On the features of decameter radio astronomy

We consider some effects of the VHF radio wave propagation in randomly irregular plasma near the Earth. Applications of these effects to main problems of decameter radio astronomy are discussed. In particular, we show that significant measurement errors of about tens to hundreds of percent for the intensity of the VHF radio emission from an extra-terrestrial source may occur due to scattering and focusing/defocusing of the radiation in the ionosphere if VHF radio astronomical facilities operate at middle latitudes. We find that the angular resolution of discrete radio sources observed using radio interferometry and the well-known scintillation methods cannot be better than about a degree due to the effect of the developed ionospheric turbulence. We propose a modified scintillation method based on the spectral analysis of radio emission from discrete sources, which allows the useful high-frequency signal corresponding to diffraction of VHF emission at weak inhomogeneities of interplanetary plasma to be separated against strong, relatively low-frequency fluctuations of the received radiation due to the effect of the developed turbulent structure of the ionosphere. We show that such a method allows the angular resolution of extra-terrestrial radio sources to be improved up to arcsecond level. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 43, No. 2, pp. 95–105, February 2000.     

Radiophysical and radio astronomical diagnostics of ionospheric effects induced by a ground-based infrasonic transmitter (preliminary results)

We present results of preliminary studies aimed at detection of weak ionospheric disturbances induced by acoustic emission of a ground-based controlled transmitter. Radio astronomical and radiophysical facilities based on the decameter radio telescope URAN-3 were used in these experiments. Three methods were applied in this study: occultation of the disturbed region by radio emission from discrete cosmic sources, scattering of sounding radio waves from the perturbed ionospheric region, and weakly-oblique sounding of the ionosphere. It is shown that the parameters of transmitted, scattered, and reflected signals are well correlated with the parameters of the acoustic radiation, and that the weak ionospheric disturbances detected in our experiments are actually induced by the acoustic radiation. G. V. Karpenko Physico-Mechanical Institute of the National Academy of Sciences of Ukraine, Khar'kov, Ukraine. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 42, No. 8, pp. 785–798, August 1999.     

The Sura-EISCAT-WIND Experiments: Ionospheric Influence on the Response of a Decameter Interferometer with a Superlong Baseline

Results of the 1999 experiments on receiving radio-frequency signals from the SURA and EISCAT facilities on the WIND spacecraft at 5475 kHz are presented. Power and frequency distortions imposed by near-Earth plasmas on the response of SURA-EISCAT, an active decameter radio interferometer with a superlong baseline of 2000 km, are studied. Quasi-oscillatory variations in the intensity of the received radiation with a period of several tenths of a second and the corresponding maximum in the intensity fluctuation spectrum are observed during synchronized operation of the facilities under both quiet and perturbed geophysical conditions including the occurrence of mid-latitude F spread. Variations in the mean frequency of the spectral line both because of the motion of the spacecraft and the large-scale traveling ionospheric disturbances are detected. The obtained results are compared with the modern theoretical concept of propagation of short radio waves in a randomly irregular ionospheric plasma. The possibility to realize the limiting angular resolution of a ground-based decameter interferometer with a superlong baseline for observations of discrete space radio sources is discussed.     

Experimental determination of physical processes in space,leading to deviations of radio synchrotron radiation spectra from the power law

We present universal formulas for spectral characteristics of cosmic radio sources of synchrotron radiation upon the presence of spectral density maxima at certain frequencies (spectra with negative curvature) taking into account most typical physical processes observed in space. On the basis of long-term observations of angular radiation structure of cosmic radio sources in the decameter wavelength range by the URAN radio interferometer system, we determine most probable physical processes resulting in spectra with extremum values for several quasars, radio galaxies, and their separate components. On the basis of these data, we estimate some parameters of cosmic medium, magnetic field, and angular sizes of compact radio sources and their components. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 51, No. 2, pp. 91–110, February 2008.     

