EXCITATION OF ELECTRON-CYCLOTRON HARMONIC-WAVES IN IONOSPHERIC MODIFICATION EXPERIMENTS

We investigate the resonance excitation of electrostatic as well as electromagnetic electron cyclotron harmonic waves by mode conversion during ionospheric F region modification experiments when the pump wave frequency is near harmonics of the electron cyclotron frequency. In the electrostatic case we calculate the bandwidth around each harmonic frequency within which excitation of electrostatic electron Bernstein waves is possible. For typical ionospheric parameters the bandwidth is found to be of the order of a few kilohertz which is in good agreement with the frequency dependence of the excitation and the quenching of the downshifted maximum (DM) feature in the recent modification experiments at Tromso (Norway) and Arecibo (Puerto Rico). In the electromagnetic case there exist frequency ranges in which the fast branch of the X mode pump wave can be directly converted into cyclotron harmonic waves. Excitation of the upper hybrid wave in the ionospheric E region by means of the fast X mode pump wave seems to be possible for certain parameter ranges. When the pump frequency is near the second harmonic, mode conversion is found to be enhanced in both the cases. Relevance of these results to the various spectral features like the DM, the broad upshifted maximum, and the broad symmetrical structures found in the stimulated electromagnetic emission from the ionosphere is pointed out.