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

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. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 461–476, June 2008.     

Study of large-scale irregularities generated in the ionosphericF-region by high-power HF waves

Experimental studies of the features of artificial ionospheric turbulence was performed at the “Sura” heating facility in August 1998 using numerous diagnostic tools, such as scintillation, chirp-sounding, backscattering, and stimulated electromagnetic emission (SEE) measurements, as well as sounding a HF-disturbed volume (DV) by probing waves. It has been found that generation of strong artificial large-scale irregularities (ALSIs), which manifest themselves through the F-spread on ionograms, scintillations of the satellite signal propagated through the DV, and amplitude fluctuations of the probing wave sounding the DV, is observed not only for an overdense heating, at fo≤foF₂, but also at higher frequencies fo>foF₂≥f _uh (here fo is the pump-wave frequency, foF₂ is the critical frequency of the F₂-layer for O-mode electromagnetic wave, and f _uh is the plasma frequency at the upper-hybrid resonance height). This means that transfer of the pump-wave energy in the plasma due to the development of thermal parametric (resonance) instability, rather than thermal self-focussing instability, plays the key role in the ALSI generation in the case where the O-mode HF wave is used for the overdense heating. This conclusion is also confirmed by the fact that the ALSI generation is suppressed in the gyroharmonic frequency range, which is similar to the well-studied quenching of the downshifted maximum (DM) in SEE spectra. In this paper, we discuss new ALSI features revealed by the measurements, as well as the limits by which one can control the ALSI spectrum using complex pumping schemes. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 6, pp. 497–519, June, 2000.     

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

Alternative mechanisms of generation of the stimulated electromagnetic emission (SEE) excited in the ionosphere by high-power radio waves are analyzed on the basis of measurements of the SEE spectra obtained during the pump-wave frequency sweeping near the forth (n = 4) and fifth (n = 5) harmonics of the electron gyrofrequency nf_ce [1] and their comparison with the existing physical models. A method for determination of the magnetic field strength and plasma density near the double-resonance region in the ionosphere is developed. It is shown that the generation of the broad upshifted maximum (BUM) feature in the SEE spectrum should occur several kilometers below the double-resonance altitude. A role of high-frequency plasma modes and small-scale magnetic field-aligned irregularities, excited under ionosphere pumping by a high-power radio wave, in the formation of SEE spectra is demonstrated. It is shown that the difference in the emission intensities for f₀ ≲ nf_ce and f₀ > nf_ce is related to different regions (altitudes) at which the plasma waves exist in these cases. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 553–570, July 2008.     

Features of Excitation of Stimulated Electromagnetic Emission of the Ionosphere Modified by an Oblique High-Power Radio Wave

We present the results of measuring the characteristics of the stimulated electromagnetic emission (SEE) of the ionosphere with variation in the zenith angle of a pump beam of high-power O-mode radio waves in the geomagnetic-meridian plane. The experiments were performed at the midlatitude heating facility “Sura.” It is established that the maximum intensity of the DM and BC components of SEE is observed for southward inclination angles θ ≈ 8°--12° of the antenna beam, for which the most intense generation of artificial small-scale ionospheric irregularities also takes place. Based on the results of measurements near the fourth and fifth harmonics of the electron gyrofrequency, it is found that the first component of the BUM (BUM-1) is generated only when the pump wave reaches the plasma-resonance region. This allows one to assume that, unlike the second component of the BUM (BUM-2), whose generation is determined by development of instability in the upper-hybrid resonance region, the BUM-1 generation mechanism should be related to processes of interaction between a high-power radio wave and the plasma in the plasma-resonance region. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 9, pp. 743–756, September 2005.     

The polar-ionosphere phenomena induced by high-power radio waves from the spear heating facility

We present the results of experimental studies of specific features in the behavior of small-scale artificial field-aligned irregularities (AFAIs) and the DM component in the spectra of stimulated electromagnetic emission (SEE). Analysis of experimental data shows that AFAIs in the polar ionosphere are generated under different background geophysical conditions (season, local time, the presence of sporadic layers in the E region, etc.). It is shown that AFAIs can be excited not only in the F region, but also in “thick” sporadic E _s layers of the polar ionosphere. The AFAIs were observed in some cycles of heating when the HF heater frequency exceeded the critical frequency by 0.3–0.5 MHz. Propagation paths of diagnostic HF radio waves scattered by AFAIs were modelled for geophysical conditions prevailing during the SPEAR heating experiments. Two components, namely, a narrow-banded one with a Doppler-spectrum width of up to 2 Hz and a broadband one observed in a band of up to 20 Hz, were found in the sporadic E _s layer during the AFAI excitation. Analysis of the SEE spectra shows that the behavior of the DM component in time is irregular, which is possibly due to strong variations in the critical frequency of the F ₂ layer from 3.5 to 4.6 MHz. An interesting feature observed in the SPEAR heating experiments is that the generation of the DM component was similar to the excitation of AFAIs when the heater frequency was up to 0.5 MHz higher than the critical frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 939–950, November 2008.     

