Anisotropic Structure of Small-Scale Ionospheric Turbulence

We consider various theoretical models for the spectrum of small-scale ionospheric turbulence. The particular role of the generalized model of the ionospheric-turbulence spectrum, which takes into account that the anisotropy (extension) of small-scale irregularities of the upper ionosphere along the Earth's magnetic field direction depends on the transverse scale of those irregularities, is emphasized. The results of the. rst target experiments on radio sensing of the midlatitude ionosphere by signals from on-orbit satellites at frequencies 150 and 400 MHz under conditions of increased solar activity are presented. The experiments were performed at the radiophysical facility in the Nizhny Novgorod region in 2003. We studied statistical characteristics of the amplitude fluctuations of the received signals for different angles ϑ between the line of sight from a satellite to a ground-based reception point and the Earth's magnetic field direction. It was found in the course of the experiments that the spectrum slope of amplitude fluctuations of the received radiation is a function of the angle ϑ. The obtained result agrees with the generalized model of the ionospheric-turbulence spectrum and can be an argument in favor of the pronounced anisotropic structure of small-scale electron-density irregularities of the midlatitude ionosphere under disturbed geophysical conditions. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 5, pp. 382–387, May 2005.     

Electron-Density Distribution in the Upper Ionosphere under Conditions of a Mid-Latitude F-Spread

We present the results of the last experiments aimed at studying the phenomenon of mid-latitude F-spread by radio-raying of the ionosphere using the signal from the SURA facility received onboard the NASA spacecraft WIND. A generalized model of spatio-temporal distribution of the electron density in the mid-latitude ionosphere under F-spread conditions is proposed. Based on this model describing large-scale ionospheric irregularities, we calculate the ray trajectories of HF radio waves. We also discuss the known results of comparative measurements of the variances of relative electron-density fluctuations in large-scale irregularities of the mid-latitude ionosphere, carried out by the OGO-6 spacecraft, and the frequency broadening of the ionograms of ground-based stations for the vertical sounding. It is shown that the proposed model of electron-density disturbances in the mid-latitude ionosphere in the presence of F-spread is able not only to describe well the main qualitative feature of HF radiowave propagation under disturbed geophysical conditions, but also to reproduce quite accurately the quantitative parameters of the frequency broadening of reflected HF signals under conditions of standard and developed F-spread.     

Radiowave fluctuations in the polar ionosphere on the satellite-to-satellite paths under high solar activity

We analyze ionospheric fluctuations of decimeter radio waves on occultation polar paths between the navigational GPS satellites and the satellite CHAMP. Time dependences of the variance of the signal amplitude and amplitude fluctuation spectra under high solar activity in October–November 2003 are presented. The behavior of the signal amplitude fluctuations during occultation ionospheric sounding in the polar regions in different time of the day and in the equatorial regions in the daytime are considered. Radio-wave fluctuations are related to the solar-activity manifestations. It is shown that during strong solar-flare activity, intense small-scale plasma irregularities are excited in the polar ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 3, pp. 185–193, March 2006.     

Ground-Based and Space-Borne Studies of Midlatitude F-Spread

We present the results of the first space-borne SURA-WIND experiment aimed at studying the features of short-radiowave propagation in the case of radio occultation of the near-Earth plasma in the presence of a developed midlatitude F-spread. We discover the effect of screening of a signal at frequency 4525 kHz that is 0.8 MHz higher than the minimum observed frequency for the SURA-WIND radio path determined using the ionograms obtained by the ground-based vertical-sounding station. The screening effect is explained by the effect of large-scale irregularities of the number density of ionospheric electrons of sizes about a few tens of kilometers, which lead to the appearance of electron-density depletion regions in the ionosphere stretched along the geomagnetic field. These depletion regions are separated by the ordinary plane-parallel distribution of the electron number density. The features of the frequency spectra of intensity fluctuations of the HF radiation over the SURA-WIND radio path in the presence of a developed midlatitude F-spread are considered.     

