layer. These results account for the correlation of ESF occurrence and high Kp values in the postmidnight hours when taken in conjunction with the characteristics of the penetration of magnetospheric electric fields during such magnetic activity. The results also show the crucial role of layer height in the physics. We have calculated the growth rates for specific nights in which perturbation of electric fields and plumes were observed at Jicamarca. These case studies suggest that a quantitative requirement for the generation of plumes is that about 10 e-folding times of the linear instability growth occur. We also suggest that a statistical study using ionosonde data could be very valuable in a testing whether a local version of this quantitative measure of event intensity could be useful in predicting plume activity. ¿ American Geophysical Union 1992