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.     

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.     

Background fluctuations in the GPS-microlab-1 ionospheric radio sounding experiment

We analyze the spectra of the phase and amplitude fluctuations of radio signals recorded in the course of four ionospheric radio-sounding sessions at altitudes from 70 to 120 km. Our study is aimed at determining the sources of these fluctuations. Comparing the statistical properties of fluctuations measured at two wavelengths with theoretical calculations, we conclude that only the low-frequency part of background fluctuations is of ionospheric origin, while the high-frequency fluctuations are caused by noise of the measuring system. The amplitude fluctuations are more informative in the high-frequency region, since they are mainly due to ionospheric irregularities. We discuss the possibility of separation of the components of amplitude fluctuations caused by ionospheric irregularities and the irregularities of the neutral atmosphere in the case of sounding at the altitudes at which those components have comparable values. It is shown that this problem cannot be solved by dispersion and extrapolation methods, which are used for separation of the regular ionospheric and atmospheric components of the amplitude and phase variations of radio signals. A. N. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 6, pp. 511–523, June 1999.     

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.     

Spectrum of signal scattered by anisotropic wandering irregularities

The problem of radio-wave scattering by anisotropic wandering irregularities is examined. Allowance for the strong anisotropy of the irregularities simplifies the problem and allows a simple formula to be derived for calculation of the Doppler spectrum of a scattered signal. The calculated spectra are similar in form to the experimental spectra of radio aurora.     

A diagnostic tool for the study of stochastic properties of density striations excited by powerful electromagnetic waves in the ionospheric plasma

Powerful electromagnetic waves illuminating the ionospheric plasma generate small-scale density irregularities elongated in the direction of the geomagnetic field. Stochastic motion of these irregularities results in fluctuations of HF radio signals backscattered by the illuminated region. Observations of the full wave form of the radio signals make it possible to reconstruct the distribution function of the irregularities over their velocities. An experiment with such a reconstruction is proposed.     

Strong scintillation spectra behind a phase screen containing large-scale anisotropic irregularities

Using the approximation of an anisotropic statistically-homogeneous phase screen, we consider spectra of strong scintillations. Numerical calculations are made for the model of large-scale anisotropic inhomogeneities typical of the Earth’s stratosphere. The spectrum transformation is studied for the transition from weak scintillations to the asymptotic regime of strong scintillations. We show that with increasing level of the scintillations, their spectra rapidly broaden to the region of large wave numbers which exceed both the inverse internal scale of the irregularities and the inverse radius of the Fresnel zone by orders of magnitude. Notable deviations of the two-dimensional spectra from the predictions based on perturbation theory are shown to occur for scintillation variance exceeding 0.1. The obtained two-dimensional spectra of scintillations give a complete picture of the behavior of one-dimensional spectra which can be retrieved from satellite observations made for different angles between the orbit plane and the direction to the source. Vertical and horizontal one-dimensional spectra are studied in detail. Approximate algebraic formulas are derived and their validity is proved by applying them to the calculation of spectra of strong scintillations for a wide (several decades) range of the wave number values. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 9, pp. 747–765, September 2007.     

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.     

Phase fluctuations model for EM wave propagation through solar scintillation at superior solar conjunction

Traveling solar wind disturbances have a significant influence on radio wave characteristics during the superior solar conjunction communication. This paper considers the impact of solar scintillation on phase fluctuations of electromagnetic (EM) wave propagation during the superior solar conjunction. Based on the Geometric Optics approximation, the close-form approximation model for phase fluctuations is developed. Both effects of anisotropic temporal variations function of plasma irregularities and their power spectrum are presented and analyzed numerically. It is found that phase fluctuations rapidly decrease with increasing Sun–Earth–Probe angle and decrease with increasing frequency at the rate of 1/f². Moreover, the role of various features of the solar wind irregularities and their influence on the EM wave characteristic parameters is studied and discussed. Finally, we study the phase fluctuations of typical cases in order to better understand the impact of phase fluctuations in future deep space communication scenarios during solar conjunction periods.     

Angle-of-arrival fluctuations for wave propagation through non-Kolmogorov turbulence

Recently the increasing experimental evidences have shown that atmospheric turbulence statistics does not obey Kolmogorov’s power spectrum model in portions of the troposphere and stratosphere. These experiments have prompted the investigations of optical wave propagation through atmospheric turbulence described by non-classical power spectra. In this paper, using an original approach and considering a non-Kolmogorov power spectrum which uses a generalized power law instead of constant standard power law value 11/3 and a generalized amplitude factor instead of constant value 0.033, the variances of the angle-of-arrival fluctuations of the plane and spherical waves are derived in weak turbulence for a horizontal path. The concise closed-form expressions are obtained and used to analyze the influence of spectral power law variation on the angle-of-arrival fluctuations.     

Effect of the outer scale on the angle of arrival variance for free-space-laser beam corrugated by non-Kolmogorov turbulence

It is well known that atmospheric turbulence causes significant variations of the arrival angle of laser beams used in free-space communications. Usually, angle-of-arrival fluctuations of an optical wave in the plane of the receiver aperture is calculated by Kolmogorov’s power spectral-density model. Unfortunately, recently increasing experimental evidence has shown that atmospheric turbulence statistics does not obey Kolmogorov’s power spectrum model in some parts of the troposphere and stratosphere. These experiments have prompted investigations of the optical-wave propagation through atmospheric turbulence described by nonclassical power spectra. In this paper, employing a new approach and considering a non-Kolmogorov power spectrum with a generalized power law instead of the constant standard power-law value 11/3 and a generalized amplitude factor instead of the constant value 0.033, we derive the variances of the angle-of-arrival fluctuations of the plane and spherical waves in a weak turbulence for the horizontal path. The concise closed-form expressions are obtained and used to analyze the influence of spectral power-law variations on the angle-of-arrival fluctuations. In addition, the outer scale effect is also analyzed.     

