INTERSTELLAR MEDIUM AND DECAMETER RADIO SPECTROSCOPY

Purpose: The analytical review of the main results of research in the new direction of the low-frequency radio astronomy, the interstellar medium radio spectroscopy at decameter waves, which had led to astrophysical discovery, recording of the radio recombination lines in absorption for highly excited states of interstellar carbon atoms (more than 600). Design/methodology/approach: The UTR-2 world-largest broadband radio telescope of decameter waves optimally connected with the digital correlation spectrum analyzers has been used. Continuous modernization of antenna system and devices allowed increasing the analysis band from 100 kHzto 24 MHz and a number of channels from 32 to 8192. The radio telescope and receiving equipment with appropriate software allowed to have a long efficient integration time enough for a large line series simultaneously with high resolution, noise immunity and relative sensitivity. Findings: A new type of interstellar spectral lines has been discovered and studied, the interstellar carbon radio recombination lines in absorption for the record high excited atoms with principal quantum numbers greater than 1000. The line parameters (intensity, shape, width, radial velocity) and their relation ship with the interstellar medium physical parameters have been determined. The temperature of line forming regions is about 100 K, the electron concentration up to 0.1 cm–3 and the size of a line forming region is about 10 pc. For the first time, radio recombination lines were observed in absorption. They have significant broadening and are amplified by the dielectronic-like recombination mechanism and are also the lowest frequency lines in atomic spectroscopy. Conclusions: The detected low-frequency carbon radio recombination lines and their observations have become a new highly effective tool for the cold partially ionized interstellar plasma diagnostics. Using them allows obtaining the information which is not available with the other astrophysical methods. For almost half a century of their research, a large amount of hardware-methodical and astrophysical results have been obtained including a record number of Galaxy objects, where there levant lines have been recorded. The domestic achievements have stimulated many theoretical and experimental studies in other countries, but the scientific achievements of Ukrainian scientists prove the best prospects for further development of this very important area of astronomical science. Key words: low-frequency radio astronomy; radio telescope; interstellar medium; radio recombination lines; carbon; hydrogen; spectral analyzer

OPERATOR METHOD IN THE PROBLEM OF A PLANE ELECTROMAGNETIC WAVE DIFFRACTION BY AN ANNULAR SLOT IN THE PLANE OR BY A RING

Purpose: The problem of a plane electromagnetic wave diffraction by an annular slot in the perfectly conducting zero thickness plane is considered. As a dual problem, the problem of diffraction by a perfectly conducting zero thickness ring is also considered. The paper aims at developing the operator method for the axially symmetric structures placed in free space. Design/methodology/approach: The problem is considered in the spectral domain. The scattered field is expressed in terms of unknown Fourier amplitudes (spectral functions). The annular slot is given as a unity of two simple discontinuities, namely of a disk and a circular hole in the plane, which interact with each other. The Fourier amplitude of the scattered field is sought as a sum of two amplitudes, the Fourier amplitude of the field of currents on the disk and Fourier amplitude of the field of currents on the perfectly conducting plane with circular hole. The operator equations are written for these amplitudes, which take into account the electromagnetic coupling of the disk and the hole in the plane. The equations use the reflection operators of a single isolated disk and a single hole in the plane. They are supposed to be known and can be obtained for example by the method of moments.The reflection operators can have singularities. After transformations, the equations are obtained, which are equivalent to the Fredholm integral equations of second kind and they can be solved numerically. Findings: The operator equations relative to the Fourier amplitudes of the field scattered by the discussed structure are obtained. The far zone scattered field for an annular slot and a ring for different values of parameters are studied. Conclusions: The rigorous solution of the problem of the electromagnetic wave diffraction by an annular slot in the plane and by a circular ring is obtained. The problem is reduced to the Fredholm integral equations of second kind. The far field distribution for different parameters is studied. The developed approach is an effective instrument for a number of problems of antenna technique to be solved. Key words: circular hole; disk; annular slot; ring; operator method; diffraction