Ultralong-range sounding of the ionospheric HF channel using an ionosonde/direction finder with chirp modulation of the signal

We present the results of experimental studies of propagation of short radio waves on a long transequatorial path of Laverton (Australia) — Rostov-on-Don, which were obtained with the help of an ionosonde/direction finder with chirp modulation of the signal. It is shown that conditions for propagation of anomalous signals by means of sideband reflection of radio waves from the Himalayan Hills and the Plateau of Iran and also due to scattering of radio waves from the high-latitude ionosphere of the northern hemisphere are realized on the given path. The propagation of radio waves is modeled with allowance for their scattering by anisotropic magnetic field-aligned irregularities of a high-latitude ionosphere, which are located on the northern wall of the main ionospheric trough of the F layer. It is shown that the results of the experiment agree well with the calculated data.     

Turbulence spectrum of the upper ionosphere

The problem of defining the spectral form of ionospheric irregularities with dimensions from hundreds to thousands of meters is considered. A generalized model is proposed for the ionospheric turbulence spectrum, taking into account both the anisotropic properties of the large-scale fraction of irregularities and the dependence of the anisotropy (elongation) of small-scale irregularities of the upper ionosphere along the Earth magnetic field direction on the transverse scale of those irregularities. Relations have been derived to determine the basic parameters of the irregularity spectrum of the uppers ionosphere (anisotropy indices for large-scale and small-scale fractions) and the depth of a thin ionospheric layer through measurement of the spectral characteristics of amplitude and phase fluctuations of orbital satellite signals. Using this model of the plasma irregularity spectrum, we can explain consistently many well-known experimental data on spectral characteristics of the phase and amplitude fluctuations of orbital satellite signals both in the high-latitude and midlatitude ionosphere. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 4, pp. 446–456, April, 1997.     

Sporadic structures in the equatorial ionosphere inferred from radio occultation data on satellite-to-satellite paths

We analyze characteristics of sporadic layers in the equatorial ionosphere using the results of radio occultation sounding on the paths between GPS satellites and the CHAMP low-orbiting satellite during the solar flare in October–November 2003. Variations in the amplitude and phase of signals during the lower-ionosphere sounding are studied. It is shown that the use of amplitude and phase data allows one to obtain parameters of the sporadic ionospheric structures. The data on the occurrence frequency, height, thickness, and intensity of the E_s layers in the daytime and nighttime equatorial ionosphere are presented. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 3, pp. 181–190, March 2008.     

Structure and dynamics of the ionospheric region with artificial small-scale irregularities according to complex measurements of the scattered radio-signal characteristics

We present the results of complex measurements of characteristics of HF radio signals scattered by artificial small-scale irregularities by means of oblique sounding using a chirp signal and by direction finding. On the basis of comparison of the experimental data and the calculations of aspect scattering of radio waves, the conclusion on the patch structure of an artificially disturbed ionosphere filled with small-scale field-aligned irregularities is drawn. The sizes of the patches amount to several ten kilometers, and they are separated by 100 or more kilometers. The drift speed of irregularities in the direction of the scattering vector in the plane perpendicular to the magnetic field is determined for several patches. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 12, pp. 1011–1025, December 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.     

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.     

Artificial periodic irregularities,hydrodynamic instabilities,and dynamic processes in the mesosphere—lower thermosphere

We present the results of measuring characteristics of the ionosphere and neutral atmosphere by the method of resonant scattering of radio waves by artificial periodic irregularities of the ionospheric plasma in the altitude range 90–120 km. It is shown that the altitude–time variations of the measured characteristics are in many respects stipulated by the propagation of atmospheric waves. Hydrodynamic instabilities in the mesosphere—lower thermosphere are analyzed. Criteria of development of different-type instabilities are presented. Contribution of different processes to the dynamics of the medium is estimated on the basis of the measurement results.     

Results of Determining the Electron Number Density in the Ionospheric E Region from Relaxation Times of Artificial Periodic Irregularities of Different Scales

We present the first results of determining the electron number density in the ionospheric E region by a novel technique based on the creation of artificial periodic irregularities when the ionosphere is affected by powerful radio emission at two frequencies. Using the results of the measurements performed in October 2006 during heating of the ionosphere by the “Sura” facility radiation at frequencies 4.7 and 5.6 MHz, we obtained the electron number density profiles in an altitude range of 100 to 110 km. Features of the procedure of measurement and calculation of the electron number density are described in detail. It is shown that the method can be used for a study of the irregular structure of the lower ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 477–484, June 2008.     

