New and detailed experimental and theoretical results concerning the prominent downshifted maximum (DM) feature in spectra of stimulated electromagnetic emissions are reported. The experimental results were obtained at the Sura ionospheric modification facility in Russia by transmitting a powerful high‐frequency ordinary mode pump wave into the ionospheric F region. We present detailed experimental results of the dependence of the DM on the pump frequency. Different frequency components of the DM have slightly different growth times after pump turn‐on and are suppressed in different pump frequency ranges at harmonics of the electron cyclotron frequency. The pump frequency range near the cyclotron harmonics in which the DM does not develop, decreases significantly with increasing harmonic, from several kilohertz at the fourth harmonic to an extremely narrow range of 0.2 kHz or less at the seventh harmonic. We discuss conditions for plasma wave propagation to explain this dependence on the cyclotron harmonics.

Experimental results concerning the spectrum of stimulated electromagnetic emissions (SEE) in the sidebands of a powerful high‐frequency electromagnetic ordinary mode pump wave are presented. The pump wave was vertically injected into the ionospheric F region from the Sura ionospheric modification facility in Russia. We report detailed measurements of the commonly observed continuum, downshifted maximum, and broad upshifted maximum emissions for pump frequencies ranging from the fourth to above the seventh electron cyclotron harmonic in the ionospheric plasma as well as observations of a new wideband emission occurring primarily in the upper sideband. The dependence of the SEE spectral structure on the pump frequency in relation to electron cyclotron harmonics is emphasized. All spectral features in the sidebands of the pump exhibit cyclotron harmonic effects.

[1] We present experimental results concerning the ponderomotive narrow continuum (NC p ) in stimulated electromagnetic emission spectra, generated in the ionospheric F region plasma by a powerful O mode electromagnetic wave. It is found that the generation of the NC p is determined by the development of the parametric decay instability in the initial stage of the interaction of the HF pump wave with the plasma. The dependence of the NC p temporal and spectral characteristics on pump power, pump frequency, time of day, and pump schedule as well as its gyrofeatures are considered. A possible mechanism for the NC p generation is discussed.

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We present measurements of stimulated electromagnetic emission (SEE) excited in the ionosphere by a powerful high-frequency ordinary mode radio wave vertically injected from a ground-based transmitter, at frequencies fo near high harmonics of the ionospheric electron cyclotron frequency f c . A prominent lower-sideband SEE feature was found to be absent in an extremely narrow fo range of 4/o//o~ 2x 10 ~5. This absorption resonance width allows an estimate of f c with an accuracy of a few tens of Hz, corresponding to a magnetic field magnitude accuracy of about 1 nT.