Induced sideband modes, due to stimulated electromagnetic emissions in the ionosphere, have been observed around the frequency of a high-intensity HF radio wave transmitted from the Heating ionospheric modification facility near Tromsø, Norway. The new observational technique used amounts to analyzing the received HF spectrum directly without utilizing any diagnostic radar. It appears that we have in this way been able to identify the parameteric decay instability and, possibly, the stimulated Brillouin backscattering of the strong HF pump wave. A series of narrow spectral peaks induced in the upper sideband by Heating transmission in extraordinary mode may indicate the excitation of ion Bernstein modes. At times, very strong spectral features, upshifted from the 5.4-MHz pump by typically 20–45 kHz, are observed. These features are still lacking a convincing theoretical explanation. By monitoring a few fixed frequencies in both the lower and the upper sidebands of the ionospherically reflected modifying wave, we have found that the intensity of the stimulated emissions exhibits a conspicuous long-term as well as short-term variability.
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