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.     

Atmospheric Turbulence and Internal Gravity Waves Examined by the Method of Artificial Periodic Irregularities

A method for studying the Earth’s ionosphere at altitudes of the mesosphere and lower thermosphere based on creating artificial periodic irregularities in the ionospheric plasma by means of powerful radio waves is breafly described. Methods for determining the temperature and density of the neutral component and the velocity of vertical and turbulent motions by measuring the characteristics of the signal backscattered by the irregularities are described. The results of experiments performed on a SURA heating facility aimed at a comprehensive investigation of the natural processes occurring in the Earth’s lower ionosphere due to the propagation of atmospheric waves and turbulent phenomena are examined. Based on measurements of the amplitude and phase of the signal scattered by periodic irregularities, the most important characteristics of the neutral and plasma components of the Earth’s atmosphere at altitudes of the mesosphere and lower thermosphere are determined. Further research on the subject is discussed.     

内波、潮导致的声简正波幅度起伏及其深度分布

分析2005黄海声传播—内波实验传播起伏数据,观察（1）号声简正波幅度起伏及其深度分布特征。非线性内波（IW）通过接收阵可以导致脉冲峰值约5 dB起伏,幅度起伏垂向分布与简正波本征函数的微分分布吻合良好,（M2）潮致信号幅度起伏也满足相同深度分布特性。然而内波和潮致起伏的垂向相位变化明显不同:前者本征函数峰值两侧起伏信号反相,而后者近乎同相。前者由接收器与简正波分布的相对移动决定,而后者取决于简正波和水听器位置变化的复合效应。      

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.     

Phase Fluctuations of Radio Waves in the Saturated-Scintillation Regime

We consider the problem of phase fluctuations of radio waves behind a strong phase screen and in an optically thick layer. It is shown that the phase-fluctuation distribution of the received radiation at an observation point located in the saturated-scintillation area behind a turbulent phase screen is almost identical to the normal distribution of phase fluctuations of the wave on the screen. Amplitude and phase fluctuations of the received radiation are uncorrelated both for single-point and space-diversity reception if, in the latter case, the distance between the observation points exceeds the spatial scale of the diffraction component of the complex field of the received signal. Expressions for the mean square and the structural function of phase fluctuations of radio waves behind a turbulent phase screen in the saturated-scintillation regime are obtained. It is shown that the structural function of phase fluctuations in the diffraction component of the scattered field, which is exactly the function that forms saturated scintillations of the received radiation, almost coincides with the structural function of phase fluctuations on the screen. It is also shown that the diffraction effects can be neglected and the geometric-optical approximation should be used when calculating statistical characteristics of phase fluctuations of a plane wave in the saturated-scintillation area in an optically thick layer with large-scale refractive-index irregularities. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 48, No. 4, pp. 275–282, April 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.     

Magnetic zenith effect and some features of the multifractal structure of the small-scale artificial ionospheric turbulence

We present the results of the experiment on studying the multifractal structure (with inhomogeneity sizes from tens to hundreds of meters across the Earth’s magnetic field) of the artificial ionospheric turbulence when the midlatitude ionosphere is affected by high-power HF radio waves. The experimental studies were performed on the basis of the “Sura” heating facility with the help of radio sounding of the disturbed region of the ionospheric plasma by signals from the Earth’s orbital satellites. The influence of the magnetic zenith effect on measured multifractal characteristics of the small-scale artificial turbulence of the midlatitude ionosphere was examined. In the case of vertical radio sounding of the disturbed ionosphere region, the measured multipower and generalized multifractal spectra of turbulence coincide well with similar multifractal characteristics of the ionospheric turbulence under natural conditions. This result is explained by the fact that the scattering of signals by weak quasi-isotropic small-scale inhomogeneities of the electron number density in a thick layer with a typical size of several hundred kilometers above the region of reflection of high-power HF radio waves gives the major contribution to the observed amplitude fluctuations of received signals. In the case of oblique sounding of the disturbance region at small angles between the line of sight to the satellite and the direction of the Earth’s magnetic field, the nonuniform structure of the small-scale turbulence with a relatively narrow multipower spectrum and small variations in the generalized multifractal spectrum of the electron number density was detected. Such a fairly well ordered structure of the turbulence is explained by the influence of the magnetic zenith effect on the generation of anisotropic small-scale artificial turbulence in a thin layer having a typical size of several ten kilometers and located below the pump-wave reflection height in the upper ionosphere.     

Multipower spectrum of small-scale ionospheric turbulence

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

Frequency dependence and intensity fluctuations due to shallow water internal waves

A theory and experimental results for sound propagation through an anisotropic shallow water environment are presented to examine the frequency dependence of the scintillation index in the presence of internal waves. The theory of horizontal rays and vertical modes is used to establish the azimutal and frequency behavior of the sound intensity fluctuations, specifically for shallow water broadband acoustic signals propagating through internal waves. This theory is then used to examine the frequency dependent, anisotropic acoustic field measured during the SWARM'95 experiment. The frequency dependent modal scintillation index is described for the frequency range of 30-200 Hz on the New Jersey continental shelf.