Effect of limiting polarization in the Jovian inner magnetosphere

Jovian decameter emission is known to exhibit almost total polarization. We consider the elliptical polarization to be a consequence of linear-mode coupling in the Jovian magnetosphere outside the source region. We determine conditions of emission propagation along the ray path that are necessary for self-consistent explanatation of the polarization observations and show that the ellipticity (axial ratio of the polarization ellipse) is determined by the magnetospheric plasma density n_e in a small region a distance of about half the Jovian radius from the radiation source. The plasma density in the region is quite low, n_e<0.4 cm⁻³, and the geometrical-optics approximation of emssion propagation in front of the region converts to the vacuum approximation behind it. The latter means that the linear-mode coupling in the Jovian inner magnetosphere is manifested as the effect of limiting polarization. Sources of decameter emission emitting at different frequencies f are located at heights corresponding to gyrofrequency levels f _Be ≅f and at magnetic-force lines that belong to L-shells passing through the satellite Io. The location of the transitional region in the Jovian magnetosphere varies depending on the emission frequency and the time. For each given decameter radio emission storm occupying some region in frequencytime space, we have a number of transitional regions located in a certain region of the Jovian magnetosphere—the interaction region of the magnetosphere (IRM) for the given emission storm. The distribution of magnetospheric plasma in an IRM is found from data of observations of the polarization ellipiicity of the given decameter radio emission storm. By matching the calculated ellipticity of emission with the observed ellipticity at every point of frequency-time space of the emission dynamic spectrum one finds a recurrent relation between the local values of the magnetospheric plasma density N_c and the planetary magnetic field B in the IRM, which allows evaluation of the distribution of plasma density if a definite model of the Jovian magnetic field has been adopted. Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod, Russia; Space Research Institute, Austrian Academy of Sciences, Austria. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 2, pp. 177–193, February, 1998.     

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.     

Energy Spectrum of Ultrahigh-Energy Cosmic Rays according to Data from Ground-Based Scintillation Detectors of the Yakutsk EAS Array

Results obtained from an analysis of the energy spectrum of cosmic rays with energies in the region of E₀ ≥ 10¹⁷ eV over the period of continuous observations from 1974 to 2017 are presented. A refined expression for estimating the primary-particle energy is used for individual events. This expression is derived from calculations aimed at determining the responses of the ground-based and underground scintillation detectors of the Yakutsk array for studying extensive air showers (EAS) and performed within theQGSJET-01-d, QGSJET-II-04, SIBYLL-2.1, and EPOS-LHCmodels by employing the CORSIKA code package. The new estimate of E₀ is substantially lower than its counterpart used earlier.     

Technique and Instrumentation for Precision Measurements of the Intensities of Extraterrestrial Radio Sources

We consider methods for decreasing the measurement error of the intensities of radio sources. The design of a two-temperature standard of noise radiation for calibrating antennas in the case of absolute measurements of the radiation flux densities of radio sources by the method of a black disk is described. We consider a method for taking into account the nonuniformity of the intensity distribution around the studied cosmic source, which facilitates choosing reference regions.     

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.     

Poynting flux impulses and the ionospheric switching of geomagnetic substorms

Observations were made of impulse events in Poynting flux calculated from electric and magnetic disturbances encountered by the Polar satellite when on high-latitude field-lines in the magnetotail. These were found to be coincident within±6 min with impulsive spikes in cosmic radio background absorption in the D region of the ionosphere as detected by the Imaging Riometer for Ionospheric Studies riometer in Finland. They were also coincident with substorm onset at the same geomagnetic latitude as determined by a change of gradient in International Monitor for Auroral Geomagnetic Effects’ X-component magnetograms. The interpretation of the observations was that magnetospheric compression waves from the geomagnetic equator region of the magnetotail were coupling to progressively initiate field-guided Alfvén shear waves towards higher geomagnetic latitudes over a large volume of the magnetosphere. The study suggested that they were then able either directly or indirectly to ionise the D region of the ionosphere and in the process to cut deep electrically conducting channels between the magnetosphere and the ionosphere through which currents could flow and initiate the characteristic signature of geomagnetic substorms in ground magnetograms.     

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 the scintillations of radiation from an extended source in a randomly inhomogeneous medium

Using the method of refractive scattering of radio waves (RSRW), we solve the problem of space correlation of scintillations of radiation from an extended source in a randomly inhomogeneous medium. General expressions are obtained for the index and radius of the space correlation of fluctuations of intensity of radiation from a source with finite angular dimensions as it propagates though a multilayered medium with refractive index fluctuations. The RSRW method is similar to the diffraction calculation of space correlation of scintillations of an extended source in a thick layer with inhomogeneities. We note that under certain conditions an increase in the number of inhomogeneous layers on the radio wave propagation path leads to a pronounced decrease in the scintillation index and an increase in the space correlation of fluctuations of intensity of received radiation from an extended source. The results obtained have a simple geometric-optical interpretation. We indicate a specific feature in determining the angular dimensions of extended radio sources using the known method of scintillations. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, No. 5, pp. 586–593, May, 1997.     