FEATURES OF IONOSPHERIC EFFECTS FROM THE PARTIAL SOLAR ECLIPSE OVER THE CITY OF KHARKIV ON 10 JUNE 2021

Purpose: Solar eclipses pertain to high-energy sources of disturbance in the subsystems of the Sun–interplanetary-medium–magnetosphere–ionosphere–atmosphere–Earth and the Earth–atmosphere–ionosphere–magnetosphere systems. During the solar eclipse, the coupling between the subsystems in these systems activates, and the parameters of the dynamic processes become disturbed. Investigation of these processes contributes to understanding of the structure and dynamics of the subsystems. The ionospheric response to the solar eclipse depends on the season, local time, magnitude of the solar eclipse, phase of the solar cycle, the observation site, the state of space weather, etc. Therefore, the study of the effects, which each new solar eclipse has on the ionosphere remains an urgent geophysics and radio physics problem. The purpose of this paper is to describe the radio wave characteristics and ionospheric parameters, which accompanied the partial solar eclipse of 10 June 2021 over the City of Kharkiv. Design/methodology/approach: To make observations, the means of the HF Doppler measurements at vertical and oblique incidence available at the V. N. Karazin Kharkiv National University Radiophysical Observatory were employed. The data obtained at the “Lviv” Magnetic Observatory were used for making intercomparison. Findings: The radiophysical observations have been made of the dynamic processes acting in the ionosphere during the solar eclipse of 10 June 2021 and on the reference days. The temporal variations in the Doppler frequency shift observed at vertical and oblique radio paths have been found to be, as a whole, similar. Generally speaking, the Doppler spectra over these radio propagation paths were different. Over the oblique radio paths, the number of rays was greater. The solar eclipse was accompanied by wave activity enhancement in the atmosphere and ionosphere. At least three wave trains were observed. The values of the periods (about 5–12 min) and the relative amplitudes of perturbations in the electron density (δN≈0.3–0.6 %) give evidence that the wave disturbances were caused by atmospheric gravity waves. The amplitude of the 6–8-min period geomagnetic variations has been estimated to be 0.5–1 nT. Approximately the same value has been recorded in the X component of the geomagnetic field at the nearest Magnetic Observatory. The aperiodic effect of the solar eclipse has appeared to be too small (less than 0.01 Hz) to be observed confidently. The smallness of the effect was predetermined by an insignificant magnitude of the partial eclipse over the City of Kharkiv (no more than 0.11). Conclusions: The features of the solar eclipse of 10 June 2021 include an insignificant magnitude of the aperiodic effect and an enhancement in wave activity in the atmosphere and ionosphere. Key words: solar eclipse; ionosphere; Doppler spectrum; Doppler frequency shift; electron density; geomagnetic field; atmospheric gravity wave

