The role of parametric decay instabilities in generating ionospheric irregularities

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 λ of the ionospheric irregularities as |Eth| ∝ λ−2 in the small‐scale range (<15 m) but becomes an increasing function as |Eth| ∝ λ in the large‐scale range. The minimum threshold field (∼2 mV/m) occurs at λ ∼ 20 m. The growth rate γ of the instabilities maximizes at λ ∼ 6 m and decreases drastically as γ ∝ λ−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.