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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. Optical emissions excited by high-power radio waves in the ionosphere can be used to measure a wide variety of parameters in the thermosphere. Powerful highfrequency (HF) radio waves produce energetic electrons in the region where the waves reflect in the F region. These hot or suprathermal electrons collide with atomic oxygen atoms to produce localized regions of metastable O(1D) and O(1S) atoms. These metastables subsequently radiate 630.0 and 557.7 nm, respectively, to produce clouds of HF pumped artificial airglow (HPAA). The shapes of the HPAA clouds are determined by the structure of large-scale (•10 km) plasma irregularities that occur naturally or that develop during ionospheric heating. When the HF wave is operated continuously, the motion of the airglow clouds follows the E x B drift of the plasma. When the HF wave is turned off, the airglow clouds decay by collisional quenching and radiation, expand by neutral diffusion, and drift in response to neutral winds. Images of HPAA clouds, obtained using both continuous and stepped radio wave transmissions, are processed to yield the electric fields, neutral wind vectors, and diffusion coefficients in the upper atmosphere. This technique is illustrated using data that were obtained in March 1993 Paper number 1999JA000366.0148-0227/00/1999JA000366 $ 09.00 been employed. This paper outlines the use of optical emissions produced artificially by high-power radio waves for determining many properties of the upper atmosphere.High-power radio waves that impinge on the ionosphere can produce plasma irregularities and enhanced optical emissions that can be employed for remote sensing of the upper atmosphere. Artificial plasma structures take the form of (1) Artificial ionospheric irregularities and airglow clouds can be used as tracers to study portions of the upper atmosphere and ionosphere. Both radio and optical techniques have been 10,657

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