FUNCTIONAL DAMAGE OF RADIO ELECTRONIC SYSTEMS

Purpose: The most important problem of any state is protection of the control and management systems used for the country, national armed forces, high-risk facilities (nuclear power plants, large chemical plants, airports, etc.). Here, the fact that the means of attack can be deployed on ballistic and cruise missiles, aircraft, and drones should be accounted for. The flight altitude of these vehicles varies from ≈300 km to ≈ 10 m. Any attack vehicle is equipped with complex avionics consisting of circuit elements sensitive to electromagnetic fields. Since the 1980s, a new scientific and engineering direction has been developing, being termed as a “functional damage to avionics”. It is based on the creation of powerful means of electromagnetic radiation possessing the energetic capabilities of incapacitating avionics at significant distances (from ~ 100 m to ~ 1000 km). The purpose of this work is to analyze the possible functional damage to avionics with account for the tendencies in avionics technologies. Design/methodology/approach: The analysis is made on the capability of inflicting functional damage to avionics accounting for the modern trends in developing the powerful means of electromagnetic energy generation in the microwave and shorter wavelength ranges, miniaturization and integration of avionics circuit elements. The regression is constructed for the critical energy time dependence. It has been determined that for decades the critical energy required to damage the circuit elements shows a tendency to decrease. This is due to the further miniaturization and integration of microcircuits according to the Moore’s law, which is still valid for now. For a number of circuit elements, the critical energy is found to be in the range of 10-11–10-10 J. At the same time, a reverse tendency arises to protect avionics from being functionally damaged. In this case, the critical energy makes 10-7–10-6 J and greater. From the derived version of the basic equation of functional damage to avionics, the maximum distance at which the damage is possible with the energetics of the existing radio systems is estimated. For the ground-based facilities, this distance can attain hundreds of kilometers. For mobile vehicles, it can reach 10–100 km. Combining target detection, identification and avionics damage capabilities in one radio system has been validated and advised. The transition from the first mode of operation to the second one occurs at shorter distances with an increase of 2–3 orders of magnitude in the pulse energy. Findings: The regression equation has been obtained for the time dependence of the critical energy required for inflicting functional damage to avionics. Its constant decrease has been confirmed. Such a behavior is closely related to the Moore’s law, which characterizes the degree of miniaturization and integration of avionics circuit elements. It has been predicted that for a number of instruments the critical energy can be smaller than 10-11–10-10 J. A version of the basic equation of functional damage to avionics has been obtained. The maximum distance for a modern radio system to damage the avionics has been shown to attain many hundreds of kilometers. For the radio systems installed on mobile vehicles, this distance makes 10–100 km. Target detection, tracking and identification, as well as avionics damage capabilities, have been proved to be rationally combined in one radio system. To cause damage at a corresponding range, the pulse energy needs to be increased by a factor of 102–103. Conclusions: There are all science and technology prerequisites for developing effective radio systems inflicting functional damage to avionics and for the state defense and protection, armed forces, and high-risk facility controlling systems. Key words: functional damage; avionics; critical energy; Moore’s law; functional damage equation; radiolocation equation; detection and destruction range

A FABRY-PEROT METARESONATOR SUPPORTING TRAPPED-MODE RESONANCES

Purpose: Investigation of the electrodynamic properties of a Fabry-Perot metaresonator formed by two parallel perfectly conducting, two-dimensionally periodic, two-element screens of finite thickness with rectangular holes. The resonator is excited by a plane linearly polarized electromagnetic wave. The basic cell of each of the screens used as the metaresonator mirrors contains two lengths of rectangular waveguides of different transverse sections. Design/methodology/approach: An operator method for solving the 3D problems of electromagnetic wave diffraction by multielement two-dimensionally periodic structures is used in the study. The computation algorithm uses the partial domain technique and the method of generalized scattering matrices. Findings: As follows from the results of the numerical modeling made, the magnitude of the plane wave reflected from the metaresonator turns to zero at fixed frequencies lying below the cutoff frequencies for the rectangular waveguide sections embedded in the resonator mirrors. The effect of the total electromagnetic wave transmission through the metaresonator at the first lower frequency is characterized by a strong localization of the electromagnetic field in the resonator volume. The reason is excitation of the metaresonator by the exponentially descending field penetrating inside the resonator through the evanescent holes at the resonance frequency. The second low-frequency resonance of the total electromagnetic wave transmission through the metaresonator is associated with the trapped-mode resonance, which is observed in multielement two-dimensionally periodic structures. This case is characterized by a strong localization of the electromagnetic field from both sides near the metaresonator mirror surfaces. Conclusions: The unique electrodynamic properties of the metaresonator can find application in the devices for measuring the electrophysical parameters of composite materials with high losses. The effect of strong localization of the electromagnetic field both in the resonator volume and near the mirror surfaces can be used for monitoring the gaseous substances in crowded places. Key words: two-dimensionally periodic screen; rectangular waveguide; Fabry-Perot metaresonator; reflection factor; evanescent waveguide; trapped-mode resonance

