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.     

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.     

On the Problem of True Values of the Spectrum Indices for Small-scale Ionospheric Turbulence

We consider the problem of obtaining reliable values of the local-spectrum indices of the electron number density fluctuations for small-scale ionospheric turbulence. It is shown that the use of a multifractal analysis in combination with the synchronous correlation processing of the received signals in the experiments on remote radio sounding of the ionosphere by satellite signals permits one to solve the posed problem. In this case, the true values of the local-spectrum indices of small-scale ionospheric turbulence, which are measured in such specialized experiments under natural conditions and during modification of the ionosphere by high-power HF radio emission, can differ notably from their standard values obtained within the framework of the classical method of radio scintillations, in which only correlation processing of the data is used. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 7, pp. 571–574, July 2008.     

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.     

Multipower spectrum of small-scale ionospheric turbulence

We consider the problem of diagnostics of the local structure of small-scale ionospheric turbulence using the multifractal analysis of received signals from the Earth’s orbital satellites after the radio sounding of the inhomogeneous ionosphere by these signals. In particular, it is shown that analysis of the multifractal structure of the received-signal amplitude records by the method of multidimensional structural functions allows one to determine the indices of the multipower local spectra of the small-scale ionospheric turbulence, which are inherent in it due to the nonuniform spatial distribution of small-scale fluctuations of the electron number density. It is noted that information on the multipower spectrum of small-scale ionospheric turbulence is not available for the conventional radio scintillation method based on the classical spectral analysis of received signals during the remote radio sounding of the ionosphere. At the same time, the method of multidimensional structural functions is efficient under conditions of actual nonstationarity of the process of scattering of the HF radio waves by the randomly inhomogeneous ionospheric plasma. The method of multidimensional structural functions is used for the multifractal processing of received signals of orbital satellites during special experiments on radio sounding of the midlatitude ionosphere under natural conditions and its modification by high-power HF radio waves. First data on the indices of the multipower local spectra of small-scale ionospheric turbulence are obtained. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 1, pp. 14–22, January 2009.     

Variations in the ionospheric wave perturbation spectrum during periodic heating of the plasma by high-power high-frequency radio waves

We present the results of spectral analysis of temporal variations in the Doppler frequency shift of the ionosphere-reflected signals from a high-frequency vertical ionospheric sounding radar located near the city of Kharkov in the days of exposure of the ionospheric plasma to the high-power radio emission of the Sura facility (Nizhny Novgorod) and in the reference day in the absence of such an exposure. It was established that the spectral characteristics of quasiperiodic variations in the Doppler frequency shift in the range of periods 10–60 min in the days of work of the facility and in the reference day differed significantly. This is considered as evidence in favor of the generation (amplification) of acoustic–gravity waves propagating at the ionospheric altitudes by high-power periodic high-frequency radiation of the Sura facility.     

Feature of the multifractal structure of small-scale ionospheric turbulence during the solar eclipse of August 1, 2008

Using the method of radio sounding of the mid-latitude ionosphere by the satellite signals, we study the multifractal structure of small-scale ionospheric turbulence during a solar eclipse. The measured multipower and generalized multifractal spectra of small-scale ionospheric turbulence at the initial and closing stages of the eclipse turn out to be almost identical on the space radio paths with different orientations. This is indicative of a sufficiently high stability of the nonuniform spatio-temporal distribution of small-scale fluctuations of the ionospheric electron number density under conditions of geophysical disturbances due to global physical processes in the ionospheric plasma during a solar eclipse. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 4, pp. 302–306, April 2009.     

Artificial ionospheric source of low-frequency electromagnetic fields in the problems of environment sounding

We theoretically estimate the comparative efficiency of the conventional methods of sounding of the environmental processes and objects and the new techniques proposed in the literature, which use the effect of low-frequency field generation when natural ionospheric current systems are modulated by powerful radio emission from ground-based transmitters. Diagnostics of internal gravity waves, electromagnetic sounding of the Earth, diagnostics of the ionospheric Alfv′en resonator, and magnetosphere location are considered. It is shown that except for the magnetosphere location, all analyzed techniques are inferior to the conventional methods in complexity, information capacity, and efficiency. Under favorable geophysical and geographical conditions, an ionospheric source of SLF and ULF electromagnetic fields can provide the most effective injection of waves into the magnetosphere. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 12, pp. 1026–1034, December 2008.     

Some results of measuring the characteristics of electromagnetic and plasma disturbances stimulated in the outer ionosphere by high-power high-frequency radio emission from the “Sura” facility

We present the results of measuring the characteristics of electromagnetic and plasma disturbances at altitudes of about 700 km, obtained by using the onboard equipment of the French microsatellite DEMETER during its passage through the magnetic tube resting upon the region of intense generation of artificial ionospheric turbulence created due to modification of the ionospheric F₂ region by high-power radio emission from the “Sura” facility. It is shown that an artificial density duct emerging from the disturbed region and extending to the Earth’s magnetosphere can be formed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 709–721, August 2007.     

