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.     

Artificial periodic inhomogeneities and a model for the lower part of theD region

The measured height profiles of the relaxation time and amplitudes of artificial periodic inhomogeneities (API) in the ionospheric D region are compared with the model. It is shown that the height dependences of the relaxation time of APIs and the amplitude of the scattered signal can be explained taking into account the hieght profile of the atmosphere density. Most probably, the upper boundary of APIs in the D region is due to the increase in the atomic oxygen density and can serve as an altitude indicator. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 5, pp. 431–437, May 1999.     

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.     

Artificial Periodic Inhomogeneities of the Ionosperic Plasma as a Promising Line in the Ionospheric Research

This paper is dedicated to a new method of ionospheric studies, developed at the Radiophysical Research Institute (NIRFI) and based on the creation of artificial periodic inhomogeneities (APIs) of the ionospheric plasma. We review the techniques and present the results of determination of the basic parameters of the ionosphere and atmosphere.     

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.     

Chirp ionosonde as a gauge for impulse characteristic and transfer function of the radio channel

When the ionosphere is sounded by a chirp signal, the information source is the amplitude and phase modulation of the signal due to its propagation in the ionospheric radio channel. We propose the method of received-signal demodulation which allows us to use the chirp ionosonde as a gauge for impulse characteristic and transfer function of the ionospheric radio channel. Operation of the chirp ionosonde is numerically simulated and estimates of intrinsic accuracy in measuring the mentioned characteristics are obtained.     

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.     

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 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.     

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.     

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 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.     

Results of Determining the Electron Number Density in the Ionospheric E Region from Relaxation Times of Artificial Periodic Irregularities of Different Scales

We present the first results of determining the electron number density in the ionospheric E region by a novel technique based on the creation of artificial periodic irregularities when the ionosphere is affected by powerful radio emission at two frequencies. Using the results of the measurements performed in October 2006 during heating of the ionosphere by the “Sura” facility radiation at frequencies 4.7 and 5.6 MHz, we obtained the electron number density profiles in an altitude range of 100 to 110 km. Features of the procedure of measurement and calculation of the electron number density are described in detail. It is shown that the method can be used for a study of the irregular structure of the lower ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 6, pp. 477–484, 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.     

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.     

Spatial Correlation of a HF Wave Field in the Case of Oblique Reflection from the Ionosphere

We consider the spatial statistical characteristics of an HF signal and the interdependence of the correlation functions of the field amplitude and phase in the case of oblique reflection from the ionosphere. The amplitude and phase parameters are expressed either in terms of the parameter of ionospheric turbidity, which can be determined most easily, or only in terms of the correlation coefficient of amplitudes at different points. We obtain the expressions for the amplitude and phase correlation functions, which describe well the experimental data.     

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.