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.     

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.     

Excitation of short-wave oscillations in the ionospheric plasma by the field of a high-power radio wave with extraordinary polarization due to induced scattering by ions

We analyze excitation of electron-cyclotron or upper hybrid oscillations of the ionospheric plasma at a frequency that is close to the pump frequency as a result of induced scattering of a high-power radio wave with extraordinary polarization by ions. The excited oscillations have a small wavelength of the order of the Larmor electron radius, which allows them to propagate near and below the reflection level of an extraordinary radio wave. We found the instability increment and threshold field, which results from collisional absorption of plasma waves. It is shown that the threshold field is minimal near the reflection level of an extraordinary radio wave when the radio wave frequency f₀ is between electron harmonics nf_Be with n≥2. In an ionospheric F layer it is of the order of 1 W/m. Such fields are easily obtained in ionospheric heating experiments allowing for radio-wave field swelling in the reflection region. In the vicinity of electron harmonics f_o≅nf_Be, the threshold field is increasing. For f_o<nf_Be with f_o≅nf_Be the instability does not develop because of the absence of plasma oscillations with a frequency that is close to the pump frequency (the latter also refers to the case f_o<2f_Be). The expressions obtained are generalized to the case in which the instability under consideration is excited by the field of an ordinary radio wave in the region of its quasilongitudinal propagation. We discuss the possibility of using the emergence of very short-wave plasma oscillations for explaining the experimentally observed phenomena. Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow Region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 5, pp. 541–560, May, 1997.     

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.     

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.     

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.     

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.     

Artificial ionosphere layers for pumping-wave frequencies near the fourth electron gyroharmonic in experiments at the HAARP facility

In this paper we consider the action (in the magnetic-zenith direction) of powerful high frequency (HF) radiation of ordinary polarization on the ionosphere F region. We deal with frequencies f ₀ > 4f _ce (f _ce is the electron cyclotron frequency) of 1.7 GW equivalent radiated power. This action results in the appearance in the ionosphere of an artificial ionization layer. The layer descends with respect to the basic (unperturbed) layer at a rate of ～500 m s^?1 down to the altitude, where f ₀ ≈ 4f _ce .     

Influence of an artificially disturbed ionosphere on the mesospheric ozone

The results of microwave measurements of ozone emission in the middle atmosphere during modification of the ionosphere by high-power short radio waves on March 14–19, 2009 and on March 27–28, 2011. The modification was carried out using a SURA heating facility (Radiophysical Research Institute) in conjunction with two microwave ozonometers aimed at different parts of the sky. A new physical phenomenon of decrease of the intensity of microwave emission from the mesosphere in the ozone line upon the modification of the ionosphere with high-power short radio waves is discovered.     

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.     

Sounding of the ionosphere disturbed by the “Sura” heating facility radiation using signals of the GPS satellites

We present the results of experimental studies of the properties of the plasma-density disturbances created during heating of the ionospheric F₂ region by high-power HF radio waves radiated by the “Sura” heating facility (Radiophysical Research Institute, Nizhny Novgorod). These experiments are specific in that they were performed in a sunlit (daytime) ionosphere when the generation of ionospheric turbulence has specific features and the turbulence intensity level is low enough. The plasma-density disturbances induced by high-power HF radio emission were sounded by signals of the GPS satellites, the line of sight to which crossed different parts of the disturbed ionosphere region. Threshold powers of the excitation of artificial plasma-density variations as well as spatial, temporal, spectral, and energy characteristics of the generated disturbances are determined.     

Properties of the thermal narrow-continuum component in the spectrum of stimulated ionospheric emission

We present the results of experimental studies of the properties of the thermal narrow-continuum component NC_th of stimulated electromagnetic emission (SEE) of the ionosphere observed in a narrow region between the pump frequency and the downshifted maximum (DM). Spectral and dynamic characteristics of the NC_th component are considered and the dependence of its properties on the frequency and power of the pumping wave as well as on the zenith angle of a high-power radio-wave beam in the geomagnetic-meridian plane is analyzed. It is proved that the NC_th generation is determined by the thermal (resonant) parametric instability near the level of the upper-hybrid resonance for a high-power radio wave. New experimental data on the properties of the main DM thermal component in the spectrum of stimulated electromagnetic emission, which we used as the reference component in most measurements, are presented. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 12, No. 48, pp. 1013–1032, December 2005.     

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

We present the results of experimental studies of the evolution of stimulated electromagnetic emission of the ionosphere (SEE) under the F-layer modification by powerful HF radio waves in a broad range of the pump wave frequencies. We compare the parameters of the overshoot effect in SEE evolution to the observations of anomalous attenuation and field-aligned scattering of radio waves. We show the overshoot effect to result from the anomalous attenuation under scattering at artificial small-scale ionospheric irregularities. We found the characteristic time scales of the overshoot effect development to decrease and its value to increase as the pump wave frequency decreases from 6 to 4 MHz; this is attributed to the observed increase of the irregularities amplitude at scales m. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 7, pp. 619–634, July 1999.     

