The energy injection into waves with a zero group velocity

The frequency maxima of electron cyclotron harmonic (ECH) waves provide strong responses to sounding in various plasma regimes in the solar system. The frequency maxima correspond to waves for which the group velocity and thus the energy propagation velocity in the plasma frame of reference is zero. A particle-in-cell (PIC) code is employed to show that the propagation of wave energy at a non-zero velocity, necessary to couple energy from a stationary antenna to the plasma, is accomplished by propagating wave precursors. The undamped waves at the frequency maxima of the ECH branches are nonpropagating hence the waves remain localized. It is demonstrated that the nonpropagating waves, built up by the wave precursors, are standing waves. The standing wave generation is followed from the linear to nonlinear regimes. For nonlinear emission amplitudes the emission causes a plasma density depletion close to the antenna. The depletion is shown to trigger a modulational instability in which the ECH wave collapses. The generated nonlinear standing wave also develops an electromagnetic component which couples the electrostatic ECH waves to the fast extraordinary wave. (C) 1999 American Institute of Physics. [S1070-664X(99)00707-7]