Features of propagation of HF signals on mid-latitude paths under conditions of geomagnetic disturbances

We present the results of experimental studies of the features of HF-signal propagation on oblique-sounding paths in the Eurasian longitude sector between England and Magadan during geomagnetic disturbances. The joint analysis of the satellite data and the data of vertical and oblique sounding of the ionosphere shows that the appearance of additional signals during magnetic disturbances can be stipulated by refraction of radio waves in the region of the auroral oval and the main ionospheric trough as well as by scattering from small-scale magnetic-field-aligned irregularities near the equatorial boundary of the auroral oval. Based on calculations and comparison with experimental data of oblique sounding of the mid-latitude ionosphere, we identify the lateral spreading signals registered on the Magadan — Irkutsk and Inskip (England) — Rostov-on-Don paths as signals scattered by small-scale field-aligned irregularities whose location coincides with the southern boundary of the auroral oval. We show that the Russian and global chirp-ionosonde networks are promising for studying the dynamics of the main ionospheric trough and the auroral oval during geomagnetic disturbances as a manifestation of space weather in the near-Earth environment.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 12, pp. 1041–1056, December, 2004.     

Monitoring of wave-like disturbances by means of oblique sounding of the ionosphere

We present the results of experimental studies of ionospheric wave-like disturbances on the basis of oblique chirp sounding data for the mid-latitude paths Cyprus-Rostov-on-Don and Inskip-Rostov-on-Don. Twenty-four-hour measurements were performed in January-May and October–November, 2005 and in January, 2006. The spectral analysis of variations in the maximum observed frequency (MOF) is performed. It is shown that the spectra of the MOF fluctuations have a well pronounced line structure. The amplitude-dominant spectral harmonics are concentrated near the frequencies 0.18–0.8 mHz (20–90-min periods). We give the results of modeling of radio waves propagation in the presence of traveling ionospheric disturbances and compare the calculated and experimental data. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radio.zika, Vol. 49, No. 12, pp. 1015–1029, December 2006.     

Detection of magneto-gravity waves in the ionosphere by analysis of the maximum observable frequencies on oblique-sounding paths

The existence of magneto-gravity waves stipulated by the substorm activity can lead to the occurrence of traveling ionospheric disturbances, whose velocity exceeds the sound speed. For detection of magneto-gravity waves, we used experimental data on propagation of decameter radio waves on the midlatitude and subauroral oblique sounding paths Inskip–Rostov-on-Don, Cyprus–Rostovon-Don, Irkutsk–Rostov-on-Don, and Norilsk–Rostov-on-Don in December 2006 and March 2007 under conditions of weak geomagnetic disturbance. Time delays between the AE indices of polar electrojets and the maximum observable frequencies for the considered paths were established by calculation of linear correlations. These delays correspond to the times required for transport of gravity disturbances from the auroral region to the reflection points of radio waves on respective paths. Among the obtained time shifts, we mention the 5–10-min ones which correspond to increased velocity of the disturbances compared with the usual velocities of acoustic-gravity waves for the paths under study. Such cases can be related to the transport of magneto-gravity waves. Study of the spectral composition of the AE-index disturbances and recorded maximum observable frequencies shows consistency of their spectral features in the cases of increased correlation for small time delays. It is also found that the spectral features of the AE-index disturbances coincide with the spectral features of the disturbances of the horizontal component of the geomagnetic field on ground-based magnetic stations. According to calculated dispersion curves, the frequencies of magneto-gravity waves were estimated, and they turned out to be equal to ω ≈ (1–2) ・ 10⁻⁴ Hz.     

Radio-frequency component of transient radiation of an extensive air shower

The mechanism of radio emission induced by the transient radiation of oppositely charged particles from an extensive air shower in the geomagnetic field was studied for the first time. For showers with an energy of ∼10²² eV, the electric field strength at a distance of 500 km from the shower axis was found to be 60 μV/mMHz. Such showers attain their maximum at sea level. The spectral intensity of emission is maximum at frequencies of about 1 MHz (at these frequencies, the intensity of atmospheric disturbances is minimum). These specific features of radio emission can be used in experiments for radio detection of high-energy cosmic rays. An experimental setup of such detection is suggested.     

Magnetogravity waves in the ionosphere under conditions of finite conductivity

We obtain dispersion relations for magnetogravity waves in the ionosphere with allowance for the combined influence of magnetic field, gravity, and finite conductivity within the framework of the hydrodynamic approximation. The required conditions are fulfilled in the ionosphere at altitudes over or about 250 km. The auroral electrojet is considered as a source of magnetogravity waves which are frequently observed as traveling ionospheric disturbances. The contribution of magnetogravity waves to the ionospheric disturbances is determined on the basis of analyzing the data from the vertical sounding of the ionospheric F₂ layer and the geomagnetic disturbances along the chosen magnetic meridian and on its sides. The features of the obtained dynamic spectra of magnetogravity waves agree with the characteristic frequencies and velocities determined by the calculated dispersion curves. As a result, we confirm the fact that magnetogravity waves stipulate some traveling ionospheric disturbances and can be used for diagnostics of the ionospheric parameters.     

