Trapped particle effect on the velocity of circularly polarized electromagnetic waves in an isotropic plasma

An exact solution of Maxwell–Vlasov equations describing a stationary transverse wave with trapped charged particles is analyzed. In the absence of a background plasma, the phase velocity of such a wave is always less than speed of light. Physical structure of the wave looks most simply in the wave frame of reference. In this frame, the constructed solution corresponds to one of the simplest force-free equilibrium plasma configurations of the type of uniformly sheared current sheets and self-consistent magnetostatic field. In the presence of the background plasma, a simple condition for existence of the waves with sublight velocities is established.