Luo Y., Chernogor L.F., Garmash K.P. «Geomagnetic effect of Turkish earthquake of January 24.2020» Радиофизика и радиоастрономия (Украина), 25, № 4, с. 276-289 (2020)

Purpose: The main cause of geomagnetic disturbances are cosmic sources, processes acting in the solar wind and in the interplanetary medium, as well as large celestial bodies entering the terrestrial atmosphere. Earthquakes (EQs) also act to produce geomagnetic effects. In accordance with the systems paradigm, the Earth–atmosphere–ionosphere–magnetosphere system (EAIMS) constitute a unified system, where positive and negative couplings among the subsystems, as well as feedbacks and precondition among the system components take place. The mechanisms for the action of EQs and processes acting in the lithosphere on the geomagnetic field are poorly understood. It is considered that the EQ action is caused by cracking of rocks, fluctuating motion in the pore fluid, static electricity discharges, etc. In the course of EQs, the seismic, acoustic, atmospheric gravity waves (AGWs), and magnetohydrodynamic (MHD) waves are generated. The purpose of this paper is to describe the magnetic effects of the EQ, which took place in Turkey on 24 January 2020. Design/methodology/approach: The measurements are taken with the fluxmeter magnetometer delivering 0.5–500 pT sensitivity in the 1–1000 s period range, respectively, and in a wide enough studied frequency band within 0.001 to 1 Hz. The EM-II magnetometer with the embedded microcontroller digitizes the magnetometer signals and performs preliminary filtering over 0.5 s time intervals, while the external flash memory is used to store the filtered out magnetometer signals and the times of their acquisition. To investigate quasi-periodic processes in detail, the temporal variations in the level of the H and D components of the geomagnetic field were applied to the systems spectral analysis, which makes use of the short-time Fourier transform, the wavelet transform using the Morlet wavelet as a basis function, and the Fourier transform in a sliding window with a width adjusted to be equal to a fixed number of harmonic periods. Findings: The train of oscillations in the level of the D component observed 25.5 h before the EQ on 23 January 2020 is supposed to be associated with the magnetic precursor. The bidirectional pulse in the H component observed on 24 January 2020 could be due to the piston action of the EQ, which had generated an MHD pulse. The quasi-periodic variations in the level of the H and D components of the geomagnetic field, which followed 75 min after the EQ, were caused by a magnetic disturbance produced by the traveling ionospheric disturbances due to the AGWs launched by the EQ. The magnetic effect amplitude was estimated to be close to 0.3 nT, and the quasi-period to be 700–900 s. The amplitude of the disturbances in the electron density in the AGW field was estimated to be about 8% and the period of 700-900 s. Damping oscillations in both components of the magnetic field were detected to occur with a period of approximately 120 s. This effect is supposed to be due to the shock wave generated in the atmosphere in the course of the EQ. Conclusions: The magnetic variations associated with the EQ and occurring before and during the EQ have been studied in the 1–1000 s period range.

Ulyanov O.M., Zakharenko V.V., Alekseev E.A., Reznichenko O.M., Kulahin I.O., Budnikov V.V., Prisiazhnii V.I., Poikhalo A.V., Voytyuk V.V., Mamarev V.N., Ozhinskyi V.V., Vlasenko V.P., Chmil V.M., Sunduchkov I.K., Berdar M.M., Lebed V.I., Palamar M.I., Chaikovskii A.V., Pasternak Yu V., Strembitskii M.A., Natarov M.P., Steshenko S.O., Glamazdin V.V., Shubnyi O.I., Kyrylenko A.O., Kulyk D.Yu. «The RT-32 radio telescope construction based on the MARK-4b antenna system. 3. Local oscillators and self-noise of the receiving system» Радиофизика и радиоастрономия (Украина), 25, № 3, с. 175-192 (2020)

