Dispersion measure and rotation measure are integral characteristics of the interstellar medium. For the weakly anisotropic plasma of the interstellar medium, the frequency dependence of these parameters can be ignored. However, both of these parameters become frequency dependent in the case of magneto-active plasma, which is present in the solar corona, the Jupiter-Io magnetic tube, etc. The presence of such a dependence makes it possible to study magneto-active plasma using line polarized radio pulses of pulsars as a probe. In this article, we analyze the frequency dependence of the dispersion measure and the rotation measure in the magneto-active plasma of the solar corona. We show that the frequency dependence of both parameters arises due to the difference in the group/phase velocities of ordinary and extraordinary wave trains under conditions of a quasi-longitudinal propagation, their quasi-transverse propagation and, in the general case, for an arbitrary angle between the wave vector and the magnetic field vector. We analyze the refractive indices under the conditions of quasi-longitudinal and quasi-transverse birefringence of ordinary and extraordinary waves in a magneto-active plasma. It is shown that in any propagation conditions the dispersion measure for a mixture of ordinary and extraordinary waves will always be higher at low frequencies than at high frequencies in the presence of an extended magneto-active plasma. Line polarized pulsed radiation of pulsars is shown to be an excellent probe of the cosmic magneto-active plasma. The analysis of polarization separation of ordinary and extraordinary waves of pulsar pulses will allow determining more accurately the parameters of the magneto-active plasma. We suggest using a combination of low-frequency and high-frequency radio telescopes for future studies of the solar corona or other sources with magneto-active plasma.

Context. Cassiopeia A occupies an important place among supernova remnants (SNRs) in low-frequency radio astronomy. The analysis of its continuum spectrum from low frequency observations reveals the evolution of the SNR absorption properties over time and suggests a method for probing unshocked ejecta and the SNR interaction with the circumstellar medium (CSM). Aims. In this paper we present low-frequency measurements of the integrated spectrum of Cassiopeia A to find the typical values of free-free absorption parameters towards this SNR in the middle of 2023. We also add new results to track its slowly evolving and decreasing integrated flux density. Methods. We used the New Extension in Nan\c{c}ay Upgrading LOFAR (NenuFAR) and the Ukrainian Radio Interferometer of NASU (URAN-2, Poltava) for measuring the continuum spectrum of Cassiopeia A within the frequency range of 8-66 MHz. The radio flux density of Cassiopeia A has been obtained on June-July, 2023 with two sub-arrays for each radio telescope, used as a two-element correlation interferometer. Results. We measured magnitudes of emission measure, electron temperature and an average number of charges of the ions for both internal and external absorbing ionized gas towards Cassiopeia A from its integrated spectrum. Generally, their values are comparable to those presented by Stanislavsky et al. (2023), but their slight changes show the evolution of free-free absorption parameters in this SNR. Based on high accuracy of the measurements, we have detected the SNR-CSM interaction. Conclusions. The integrated flux-density spectrum of Cassiopeia A obtained with the NenuFAR and URAN-2 interferometric observations opens up new possibilities for continuous monitoring the ionized gas properties in and around Cassiopeia A to observe theevolution of unshocked ejecta and the SNR-CSM interaction in future studies.

An analysis of the observational polarization properties of the decameter spikes is presented in the paper. It is shown that decameter spikes possess high degree of circular polarization with average value of about 60%. In the frames of “leading spot” theory we associated the spikes activity with a certain active region on the solar disk and determined the mode of the emission. Supposing plasma emission mechanism we link and determine coronal plasma and fast electron beam parameters.

Spectral properties of a single type III burst in the wide frequency band from 10 to 70 MHz are studied in detail. It is shown that electrons corresponding to different levels of type III emission move with different velocities. Moreover, these electron velocities decrease from the maximum value, which corresponds to the 0.1 level of the maximum type III flux at its front, to the minimum value, corresponding to the 0.1 level of the maximum type III flux at its back. The velocity of electrons corresponding to the maximum type III flux was approximately 0.31 c. This value equals 0.6 of maximum velocity, and, namely, it was predicted by the gas dynamic theory of electron propagation through the coronal plasma. In addition, we adduce arguments that the type III radio emission is the harmonic emission. In supposition that type III electrons move through the Newkirk coronal plasma, we find electron velocities for every level of the type III burst. The duration dependence on frequency obtained from the observations is close to Elgaroy–Lingstad dependence. We discuss the contribution of electron velocity dispersion to the type III burst duration. In addition, we derived type III flux dependence on frequency in the frequency bands of 10–33 MHz and 33–62 MHz.

