Structure and dynamics of the ionospheric region with artificial small-scale irregularities according to complex measurements of the scattered radio-signal characteristics

We present the results of complex measurements of characteristics of HF radio signals scattered by artificial small-scale irregularities by means of oblique sounding using a chirp signal and by direction finding. On the basis of comparison of the experimental data and the calculations of aspect scattering of radio waves, the conclusion on the patch structure of an artificially disturbed ionosphere filled with small-scale field-aligned irregularities is drawn. The sizes of the patches amount to several ten kilometers, and they are separated by 100 or more kilometers. The drift speed of irregularities in the direction of the scattering vector in the plane perpendicular to the magnetic field is determined for several patches. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 12, pp. 1011–1025, December 2008.     

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.     

Oblique chirp sounding of the modified ionosphere. Experiment and simulation

We present the results of experimental studies of the influence of artificial ionospheric disturbances on HF signals used for oblique sounding of the disturbed volume. The measurements have been performed by a chirp ionosonde over the path Yoshkar-Ola-“Sura”-niznhy Novgorod with length 234 km. We found the 2F2 mode to disappear (attenuation up to 20 dB) when the ionosphere is influenced by a vertical powerful radiation in the ordinary mode with long (15 min each) heating and pause intervals. Modeling of the observed effect was carried out. The calculations agree well with experimental data if the traveling ionospheric disturbances (TID) with vertical and horizontal scales l_z∼20 km and l_x∼50 km, respectively, and the relative disturbance of the electron density δN∼0.2–0.3 are amplified (generated) during the ionosphere heating. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 4, pp. 303–313, April 1999.     

Some distinctive features in the behavior of small-scale artificial ionospheric irregularities at mid-and high latitudes

We present the results of experimental studies of some features in the behavior of small-scale artificial irregularities (SSAIs) at mid-and high latitudes based on the “Sura” and EISCAT/HEATING HF facilities. Observations were performed by the method of aspect scattering using a network of diagnostic paths having a common reception point located near St. Petersburg. We found that an extremely long duration of the second (slow) stage of SSAI relaxation of up to 5 min occurs in the evening hours when the ionosphere above the “Sura” facility is illuminated by the Sun, but the solar terminator travels through the magnetically conjugated ionosphere. The conjecture is made that the processes initiated by the terminator are mostly responsible for secondary ionospheric turbulence maintaining the irregularities above “Sura.” A drastic increase in the Doppler spectra width of the scattered signals is revealed when the magnetically conjugate point of the ionosphere is located on the shade side of the terminator, but the ionosphere above the “Sura” facility is still lighted. It is assumed that the “ run away” of photoelectrons from the day to the night side could reduce the threshold of excitation of artificial irregularities, leading to an increase in their intensity. The presence of fairly intense scattered signals was detected from the “Sura” and EISCAT/HEATING experimental results both under conditions of pulsed HF heating after continuous heater-on periods and cycled HF heating by short pulses. In the case of pulsed heating by short pulses with duration τ_p < 100 ms and average radiated power P_a below the threshold power P_thr of the SSAI generation cutoff the irregularities can be maintained due only to striction parametric instabilities. The excitation of irregularites under the cycled HF pumping with the pulse duration τ_p = 384 ms for P_a comparable with P_thr was detected. The aspect-angle dependence, or the so-called magnetic zenith effect, was found in the SSAI intensity. The residual turbulence aftereffects played a significant role in the SSAI development. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 678–694, August 2007.     

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.     

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.     

Influence of small-scale irregularities on features of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part II. Relaxation stage

We present the results of experimental studies of the evolution of diagnostic stimulated electromagnetic emission of the ionosphere (DSEE) in a broad range of the pump and probing wave frequencies under sounding of the F-layer disturbed by powerful HF radio waves. We study the parameters of the overshoot effect in SEE evolution during the relaxation stage of artificial small-scale irregularities and show that the amplification of DSEE (inverse overshoot effect) is related to the relaxation of the anomalous attenuation due to scattering at these irregularities. We have found that the characteristic time scales of development of the overshoot effect decrease and its value increases as the pump wave frequency decreases from 6 to 4 MHz and as the probed volume moves from the edge to the central part of the disturbed volume. We relate the effects observed to the increasing intensity of the irregularities with scales l_∼∼3−10 m. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 810–824, August 1999.     

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.     

Influence of small-scale irregularities on the features of the overshoot effect in the temporal evolution of stimulated electromagnetic emission. Part I. Development stage

We present the results of experimental studies of the evolution of stimulated electromagnetic emission of the ionosphere (SEE) under the F-layer modification by powerful HF radio waves in a broad range of the pump wave frequencies. We compare the parameters of the overshoot effect in SEE evolution to the observations of anomalous attenuation and field-aligned scattering of radio waves. We show the overshoot effect to result from the anomalous attenuation under scattering at artificial small-scale ionospheric irregularities. We found the characteristic time scales of the overshoot effect development to decrease and its value to increase as the pump wave frequency decreases from 6 to 4 MHz; this is attributed to the observed increase of the irregularities amplitude at scales m. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 7, pp. 619–634, July 1999.     

