On propagation of decametric-decimentric radiowaves under conditions of middle-latitude F-spread

In this paper we analyze the influence of the large-scale inhomogeneous structure of a middlelatitude ionosphere with F-spread on the propagation of decametric-decimetric radiowaves. The phase fluctuations of short waves in the presence of middle-latitude F-spread are considered. A general integral formula for the frequency dispersion of phase fluctuations of short radiowaves during their propagation in a three-dimensional randomly-inhomogeneous ionosphere with an arbitrary electron-density distribution is obtained. For some of the simplest types of ray trajectories we find analytical solutions which allow us to obtain simple estimates of the frequency correlation of phase fluctuations of short radiowaves under conditions of middle-latitude F-spread. The amplitude fluctuations of short radiowaves under conditions of ionospheric F-spread are considered by the example of wave propagation with strong backscattering in a layered randomly inhomogeneous medium. It is shown that the presence of a strongly developed large-scale structure of electron density in a middle-latitude ionosphere with F-spread can cause reflected short-wave signals with relatively large delays and abnormally high fluctuation levels. The fluctuational and quasiregular effects accompanying the transionospheric propagation of metric-decimetric radiowaves are discussed. In particular, it is shown that under conditions of middle-latitude F-spread the inhomogeneous structure of the ionospheric plasma does not have a significant influence on the fluctuational or quasi-regular propagation characteristics of decametric-decimetric radiowaves.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 926–938, September, 1995.     

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.     

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.     

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.     

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.     

Allowance for the solar-magnetospheric connections in a problem of predicting the critical frequency of the subauroral ionosphere

The cause-and-effect relation between variations in the critical frequency of the F ₂ subauroral ionospheric layer and the key solar-magnetospheric parameters is studied. The features of the dependence of the critical frequency of the subauroral ionosphere on the value and components of the interplanetary magnetic field, the solar-wind parameters, the X-ray and ultraviolet radiation intensities, the solar zenith angle, and the intensity of precipitation of low-energy particles are established. Within the framework of this study, codes allowing one to solve the problems of predicting the critical frequency of the ionosphere by the method of artificial neural networks has been developed. The neural-network forecasting experiments reveal the characteristic times of the ionospheric response to magnetospheric disturbancs. The found optimal architecture of the neural network allows us to predict the time series of the critical frequency of the high-latitude ionosphere for the interval 0.5–3 h with an accuracy of up to 93%. The obtained linear and nonlinear dependences of the critical frequency of the ionospheric F ₂ layer on the geophysical parameters can be used for developing ionospheric models required for determining the maximum usable frequency of HF radio communication. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 2, pp. 109–117, February 2009.     

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.     

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.     

On the spectrum of large-scale inhomogeneities of the upper ionosphere

The possibility of measuring the large-scale turbulence structure of the upper ionosphere by vertical and oblique short-wave (SW) radio-sounding techniques is considered. General expressions have been derived for the phase fluctuation spectrum of a short-wave signal reflected at the ionospheric layer with an arbitrary regular permittivity profile and given spectrum of inhomogeneities. We have analyzed a number of particular cases which are most typical of phase measurements in the vertical and oblique SW radio-sounding of the randomly inhomogeneous ionosphere. It is shown that when these methods are used the phase fluctuation spectra of reflected signals may critically depend on the form of the ionospheric electron density profiles. The correct interpretation of the measurement data requires use of stations of synchronous vertical and oblique sounding to obtain proper ionograms and calculate the current spectra of a regular permittivity distribution of the ionosphere. Specific difficulties in interpreting the phase measurements of ionospheric inhomogeneity spectra by vertical and oblique radio-sounding methods are mentioned.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 7, pp. 653–659, July, 1995.     

Some results of synchronous experimental study of artificial F-spread at radiophysical test sites in nizhny novgorod province

We show some results of experiments on synchronous sounding of the ionosphere by short-wave signals at the Radiophysical Research Institute’s test site in Zimenki and Vasil’sursk, Nizhniy Novgorod province, during ionospheric modification by high-power short radiowaves from transmitters of the “Sura” facility in Vasil’sursk. In the course of experiments we proved directly the decisive role of large-scale inhomogeneities of the ionospheric plasma with dimensions of from several kilometers to several dozens of kilometers in the formation of artificial F- spread. The small-scale inhomogeneities with dimensions smaller than 1 km, which are localized in a relatively thin layer near the reflection level of a high-power short radiowave, emerged only at sounding waves, propagating (reflected) in the immediate vicinity of the center of the heating region. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 6, pp. 688–692, June, 1997.     

