Optical Mixing in a Magnetoactive Plasma

Abstract

The nonlinear interactions of two electromagnetic waves resulting in the excitation of electrostatic oscillations of a homogeneous plasma in a strong magnetic field are studied. Starting from the cold plasma equations for electrons, and neglecting ion motions, a general expression for the second-order density perturbation of the plasma in the presence of two first-order electromagnetic waves is derived. One basic assumption made is that the frequencies of the electromagnetic waves are much larger than the plasma frequency and than the electron cyclotron frequency. The second-order density perturbation is explicitly calculated in the two cases where the difference frequency and wave vector of the interacting waves satisfy the dispersion relation for plasma oscillations and for the upper hybrid resonance. The scattering cross section for electromagnetic radiation per electron due to the excited electrostatic oscillation is calculated and compared to cross section due to thermal fluctuations. The results are applied to a laboratory Q device using ruby lasers as sources of the electromagnetic waves and to an ionospheric plasma using radio waves. The effect of finite beamwidth and of nonuniform plasma density is also considered. It is found that the excited oscillations near either of the two resonances considered in the ionosphere should easily be observed by use of a satellite receiving part of the scattered radiation. This method of excitation and detection is suggested for sounding of ionospheric plasmas.