Modelling of HF wave propagation in the vicinity of quasicritical rays in the disturbed ionosphere

The results of experimental studies of the fine structure of a signal in the vicinity of the maximum observed frequency (MOF) on the Khabarovsk—Nizhny Novgorod path of oblique chirp sounding (OCS) are presented. Additional tracks were observed in the region between the high-angle and low-angle rays during magneto-ionospheric disturbances. Under strong disturbances the ionograms were of a spreading type in the vicinity of the MOF. The observed effect was modelled in the presence of travelling ionospheric disturbances (TID) with different parameters. It is shown that the stratification of the high-angle ray into several additional tracks is a maximum for TID with vertical scales km whose wave fronts make angles about 0–10° with the horizontal line. The possibilities of using the Pedersen mode as a probing wave for diagnostics of the fine structure of the ionosphere in the vicinity of the F-layer maximum are discussed. Radiophysical Research Institute, Nizhny Novgorod; State Technical University of Ioshkar-Ola, Mari Republic; Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation, Russian Academy of Sciences, Troitsk, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 1, pp. 3–12, January, 1998.     

Field-aligned scattering of HF radio waves under conditions of action of high-power oblique radio waves on the ionosphere

We present the results of experimental studies of the features of field-aligned scattering of HF radio waves for different angles of incidence of high-power radio waves on the ionosphere. The amplitude, Doppler frequency shift, and azimuthal and vertical arrival angles of the scattered signals are measured. Calculated ray paths in the magnetoactive ionosphere are compared with experimental data on the scattered-signal reception under conditions of action of high-power oblique radio waves on the ionosphere. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 8, pp. 669–677, August 2007.     

The polar-ionosphere phenomena induced by high-power radio waves from the spear heating facility

We present the results of experimental studies of specific features in the behavior of small-scale artificial field-aligned irregularities (AFAIs) and the DM component in the spectra of stimulated electromagnetic emission (SEE). Analysis of experimental data shows that AFAIs in the polar ionosphere are generated under different background geophysical conditions (season, local time, the presence of sporadic layers in the E region, etc.). It is shown that AFAIs can be excited not only in the F region, but also in “thick” sporadic E _s layers of the polar ionosphere. The AFAIs were observed in some cycles of heating when the HF heater frequency exceeded the critical frequency by 0.3–0.5 MHz. Propagation paths of diagnostic HF radio waves scattered by AFAIs were modelled for geophysical conditions prevailing during the SPEAR heating experiments. Two components, namely, a narrow-banded one with a Doppler-spectrum width of up to 2 Hz and a broadband one observed in a band of up to 20 Hz, were found in the sporadic E _s layer during the AFAI excitation. Analysis of the SEE spectra shows that the behavior of the DM component in time is irregular, which is possibly due to strong variations in the critical frequency of the F ₂ layer from 3.5 to 4.6 MHz. An interesting feature observed in the SPEAR heating experiments is that the generation of the DM component was similar to the excitation of AFAIs when the heater frequency was up to 0.5 MHz higher than the critical frequency. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 11, pp. 939–950, November 2008.     

Sounding of an artificially perturbed ionosphere by means of an ionosonde/position finder with chirp modulation of the signal

We describe the operation of an ionosonde/position finder with chirp modulation of the signal. The first results of measuring the characteristics of short-wave radio signals scattered by artificial small-scale inhomogeneities, which were obtained by means of an ionosonde/position finder on the IZMIRAN—“SURA”—Rostov-on-Don path are presented. It was found that under certain ionospheric conditions, the angular and frequency selection of the scattered signals take place, in which case the signals are observed simultaneously in several frequency intervals (mainly, in three, namely, 6–9.5 MHz, 10–12 MHz, and 15–18 MHz) with different angles of incidence of radio waves in the vertical plane. In this case, the incidence angles were 20◦–35◦, 18◦–32◦, and 10◦–20◦ from the horizon for the first, second, and third frequency interval, respectively. Ionograms of oblique sounding were modeled allowing for the scattering of radio waves by artificial small-scale inhomogeneities. It is shown that at frequencies from 10 to 12 MHz, aspect conditions are fulfilled for the signals ducting along the high-angle beam (Pedersen mode). At frequencies 15–18 MHz (higher than the maximum observable frequency of the forward signal on the path IZMIRAN—Rostov-on-Don), aspect scattering conditions are fulfilled for the signals incident on a scattering area in the ascending part of the trajectory. At low frequencies 6–9.5 MHz (below the maximum observed frequency of the forward signal on the IZMIRAN—Rostov-on-Don path), the observable additional signals are caused by the scattering of radio waves by artificial inhomogeneities with subsequent relfection of the scattered signal from the Earth on the “SURA”—Rostov-on-Don path. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 52, No. 4, pp. 267–278, April 2009.     

