Parametric amplification of Alfven waves

Parametric excitation of Alfven waves was predicted by Montgomery and Harding (1966). They showed that torsional Alfven waves of frequency omega are excited in a magnetised plasma by sinusoidally perturbing the external magnetic field at a frequency of 2 omega . This paper presents some of the theory for, and reports the observation of, the amplification of Alfven waves launched into a perturbation region. This effect was found to depend on the relative frequencies of the wave and the perturbation field, on the strength of the perturbation field, and on the phase of the wave entering the perturbation region.