Spectra of Stimulated Electromagnetic Emission of the Ionosphere Upon Sweeping of the Pump Wave Frequency Near Gyroharmonics. I. Experimental Results

Abstract

We present the results of experimental studies of the spectra of the stimulated electromagnetic emission excited in the ionosphere by powerful radio waves during the pump wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron cyclotron frequency nf ce . The frequency sweep was carried out for long (continuous) pumping in vertical and inclined directions (at 14 • and 18 • south of the zenith), as well as for the pulse diagnostic wave both with and without additional pumping far from the gyroharmonics. The dependences of the spectral features of the stimulated electromagnetic emission on the ratio between the pump-wave frequency f 0 (or on the diagnostic-wave frequency f DW ) and nf ce were analyzed. It is found that near the multiple gyroresonance, different spectral features of the stimulated emission are quenched at the same frequency for different pump-wave frequencies. For a sufficiently large inclination of the pump wave beam from the vertical direction, the intensity of the stimulated electromagnetic emission is notably decreased for f 0 nf ce as compared with f 0 > nf ce .

Spectra of Stimulated Electromagnetic Emission of the Ionosphere Sweeping of the Pump Wave Frequency Near Gyroharmonics. II. Discussion of the Results

“…The parameters for these calculations were chosen typical of the ionospheric plasma, namely, the electron gyrofrequency and the electron-density gradient correspond to the experimental conditions of September 18, 2004; see Sec. 3.3 in [1]). The quantity kρ e (z) was calculated from the more exact dispersion relation…”

“…In our recent paper [1], we described the techniques and presented the results of studying spectral properties of the stimulated electromagnetic emission (SEE) of the ionosphere in the case of fast sweeping of the pump-wave frequency f 0 and diagnostic-wave frequency f DW near the fourth (n = 4) and fifth (n = 5) harmonics of the electron gyrofrequency nf ce , performed in 2004 at the "Sura" facility. Recall that the SEE is the noise-like low-intensity component in the spectrum of the pump-wave signal reflected from the ionosphere.…”

“…The developed methods and the use of a fast wideband receiver for the reception and detection of SEE allowed us to perform the measurements fairly rapidly and minimize the influence of both natural variations in the ionospheric conditions (first of all, the altitude of the interaction of the pump wave with the ionospheric plasma and, hence, the value of f ce ) and variations in the intensity of the small-scale field-aligned inhomogeneities, on the SEE properties. In [1], the following SEE spectral features showed in Fig. 1 were studied: the downshifted maximum (DM) and its sideband 2DM, the upshifted maximum (UM) located in the spectrum almost symmetrically to the DM with respect to the pump-wave frequency, the narrow continuum (NC), i.e., both the ponderomotive narrow continuum (NC p ) at the initial stage of pumping and the thermal narrow continuum (NC t ) for the long-term pumping, the broad continuum (BC), and the broad upshifted maximum (BUM).…”

“…The BUM exists for f 0 f 0 DM and, according to [3], has the two-component structure, i.e., BUM1 with the intensity peak shifted by ∆f BUM1 ≈ 12-20 kHz with respect to the pump-wave frequency in the pump-wave frequency range f 0 f 0 DM + (20-25) kHz, and BUM2 for f 0 f 0 DM + (20)(21)(22)(23)(24)(25) kHz with the intensity-peak shift linearly increasing with f 0 according to Eq. (6) in [1]. Hereafter, we use the same notations as in [1], namely, f 0 DM , f 0 UM , f 0 NC , and f DM , f UM , f NC for the pump-wave and SEE frequencies, respectively, at which the corresponding SEE components undergo the maximum suppression when passing through the gyroharmonic, and f 0min for the pump-wave frequency at which the minimum total SEE intensity is observed.…”

“…(6) in [1]. Hereafter, we use the same notations as in [1], namely, f 0 DM , f 0 UM , f 0 NC , and f DM , f UM , f NC for the pump-wave and SEE frequencies, respectively, at which the corresponding SEE components undergo the maximum suppression when passing through the gyroharmonic, and f 0min for the pump-wave frequency at which the minimum total SEE intensity is observed. In this paper, the experimental results presented in [1] are discussed on the basis of existing physical models of the processes of interaction of high-power HF radio waves with the ionosphere and generation of different SEE components.…”