Conditions of plasma irregularity (or Spread F) regeneration under the Rayleigh--Taylor mechanism in the nighttime equatorial ionosphere are examined from ionogram data registered over Fortaleza (4¿ S, 38¿ W, mag. dip lat. -1.7¿) Brazil for a one year period. The results show that local ionospheric conditions such as the bottomside F-region electron density scale length and the ion-neutral collision frequency at the base of the layer, at sunset hours, determine the generation of the spread F irregularities over the equator in the post sunset hours.