Experimental Studies of the Effects Observed During the Nonlinear Interaction of Two High-Power Radio Waves in a Magnetoplasma

We present the results of experiments on modification of the ionospheric F region by two high-power (P_eff 20 MW) O-mode electromagnetic waves. The experiments were performed at the Sura heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia) in May 23–27, 1993 at the pump frequencies near the 4th, 5th, or 6th harmonics of the electron gyrofrequency. Ionospheric perturbations were diagnosed by measuring the stationary spectral characteristics of the stimulated electromagnetic emission (SEE) of the ionospheric plasma. We determine the features of variation in the spectral characteristics of particular SEE components during the simultaneous heating of the ionospheric plasma by two radio waves in comparison with the case of a monochromatic pump wave. We observed the effect of enhanced generation of the broad up-shifted maximum (BUM) by the higher-frequency pump wave. This is accompanied by strong suppression of the BUM induced by the lower-frequency pump wave. It is shown that the effects observed during the two-frequency heating of the ionosphere have well pronounced gyroharmonic properties, i.e., depend on both the electron-gyroharmonic number and the frequency detuning of the pump waves from a harmonic of the electron gyrofrequency. We also pointed out that a change in the properties of artificial small-scale ionospheric irregularities (striations) excited by high-power radio waves is not the cause of a change in the properties of the down-shifted maximum and the BUM during the two-frequency modification of the ionospheric plasma. Ways for the further development of these studies are discussed.__________Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 2, pp. 110–133, February 2005.     

On the study of artificial ionospheric turbulence by means of stimulated electromagnetic emission

Results of measurements of the characteristics of stimulated electromagnetic emission induced in ionospheric plasma by pulsed high-power radio waves (diagnostic emission) presented. The dependences of the properties of diagnostic emission on the parameters of the diagnostic wave, ionospheric conditions, and the level of development of artificial ionospheric turbulence are determined, and criteria are given for selection of optimal diagnostic conditions for ionospheric-plasma sounding. Results of experiments on the sounding of artificial ionospheric turbulence by means of diagnostic emission are provided. It is established that with transition from daytime to evening conditions, the characteristic times of emission decay are sharply increased. This is attributed to an increase in the natural perturbation of the F-region.Scientific-Research Radio-Physics Institute, Nizhny Novgord. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 7, pp. 909–928, July, 1994.     

Stimulated raman scattering from plasma modes in magnetoactive semiconductors

Stimulated scattering off electron plasma mode is investigated analytically for the case when the pump wave is an intense circularly polarised electromagnetic wave propagating parallel to a homogeneous dc magnetic field in an isotropic semiconductor-plasma. The threshold electric field of the pump necessary for the stimulated Raman scattering and the growth rate of the parametrically unstable mode have been obtained for two cases (i)B ₀=0 and (ii) B₀ ≠ 0. It is seen that the magnetic field does not significantly affect the threshold electric field as well as the growth rate provided the cyclotron frequency is small compared to the frequency of the pump wave. The threshold conditions are also found to be insensitive to the electron thermal velocity.     

First observations of stimulated electromagnetic emission in the ionosphere modified by the spear heating facility on Spitsbergen

We present the first results of observations of the stimulated electromagnetic emission (SEE) in the ionosphere modified by the Space Plasma Exploration by Active Radar (SPEAR) heating facility. Observation of the SEE is the key method of ground-based diagnostics of the ionospheric plasma disturbances due to high-power HF radiation. The presented results were obtained during the heating campaign performed at the SPEAR facility in February–March 2007. Prominent SEE special features were observed in periods in which the critical frequency of the F ₂ layer was higher than the pump-wave frequency (4.45 MHz). As an example, such special features as the downshifted maximum and the broad continuum in the region of negative detunings from the pump-wave frequency are presented. Observations clearly demonstrate that the ionosphere was efficiently excited by the SPEAR heating facility despite the comparatively low pump-wave power. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 951–955, November 2008.     

