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.     

Some features of the fractal structure of the developed small-scale ionospheric turbulence

We present the results of the first studies of the fractal structure of the developed small-scale ionospheric turbulence (SSIT) during special experiments on radio-raying of the midlatitude ionosphere by signals from orbital satellites in 2005–2006. It is established that under conditions of developed turbulence, typical values of the fractal dimension of the space occupied by natural SSIT inhomogeneities are, as a rule, close to the topological dimension of their embedding space, and the true values of the spectral index of isotropic SSIT only slightly differ from the corresponding generally accepted nominal values in the embedding space. Nevertheless, even small differences in the mentioned parameters detected in the experiment witness a sharply nonuniform distribution of the local fractal structures of the developed SSIT in space. We propose a stochastic model of the nonstationary process for fast amplitude fluctuations of signals during their propagation in the ionosphere with nonuniform spatial distribution of small-scale electron-density fluctuations. Eventually, namely this nonuniform distribution of small-scale electron-density fluctuations leads to the specific multifractal structure of the amplitude records of received signals. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 4, pp. 287–294, April 2008.     

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.     

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.     

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.     

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.     

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.     

On the fractal structure of small-scale traveling ionospheric disturbances

We present the results of the studies of fractal properties of the small-scale inhomogeneities of traveling ionospheric disturbances in special experiments on radio-raying of the midlatitude ionosphere by signals from orbital satellites in 2004–2006. Along with the conventional correlation processing of the received signals, we performed their multifractal analysis, as well as fractal processing of signals by the correlation-integral method. Important information on fractal properties of the small-scale turbulence for the least studied part of the upper-ionosphere inhomogeneity spectrum in the interval of characteristic scales l ≈ 1–10 km is obtained. In particular, it is noted that the fractal structure of these inhomogeneities can be originated from the nonlinear “destruction” of several large-scale sinusoidal structures in a quasistable traveling disturbance. It is also noted that the multifractal spectra of amplitude fluctuations of the received signals obtained in the experiments in different years, in different time of the day, and in different seasons of the observations are quite similar. This is evidence that intermittency is a universal property of the plasma turbulence, at least for the midlatitude upper ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 1, pp. 22–30, January 2008.     

Multifractal Structure of Intermittency in a Developed Ionospheric Turbulence

We present the results of studying the multifractal structure of intermittency in a developed ionospheric turbulence during special experiments on radio-raying of the midlatitude ionosphere by signals from orbital satellites in 2005–2006. It is shown, in particular, that the determination of multidimensional structural functions of the energy fluctuations of received signals permits one to obtain the necessary information on multifractal spectra of the studied process of radio-wave scattering in the ionosphere. Experimental data on multifractal spectra of slow fluctuations in the received-signal energy under conditions of a developed small-scale turbulence are compared with the existing concept of the radio-wave scattering within the framework of the statistical theory of radio-wave propagation in the ionosphere. It is inferred that under conditions of a developed ionospheric turbulence, the multifractal structure of the intermittency of slow fluctuations in the received-signal energy is a consequence of the intermittency of small-scale fluctuations in the electron number density of the ionospheric plasma on relatively large spatial scales of about several ten kilometers. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 485–493, June 2008.     

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.     

A Lorentz-type mechanism for generation of small-scale irregularities in the heater-modified ionosphere

We consider Lorentz-type mechanism for growth of LF small-scale turbulence due to generation of local field-aligned electric fields (FAEF) in the ionospheric F-region modified by powerful radio waves. The FAEF are induced by Lorentz-type forces caused by the large-scale pressure structure of the heated volume. We found that small- scale structuring of the large-scale depleted region (the patch) is a function of altitude and that the cross-field scale of small-scale irregularities is definitely determined by the gradient scale length in plasma density. This mechanism allows us to explain the generation of irregularities with scale lengths of 6 m or longer and observations of aftereffects within 30 seconds or longer after the pump switch-off if they are defined by the lifetime of the induced local sources. The predictions of the Lorentz-type mechanism are shown to be consistent with the measurements of the significant growth of DSEE typical times, related to relaxation of heater-induced small-scale irregularities, under conditions of strong natural turbulence observed as F-spread in the ionograms.Published from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 3, pp. 318–328, March, 1996.     

On small-scale plasma inhomogeneities of the traveling ionospheric disturbances

We report on the results of the first special experiment on radio sounding of the midlatitude ionosphere by signals from in-orbit satellites at a frequency of 150 MHz under quiet geophysical conditions. Along with the conventional correlation processing, fractal processing of the received signals was also performed. Using the fractal approach, we obtained first data on the sources and generation mechanisms of small-scale plasma inhomogeneities of the traveling ionospheric disturbances (TIDs) in the upper ionosphere. It is noted that the phenomenon of nonlinear “ breaking” of the acoustic-gravity waves entering the ionosphere from the underlying atmosphere plays the crucial role in the formation of plasma inhomogeneities of the TIDs. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 7, pp. 561–569, July 2006.     

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.     