Sura-wind radar: Studies of nonlinear effects using VHF-wave occultion of the ionosphere

We present experimental results on reception of VHF signals of the transmitting facility “Sura” of the Radiophysical Research Institute by the NASA spacecraft WIND. The experiments were performed during daytime during the summer seasons 1997–1998. The dependence of power and spectral characteristics of the VHF radio waves on the power of the sounding radiation is analyzed. We find that, in a wide range of effective powers of the VHF radiation from about 40 kW to 160 MW, the phenomena observed in the radiation received onboard WIND (such as focusing, scintillations, or frequency deviations of signals) does not usually depend on the operation mode of the “Sura” facility. At the same time, broadening of the radiation pattern of the facility and decrease of the mean level of the received signal by about 6 dB toward the direction of maximum of the radiation pattern were observed at the peak radiation power of the facility. The experimental results are compared to present concepts on effects of self-focusing instability and nonlinear defocusing of VHF radio waves in the ionospheric plasma. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 799–809, August 1999.     

Diagnostics of the HF-pumped ionospheric region using wide-band radio emission

We present the results of comprehensive studies of the spatio-temporal structure of the artificial ionospheric plasma turbulence created by powerful radio emission over the “Sura” heating facility. The elaborated methods of the transmitter-receiver operation control, wide-band signal recording, digital filtering and spectral analysis of the stimulated electromagnetic emissions and short radio pulses are used for diagnostics. Novel data are obtained on the evolution of the Langmuir and upper-hybrid turbulence spectrum, the altitude distribution of the plasma wave decay rates, and the amplitude-phase characteristics of the test signals under wide-band sounding of the ionospheric plasma. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 649–668, August 2007.     

On the features of decameter radio astronomy

We consider some effects of the VHF radio wave propagation in randomly irregular plasma near the Earth. Applications of these effects to main problems of decameter radio astronomy are discussed. In particular, we show that significant measurement errors of about tens to hundreds of percent for the intensity of the VHF radio emission from an extra-terrestrial source may occur due to scattering and focusing/defocusing of the radiation in the ionosphere if VHF radio astronomical facilities operate at middle latitudes. We find that the angular resolution of discrete radio sources observed using radio interferometry and the well-known scintillation methods cannot be better than about a degree due to the effect of the developed ionospheric turbulence. We propose a modified scintillation method based on the spectral analysis of radio emission from discrete sources, which allows the useful high-frequency signal corresponding to diffraction of VHF emission at weak inhomogeneities of interplanetary plasma to be separated against strong, relatively low-frequency fluctuations of the received radiation due to the effect of the developed turbulent structure of the ionosphere. We show that such a method allows the angular resolution of extra-terrestrial radio sources to be improved up to arcsecond level. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 2, pp. 95–105, February 2000.     

Variations in the parameters of scattered signals and the ionosphere connected with plasma modification by high-power radio waves

We describe the results of using the incoherent scatter technique to observe time-altitude variations in regular parameters of the ionospheric plasma and wave disturbances, which accompanied periodic modification of the near-Earth plasma by radio waves emitted by the “Sura” facility. A distinctive feature of the experiments was that the processes in the ionosphere were diagnosed at a distance of about 1000 km from the facility. It was found that the spectrum composition of wave disturbances in the electron density was changing noticeably during the active experiment. Quasi-periodic processes in the ionosphere were observed with a delay of about 40–60 min. The relative amplitude of wave disturbances was equal to 0.02–0.10, and the periods were equal to 30, 60, 120, and 150–180 min. The observed effect can be explained by the generation and/or amplification of traveling ionospheric disturbances. The results of theoretical estimations agree well with the observational data.     

Dynamical Characteristics of Traveling Wave Packets of Total Electron Content Disturbances

Using the COPHASE method and the GPS interferometry method for travelling ionospheric disturbances, we analyze in detail the spatio-temporal properties of travelling wave packets (TWP) of total electron content (TEC) disturbances. The analysis is performed on the example of a clearest TWP manifestation observed in California, USA, in October 18, 2001, using the GLOBDET technique, developed at the Institute of Solar-Terrestrial Physics of the Siberian Branch of RAS for global detection and monitoring of natural and technogenic ionospheric disturbances on the basis of TEC variations retrieved from the global network of GPS receivers. In the time domain, TWPs are quasi-periodic TEC oscillations of duration about 1 h, period of 10–20 min, and amplitude exceeding that of the background TEC fluctuations by at least one order of magnitude. The velocity and direction of TWP motion are similar to those of mid-latitude mesoscale travelling ionospheric disturbances, as obtained earlier from the analysis of phase parameters of HF radio signals and the signals of geostationary satellites and discrete space radio sources.