Context. Shock waves in the solar corona are associated with solar flares and coronal mass ejections (CMEs). Type II solar bursts are radio signatures of shock waves in the solar corona. They are driven by solar flares or CMEs. Despite extensive studies, the intricate spectral patterns observed in type II solar bursts occasionally pose new challenges for the theory of electron acceleration in shocks. Aims. We study a newly identified feature in type II solar bursts called spectral cleaving. This feature is characterized by the actual branching of a type II radio emission lane in radio spectral data. Methods. We analyzed the type II burst exhibiting spectral cleaving in high-fidelity dynamic spectra obtained using the URAN-2 radio telescope (8.25-33 MHz; Poltava region, Ukraine) on 2011 February 14. The high-resolution spectrograms were examined to ascertain its spectral morphology. Results. Our research represents the first recognition of spectral cleaving as a peculiarity of type II bursts that is yet to be classified. This effect occurs due to the shift (or migration) of radio source(s) along a shock front, which in turn is caused by changes in the magnetic field orientation ahead of the propagating shock front. Conclusions. The spectral cleaving observed in solar type II bursts reveals a distinct phenomenon that indicates complex interactions between shock waves and magnetic fields in the solar corona. This discovery enhances our understanding of the mechanisms behind solar radio emissions and emphasizes the need for further observational studies to verify these findings.

Introduction: The properties of the S-bursts observed during the storm on 20–21 June 2022 in frequency band 10–32 MHz by the radio telescope URAN-2 are discussed in this paper. The storm was highly populated with other solar bursts, such as Type III bursts and drift pairs. The occurrence rate of S-bursts was very high reaching 60 bursts per minute. All observed S-bursts were characterized by low fluxes with respect to the background radio emission. Thus special processing methods are used to retrieve spectral properties of the bursts. Some individual “long” S-bursts covered the whole frequency band of the URAN-2 radio telescope from 10 to 32 MHz. Such extended in frequency S-bursts were recorded for the first time. 50 extended S-bursts were selected for the further analysis.Methods: The S-bursts dynamic spectra with time-frequency resolutions of 100 ms and 4 kHz as well as single-frequency profiles were used in the analysis. Due to low S-bursts intensities the drift rates were estimated from the time-differentiated dynamic spectra, highlighting the tracks of the bursts maxima. Polarization dynamic spectra were used for measuring the degree and sense of the S-bursts circular polarization. Individual S-bursts tracks were used for instant coronal inhomogeneities diagnostics. Mean S-bursts parameters retrieved from the statistical processing of the set of 50 bursts were compared with previously obtained ones.Results: We concluded that by the mean durations, drift rates, frequency extent and the polarization all observed S-bursts could be divided into two separate groups, the “short” and the “long” S-bursts. The power-law index of the drift rate-frequency dependence averaged over all 50 selected bursts was found to be 1.7. It was shown that sources of S-bursts most likely move through the Newkirk corona with the velocities of 0.06–0.08c. The power-law dependence of the “long” S-bursts durations on frequency in frequency band of 12–30 MHz was obtained. Its index equal to −0.61 appeared to be very close to that for Type III bursts. From this dependence the electron velocity dispersion in the beam, responsible for S-bursts generation was calculated. Its value of 0.02 indicates that the beams, responsible for S-bursts generation are almost monoenergetic.Discussion: It is assumed that non-monotonic appearance of individual S-bursts tracks on the dynamic spectrum reflects density inhomogeneities encountered by the sources on their paths. From the dynamic spectra of such S-bursts the characteristic size and amplitude of these coronal inhomogeneities were detected. From the S-bursts durations and the velocities of their sources the longitudinal sizes of the latter were estimated. It was then shown that the sizes of small-scale coronal inhomogeneities were comparable to those of “long” S-bursts sources. Thus we concluded that individual tracks of the “long” S-bursts can be used for fie diagnostics the coronal plasma at heliocentric heights range from 1.7 to 3.2 Rs, where Rs is the solar radius. On the other hand, these tracks being ensemble-averaged give the information about the long-term large scale properties of the corona.

