Parametric excitation of low-frequency circularly polarized Alfven wave

It is shown that a circularly polarized electromagnetic wave with frequency ω0 near the electron plasma frequency propagating along a d.c. magnetic field decays into a Langmuir wave and a circularly polarized Alfven wave. It is found that the pump and Alfven waves have the same helicity. Explicit expressions for the growth rate and threshold are obtained. (author)