NONLINEAR COUPLING OF MHD WAVES IN INHOMOGENEOUS STEADY FLOWS

The nonlinear coupling of MHD waves in a cold (beta = 0) compressible plasma with a smoothly inhomogeneous low-speed steady flow directed al ong the magnetic field is considered. The effect is similar to Alfven wave phase mixing in a static, inhomogeneous medium and leads to the p roduction of steep transversal gradients in the plasma parameters, whi ch increases dissipation. Transversal gradients in the total pressure, produced by phase mixing, lead to the secular generation of obliquely propagating fast magnetosonic waves, at double the frequency and the wavenumber of the source Alfven waves. The efficiency of the generatio n is defined by the Alfven wave amplitude and the transversal spatial scale of the flow inhomogeneity. The secular growth of density perturb ations, connected with fast waves, takes place for flow speeds that ar e considerably below the thresholds of the Kelvin-Helmholtz and negati ve energy wave instabilities. The initial stage of the nonlinear gener ation of the fast waves is considered analytically and illustrated by numerical simulations.