Scintillation index of a stochastic electromagnetic beam propagating in random media

We study the behavior of the scintillation index (the normalized variance of fluctuating intensity) of a wide-sense statistically stationary, quasi-monochromatic, electromagnetic beam propagating in a homogeneous isotropic medium. In particular, we show that in the case when the beam is treated electromagnetically apart from the correlation properties of the medium in which the beam travels not only its degree of coherence but also its degree of polarization in the source plane can affect the values of the scintillation index along the propagation path. We find that, generally, beams generated by unpolarized sources have reduced level of scintillation, compared with beams generated by fully polarized sources, provided they have the same intensity distribution and the same state of coherence in the source plane. An example illustrating the theory is considered which examines how the scintillation index of an electromagnetic Gaussian Schell-model beam propagates in the turbulent atmosphere. These results may find applications in optical communications through random media and in remote sensing.     

Artificial focusing system in the ionosphere

We propose a method for the creation of an artificial focusing radio wave system in the ionospheric E layer on the basis of the recombination effect by production of a bagel-shaped disturbance region in the ionosphere in the two-dimensional case and a two-band disturbance region in the one-dimensional case. Two-band disturbance of the ionosphere was achieved by appropriate reconstruction of the antenna system of the SURA facility. First experiments on the diagnostics of a focusing ionospheric system have been performed by observations from the NASA WIND space vehicle. Radiophysical Research Institute, Nizhny Novgorod, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 40, Nos. 1-2, pp. 101–110, January–February, 1997.     

Studies of the Irregular Structure of the Ionosphere using Scattering of Radio Waves on Artificial Periodic Inhomogeneities

We present new results of our studies of the irregular structure of the ionosphere using artificial periodic inhomogeneities (APIs) of the ionospheric plasma. The observations were carried out from 9:00 to 17:00 in August 10–12, 1999 with a height step of 0.7 km and digital registration and real-time processing of the signal quadratures. It is shown that in many cases, the amplitude of the scattered signal is determined by the interference of radio waves scattered on APIs and on natural ionospheric formations including sporadic layers and large-scale natural irregularities. This allows one to study the irregular structure of the lower ionosphere by analyzing height-time dependences of the amplitude and phase of the scattered signal.     

Spectrum of long-term cosmic ray variations during the sunspot minimum in 2009

A method for determining the long-term spectrum of cosmic ray (CR) variations from observations with different ground-based (neutron monitors) and near-earth (stratosphere and satellite) detectors was presented in our previous works. In this paper, muon telescope data (Nagoya) are added, expanding the analyzed energy range and allowing us to determine the rigidity spectrum for 1974–2011. The data from the above devices for these years can be used jointly. The rigidity spectrum for the sunspot minimum of the 24th cycle was studied and compared with the CR intensity for the sunspot minima of other cycles. The reasons for abnormally high CR density during the minimum of the 24th cycle and other features of this period are discussed.     

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.     

A 151 MHZ radioastronomical antenna of wave channel elements

We describe a simple, inexpensive multibeam receiving antenna array with wavelength λ=2 m for radioastronomical studies of the solar wind by interplanetary scintillation observations of a large number of weak space radio sources. Radioastronomical Observatory of the Astrocosmic Center of the Lebedev Physical Institute, Pushchino, Moscow region, Russia. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 41, No. 7, pp. 824–840, July, 1998.     

The moon as a cosmic source of linearly polarized radio emission

We analyze parameters of the partially linearly polarized thermal radio emission from the Moon taking the effects of radiative heat transfer and surface roughness into account. The distributions of the Stokes parameters I, Q, and U over the visible lunar disk are considered. The polarization parameters of the integral radio emission as functions of the frequency and the Moon phase are obtained by integrating the Q and U distributions. We consider the possibility of using the Moon as a reference source of partially linearly polarized radiation for space-borne and ground-based projects aimed at studying polarization of the Galactic radio emission and the cosmic microwave background. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 7, pp. 593–606, July 2007.     

Analysis of ground level enhancement on January 6, 2014

Variations in the rigidity spectrum and anisotropy of cosmic rays during ground level enhancement (GLE) on January 6, 2014, is investigated using ground-based observations of cosmic rays (CRs) from the worldwide network of stations and spacecraft measurements obtained via a spectrographic global survey. The CR rigidity spectrum and relative variations in the intensity of CRs with rigidity of 4 GV are presented in the solar–ecliptic geocentric coordinate system in certain periods of the investigated event. It is shown that protons were accelerated during this GLE up to a particle rigidity of R ~ 2.4 GV. In the ~0.3 to ~2.4 GV range of rigidity, the CR differential rigidity spectra during the considered event were described by neither a power function nor an exponential function of particle rigidity. At the time of GLE, the Earth was in a loop-like IMF structure.