Nonlinear Phenomena in a Plasma Located in an Alternating Electromagnetic Field

A PLASMA (IONOSPHERE) IHE nonlinearity of electrodynamic processes in a plasma manifests itself particularly clearly in the propagation of sufficiently strong radio waves.Thus, the propagation of one wave in a plasma gives rise to a nonlinear "self interaction" effect wherein the frequency spectrum of the wave (appearance of harmonics of the principal frequency), its absorption, and its phase all change.When several waves are propagated, the superposition principle is violated: the incident and reflected, the ordinary and extraordinary, or in general any two waves, cease being independent -they interact in a nonlinear manner because they themselves change the property of the medium (plasma) in which they propagate.tThe ordinary theory of propagation of radio waves in a plasma (in the ionosphere or in the corona of the sun) 15 neglects the influence of the wave on the plasma.This is true, as a first approximation, if the field of the wave is weak, i.e., if its amplitude satisfies the condition Eo ^T 6 eff (Ш* (0.1)The influence of the field of a weak wave on a plasma can be taken into account in the next approximation and should naturally result only in small corrections, bi spite of this, nonlinear effects can be observed even for weak waves, and are of practical significance.For example, cross modulation of radio waves in the ionosphere is readily observed even if condition (0.1) is satisfied (see Sec. 3.4).Naturally, the effect of strong radio waves (E 0 and more so of very strong ones (E 0 » Ep), on the plasma can no longer be neglected.Therefore ordinary Eph *For part 1 see Usp.Fiz Nauk 70, 202 (1960), Soviet Physics -Uspekhi 3, 147, (1960).tIn an inhomogeneous medium, the term "interaction of waves" is also used in a different sense."Here we take interaction to mean only the effects connected with nonlinearity.