Abstract We discuss properties of Type IV burst observed by radio telescopes URAN-2 (Ukrainian Radio Interferometer of NASU-2) and NDA (Nançay Decameter Array) on 13 July 2022. This burst was registered at frequencies 26–80 MHz and lasted from 07:00 to 12:00 UT. It had the fine structure in the form of sub-bursts similar to decameter Type III-like bursts, with negative frequency drift rates from − 5MHz/s to -21MHZ/s and sometimes with positive drifts. Their durations were from 1 s to 2.6 s in most cases and maximum fluxes were up to 700 s.f.u. At the same time maximum polarization of Type IV burst was 100%. It is very unusual for decameter Type IV bursts because as a rule this value is about 40%. This Type IV burst was not associated with any CME. We discuss properties of this Type IV burst in the plasma model of radio emission.

Subject and Purpose. The sporadic radio emissions coming from the Sun in a broad frequency range contain a lot of important information concerning the solar corona, parameters of the radio frequency sources therein, and the parameter variations resulting from active processes on and about the Sun. These have been the reasons for recent launches of the space missions intended for stud- ying the Sun and its corona, such as the Parker Solar Probe (PSP) and the Solar Orbiter. The present work is aimed at demonstrating effectiveness of the ground-based support for the space missions, the PSP before all, which is provided by the large Ukrainian radio telescopes of the decameter wavelength range. Another purpose has been cross-calibration of the space-borne radiometer against calibrated data from a ground-based instrument. Methods and Methodology. One of the remote diagnosis techniques widely used with respect to the solar corona is to analyze parameters of the radio frequency emissions from sources lying at a variety of altitudes within the corona. The methodology of such joint, space-borne/terrestrial investigations suggests simultaneous observations of certain individual events during closest approach of the space probe PSP to the Sun, with analysis over a widest possible frequency range. The data obtainable within overlapping fre- quency bands are proposed for calibrating the on-board radio receivers of the space probe. Results. The methodology proposed for joint, space-based / terrestrial observations has been substantiated. Data from the UTR-2 and URAN-2 radio telescopes and the space probe PSP have been used to plot the dynamic and the polarization spectra of the June 9, 2020 solar bursts, with identification and comparison of the relevant individual events. A joint dynamic spectrum of these bursts has been obtained for the frequency band of 0.5 to 32 MHz. The calibrated data from the ground-based radio telescopes have allowed performing cross-calibration of the HF receiver in the FIELDS-PSP data taking module within the frequency band of 10 to 18 MHz. Conclusions. The paper has provided evidence of an effective ground-based support for the space mission PSP on the part of large Ukrainian radio telescopes. Examples of joint observations have been given, and a methodology described which is employed for cross-calibrating the HF receivers of the FIELD-PSP module. Prospects are outlined of further ground-based support for solar space research missions.

We present results of observations of Type IV burst with unusual high polarization equalled in maximum phase about 100%. This burst was registered both by URAN-2 and NDA radio telescopes on 13 July 2022. It continued for about 5 hours and consisted of sub-bursts with high frequency drift rates and had short durations. We associate it with the weak CME, which propagated in the East direction in the form of thread-like structure. Theoretical description of sub-burst in the plasma mechanism of radio emission allow explaining their high polarization, high drift rates and short durations.

Subject and Purpose. Results are presented of the solar corona investigations performed with the world famous Ukrainian radio telescopes. The work has been aimed at offering a consistent review of recent achievements in observations of a variety of low-frequency radio emissions from the Sun. Methods and Methodology. The studies of the quiet (thermal) and sporadic (burst-like) radio emissions from the Sun have been carried out with the decameter-wavelength radio telescopes UTR-2, GURT and URAN-2. Specific features of the low-frequency solar radio emissions from a variety of sources are presented, with characterization of the optimized techniques that were applied in each case for evaluating physical parameters of the corona in the areas of decameter-wavelength radio wave generation. Results. The analysis of temporal, frequency and spatial characteristics of solar radio emissions has allowed suggesting a number of models for the coronal electron density distribution, and evaluating magnetic field strengths in the corona. Also, our experimental results have proven to be consistent with the observational data obtained in different frequency ranges and with the use of both ground based and space-borne instruments. Conclusions. The radio observations performed with Ukrainian radio telescopes have permitted studying, with high temporal, frequency and spatial resolutions, solar radio frequency emissions from various localized sources. Along with the large effective area and high sensitivity of the antennas, this permits application of a wide range of methods and tools aimed at detecting and analyzing solar bursts, of both strong and weak intensity, against the background of terrestrial interference of natural or artificial origin.