Предмет и цель работы: Исследование с высоким разрешением спектральных линий космических радиоисточников требует низких собственных шумов приемной системы радиотелескопа. Они обеспечиваются как входными криогенными усилителями, так и низкими фазовыми шумами гетеродинов. Для выполнения спектральных исследований необходимо иметь возможность перестроения частот гетеродинов с малым частотным шагом. В работе приведены результаты разработки синтезаторов частоты, которые одновременно обеспечивают как очень малый частотный шаг, так и низкий уровень фазовых шумов. Приведены также результаты измерений собственных шумов криогенных приемных систем радиотелескопа РТ-32. Методы и методология: Приемные системы РТ-32 созданы по схемам супергетеродинных приемников с двумя степенями преобразования частоты. Настройка приемной системы с частотным шагом 10 или 20 МГц обеспечивается гетеродина первого преобразования частоты, а точная настройка происходит благодаря сверхвысокой разрешающей способности (0.0001 МГц) гетеродинов второго преобразования частоты, созданные на основе синтезаторов прямого цифрового синтеза. Результаты: Показано, что применение синтезаторов прямого цифрового синтеза возможно только с низкими значениями коэффициентов умножения частоты, а также при тщательной фильтрации всех опорных сигналов. Измерение параметров гетеродинов проводилось с помощью спектра N9951A (Keysight Technologies), который имеет высокое разрешение и широкий динамический диапазон. Для измерений шумовых характеристик радиоприемной системы радиотелескопа была изготовлена специально согласованная нагрузка с возможностью охлаждения до температуры жидкого азота. Измерения шумовой температуры было проведено в различных разрезах приемного тракта РТ-32. Сопоставление таких измерений в различных конфигурациях дает возможность сделать предварительную оценку собственных шумов РТ-32 в С и K диапазонах. Вывод: Результаты измерений собственных шумов радиоприемных систем и фазовых шумов гетеродинов радиотелескопа РТ-32 показывают, что радиотелескоп в С-диапазоне способен выполнять высокочувствительные исследования как в широкой полосе частот, так и в узкой полосе частот с высокой спектральной разрешающей способностью. В K-диапазоне собственные шумы сопоставимы (≈60–80 К) с внешними шумами, что также дает возможность исследовать излучение мазерных источников.

Gorbunov A.A., Ryabov M.I., Sukharev A.L., Bezrukovs V.V. «Periodic and sporadic variations in the spectral flux density of the Cas A supernova remnant» Радиофизика и радиоастрономия (Украина), 25, № 4, с. 268-275 (2020)

Purpose: Based on the long-term study data in all radio spectrum ranges, the nature of deviations of spectral flux density of the Cas A supernova remnant from the tendency of its secular decrease is considered. The aim of this work is determining the presence of quasiperiodic variations and sporadic changes in the Cas A spectral radiation flux density depending on frequency. Design/methodology/approach: The main database is using the published results obtained with the method of absolute measurements of the Cas A spectral radiation flux density in a wide range from millimeter to decimeter wavelengths, as well as the results obtained with the method of relative measurements of the ratio of the flux densities of the Cas A supernova remnant and radio galaxy Cyg A, this latter being used as a reference source in the meter wavelength range. For making comparison with the aforesaid data obtained with different methods, the results of a long-term monitoring (since 1987) of the variation of the ratio of the spectral flux densities of Cas A and Cyg A made with the URAN-4 radio telescope of the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine at 25 MHz were used. Findings: As a result of the analysis of the observed data for the Cas A radiation flux density in the entire radio wavelength range, the existence of quasiperiodic variations in the range from millimeter to meter wavelengths within 2 to 9 years has been noted. The reason for the detected quasiperiodic variations can be the processes in the Cas A remnant itself. In the decameter wavelength range, according to monitoring data obtained with the URAN-4 radio telescope, the seasonal-diurnal and long-term variations are noted, being associated with changes in the ionosphere state in the solar activity cycle, with some weak appearance of the secular decrease of the Cas A radiation flux. The presence of sporadic variations in the ratio of the spectral flux densities of Cas A and Cyg A is associated with the effect of the increased solar activity. For explaining the lowering of the Cas A spectral flux density to the Cyg A level and maintaining the excess of the Cas A flux at decameter waves, the quasi-simultaneous observations made with radiotelescopes for different wavelength ranges will be required. Conclusions: The evolution of the Cas A supernova remnant remains the focus of interest of current research efforts. An intriguing moment was the discovery of a point X-ray source in the center of radio source Cas A as a possible supernova remnant. The role of this source in the Cas A radio flux secular decrease and in its variations needs to be clarified. A detailed analysis of the long-term data and making quasi-simultaneous observations will allow to reveal the processes occurring in the radio source itself and to determine the influence of the ionosphere state on the results of measurements. A joint program is suggested for observations of Cas A and Cyg A flux variations with the RT-32 and RT-16 radio telescopes, the LOFAR element of the Ventspils International Radio Astronomy Center (Latvia), and the URAN, UTR-2 and GURT radio telescopes of the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine.