Although it is generally accepted that equatorial spread F (ESF) is due to nonliner evolution of the Rayleigh-Taylor instability, a number of important properties of the process remain unexplained. In particular, we investigate two as yet unexplained features of ESF: the common dominance of very large scale features (≥20 km) and their large amplitude. Although associated for years with spread F we show here for the first time that atmospheric gravity waves can initiate the Rayleigh-Taylor instability. In agreement with other analytical theories and computer simulations we find that the initiated instability will be saturated by nonlinear coupling of unstable modes to damped modes if the amplitude of the seed gravity wave is small. However, if the amplitude of the seed gravity wave is large enough, the relative plasma density perturbations can reach 50% or more, implying essentially no saturation. The required initial amplitudes are not unreasonable. In the latter case, significant enhancements and depletions of the plasma density occur within several hundred seconds. The analysis presented here demonstrates the possibility that large-scale spread F is triggered by gravity waves with the Rayleigh-Taylor instability as a source of amplification. In a separate calculation we also show that large-amplitude plasma perturbations can be produced by an explosive instability of the Rayleigh-Taylor modes. It is found that the condition for explosive growth of the Rayleigh-Taylor modes can be satisfied in the ionospheric F region. We propose that the explosive mode coupling of the Rayleigh-Taylor instability is a possible mechanism for production of large-amplitude bottomside sinusoidal structures. ¿ American Geophysical Union 1993