Modification of the earth’s ionosphere by high-power HF radio emission: Artificial periodic inhomogeneities and the sporadic <Emphasis Type="Italic">E</Emphasis> layer

We present the results of new studies of the sporadic E layer in the case of heating of the ionosphere by high-power HF radio emission. The measurements were performed at the “Sura” heating facility. Ionosphere was modified by high-power radio emission from the “Sura” facility and was sounded by the probing radio waves of the same frequency and mode. The heating of the ionosphere resulted in the formation of artificial periodic inhomogeneities, and an increase in the intensity of all signals scattered by the D, E, and F regions and the sporadic E layer by 5–20 dB was observed. The increase was observed during heating of the ionosphere by each magnetoionic component, but was smaller for heating by an ordinary-mode wave. This effect was resonant and disappeared as a result of the frequency detuning down to 85 kHz. During the ionospheric modification, the signal-intensity increased due to modulation of the natural profile of the electron number density by the artificial periodic structure. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 695–708, August 2007.     

Thermogravitational convection in ionospheric plasma modified by high-power decametric radiowaves

Quality theoretical analysis allows us to assume that artificial plasma inhomogeneities in the ionosphere modified by high-power decametric radiowaves could be space modes of thermogravitational convection. Effects observed in the scattering of test 10-m radiowaves are interpreted as anindication of thermogravitational convection in heated ionospheric plasma.Translated from Izvestiya Vyssikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 568–578, May, 1996.     

Mimic Simulations of Radio-Wave Propagation in a Randomly Irregular Ionosphere

We present the technique and results of mimic simulations of radio-wave propagation in a randomly irregular ionosphere with allowance for the Earth's sphericity and the background ionosphere. Based on consideration of the probability distributions of the angle of reception and of the corresponding amplitude, eikonal, and angle of radiation, obtained by the mimic modeling, we conclude that the most probable ray path is symmetric with respect to the region of its reflection from the ionosphere and that the mean reception angle and the corresponding mean radiation angle are equal. The simulations yield the statistical characteristics of a wave, such as the variances of the reception angle and the eikonal, as well as the correlation functions of the eikonal and the field. The simulation results concerning the variances of reception angles and eikonal are compared with the results of the first approximation of the perturbation theory. It is shown that the eikonal fluctuations in the irregularity-free space, caused by fluctuations of angles of the lower rays escaping from an ionospheric layer with random irregularities, should be taken into account.     

On one feature of the F-spread phenomenon in the midlatitude ionosphere

We report on the results of observations of reflected SW signals by vertical sounding (VS) of the ionosphere in Zimenki, Nizhny Novgorod region, in the summers of 1994 and 1995. We discovered an anomalous increase in the multipath propagation of the received SW signals during ionospheric F-spread: The duration of a multipath reflected signal increased considerably when we used a highly directional antenna compared to the case of a weakly directional antenna. The fast amplitude fluctuation index increased with increasing delay of the reflected SW signals. The observations are interpreted in terms of a stochastic model of midlatitude F-spread as the phenomenon of multipath radio wave scattering (reflection) from large-scale electron density inhomogeneities under conditions of a strongly developed inhomogeneous structure of the ionosphere.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 10, pp. 1064–1070, October, 1995.     

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.     

A stochastic model of the F-spread phenomenon in the ionosphere

A stochastic model of the F-spread phenomenon in the ionosphere is proposed. The model is based on the idea that under F-spred conditions the electron-density distribution in the F-region of the ionosphere is a random process that is locally stationary in space and time and assumes multiple-ray propagation of short waves with independent phase fluctuations in individual rays. Some peculiarities of F-spred ionograms are analyzed. The problem of intensity fluctuations of short radiowaves during F-spred in the ionosphere is considered.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 12, pp. 1227–1240, December, 1995.     

On the excitation of the ionosphere by high-power radio waves in the geomagnetic equator region

We consider the features of the modification of the equatorial ionosphere by high-power radiowaves. A possibility for creating strong (compared to mid-latitude) quasi-periodic ionospheric F-layer irregularities, formed by the field of the high-power standing wave, producing artificial bubbles, and influencing the natural equatorial “bubbles” is discussed. The peculiarities of exciting the striction and thermal parametric instabilities in the equatorial ionosphere are considered. The problem of the modulation of the equatorial current jet is briefly discussed. Radiophysical Research Institute, Nizhny Novgorod, Russia; Swedish Institute of Space Physics, Uppsala Division, Uppsala, Sweden. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1–2, pp. 250–262, January–February, 1997.     

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.     

Ambiguity of the Reconstruction of Plasma Frequency Profiles from a Given Height-Frequency Characteristic and Their Discernibility for Oblique Propagation of HF Radio Waves in an Isotropic Ionosphere

We specify the formulation of the problem of reconstructing the plasma frequency height distribution from a given height-frequency characteristic (HFC) for a spherically symmetric isotropic ionosphere. We propose a numerical algorithm for solving the problem of correctly processing the region of near-critical frequencies of ionospheric layers. On this basis, we study the discernibility of plasma frequency profiles (PFPs) satisfying a given HFC when the properties of an obliquely propagating HF signal are analyzed.