Abstract
By Whitham's method of averaged Lagrangian developed by Dysthe, two coupled nonlinear Schrödiriger equations are obtained for the evolution of the amplitude of two plane polarized transverse waves in a cold plasma, both propagating in the same direction. Relativistic corrections are included and amplitude-dependent frequency shifts are derived. It is found that two transverse waves are modulationally unstable. The relativistic terms play an important part in this instability.

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