A. V. Rakhlin

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.

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.

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.

Sura-wind radar: Studies of nonlinear effects using VHF-wave occultion of the ionosphere

We present experimental results on reception of VHF signals of the transmitting facility “Sura” of the Radiophysical Research Institute by the NASA spacecraft WIND. The experiments were performed during daytime during the summer seasons 1997–1998. The dependence of power and spectral characteristics of the VHF radio waves on the power of the sounding radiation is analyzed. We find that, in a wide range of effective powers of the VHF radiation from about 40 kW to 160 MW, the phenomena observed in the radiation received onboard WIND (such as focusing, scintillations, or frequency deviations of signals) does not usually depend on the operation mode of the “Sura” facility. At the same time, broadening of the radiation pattern of the facility and decrease of the mean level of the received signal by about 6 dB toward the direction of maximum of the radiation pattern were observed at the peak radiation power of the facility. The experimental results are compared to present concepts on effects of self-focusing instability and nonlinear defocusing of VHF radio waves in the ionospheric plasma.

Amplitude fluctuations in the theory of radio-wave refraction scattering

We consider some aspects of the theory of radio-wave scattering with respect to studies of radiation intensity fluctuations behind a random phase screen. The expressions for a piecewise approximation of the function of distribution of intensity fluctuations in the near zone and in the region of random focusing behind the screen are found. The expressions obtained are used to calculate the radio-wave scintillation index in these regions. The corresponding calculations are performed with allowance for the finite dimensions of external and internal scales of turbulent inhomogeneities of the phase screen. It is shown that the radio-wave refraction scattering, as a rule, is characterized by a pronounced random focusing of the radiation intensity behind the phase screen. However, under strongly developed turbulence of the radio-wave propagation medium (phase screen) the above phenomenon may be absent.

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.

Behavior characteristics of the index of middle-latitude F-spread

We present the results of analysis of data on ionospheric vertical sounding in Zimenki (Nizhny Novgorod) over the period from 1964 to 1992. The January ionograms are used for constructing the plots of daily indices of F-spread for an ionospheric layer. The dependence of the average monthly (January) index of F-spread on the solar activity is obtained. The index of F-spread is found to grow with time. The technogenic nature of this increase is assumed.

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.

On a Modification of the Scintillation Method

We solve the problem of diffraction of fluctuating radiation by an optically thin irregular layer (phase screen) with developed turbulent structure. It is shown that in the case of diffraction of radiation with saturated fluctuations and a narrow-band frequency spectrum by a weakly turbulent moving phase screen, the measured frequency spectrum of intensity fluctuations in the observation plane allows one to obtain information on the form of the spectrum of irregularities of an optically thin irregular layer in a wide size range significantly exceeding the size of the first Fresnel zone. Similarly to the well-known phase method of diagnostics of randomly irregular media, the conventional scintillation method modified in such a way yields undistorted information on the form of the irregularity spectrum. However, in contrast to the phase method, it also allows one to obtain data on the drift velocity of irregularities in the studied irregular layer.

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.