Computer simulations of the intermediate wavelength (100--1000 m) collisional Rayleigh-Taylor instability in local unstable regions of the postsunset bottomside (300 km) equatorial F region ionosphere have been performed. For ambient electron density gradient scale lengths L=5, 10, 15 km we find that the linearly unstable horizontal modes saturate by nonlinear generation of linearly damped vertical modes with the result that in the nonlinear regime, power laws are observed in the horizontal P(kx) ∝ kx-n and vertical P(ky) ∝ ky-n one-dimensional power spectra with n=2--2.5. These results are consistent both with in situ experimental data and with theoretical prediction.