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.

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

Electromagnetic emission, stimulated by a powerful high frequency radio wave injected into the ionospheric F region, exhibits a rich spectral structure in the sidebands of the reflected pump wave. Results from such experiments at the Heating facility near Troms0, Norway, are presented. Spectral features in a steady state, a few seconds after the onset of the pump, are shown to depend on ionospheric conditions, the pump frequency, and power. Different types of stimulated radiation spectra may be obtained when the ionospheric critical frequency is well above or near the pump frequency. Further, the appearance of the spectral features is very sensitive to the pump frequency near harmonics of the F region electron cyclotron frequency. Our interpretation is that the considered stimulated electromagnetic emissions are generated primarily in two interaction regions, the plasma resonance region at the pump reflection height and the upper hybrid resonance region, typically a few kilometers below the reflection height.

In September 1983 a series of ionospheric modification experiments was performed in Scandinavia using the high‐frequency (HF) heating facility located near Tromsø, Norway. The experiments were designed to study the production of geomagnetic field‐aligned irregularities in the auroral E region by a powerful HF radio wave. In this initial report, observations of 3.2‐m irregularities made with a mobile 46.9‐MHz field radar are presented. When ionospheric conditions are correct, irregularities having peak cross sections of about 104 m² are excited in the E region over Tromsø. The present results are consistent with theoretical studies which indicate that it is easier to generate short‐scale field‐aligned irregularities at locations where the geomagnetic dip angle is large. When the HF‐induced radar echo at 46.9 MHz is strong, the e‐folding growth times of the echo power are typically between 50 and 150 ms. However, when the E region echo over Tromsø is weak, the growth period can range from seconds to tens of seconds. The irregularities generated by the HF wave often exhibit dynamic structure both in time and in space. In general, the observations provide experimental support for instability theories that employ interactions near the upper hybrid resonance to drive the artificial irregularities. The results also indicate that naturally occurring irregularities may at times play a key role in the excitation of HF‐induced irregularities in the auroral E region. Additional processes are probably required to explain the slower irregularity growth rates often observed when radar echoes are relatively weak and occasionally observed even for relatively strong signals.

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.