On fractal properties of small-scale ionospheric irregularities

We consider the problem of relating the local structure of small-scale ionospheric turbulence to the measured frequency-spectrum indices and fractal dimensions of amplitude records of the signals received on the Earth during remote sensing of the ionosphere onboard the satellites. It is shown that knowledge of these parameters permits one to determine the true values of the local-spectrum indices of the electron-density fluctuations for isotropic small-scale turbulence of the ionosphere both under natural conditions and during its modification by high-power short-wave radiation as well as to specify fractal dimensions of space filled by small-scale irregularities of the turbulent structures in the ionosphere. We show the necessity of detailed experimental studies of the fractal properties of small-scale ionospheric irregularities of both natural and artificial origin by using a multifractal analysis in combination with the synchronous correlation processing of received signals during remote sensing of the ionosphere. This will give important information on the local structure of small-scale ionospheric turbulence inaccessible for studies within the framework of the classical method of radio scintillation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 4, pp. 300–308, April 2007.     

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.     

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.     

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.     

Fractal structure of artificial ionospheric turbulence

We show the results of the first experimental studies of the multifractal structure of the developed artificial ionospheric turbulence. As a result of the special multifractal analysis of the recorded amplitudes of signals from the orbital satellites, which were obtained during the experiments on radio tomography of the irregularities excited in the ionosphere by the powerful mid-latitude heating facility “Sura,” it is found that the characteristic multifractal structure of these records is caused by the nonuniform spatial distribution of the small-scale fluctuations of the electron number density in the artificial irregularities of the ionospheric plasma. Comparative analysis is performed for the multifractal spectra of fluctuations of both the amplitudes and energies of signals received from the orbital satellites upon radio transmission probing of the region of artificial ionospheric turbulence by these signals at three observation points located near the “Sura” heating facility and spaced apart to a distance of about 100–150 km. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 970–976, November 2008.     

Amplitude and Phase Fluctuations of Decimeter Radio Waves Raying the Atmosphere via Satellite-to-Satellite Paths

We present the results of experimental studies of the atmospheric phase and amplitude fluctuations of decimeter radio waves in radio occultation measurements using paths connecting the MICROLAB satellite and the satellites of the GPS navigation system. The dependences of the amplitude- and phase-fluctuation variance on the minimum altitude of the ray trajectory and the frequency spectra of the fluctuations are presented. The experimental data are compared with the theory of radio-wave propagation in random media. We determine the spectral index of irregularities of the atmospheric refractive index, the external scale of the irregularities, and the variance of the refractive-index fluctuations. It is shown that the radio occultation technique allows one to monitor small-scale irregularities of the atmosphere.     

Generation of ionospheric irregularities upon propagation of solitary internal gravity wave during the major magnetic storm of October 29–31, 2003

Using the technique of global detection of ionospheric disturbances, based on processing the data of the global GPS-receiver network, we obtain experimental proof of the existence of a solitary wave (soliton) in the atmosphere during the main phase of the major magnetic storm of October 30, 2003. The soliton with a characteristic duration of about 40 min and a relative amplitude of up to 40%, originated at the moment of the maximum disturbance of the Earth’s magnetic field, traveled without changing its shape at a distance of up to 4500 km with a velocity of 1400 m/s, which exceeded the atmospheric sound velocity at the heights of the main electron-density maximum in the ionosphere (about 300 km) by a factor of 1.5. The intensity of variations in the total electron content in the period range 1–10 min increases by an order of magnitude as the soliton propagates from the North-East to the South-West of the USA in the regions with the maximum amplitude of the large-scale disturbance. This corresponds to enhancement of ionospheric irregularities with scales from 10 to 100 km, and also of small-scale irregularities (SSI) with scales of 100 to 1000 m, since the spectrum of the ionospheric irregularities has a power-law shape. Spatio-temporal characteristics of the density distribution of phase slips of GPS signals are close to the corresponding characteristics of the SSI intensity. This agrees with the existing concept that the phase slips result from scattering of GPS radio signals by SSIs. Both the SSI amplitude and the density of phase slips decrease as the soliton decays in amplitude. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 2, pp. 89–104, February 2006.     

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.