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.

The broad up-shifted maximum (BUM) is one of the most prominent stimulated electromagnetic emission features and has been the subject of intensive investigation in past ionospheric modification experiments. The spectral properties representing the BUM have been regarded as belonging to one uniform feature. Here we present experimental evidence that the BUM actually consists of two separate components, and we elaborate their characteristic properties. [S0031-9007(98)06868-9] PACS numbers: 94.20.Bb, 52.25.Sw, 52.35.Ra, 52.40.Db Since the first observations of stimulated electromagnetic emissions (SEE), excited in the ionospheric F region plasma by a powerful hf electromagnetic wave [1], the investigation of their features has become one of the leading methods to study the properties of artificial turbulence in the ionospheric plasma. The physical strength of the SEE phenomenon has its origin in the compound structure of the spectra and in the individual temporal evolutions of different spectral components, reflecting the development of and competition between various wave interaction processes, from their initial growth to their nonlinear saturation. These experiments, although performed in a geophysical environment, have the character of pure plasma physics experiments, and their results are transferable to other than ionospheric plasmas.Recently, considerable attention has been focused on investigations of the SEE features when the pump wave frequency f 0 is close to a harmonic of the electronic cyclotron frequency, nf ce . The experiments performed have shown that in a narrow frequency band for f 0 Ӎ nf ce , the SEE features are very sensitive to the pump frequency offset from nf ce , df ϵ f 0 2 nf ce [2][3][4][5][6][7]. The main results obtained in these experiments are a weakening and quenching of the down-shifted maximum (DM) in the SEE spectra when f 0 is very close to nf ce , and the appearance of a broad up-shifted maximum (BUM) when f 0 is slightly higher than nf ce . The DM is a spectral maximum occurring at a frequency offset of approximately 210 kHz from the pump frequency. The BUM is a SEE feature that exists on the up-shifted side and may reach out to 200 kHz above the pump frequency. The suppression of the DM has been proposed to occur when the pump frequency coincides exactly with a harmonic of the electron cyclotron frequency in the upper hybrid resonance region and results from strong cyclotron damping of plasma waves. The narrowness of the DM resonance absorption provides a possibility to determine experimentally the magnitude of the gyroharmonic frequency with an accuracy of a few kHz [5]. Investigations of the BUM features have revealed that the frequency of the BUM peak intensity ͑ f BUM ͒ versus pump frequency closely follows the relation f BUM 2f 0 2 nf ce or Df BUM df, where Df BUM is the shift of the BUM spectral peak from f 0 [2]. This has been taken as a hint that the BUM might be generated through a four-wave interaction process, involving two pump photons (or upper hybrid plasmons)...

Abstract. Steady state spectral features of stimulated electromagnetic emissions (SEEs) for their major emission components (DM, NC, BC, BUM, and BUS) are studied in a wide pump wave frequency range, from 4.3 to 9.5 MHz, i.e., from slightly above the 3 rd to slightly above the 7 th gyroharmonic frequency. Based on these systematic experimental data, new peculiarities in the behaviour of the SEE intensity and of the spectral properties, in relation to the gyroharmonic mode number, have been found. The experimental results, discussed in the paper, were collected during the years 1996-2000 at the Sura heating fa,cility in Russia by modification of the ionospheric F region, using ordinary mode HF pump waves.

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