50 YEARS OF RESEARCH IN CONTINUUM AT THE UTR-2 RADIO TELESCOPE

Purpose: The results of research in continuum decameter-wave radio emission of the Galaxy background, ionized hydrogen regions, supernova remnants, extragalactic discrete sources, extended galaxies, galactic clusters, extragalactic background are given. The aim of this work is reviewing the results achieved for over 50-years of the UTR-2 radiotelescope research of our Galaxy and its population, as well as extragalactic radio sources in the continuum radio emission spectrum at extremely low frequencies for the ground based observations. Design/methodology/approach: The review, analysis, collection of archival data in various publications related to the subjectof this work. Findings: The basic results of studying the ionized hydrogen regions, supernova remnants, Galaxy background emission and its large-scale structure are given, and the maps of these sources are obtained. The catalog of extragalactic discrete radio sources of the most Northern sky part and the cosmological conclusions based on its analysis are described; the estimate of the isotropic extragalactic background brightness temperature is obtained; for the first time, the observational results for the Andromeda galaxy and two galactic clusters Coma and A2255 are given briefly. Conclusions: All the results presented here emphasize the uniqueness and importance of research in the decameter wavelength range, and the large area, flexibility of structure, continuous improvement make the UTR-2 radio telescope an indispensable tool for solving the most important tasks of modern radio astronomy, despite its respectable age. For example, only in the range of 10 to 30 MHz the ionized part of the most common element in the universe, the hydrogen, becomes optically thick and begins to absorb the synchrotron emission on the line of sight, which allows rather easy separation of thermal and non-thermal components of radioemission. This property allows to determine the ionized hydrogen regions’ electron temperature and the electron concentration on the line of sight independently in studying the hydrogen emission regions. When studying the supernova remnants, we can determine the ionized matter location by their spectrum drops ‒ before, inside or behind the remnant. Based on the HB3 supernova remnant radio imagies, an assumption was made on the existence of an ionized hydrogen relic shell aroundit, being caused by the initial ultraviolet flash of a supernova. For the first time, the maps of the Northern sky large-scale structure in the declination range from ‒15° to +85° at extremely low frequencies 10, 12.6, 14.7, 16.7, 20 and 25 MHz for the ground-based observations are published, which, besides their own scientific value, may allow to correct the UTR-2 radio telescope imaging results. Using the full-resolution UTR-2 maps and the developed method of multifrequency T‒T diagrams, it was possible to separate the background radiation into galactic and extragalactic components and construct the spectrum of the latter. From the analysis of the most complete decameter wavelength range catalog of discrete sources, it follows that there is a gap in the redshift spatial distribution for all classes of extragalactic sources. The existence of an ionized hydrogen ring in the Andromeda Nebula disk has been suggested. It is shown that the main partof the galaxy clusters decameter-wave emission comes from haloes and relics. Key words: decameter range; discrete sources; supernovaremnants; catalog; continuum radio emission; HII regions; UTR-2; background emission

PULSED POWER TO MICROWAVES CONVERSION IN NONLINEAR TRANSMISSION LINES

Purpose: Experimental results and numerical simulations are presented, concerning effects of microwave generation in coaxial transmission lines which are fed with unipolar, high voltage electric pulses. The work is aimed at clarifying the relative importance of several mechanisms that could be responsible for the appearance of microwave-frequency oscillations in the course of pulse propagation through the guiding structure. Design/methodology/approach: Dispersive and filtering properties of coaxial waveguides that involve three structural sections are discussed. These latter follow one another along the axis of symmetry. Two identical sections at the input and output are filled with an isotropic liquid dielectric, while the middle part may, in addition, be either partially or fully filled with a non-conductive gyrotropic material. The inserted core represents a set of ferrite rings showing a nonlinear response to the initial high voltage, pulsed excitation. Throughout the series of measurements, the diameters of the inner conductor and of the ferrite core were kept constant. The outer conductor’s diameter was varied to permit analysis of the effect of that size proper and of the degree to which the cross-section is fi lled with ferrite. The gyrotropic properties of the ferrimagnetic material were realized through application of a magnetic bias field from an external coil. The measurements were made for a variety of pulsed voltage magnitudes from the range of hundreds of kilovolts, and magnetic bias fields of tens kiloamperes per meter. Findings: As observed in our experiments, as well as in papers by other writers, a unipolar pulse coming from the radially uniform front-end section, further on gives rise to quasi-monochromatic voltage oscillations. These appear as soon as the pulse has advanced a sufficient distance into the radially nonuniform portion of the guide. The oscillations may consist of a small number of quasi-periods, which suggests a large spectral line width. However, by properly selecting geometric parameters of the wave guiding line and the characteristics of the initial pulsed waveform it proves possible to obtain output frequencies of about units of gigahertz and pulse powers at subgigawatt levels. Conclusions: The frequencies and amplitudes of the appearing oscillations, as well as their spectral widths, are governed by the complex of dispersive and non-linear properties of the guiding structure. The diameters of the inner and outer coaxial conductors in the line, diameter of the ferrimagnetic insert and its intrinsic linear dispersion determine the set of waveguide modes capable of propagating through the line. An oscillating part of the waveform may appear and get separated from the main body of the pulse if it has originated at a higher frequency than the cut-off value for a different mode than the initial TEM. Key words: unipolar pulse, coaxial transmission line, microwave frequency oscillations, dispersion laws, waveguide modes

