Theory of resonant and stimulated excitation of magnetic-moment fields in wave-plasma interactions

Abstract

Theories of two nonlinear processes of magnetic-moment-field generation in wave-plasma interactions are presented here. These processes are (i) resonant excitation of a moment field (REMF) and (ii) stimulated excitation of a moment field (SEMF). This field generally evolves from the wave-induced bending of the direction of motion of constituents of a plasma. It is important when it has a large value, and when it grows to large values with time, because then it effectively controls the wave-induced features of a plasma. Specifically, this growing field gives rise to a strong anisotropy of the plasma in the region of the common direction of propagation of the involved waves, which leads to enhancement of synchrotron and bremmsstrahlung losses, and filamentation. The REMF is a static field of resonance when the beat frequency of two waves equals the frequency of another wave, all propagating in the same direction. The three possible cases of such interaction, involving waves of only high frequencies, with unmagnetized plasmas, for which the REMF formula has been calculated, are (a) two transverse waves and one longitudinal wave, (b) two longitudinal waves and one transverse wave, and (c) three transverse waves. The SEMF is a parametrically stimulated nonoscillating, exponentially temporally growing field of stimulated Brillouin scattering from a signal Alfven wave, a pump Alfven wave, and a signal sound wave. A second simultaneous resonance occurs only for weak nonlinearity and finite electrical conductivity, when the signal Alfven wave frequency equals the parametric frequency shift. This, being a slow process of transfer of plasma kinetic energy to field energy, can be a strong candidate for evolution of the field in plasma configurations of outer space.