Parametric Instabilities and Wave Coupling in Alfvén Simple Waves


G. M. Webb

LPL, University of Arizona, Tucson AZ 85721

M. Brio

Dept. of Mathematics, University of Arizona

A.R. Zakharian

ACMS,  University of Arizona

A wave coupling formalism for magnetohydrodynamic (MHD) waves in a non-uniform
background flow is used to study parametric instabilities of the large amplitude,
circularly polarized, simple plane Alfvén wave in one Cartesian space dimension.
The large amplitude Alfvén wave (the pump wave) is regarded as the background flow,
and the seven small amplitude MHD waves (the backward and forward fast and slow
magnetoacoustic waves; the backward and forward Alfvén waves and the entropy wave)
interact with the pump wave via wave coupling coefficients which depend on the gradients
and time dependence of the background flow. The dispersion equation for the waves
D(k,\omega)=0 obtained from the wave coupling equations reduces to that obtained
by previous authors. The general solution of the initial value problem for the waves is
obtained by Fourier and Laplace transforms. The dispersion function D(k,\omega) is a fifth order
polynomial in both the wave number k and the frequency \omega. The regions of
instability and the neutral stability curves are determined.
The instabilities depend parametrically on the pump wave amplitude and on the plasma beta.
The wave interaction equations are also studied from the perspective of a single master
wave equation, with multiple wave modes, and with a source term due to the entropy wave.
The wave hierarchies for short and long wavelength waves of the master wave equation
are used to discuss wave stability. By expanding the dispersion equation for the different
long wavelength eigenmodes about k=0, yields either the linearized Korteweg deVries
equation or the Schröidinger equation as the generic wave equation at long wavelengths.
The equations exhibit an absolute instability for hump or pulse-like
initial data. Some preliminary results on envelope equations are discussed.




Last modified: Thu Jan 18 13:55:17 MST 2001