A SYNTHESIS OF TEMPORAL VARIATIONS IN DOPPLER SPECTRA RECORDED AT A QUASI-VERTICAL INCIDENCE BY THE HF DOPPLER RADAR WITH SPACED RECEIVERS

Purpose: The ionospheric channel is widely used for the communication, radio navigation, radar, direction finding, radio astronomy, and remote radio probing systems. The radio channel parameters are characterized by nonstationarity due to the dynamic processes in the ionosphere, and therefore their study is one of the topical problems of space radio physics and earth-space radio physics of geospace. This work aims at presenting the results of synthesis of temporal variations in the Doppler spectra obtained by the Doppler probing of the ionosphere at vertical and quasi-vertical incidence. Design/methotology/approach: One of the most effective methods of ionosphere research is the Doppler sounding technique. It has a high time resolution (about 10 s), a Doppler shift resolution (0.01–0.1 Hz), and the accuracy of Doppler shift measurements (~0.01 Hz) that permits monitoring the variations in the ionospheric electron density (10–4–10–3) or the study of the ionospheric plasma motion with the speed of 0.1-1 m/s and greater. The solution of the inverse radio physical problem, consisting in determination of the ionosphere parameters, often means solving the direct radio physical problem. In the Doppler sounding technique, it belongs with the construction of variations in Doppler spectra and comparing them with the Doppler spectra measurements. Findings: For the radio wave ordinary component, three echoes being produced by three rays are observed. Influence of the geomagnetic fi eld and large horizontal gradients in the electron density of δ≥10 % give rise to complex ray structures with caustic surfaces. The ionospheric disturbances traveling along the magnetic meridian form the skip zones. The longitudinal and transverse displacement of the ray reflection point attains a few tens of kilometers along the vil. Haidary to vil. Hrakove quasi-vertical radiowave propagation path, for which the great circle range is 50 km. For the vertical incidence, the signal azimuth at the receiver coincides with the traveling ionospheric disturbance azimuth. The synthesis of temporal variations in the HF Doppler spectra has been made and compared with the temporal variations in the Doppler spectra recorded with the V. N. Karazin Kharkiv National University radar. The estimate of δ=15 % obtained confirms the existence of large horizontal gradients in electron density. Conclusions: Temporal variations in Doppler spectra and in azimuth have been calculated for the vertical and quasi-vertical incidence with allowance for large horizontal gradients of the electron density caused by traveling ionospheric disturbances. Key words: ionosphere, Doppler sounding at oblique incidence, synthesis of temporal variations in HF Doppler spectra, traveling ionospheric disturbances, electron density