Influence of small-scale irregularities on features of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part II. Relaxation stage

We present the results of experimental studies of the evolution of diagnostic stimulated electromagnetic emission of the ionosphere (DSEE) in a broad range of the pump and probing wave frequencies under sounding of the F-layer disturbed by powerful HF radio waves. We study the parameters of the overshoot effect in SEE evolution during the relaxation stage of artificial small-scale irregularities and show that the amplification of DSEE (inverse overshoot effect) is related to the relaxation of the anomalous attenuation due to scattering at these irregularities. We have found that the characteristic time scales of development of the overshoot effect decrease and its value increases as the pump wave frequency decreases from 6 to 4 MHz and as the probed volume moves from the edge to the central part of the disturbed volume. We relate the effects observed to the increasing intensity of the irregularities with scales l_∼∼3−10 m. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 810–824, August 1999.     

Gyroharmonic properties of the middle-scale artificial ionospheric turbulence during heating of the ionospheric F 2 layer by a high-power O-mode radio wave

We present the results of studying the properties of artificial F-spread that appears on ionograms during heating of the ionospheric F₂ region by a high-power O-mode radio wave. It is shown that the regions of resonant interaction of a high-power radio wave with plasma, where the pump-wave energy is almost totally absorbed and the plasma is subject to intense heating, affect significantly the development of a self-focusing instability of a high-power radio wave and the generation of middle-scale (with characteristic scales across the magnetic field l_⊥ ≈ 0.4–1.5 km) artifical ionospheric inhomogeneities. It is established that the intensity of such inhomogeneities depends on the sign and magnitude of detuning of the pump-wave frequency with respect to the electron gyroresonance harmonic frequency Δf = f_PW − nf_ce and has the minimum value for Δf ≈ −20 kHz, thus demonstrating the asymmetry of the gyroharmonic properties of their excitation mechanism. Relationship between the observed phenomena and known characteristics of the artificial ionospheric turbulence for f_PW ≈ nf_ce is analyzed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 5, pp. 367–375, May 2008.     

Superradiant and stimulated superradiant emission in a prebunched beam free-electron maser

An electron beam, prebunched at the synchronous free-electron laser frequency and passing through a magnetic undulator, emits coherent (superradiant) synchrotron undulator radiation at the bunching frequency. If an external electromagnetic wave is introduced into the interaction region, at the same frequency and at a proper phase, the radiation process will be stimulated (stimulated prebunched beam radiation). We report first experimental measurements of stimulated superradiant emission in a prebunched free-electron maser. Measurements are in good agreement with theory.     

Analysis of Gain Volume in Random Laser in a Spherical Multiple Scattering Medium

We performed numerical simulation of a random laser in a spherical multiple scattering medium in order to examine the saturation absorption in the excitation process and the resultant increasing process of the gain volume under the random laser action. The size of the gain volume increases with 1/3 power of the excitation pulse energy in the weak excitation region, and then approaches a critical value L_th when the excitation pulse energy is increased above the threshold. The dependence of L_th, as well as the dependence of the threshold excitation pulse energy on the dye concentration, transport mean free path, stimulated emission cross section and excited state lifetime were also examined. These results of the simulation well correspond to the experimental results by a two-beam spatial-correlation method.     

On specific features of diurnal variations in characteristics of the diagnostic stimulated electromagnetic emission and their relation to the evolution of artificial ionospheric irregularities

We analyze variations in characteristics of the diagnostic stimulated electromagnetic emission of the ionosphere in the evening hours including the times of sunset both on the Earth’s surface at the observation point and in the ionosphere over it. It is found that an increase in typical times of evolution of the diagnostic emission begins to be recorded just before the sunset on the Earth’s surface when the ionosphere is illuminated and its parameters are not significantly changed yet. We state that the typical times of evolution of the diagnostic emission increase when the pump-wave frequency approaches the critical frequency of the ionospheric F₂ layer, but such an effect is not as significant as when passing from the illuminated to the unilluminated ionosphere. It is established that at the stage of diagnostic sounding the pump-wave pulse power does not exert any notable influence on the first (fast) stage of relaxation of small-scale artificial ionospheric irregularities, but can increase the decay time of the irregularities at the second (slow) stage of relaxation. Capabilities of the method for a study of artificial plasma turbulence using the diagnostic stimulated electromagnetic emission are discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 4, pp. 273–286, April 2008.     

