DOWNSHIFTED MAXIMUM FEATURES IN STIMULATED ELECTROMAGNETIC EMISSION-SPECTRA

New and detailed experimental and theoretical results concerning the p rominent downshifted maximum (DM) feature in spectra of stimulated ele ctromagnetic emissions are reported. The experimental results were obt ained at the Sura ionospheric modification facility in Russia by trans mitting a powerful high-frequency ordinary mode pump wave into the ion ospheric F region. We present detailed experimental results of the dep endence of the DM on the pump frequency. Different frequency component s of the DM have slightly different growth times after pump turn-on an d are suppressed in different pump frequency ranges at harmonics of th e electron cyclotron frequency. The pump frequency range near the cycl otron harmonics in which the DM does not develop, decreases significan tly with increasing harmonic, from several kilohertz at the fourth har monic to an extremely narrow range of 0.2 kHz or less at the seventh h armonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.