On fractal structure of large-scale electron-density inhomogeneities of traveling disturbances in the midlatitude ionosphere

We present the results of studies of the multifractal structure of slow (of duration τ ≈ 10 s) fluctuations of the received-signal amplitudes in special experiments on radio-raying of the midlatitude ionosphere by signals from orbital satellites in 2004–2006. It is shown, in particular, that the method of multifractal analysis of amplitude records of the received signals yields information on the spectrum of large-scale ionospheric inhomogeneities, which is inaccessible for the classical method of radio scintillations. From the results of measurements with the use of multifractal processing of experimental data, we found that large-scale (tens of kilometers) quasiregular electron-density inhomogeneities of traveling ionospheric disturbances (TIDs) have a power-law spectrum. It is exactly the power-law form of the spatial spectrum of large-scale inhomogeneities of TIDs that can be the reason for the observed multifractal structure of the intermittency of slow fluctuations of the received-signal amplitudes. However, under conditions of a developed small-scale turbulence of TIDs, the observed multifractal structure of the received signals is, as a rule, stipulated by the spatial inhomogeneity of the variance of the integral electron-density fluctuations of small-scale inhomogeneities on scales comparable with the sizes of large-scale inhomogeneities of TIDs. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 3, pp. 191–198, March 2008.     

Radar observations of artificial ionospheric turbulence during a magnetic storm

We present the results of radar observations of artificial ionospheric turbulence (AIT) created due to modification of the ionosphere by high-power radio emission from the Sura heating facility (Nizhny Novgorod region, Russia). Measurements were carried out in August 18–22, 2003 in the evening time (16:00–20:00 UT) with the use of over-the-horizon chirp HF radars on the Khabarovsk-Rostov-on-Don, Irkutsk-Rostov-on-Don, Inskip (England)-Rostov-on-Don paths, and also on the Moscow-Rostov-on-Don path for which reference signals of the standard-time RVM station were received. It is found that conditions for propagation of HF signals through the upper ionosphere at frequencies exceeding the maximum usable frequency for standard hop propagation through the F region were realized on the Irkutsk-Sura path in the presence of the powerful sporadic E_s layer. The presence of such signals was revealed at the Rostov-on-Don station by receiving radio waves which escape from the altitudes of the ionospheric F region due to scattering by artificial small-scale magnetic-field-aligned irregularities. We studied the ionospheric effects of a magnetic storm occurring during the experiment by using the measurement data of the Doppler frequency shift of signals scattered by artificial ionospheric turbulence. It is shown that during a magnetic storm, the electric field and the drift velocity of irregularities at the altitudes of the F layer over the Sura facility reach values of 8.6 mV/m and 186 m/s, respectively, which are typical of the high-latitude ionosphere. We consider the relation between quasi-periodic oscillations of the Doppler frequency shift of the scattered signal and propagation of magnetohydrodynamic waves excited during the magnetospheric storm.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 47, No. 9, pp. 722–738, September, 2004.     

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.     

Studies of the Irregular Structure of the Ionosphere using Scattering of Radio Waves on Artificial Periodic Inhomogeneities

We present new results of our studies of the irregular structure of the ionosphere using artificial periodic inhomogeneities (APIs) of the ionospheric plasma. The observations were carried out from 9:00 to 17:00 in August 10–12, 1999 with a height step of 0.7 km and digital registration and real-time processing of the signal quadratures. It is shown that in many cases, the amplitude of the scattered signal is determined by the interference of radio waves scattered on APIs and on natural ionospheric formations including sporadic layers and large-scale natural irregularities. This allows one to study the irregular structure of the lower ionosphere by analyzing height-time dependences of the amplitude and phase of the scattered signal.     

Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes

We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs) at mid-and high latitudes based on the “Sura” and EISCAT/HEATING HF facilities. Observations were performed by the method of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when the ionosphere above the “Sura” facility is illuminated by the Sun, but the solar terminator travels through the magnetically conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary ionospheric turbulence maintaining the irregularities above “Sura.” A drastic increase in the Doppler spectra width of the scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the terminator, but the ionosphere above the “Sura” facility is still lighted. It is assumed that the “ run away” of photoelectrons from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase in their intensity. The presence of fairly intense scattered signals was detected from the “Sura” and EISCAT/HEATING experimental results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses. In the case of pulsed heating by short pulses with duration τ_p < 100 ms and average radiated power P_a below the threshold power P_thr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation of irregularites under the cycled HF pumping with the pulse duration τ_p = 384 ms for P_a comparable with P_thr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The residual turbulence aftereffects played a significant role in the SSAI development. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 678–694, August 2007.     

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.     

Observations of the magnetic-zenith effect using GPS/GLONASS satellite signals

We present the results of first studies of the modification of the ionosphere by high-power HF radiation, which were obtained using signals of high-orbit GPS/GLONASS navigation satellites. Enhancement of the ionospheric modification in the magnetic-field direction was observed for the first time. This leads to a total decrease in electron number density and the formation of electron density irregularities near the magnetic-zenith direction. The efficiency of using GSP/GLONASS satellite signals for the studies of the ionosphere modified by HF radiation is demonstrated. Prospects for further studies in this field are discussed. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 934–938, November 2008.