NONLINEAR EXCITATION OF LANGMUIR AND ALFVEN WAVES BY AURORAL WHISTLERWAVES IN THE PLANETARY MAGNETOSPHERE

A new excitation mechanism of auroral Langmuir and Alfven waves in the planetary magnetosphere is proposed. It is shown that a large-amplitu de electromagnetic whistler wave propagating along the magnetic field lines can nonlinearly generate Langmuir and Alfven waves through three -wave parametric instabilities, provided the interacting waves satisfy the phase matching and wave helicity conservation conditions. The pro posed mechanism can account for the observation of Langmuir-Alfven-whi stler (LAW) auroral events in the Earth's magnetosphere. It is suggest ed that similar parametric processes are likely to occur in other fiel d-aligned current regions of astrophysical plasmas wherein the electro n plasma frequency is of the order of or less than the electron cyclot ron frequency.