Modulational stability of two electrostatic waves in a warm isotropic plasma

The modulational stability of two electrostatic waves propagating in a warm isotropic plasma is studied. The method of the averaged Lagrangian is employed, with a kinetic formulation. Coupled nonlinear Schroedinger equations describing the slow space-time evolution of the amplitudes of the interacting waves are derived. The resulting nonlinear dispersion relation is analysed and regimes where the modulational instability occurs are identified. The paper concludes with a brief examination of solitary-wave solutions. (orig.)