Artificial periodic irregularities,wave phenomena in the lower ionosphere,and the sporadic <Emphasis Type="Italic">E</Emphasis> layer

We present new results of studying the artificial periodic irregularities produced in the ionosphere by the radio emission of the high-power heating facility “Sura” during the period of observations of the sporadic E layer. Basing on the measurement results for the velocity of the vertical plasma motion and the electron profile density in the E region of the ionosphere, which were obtained by the method of resonance scattering of radiowaves by an artificially produced periodic structure, we analyze the possibility to produce a sporadic E layer by driving metal ions affected by the vertical shear of the velocity, which occurs at these altitudes due to the propagation of internal gravity waves. The parameters of these waves are evaluated, as well as the mass and concentration of metal ions in the sporadic E layer.     

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.     

Satellite measurements of plasma-density perturbations induced in the topside ionosphere by high-power HF radio waves from the “Sura” heating facility

We report on the French DEMETER and American DMSP satellite measurements of largescale field-aligned plasma-density perturbations (ducts) induced in the topside ionosphere by the ionospheric F ₂-layer pumping by means of high-power HF radio waves from the “Sura” heating facility. Characteristics of such plasma perturbations and conditions of their formation are determined. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 915–924, November 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.     

Magnetic zenith effect and some features of the multifractal structure of the small-scale artificial ionospheric turbulence

We present the results of the experiment on studying the multifractal structure (with inhomogeneity sizes from tens to hundreds of meters across the Earth’s magnetic field) of the artificial ionospheric turbulence when the midlatitude ionosphere is affected by high-power HF radio waves. The experimental studies were performed on the basis of the “Sura” heating facility with the help of radio sounding of the disturbed region of the ionospheric plasma by signals from the Earth’s orbital satellites. The influence of the magnetic zenith effect on measured multifractal characteristics of the small-scale artificial turbulence of the midlatitude ionosphere was examined. In the case of vertical radio sounding of the disturbed ionosphere region, the measured multipower and generalized multifractal spectra of turbulence coincide well with similar multifractal characteristics of the ionospheric turbulence under natural conditions. This result is explained by the fact that the scattering of signals by weak quasi-isotropic small-scale inhomogeneities of the electron number density in a thick layer with a typical size of several hundred kilometers above the region of reflection of high-power HF radio waves gives the major contribution to the observed amplitude fluctuations of received signals. In the case of oblique sounding of the disturbance region at small angles between the line of sight to the satellite and the direction of the Earth’s magnetic field, the nonuniform structure of the small-scale turbulence with a relatively narrow multipower spectrum and small variations in the generalized multifractal spectrum of the electron number density was detected. Such a fairly well ordered structure of the turbulence is explained by the influence of the magnetic zenith effect on the generation of anisotropic small-scale artificial turbulence in a thin layer having a typical size of several ten kilometers and located below the pump-wave reflection height in the upper ionosphere.     

Results of Experiments on Modification of the Sporadic E-Layer by Powerful HF Radio Waves

We present the results of experiments on modification of the sporadic E-layer of the Earth's ionosphere by a powerful vertical beam of HF radio waves. The experiments were performed at the Sura heating facility (Radiophysical Research Institute, Nizhny Novgorod) from May 31 to June 4, 2001. The main results were obtained in May 31 from 17:30 to 20:00 MSK, when the E_s layer was fairly stable and the pump frequency was in the range where this layer was transparent. The diagnostics of the ionosphere was performed using X-mode probe waves in the frequency range f_pr=4.3-7.8 MHz. The ionosphere plasma heating was accompanied by an increase in the intensity of probe waves, reflected from the E_s layer, with frequencies close to or slightly higher than the critical frequency of the layer. Artificial fluctuations of the reflected signal appeared in the entire frequency range of sounding. We consider the dynamical characteristics of the observed phenomena and their dependence on the frequency f_pr and the parameters of the E_s layer.     

Field-aligned scattering of HF radio waves under conditions of action of high-power oblique radio waves on the ionosphere

We present the results of experimental studies of the features of field-aligned scattering of HF radio waves for different angles of incidence of high-power radio waves on the ionosphere. The amplitude, Doppler frequency shift, and azimuthal and vertical arrival angles of the scattered signals are measured. Calculated ray paths in the magnetoactive ionosphere are compared with experimental data on the scattered-signal reception under conditions of action of high-power oblique radio waves on the ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 669–677, August 2007.