We show that purely growing instabilities driven by the saturation spectrum of parametric decay instabilities can produce a broad spectrum of ionospheric irregularities. The threshold field ‖Eth‖ of the instabilities decreases with the scale lengths &lgr; of the ionospheric irregularities as ‖Eth‖∝&lgr;-2 in the small-scale range (<15 m) but becomes an increasing function as ‖Eth‖∝&lgr; in the large-scale range. The minimum threshold field (~2 mV/m) occurs at &lgr;~20 m. The growth rate &ggr; of the instabilities maximizes at &lgr;~6 m and decreases drastically as &ggr;∝&lgr;-2 with scale lengths larger than a few kilometers. The excitation of kilometer-scale irregularities is strictly restricted by the instabilities themselves and by the spatial inhomogeneity of the medium. These results are drawn from the analyses of four-wave interaction. Ion-neutral collisions impose no net effect on the instabilities when the excited ionospheric irregularities have a field-aligned nature.