[1] In recent years, large electron temperature increases of 300% (3000 K above background) caused by powerful HF-radio wave injection have been observed during nighttime using the EISCAT incoherent scatter radar near Tromsø in northern Norway. In a case study we examine the spatial structure of the modified region. The electron heating is accompanied by ion heating of about 100 degrees and magnetic field-aligned measurements show ion outflows increasing with height up to 300 m s À1 at 582 km. The electron density decreases by up to 20%. When the radar antenna was scanned between three elevations from near field-aligned to vertical, the strongest heating effects were always obtained in the field-aligned position. When the HF-pump beam was scanned between the same three positions, the heating was still almost always strongest in the field-aligned direction. Simultaneous images of the 630 nm O( 1 D) line in the radio-induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field-aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field-aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field-aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio-induced aurora suggests that a common mechanism, upper-hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self-focusing on field-aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important.

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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

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.