Pedersen mode ducting in a randomly stratified ionosphere

Abstract

A new HF propagation mechanism giving rise to a Pedersen mode has been discovered. The boundary frequency f _b dividing the Pedersen mode frequency range into parts with different HF propagation mechanisms is determined. A known sliding propagation mechanism characterized by an exponential decrease in the field with the distance travelled by waves along the layer prevails at frequencies f > f _b. At the low end of the Pedersen mode frequency range (f < f _b) the stable wave formation associated with a field localization effect under HF propagation in a randomly stratified ionosphere appears. The observed phenomenon is described in terms of the wave theory of diffraction. Numerical calculation of the wave field amplitude is performed using the parabolic equation method. The Pedersen mode ducting effect due to multiple re-reflection of a sliding beam at anisotropic irregularities elongated along the F2 layer with characteristic scales from hundreds of metres to several kilometres vertically and tens of kilometres horizontally is demonstrated. The irregularities with anisotropy coefficients ～1–3 also localize waves but the size of the effect is between one and two orders smaller than in a model of strong stratified irregularities with η ～10–30.     

The wideband attenuation of short radio waves on mid-latitude paths during X-ray flares in January 2005

We present the results of the ionosphere oblique chirp sounding on the Cyprus—Nizhny Novgorod, Cyprus—Rostov-on-Don, and Moscow—Rostov-on-Don mid-latitude paths during X-ray flares in January 17, 19, and 20, 2005. It is found that during strong flares the blackout of short radio waves was observed over the entire frequency range of chirp sounding on the Cyprus—Nizhny Novgorod and Cyprus—Rostov-on-Don paths. Modeling of the electron-density profiles in the lower ionosphere based on absorption of short radio waves on the Moscow—Rostov-on-Don path at different stages of the decay of the X-ray radiation intensity is carried out. It is shown that at the instant corresponding to the maximum value of the flare radiation flux, the electron density in the lower ionosphere at altitudes 60–80 km increased by a factor of about 10 and 100 for flares with radiation flux densities 5·10⁻² and 3·10⁻¹ erg/(cm ²·s) in the wavelength range 0.5–4.0 Å which took place in January 19 and 20, respectively. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 1, pp. 1–8, January 2007.     

Model of separate sample spectra of coherent-echo signals based on Irkutsk incoherent-scatter radar data

We propose a model of coherent-echo signals during the monostatic sounding of the ionosphere. The model is based on the previously obtained radar equation for separate samples of the scattered signal. The dielectric-permittivity perturbation described by a discrete set of spatial harmonics modulated in space and time is used as the scattering irregularities. The model was tested using Irkutsk incoherent-scatter radar data obtained during the coherent-echo observations in July 15 and 16, 2000. The test shows that the model is suitable for describing the observed characteristics of separate sample spectra of the coherent-echo signals. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 49, No. 6, pp. 459–477, June 2006.     

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.     

Influence of ionospheric irregularities on the form of radio signal scattering spectrum in over-the-horizon radar sounding of the rough sea surface

Abstract

This paper is concerned with the backscattering of HF radio waves from the rough sea surface, which have propagated through the ionosphere with random large-scale irregularities.

For the sake of simplicity, it is assumed in calculations that the rough sea surface is a perfectly conducting surface with the known Philips power spectrum of irregularities. Ionospheric irregularities of a random medium that are isotropic and single-scale ones, with a Gaussian spectrum, are considered within the limits of the hypothesis of frozen-in irregularities.

Within the first approximation of perturbation theory, using, as the incident wave and the Green function, their geometrical-optics approximations, we obtained the expression for the backscattering spectrum of the ionospheric chirp radio signal with a Gaussian envelope. The expression involves the parameters of the receive–transmit antenna, the signal, the propagation medium, and of the scattering surface. Numerical simulation was used to investigate the influence of all the above-mentioned parameters on the backscattering spectrum. It is shown that travel of ionospheric irregularities has the largest influence on the scattering spectrum, the signal parameters mainly determine the size of the scattering area in the range, and the form of the coherent integration window determines the form of the received signal and can distort it.     

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.     

Monitoring of wave-like disturbances by means of oblique sounding of the ionosphere

We present the results of experimental studies of ionospheric wave-like disturbances on the basis of oblique chirp sounding data for the mid-latitude paths Cyprus-Rostov-on-Don and Inskip-Rostov-on-Don. Twenty-four-hour measurements were performed in January-May and October–November, 2005 and in January, 2006. The spectral analysis of variations in the maximum observed frequency (MOF) is performed. It is shown that the spectra of the MOF fluctuations have a well pronounced line structure. The amplitude-dominant spectral harmonics are concentrated near the frequencies 0.18–0.8 mHz (20–90-min periods). We give the results of modeling of radio waves propagation in the presence of traveling ionospheric disturbances and compare the calculated and experimental data. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radio.zika, Vol. 49, No. 12, pp. 1015–1029, December 2006.