PARAMETRIC-INSTABILITY OF A LARGE-AMPLITUDE NONMONOCHROMATIC ALFVEN-WAVE

The parametric instability of a finite-amplitude Alfven wave is studie d in a one-dimensional geometry. The pump wave is an exact solution of the nonlinear magnetohydrodynamic (MHD) equations, i.e., the magnetic held perturbation has a uniform intensity and rotates in the plane pe rpendicular to the propagation direction, but its Fourier spectrum con tains several wavelengths. The weakly nonmonochromatic regime is first studied by an analytical approach. It is shown that the growth rate o f the instability decreases quadratically with a parameter that measur es the departure from the monochromatic case. The fully nonmonochromat ic case is studied by numerically solving the instability equations, w hen the phase function of the pump wave has a power-law spectrum. Thou gh the growth rate is maximum in the monochromatic case, it remains of the same order of magnitude also for wide spectrum pump waves. For qu asimonochromatic waves the correction to the growth rate depends only on the spectral index of the phase function. (C) 1996 American Institu te of Physics.