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.     

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.     

Frequency correlation of fluctuations of short-wave signals in the theory of refractive scattering of radiowaves in the ionosphere

We consider the problem of frequency correlation of phase fluctuations of HF signals reflecte from a turbulent ionospheric layer with an arbitrary profile of electron density in vertical and oblique sounding of the ionosphere. Expressions for evaluation of the coherence band of HF signals reflected from a randomly inhomogeneous ionosphere are obtained. Calculation results are compared with corresponding calculations of frequency correlation of fluctuations of HF radiowaves in Buker's theory of refractive scattering of radiowaves. It is shown, in particular, that the influence of splitting of rays for waves with different frequencies in an inhomogeneous ionosphere on the frequency distortions of HF signals prevails compared with the phenomenon of frequency decorrelation of fluctuating HF signals propagating in a free space beyond a turbulent ionospheric layer.Radiophysical Research Institute, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 9, pp. 893–900, September, 1995.     

On the nature of middle-latitude F-spread

The results of a special experiment to study the statistics of short-wave signals reflected from the ionosphere during F-spread conducted in the first half of 1994 at the proving ground of the Scientific-Research Radio-Physics Institute at Zimenki of the Nizhny Novgorod region are presented. The dependence of the index of amplitude fluctuations on the reception bandwidth was studied. The experiment results are compared with existing theoretical concepts of the origin of F-spread. The experiment favors the hypothesis that middle-latitude F-spread is result of the multiple-ray nature of short-wave signals reflected from the large-scale quasi-regular inhomogeneous structure of the ionosphere.Scientific-Research Radio-Physics Institute. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 11, pp. 1447–1451, November, 1994.     

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.     

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.     

Evolution of wave disturbances in the upper ionosphere. Part II

A theoretical analysis of the vertical propagation of large-scale wave disturbances in the F region and outer ionosphere is presented. The analysis has included all the major factors influencing the dynamics of the ionospheric plasma under mid-latitude conditions. It is shown that the disturbances propagating downward in a strongly inhomogeneous medium rise in intensity up to the heights of the F₂-layer maximum and are damped then in the lower layers of the ionosphere. This mechanism can be considered a source responsible for the inhomogeneous structure of the upper ionosphere. State University, Irkutsk, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 9, pp. 1086–1092, September, 1998.     

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.     

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.     

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.     

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 a model of artificial F-spread in the ionosphere

The results of a complex experiment on the synchronous observation of geostationary-satellite signals and of reflected HF signals of the vertical-sounding station in the course of ionosphere modification by powerful short waves are analyzed using the method of refractive scattering of radio waves. We show that the main statements of the known stochastic theory of F-spread in the ionosphere can be used in the development of a model of artificial F-spread. In particular, the artificial F-spread can be described as multiple-ray propagation of short radio waves in the ionosphere modified by high-power radio waves, the electron-density distribution of which is a random process locally stationary in space and time.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 7, pp. 876–881, July, 1996.This paper was supported by the Russian Foundation for Fundamental Research (project 95-02-03716).     

The Structure of a Chirp Signal in the Randomly Inhomogeneous Earth-Ionosphere Channel

We analyze theoretically the structure of a chirp-ionosonde signal for the cases of one- and two-hop propagation in the randomly inhomogeneous ionosphere. For the case of two-hop propagation, wave scattering by the rough ground is taken into account. Our numerical simulation showed that random ionospheric irregularities and ground roughnesses play a significant role in the formation of a signal structure. We compare numerical results with experimental data obtained at oblique ionospheric sounding.     

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.     

On the role of large-scale ionospheric inhomogeneities in the formation of middle-latitude F-spread

We present the results of a special experiment carried out in the settlement of Zimenki, Nizhniy Novgorod province, on ionospheric vertical sounding by HF signals under F-spread using practically synchronous measurements of the multiple-ray nature of normal waves reflected from magnetoactive ionospheric plasma with ordinary and extraordinary polarizations at frequencies shifted by 0.7MHz. The experiment confirms the dominating role of large-scale electron-density inhomogeneities of the moving ionospheric disturbance type in the formation of middle-latitude F-spread.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 39, No. 5, pp. 564–567, May, 1996.This work was supported by the Russian Foundation for Fundamental Research (project N. 95-02-03716 RFFI).     

Model of interaction between decameter-decimenter radio waves and a strongly inhomogeneous mid-latitude ionosphere

A model of decameter-decimeter radio wave propagation in a strongly inhomogeneous mid-latitude ionosphere is constructed using a modified method of radio wave refractive scattering. The model establishes the relationship between the basic statistical radio wave characteristics and the turbulence parameters of the upper ionosphere. Different aspects of the theory of radio wave refractive scattering are considered in application to the study of amplitude and phase fluctuations of decameter-decimeter radio waves propagating in a three-dimensional randomly inhomogeneous ionosphere with an arbitrary electron density distribution. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 11, pp. 1323–1341, November, 1997.     

Oblique Sounding and Modeling of the Ionospheric HF Channel

We present the results of experimental studies of the distance-frequency and amplitude-frequency characteristics of the ionospheric HF channel on mid-latitude paths of oblique chirp sounding. It is shown that the maximum observed frequencies (MOFs) are subject to short-period variations with the quasi-periods from 30 min to 2 h.The amplitude of the MOF variations reaches 2 MHz and can increase up to 5–8 MHz on the Cyprus—Rostov-on-Don one-hop path in the sunrise-sunset time. It is established that the MOF fluctuations are accompanied by pronounced “cusp” features occurring in the upper rays and moving with time to the region of shorter delays, i.e., from lower to higher frequencies. The amplitude-frequency characteristics of individual propagation modes undergo deep fluctuations (up to 20–30 dB)whose quasi-period and depth depend on the frequency. It is shown that the appearance of fluctuations is caused by interference of the unresolved rays within the limits of one propagation mode. Based on the modeling, it is shown that “cusps” in oblique-sounding ionograms are due to the influence of traveling ionospheric disturbances (TIDs). The TID parameters are estimated. It is shown that conditions of the formation of “ cusps” in the distance-frequency characteristics depend on the TID amplitude, the wavelength of the disturbance wave, and the direction of its phase front with respect to the propagation path. The effect of quasi-regular frequency modulation of the Pedersen mode with a period of 250–300 kHz on the Cyprus-Rostov-on-Don chirp-sounding path is found. Altitude stratification of the ionosphere near the F-layer maximum, which is responsible for the focusing and defocusing of the Pedersen mode, is estimated. It is established that the stratification scale amounts to approximately 200–250 m. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 6, pp.455–471, June 2005.     

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.     

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.     

Effects of Ionospheric Radio Wave Scattering in the Decameter Interferometry

We consider some theoretical issues concerning diffraction of radio waves in a randomly irregular ionosphere with application to the problems of long-wavelength interferometry of cosmic objects. The statistical characteristics of intensity fluctuations of the decameter radio emission from discrete sources in the case of ground-based observations by two-element interferometers with very long and small baselines are analyzed. Analytical expressions are obtained for the autocorrelation function of the radiation intensity and for the scintillation spectrum of a point source in the limiting cases of large and small phase increments in an irregular ionospheric plasma. We find that in the case of radio interferometric reception, the scintillation spectrum corresponding to observations of a source by a single antenna is transferred from the zero-frequency region to the region of the Doppler frequency of the interferometer. It is shown that decameter ground-based and space-borne radio interferometers can be used to study the angular distribution of the radio brightness of cosmic sources under conditions of both quiet and disturbed ionosphere.