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A powerful HF wave, transmitted in the O mode with a frequency not exceeding the critical F region frequency, gives rise to secondary electromagnetic radiation, filling a frequency band of several 10 kHz around the frequency of the primary wave. The spectrum of these secondary waves is richly structured. The systematically occurring spectral features are identified and described. The majority of these features can be understood by scatter processes involving Langmuir waves and low‐frequency density perturbations excited by the parametric decay instability. Other features, including a broad spectral maximum at 20 to 40 kHz on the upshifted side, are not as yet fully understood.

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Observations of electromagnetic emission stimulated by a high-frequency radio wave injected into the ionosphere from a ground-based powerful transmitter operated near harmonics of the ionospheric electron cyclotron frequency are reported. Significant changes in the spectrum of the stimulated electromagnetic radiation were obtained as the injected frequency was varied in small steps around these harmonics. The experimental results are attributed to nonlinear wave interactions involving electrostatic wave modes perpendicular to the local geomagnetic field.PACS numbers: 52.35. Mw, 52.25.Sw, 94.20.Bb A powerful high-frequency (hf) electromagnetic (em) wave in the ordinary mode, launched from the ground into the ionosphere, stimulates secondary em radiation in the sidebands of the reflected primary wave. l~5 The spectra of these emissions depend on the ionospheric conditions as well as the frequency of the primary hf (pump) wave, /o, but exhibit in the general case a clear asymmetry as expected for parametric three-wave decay processes. If, however, /o is near a harmonic of the electron cyclotron frequency, f ce , in the F region of the ionosphere, the spectral structure of the stimulated electromagnetic emission (SEE) is different and strongly dependent on /o as described in this Letter.We present experimental results from the ionospheric modification facility Heating near Tromstf, Norway, obtained by varying the pump frequency in steps of 20 kHz between 5.343 and 5.483 MHz, which is near 4f ce . The pump wave was transmitted continuously for a few minutes on each frequency and the observed SEE spectra, as they appear a few seconds after the onset of the pump, persisted throughout this period. The effective radiated power of the vertically launched pump wave was 250 MW. The corresponding energy flux at 200-km altitude is approximately 0.5 mW/m 2 , neglecting ionospheric absorption. The angle between the geomagnetic field and the downward vertical is approximately 13°. Figures 1(a)-1(c) display three 200-kHz-wide spectra around the pump frequencies of 5.443, 5.403, and 5.383 MHz, respectively. In Fig. 1(a) two distinct features, the "downshifted maximum" (DM) and "broad upshifted maximum" (BUM), can be seen at A/DM« ~9 kHz and A/BUM « +35 kHz, respectively. The DM feature, which is commonly observed for a wide range of pump frequencies, 3 is absent in Fig. 1(b) and A/BUM «+15 kHz, whereas in Fig. 1(c) A/DM« -9 kHz and the BUM is absent. The strong spectral dependence on fo is typical and systematic and has been observed in several experiments with fo^nf ce , AI "3,4,5. As seen from