Purpose: Review of investigations of cosmic sources radioemission scintillations due to interplanetary plasma irregularities made at the Institute of Radio Astronomy of the National Academy of Sciences of Ukraine, from the first observations in the mid-70s until now. Design/methodology/approach: In the course of preparation of this paper, the authors have reviewed, analyzed and summarized the information being published in the home and foreign publications, and reported at scientific conferences. Findings: The investigations of the interplanetary scintillations carried out at the Institute of Radio Astronomy, NAS Ukraine have been reviewed. A retrospective discussion has been made on how in the course of these researches the knowledge about the basic parameters of interplanetary scintillations in the decameter wavelength range, as well as that on the important parameters of the solar wind and its structure, have been obtained. Also, various methods of processing and analysis of experimental data were offered, and new means for receiving cosmic radiation were created. The place and importance of the discussed researches for the world science are shown. Conclusions: Over the years since the beginning of the research, the think tank of the Department of Low-Frequency Radio Astronomy of the Institute of Radio Astronomy, NAS Ukraine has obtained a number of new relevant results, which bring Ukraine into the cohort of world centers of interplanetary scintillation researches. The construction of a new GURT radio telescope, among other things, creates new prospects for the development of this relevant line of investigation. Key words: interplanetary scintillations; decameter wavelength range; solar wind; solar wind stream structure; coronal mass ejection

Properties of type III and type IIIb bursts in the frequency band of 8 – 80 MHz observed by the radio telescopes Ukrainian Radio Interferometer of NASU-2 (URAN-2) (Poltava) and Giant Ukrainian Radio Telescope (GURT) (Kharkiv) during the Parker Solar Probe (PSP) perihelion in April 2019 are discussed. These correspond to those that were observed by PSP at frequencies <19 MHz. We analyze dependencies of drift rates and durations on frequency for these bursts. We show that drift rate dependencies on frequency agree well with those derived from the Newkirk corona if source velocities are between 0.17 and 0.2 c for both type IIIb bursts and type III bursts under the assumption that the first ones are fundamentals and the second ones are their harmonics. However, all observational dependencies are flatter in comparison with the dependencies for a Newkirk corona. We assume that this can be related with coronal temperature decreasing at heliocentric distances from 1.35 to 6.5 solar radii. Duration dependencies of type IIIb and type III bursts on frequency in the range of 10 – 70 MHz are also obtained. We note that the durations and drift rates of type III bursts as well as their dependencies on frequency differ essentially from those for type IIIb bursts.

Purpose: A historical review of the experimental baselopment of low-frequency radio astronomy in Ukraine, its foundation half a century ago by an outstanding scientist S.Ya. Braude to the current state. Design/methodology/approach: The constant progress of electronic, computer and digital technologies, information and telecommunication technologies, theory and practice of antenna and receiving systems design, which introduction enriched the hardware and methodological ideology of construction and usage of the UTR-2, URAN, and GURT radio telescopes, have been used. Findings: The worldwide most effective national experimental radio astronomy means, the UTR-2, URAN, and GURT decameter-meter wave radiotelescopes, have been created and improved. The best combination of the systems main parameters: sensitivity; frequency band; spatial, frequency and temporal resolutions; noise immunity; uniformity of amplitude-frequency and space-frequency characteristics and multifunctionality has been provided. Conclusions: For the half a century of radio astronomical scientific and technical at the Institute of Radio Astronomy of NAS of Ukraine, the high astrophysical informativeness of the low-frequency radio astronomy and the possibility of creating a highly efficient experimental base – giant radio telescopes of decameter-meter wavebands have been proved. Today, the Ukrainian radio telescopes are well known and recognized world-wide being indispensable and most in demand by the scientific community. The founder of the decameter radio astronomy in Ukraine, the eminent scientist Semen Yakovych Braude was not mistaken when he decided to start radio astronomical explorations. The memory of him will always remain in the minds and hearts of many generations. Key words: low-frequency radio astronomy; radio telescope; phase shifter; antenna amplifier; digital signal recorder; effective area; sensitivity; resolution; noise immunity

