IonosphericE-region irregularities produced by nonlinear coupling of unstable plasma waves

The irregularities responsible for the strongest VHF echoes from the equatorial E region and the aurora appear to have been adequately explained as arising from a form of two-stream plasma instability. Many of the weaker features of the echoes from the electrojet irregularities cannot be explained on this basis, however. The waves generated by the instability will grow in amplitude until they are limited by nonlinear effects. These effects, furthermore, will cause coupling between waves that will generate new waves that cannot be directly excited. In the present paper we show that scattering from such secondary irregularities can account for many of the weak features of the VHF echoes. The theory is only qualitative, but the agreement between its predictions and most of the experimental observations is quite good.