Theoretical and experimental studies on one-dimensional magnetic systems

The present situation in theoretical and experimental studies on one-dimensional magnetic systems is fully discussed. Equal-time as well as the dynamic properties are included with an emphasis on the latter. Four model systems are examined in detail: TMMC (Heisenberg antiferromagnet), CsNiF₃ (planar ferromagnet), CoCl₂. 2NC₅H₅ (Ising ferromagnet), CuCl₂. 2NC₅H₅ (Heisenberg antiferromagnet with S = ½). The equal-time properties are quite well understood in theory and in experiment but the dynamical properties much less so. The open questions and possible investigations for the future are discussed.     

Determinations of the transient thermal lensing effect in metal cluster Polymer {WS4Cu4I2(bpe)3}n solution by the use of the Z-scan

The nonlinear optical (NLO) properties of a novel cluster Polymer {WS₄Cu₄I₂(bpe)₃}_n solution are studied by using Z-scan technique with laser pulses of 4.5 ns pulse-width at a wavelength of 532 nm. The results show that the cluster solution possesses strong nonlinear absorption and refraction. Nonlinear refraction of the cluster is composed of third-order nonlinear refraction and transient thermal effect. The thermal effect is mainly due to the strong nonlinear absorption. Numerical simulations obtained by solving simultaneously photo-acoustic and electromagnetic wave equations, agrees basically with experimental results.     

Tuning properties of NC3H quantum dot by adsorption of ammonia and carbon dioxide

Transport properties of NC₃H quantum dot by adsorption of ammonia and carbon dioxide are investigated with an ab initio method combined with a non-equilibrium Green function method. The effects of different configurations of ammonia, size of NC₃H surface and position where ammonia and carbon dioxide had been adsorbed onto NC₃H quantum dot on transport properties are revealed. In comparison with NC₃H quantum dot device, results show that the adsorption of ammonia molecule on NC₃H quantum dot in one configuration enhances the conductance of device, while adsorption in another configuration reduces the conductance. The size of NC₃H significantly altered the transport properties in both NC₃H–NH₃ and NC₃H–CO₂ system. The position of adsorption of ammonia displays obvious change on transport properties while the effects of position of carbon dioxide on transport properties are negligible.     

等离子体中电磁波传输特性理论与实验研究

本文对35 GHz和96 GHz电磁波在等离子体中的传输特性进行了理论与实验研究, 得到了电磁波衰减随等离子体密度、碰撞频率和电磁波频率的变化规律. 等离子体密度增加一个数量级时, 电磁波衰减增加一个数量级; 随着等离子体碰撞频率的增加, 电磁波衰减先增加后减小; 随着电磁波频率的增加, 衰减下降. 以激波管为实验平台进行了电磁波在等离子体中传输特性的实验研究, 实验结果和理论结果吻合较好. 理论和实验结果均表明, 提高电磁波频率是解决黑障问题的有效途径.     

Nonlinear mixing of oppositely travelling surface plasmons

A theoretical treatment of the nonlinear mixing of two contra-directional surface plasmons of frequency ω₁ and ω₂ propagating on a semi-infinite metal surface is presented. The nonlinear interaction is analysed in terms of (a) nonlinear surface currents due to the breaking of inversion symmetry and the rapid variation of the normal electric field component at the surface, and (b) the nonlinear response of the electron gas in the bulk of the metal. These currents are treated as source terms for Maxwell's equations and electromagnetic fields are found which satisfy the driven wave equation and boundary conditions at the frequencies ω₁ ± ω₂. At the difference frequency ω₁ ? ω₂ the solution fields decay exponentially into both the air and the metal. However, at the sum frequency ω₁ + ω₂, coupling to transverse electromagnetic waves in both the air and the metal is predicted under appropriate wavevector and frequency conditions. The free-space radiation field is treated in detail and the feasibility of its experimental detection is discussed. No coupling to longitudinal bulk plasmons is predicted in this model of the nonlinear interaction.     

On the frequency dependence of anomalous attenuation of sounding waves in the case of ionospheric modification by high-power radio emission

The dependence of anomalous attenuation of sounding waves on the frequency f is studied experimentally in the case of ionospheric F-layer modification by high-power radio emission. It is shown that anomalous attenuation increases for |f−f₀| < 20 kHz, f<f₀ (f₀ is the pump-wave frequency) and decreases for f>-f₀ compared with the frequencies that are more distant from f₀. This phenomenon is caused by plasma displacement and formation of a stepped electron-density profile in the region of upper hybrid resonance of the pump wave. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 8, pp. 966–977, August, 1998.