Four different two‐dimensional (perpendicular to the ambient magnetic field) plasma fluid‐type numerical simulations following the nonlinear evolution of the collisional Rayleigh‐Taylor instability in the nighttime equatorialFregion ionosphere have been performed. Realistic altitude dependent ion‐neutral collision frequencies, recombination rates, and ambient electron density profiles were used. In three cases (ESF 0, 1, 3) the electron density profile was kept constant, with a minimum bottomside background electron density gradient scale lengthL∼ 10 km, but the altitude of theFpeak was changed, withFpeak altitudes at 340, 350, and 430 km. All cases resulted in bottomside growth of the instability (spreadF) with dramatically different time scales for development. Plasma density depletions were produced on the bottomside with rise velocities, produced by nonlinear polarizationE × Bforces, of 2.5, 12, and 160 m/s and percentage depletions of 16, 40, and 85, respectively. In one case, ESF 0, the bubble did not rise to the topside, but in ESF 1 and 3, topside irregularities were produced by the bubbles (where linear theory predicts no irregularities). In these three cases, spreadFcould be described from weak to strong. In the fourth case (ESF 2) the altitude of theFpeak was 350 km, but the minimumLon the bottomside was changed to 5 km. This resulted in a bubble rise velocity of ∼23 m/s and a 60% depletion with strong bottomside and moderate topside spreadFand a time scale for development between ESF 1 and 3. Two other cases, ESF 0′ and 0″ with peaks at 330 and 300 km, respectively, and bottomsideL∼ 10 km, were investigated via linear theory. These cases resulted in extremely weak bottomside spreadFand no spreadF(entire bottomside linearly stable), respectively. These simulations show that under appropriate conditions, the collisional Rayleigh‐Taylor instability causes linear growth on the bottomside of theFregion. This causes the formation of plasma density depletions (bubbles) which rise to the topside (under appropriate conditions)Fregion by polarizationE × Bmotion. High altitude of theFpeak, small bottomside electron density gradient scale lengths, and large percentage depletions yield large vertical bubble rise velocities, with the first two conditions favoring bottomside linear growth of the instability. The numerical simulation results are in good agreement with rocket and satellite in situ measurements and radar backscatter measurements, including some of the recent results from the August 1977 coordinated ground‐based measurement campaign conducted by Defense Nucl