The decay of a Type III burst into two Type III bursts was registered during solar observations by GURT and URAN-2 radio telescopes on 2017 April 18. This was the first time such a phenomenon was observed. Newborn Type III bursts have drift rates smaller than that of a decaying Type III burst. Such decays of Type III bursts were predicted by a gas-dynamic theory of high-energy electron beams propagating through the thermal background plasma. In the frame of this theory Type III sources are beam-plasma structures moving with constant velocities. In our case the sum of velocities of newborn Type III sources equals the velocity of a decaying Type III source. The last one is 0.33c in the case of fundamental radio emission and 0.2c at the harmonic radio emission of a Type III burst. The density ratio of slow and fast newborn Type III sources is about 3.

This paper describes the statistical property of Jupiter's millisecond burst (S‐burst) beaming for Io‐related decametric (Io‐DAM) sources from a ground‐based radio telescope network. To do so, we performed simultaneous observations of Jovian Io‐DAM S‐bursts from 15 January through 4 May 2016 for a total of nine events using several radio telescopes. These radio telescopes include the Long Wavelength Array station One (LWA1) in the United States, Nançay Decameter Array (NDA) in France, and three large radio telescopes (UTR2, URAN2, and URAN3) in Ukraine. We conducted a cross‐correlation analysis of the S‐burst spectrograms in a frequency range of 10.5 to 33 MHz over effective baselines of up to 8,950 km. We found that the beaming of the S‐bursts is formed on the flashlight‐like structure within an east‐west beam width of 2.75" for Io‐A/C, 2.63" for Io‐A', and 2.75" for Io‐B/D. In parallel, the flashlight‐like beam was completely filled because the results from all usable pairs of telescopes supported this model. Hence, these beam widths directly correspond to the minimum cone thickness where a radio source emanates over large solid angles from the same direction of Jupiter, as opposed to a localized radio source emitting over small solid angles along active magnetic flux tubes that are tied to Io's orbital motion in Jupiter's rotation frame (beacon‐like structure). Additionally, this cross‐correlation technique shows a practical benefit of producing statistical profiles of S‐bursts.

Solar wind is a set of flows with different parameters (the speed, the exponent of the spectrum of heterogeneities, the width, etc.). A bimodal character of the speed distribution of the solar wind was determined in spaceborne experiments. The measurements onboard the Ulysses spacecraft confirmed that the bimodal structure of solar wind continues to persist at relatively large distances from the Sun (to several astronomical units). However, there is one more possibility to determine the stream structure of solar wind. This is the method of interplanetary scintillations. The purpose of the paper is to reconstruct the stream structure of solar wind beyond the Earth’s orbit using the data on interplanetary scintillations obtained at two observational sites. The experiments were carried out at decameter wavelengths, since they are rather strongly scattered by the rarefied interplanetary plasma beyond the Earth’s orbit. The experimental data on interplanetary scintillations analyzed in this work were obtained in synchronous observations with the UTR-2 and URAN-2 radio telescopes. The parameters of solar wind and its stream structure were determined by comparison of the characteristics of interplanetary scintillations measured in the experiment (the dependences of the harmonic velocity of the cross-spectrum of scintillations and the power spectra) to those calculated with the models. To separate the interplanetary and ionospheric scintillations, the spectral, spatial, and frequency criteria were used. The results of this analysis show that solar wind beyond the Earth’s orbit consists of several streams that replace each other on the line of sight toward the radio source. These investigations prove the reliability and efficiency of the interplanetary scintillation method for reconstructing the stream structure of solar wind.

