Energy flux associated with a signal in a moving magnetized plasma

The energy exchange between an electromagnetic signal and a homogeneous plasma which is moving along a strong magnetic field is analyzed for the case in which the initial signal is specified in the plane parallel or perpendicular to the drift velocity. Expressions for the fields excited in the far zone are found by the stationary--phase method for the first of these cases. The solutions show that at a certain time the direction of the energy flux and the sign of the flux density reverse. The reason is that the fast Fourier components of the original signal reach the observation point before the slow components, for which the energy density is negative. (The ''fast'' components are those for which the phase velocity is higher than the drift velocity.) It is also shown that in the case of longitudinal propagation of perturbations excited by a quasimonochromatic source the average energy flux and the average energy density are zero in the slightly relativistic approximation. Finally, a study is made of electromagnetic waves whose energy changes sign over time in a nondispersive medium which is moving at a velocity faster than that of light