to the F layer, in the nonlinear regime

Abstract. The Es layer instability has been suggested as a participant in the creation of frontal structures observed in both the Es and F layers of the nighttime midlatitude iono-sphere, in spite of the fact that the spatial scales of the frontal structures are very different in the two layers. The linear growth rate of the instability has a maxima in the vicin-ity of the wavelength observed for the Es layer structures (short wavelengths). However, the maxima is non-distinct, and simulations have shown that the instability is extremely nonlinear. Therefore, to understand the wavelength depen-dence of the Es layer instability it is necessary to factor in nonlinear behavior. Simulations have shown that the insta-bility is active at the wavelengths observed in the F layer, and revealed that the Es layer behavior at these long wave-lengths is so nonlinear that the common, highly localized Es layer observation techniques would likely miss the signature, which is highly visible in the F layer. However, there is cur-rently no explanation for why long wavelengths so clearly dominate short (or intermediate) wavelengths in the F layer observations, and this is a weakness in arguments that the Es layer instability participates in the creation of F-region frontal structures. Herein we remove this weakness by show-ing that longer wavelengths grow to larger amplitudes before eventual nonlinear saturation, and couple more effectively to the F-region