AN ANTENNA BASED ON A HYBRID METAL–DIELECTRIC STRUCTURE

Purpose: Nowadays, in the millimeter frequency range, the dielectric waveguides of various modifications have certain advantages over the standard metal waveguides, primarily due to the possibility of creating functional units based on them. This is due to the relative simplicity and low cost of manufacturing the dielectric waveguides and functional units using them, the high degree of their integration with active elements, the use in their manufacture of different dielectrics and polymers with a wide range of material constants and a variety of mechanical properties (in particular, some materials have a significant flexibility). After making a series of physical experiments we have found the possibility of implementing the frequency selection and radiation into free space of electromagnetic waves by a hybrid metal-dielectric structure. Design/methodology/approach: The studied electrodynamic structure belongs to the class of hybrid metal-dielectric structures. It includes a modified inverted dielectric waveguide with a periodic sequence on the dielectric plate of fifteen dielectric rods with metallized coating on one of the faces placed outwards. The structure efficiency was estimated by the voltage standing-wave ratio (VSWR) values and power attenuation in the duct. The measurements were made with the reflectometer method. To estimate the degree of electromagnetic field concentration near the rod inhomogeneities in the near zone, the mobile probe method was used. The field structures were visualized with the method of isolines. Findings: The results of a series of experimental investigations showed the possibility of matching the structure with the external waveguides in the frequency range of 26.5-32.5 GHz with the voltage standing-wave ratio (VSWR) less than 1.8. The frequency dependence of attenuation is oscillatory with clearly expressed frequency ranges with small and large attenuation values. Moreover, the dependence is almost periodic, which is typical of periodic structures. The frequency response slope in the transition zones can be quite high and reach values of 41.26 dB/GHz. The degree of concentration of the electric field near the waveguide dielectric rod and the degree of excitation of the dielectric inhomogeneities was found by directly measured electric field strength in the near zone. Measurements of energy characteristics made under the short-circuit conditions for the main guide and in the mode of matched load of the main guide showed both the ability to control the polarization characteristics and the ability to change the appearance of the pattern and its orientation in space. Conclusions: It has been experimentally proven that a hybrid metal-dielectric structure, being a modified inverted dielectric waveguide with a periodic sequence on the dielectric plate of fifteen dielectric rods with metalized coating on one of the faces placed outwards, can be effectively integrated into a standard transmission line. It is found that this structure can be matched with the external circuits in a fairly wide frequency range. It is also found that in different frequency ranges this hybrid metal-dielectric structure shows the possibility of both efficient frequency selection and radiation in free space. Antenna measurements have shown the beam pattern shape controllability. Key words: inverted dielectric waveguide, periodic sequence, voltage standing-wave ratio (VSWR), attenuation, reflectometer method, mobile probe method, directivity pattern

EVIDENCE OF THE EARTH’S INNER RADIATION BELTS DURING THE LOW SOLAR AND GEOMAGNETIC ACTIVITY OBTAINED WITH THE STEP-F INSTRUMENT

Purpose: The subject of research is the spatio-temporal charged particles in the Earth’s magnetosphere outside the South Atlantic magnetic Anomaly during the 11-year cycle of solar activity minimum. The work aims at searching for and clarifying the sustained and unstable new spatial zones of enhanced subrelativistic electron fluxes at the altitudes of the low Earth orbit satellites. Design/methodology/approach: Finding and ascertainment of new radiation belts of the Earth were made by using the data analysis from the D1e channel of recording the electrons of energies of ΔEe=180–510 keV and protons of energies of ΔEp=3.5–3.7 MeV of the satellite telescope of electrons and protons (STEP-F) aboard the “CORONAS-Photon” Earth low-orbit satellite. For the analysis, the data array with the 2 s time resolution normalized onto the active area of the position-sensitive silicon matrix detector and onto the solid angle of view of the detector head of the instrument was used. Findings: A sustained structure of three electron radiation belts in the Earth’s magnetosphere was found at the low solar and geomagnetic activity in May 2009. The two belts are known since the beginning of the space age as the Van Allen radiation belts, another additional permanent layer is formed around the drift shell with the McIlwaine parameter of L = 1.65±0.05. On some days in May 2009, the new two inner radiation belts were observed simultaneously, one of those latter being recorded between the investigated sustained belt at L≈1.65 and the Van Allen inner belt at L≈2.52. Increased particle fluxes in this unstable belt have been formed with the drift shell L≈2.06±0.14. Conclusions: The new found inner radiation belts are recorded in a wide range of geographic longitudes λ, both at the ascending and descending nodes of the satellite orbit, from λ1≈150° to λ2≈290°. Separately in the Northern or in the Southern hemispheres, outside the outer edge of the outer radiation belt, at L≥7–8, there are cases of enhanced particle fl ux density in wide range of L-shells. These shells correspond to the high-latitude region of quasi-trapped energetic charged particles. Increased particle fluxes have been recorded up to the bow shock wave border of the Earth’s magnetosphere (L≈10-12). Key words: radiation belt, STEP-F instrument, electrons, magnetosphere, drift L-shell, particle flux density