We report on the results of observations of a type IV burst made by the Ukrainian Radio interferometer of the Academy of Sciences (URAN-2) in the frequency range 22 – 33 MHz. The burst is associated with a coronal mass ejection (CME) initiated by a behind-the-limb active region (N05E151) and was also observed by the Nançay Decameter Array (NDA) radio telescope in the frequency band 30 – 60 MHz. The purpose of the article is the determination of the source of this type IV burst. After analysis of the observational data obtained with the URAN-2, the NDA, the Solar-Terrestrial Relations Observatory (STEREO) A and B spacecraft, and the Solar and Heliospheric Observatory (SOHO) spacecraft, we come to the conclusion that the source of the burst is the core of a behind-the-limb CME. We conclude that the radio emission can escape the center of the CME core at a frequency of 60 MHz and originates from the periphery of the core at a frequency of 30 MHz that is due to occultation by the solar corona at the corresponding frequencies. We find plasma densities in these regions assuming the plasma mechanism of radio emission. We show that the frequency drift of the start of the type IV burst is governed by an expansion of the CME core. The type III bursts that were observed against this type IV burst are shown to be generated by fast electrons propagating through the CME core plasma. A type II burst was registered at frequencies of 44 – 64 MHz and 3 – 16 MHz and was radiated by a shock with velocities of about \(1000~\mbox{km}\,\mbox{s}^{-1}\) and \(800~\mbox{km}\,\mbox{s}^{-1}\), respectively.

A large number of Type IIIb–III pairs, in which the first component is a Type IIIb burst and the second one is a Type III burst, are often recorded during decameter Type III burst storms. From the beginning of their observation, the question of whether the components of these pairs are the first and the second harmonics of radio emission or not has remained open. We discuss properties of decameter IIIb–III pairs in detail to answer this question. The components of these pairs, Type IIIb bursts and Type III bursts, have essentially different durations and polarizations. At the same time their frequency drift rates are rather close, provided that the drift rates of Type IIIb bursts are a little larger those of Type III bursts at the same frequency. Frequency ratios of the bursts at the same moment are close to two. This points at a harmonic connection of the components in IIIb–III pairs. At the same time there was a serious difficulty, namely why the first harmonic had fine frequency structure in the form of striae and the second harmonic did not have it. Recently Loi, Cairns, and Li ( Astrophys. J. 790 , 67, 2014 ) succeeded in solving this problem. The physical aspects of observational properties of decameter IIIb–III pairs are discussed and pros and cons of harmonic character of Type IIIb bursts and Type III bursts in IIIb–III pairs are presented. We conclude that practically all properties of the IIIb–III pair components can be understood in the framework of the harmonic relation of the components of the IIIb–III pairs.

The results of the first observations of Type IV bursts at frequencies 10-30 MHz are presented. These observations were carried out at radio telescopes UTR-2 (Kharkov, Ukraine) and URAN-2 (Poltava, Ukraine) during the period 2003-2006. Detection of Type IV bursts in wide band from 10 to 30MHz with high sensitivity and time resolution allowed to study their properties in details. These bursts have fluxes 10-2000s.f.u. at maximum phase. Their durations are about 1-2 hours and even more. Some of Type IV bursts drift from high to low frequencies with drift rates about 10kHz/s. All observed Type IV bursts have fine structures in the form of sub-bursts with durations from 2s to 20s and frequency drift rates in a majority of 1-2MHz/s. In most cases, sub-bursts with negative drift rates were registered. Sometimes sub-bursts in absorption with durations 10-200s against Type IV burst background have been observed. The Type IV burst observed on July 22, 2004 had zebra structure, in which single zebra stripes had positive, negative and infinite drift rates.

We present the results of observations of complex powerful type II burst associated with narrow Earth-directed CME, which was ejected at around 11 UT on 31 May 2013. The observations were performed by radio telescope UTR-2, which operated as local interferometer, providing the possibility of detection of the spatial parameters of the radio emission source. There are also polarization data from URAN-2 radio telescope. The CME was detected by two space-born coronagraphs SOHO/LASCO/C2 and STEREO/COR1-BEHIND, and was absolutely invisible for STEREO-AHEAD spacecraft. The associated type II burst consisted of two successive parts of quite different appearance on the dynamic spectrum. The first burst was narrow in frequency, had cloudy structure and was completely unpolarized while the second one represented rich herring-bone structure and exposed high degree of circular polarization. Both parts of the whole event reveal band splitting and well distinguished harmonic structure. The positions and sizes of the sources of the type II burst were found using cross-correlation functions of interferometer bases. The sources of the type II bursts elements were found to be of about 15 arcmin in size in average, with the smallest ones reaching as low as 10 arcmin. Corresponding brightness temperatures were estimated. In most cases these temperatures were between 10^11 and 10^12 K with maximum value as high as 10^14 K. The spatial displacement of the source was measured and model independent velocities of the type II burst sources were determined.