arXiv subjects

Eduard Kontar

Publications and source records attributed to Eduard Kontar.

At least 19 recordsLinked to original sources

arXiv2026

Solar, Heliospheric and Ionospheric Physics: Pathfinders, Precursors and SKAO Perspective

The Solar, Heliospheric and Ionospheric (SHI) Physics Science Working Group of the Square Kilometre Array Observatory (SKAO) addresses the full chain of plasma processes linking the solar corona to the terrestrial environment. This overview chapter synthesises 16 topical contributions to Advancing Astrophysics with the SKA-II, spanning the quiet and active solar atmosphere, eruptive phenomena, heliospheric turbulence and solar-wind diagnostics, ionospheric science, stellar-solar connections, and the observational frameworks required to deliver these science goals. The primary focus is on the capabilities of Array Assembly 4 (AA4), the design baseline for both SKA-LOW (50-350,MHz) and SKA-MID (0.35-15.4,GHz), which together provide continuous spectral coverage, sub-arcsecond angular resolution, full-Stokes polarimetry, and sensitivity gains of an order of magnitude over existing facilities. From resolving fine-scale coronal heating events to mapping coronal mass ejection magnetic fields and characterising multi-scale heliospheric turbulence, SKAO will deliver transformative advances in solar and space-weather science. We frame these contributions as a single end-to-end Sun-to-Earth system. We identify cross-cutting themes and gaps not fully addressed by individual chapters and outline the staged roadmap from early operations through to the full AA4 capability.

astro-ph.SR↗

arXiv2026

Probing the Solar Corona and the Solar Wind Using Angular Broadening Observations with the SKA

Angular broadening observations of compact radio sources provide a powerful method for probing the solar corona and solar wind. Such observations enable studies of the phase structure function, turbulence amplitude, intermediate-scale density fluctuations, solar-wind heating rates, and dissipation scales. When a compact radio source is observed through coronal or solar-wind plasma, several observable effects can arise: (1) the apparent source size increases because of scattering by turbulent plasma, producing angular broadening; (2) the source flux density decreases because of scattering and absorption; (3) the observed angular broadening can be anisotropic, reflecting anisotropic turbulence in the corona and solar wind; and (4) the position angle of the anisotropy, measured from north through east, can help infer the orientation of the coronal magnetic field. These effects provide insights into the physical processes governing the solar wind and its interaction with electromagnetic waves, and they offer constraints on coronal turbulence and magnetic-field structure. At present, angular broadening studies remain limited and have mostly focused on very bright radio sources such as Tau A. The unprecedented sensitivity and angular resolution of the Square Kilometre Array are expected to greatly expand the number of suitable background sources, opening a new window on the solar corona, solar wind, and heliosphere.

astro-ph.SR↗

arXiv2026

Unravelling the nonthermal emissions from the quiet solar corona with the SKA

Investigation of the nonthermal emissions from the quiet solar corona has been rather limited. This primarily stems from the fact that the emission is expected to be quite weak and very high dynamic range images are required to detect such emissions. Although, the few detections of the nonthermal emissions have all come from the radio band, the past observations had several issues, the most important being the low image fidelity and lack of simultaneous broadband observation capability. Recent observations have been able to tackle many of these prior difficulties using modern instrumentation, and have produced multiple interesting results. In spite of these improvements, multiple challenges remain. For example, most of the recent investigations were only able to focus on the brightest of these events, due to rather poor spectroscopic snapshot PSF of the instruments. The SKA, with its excellent sensitivity as well as its exquisite spectroscopic snapshot PSF would be a game-changer in this field. It would not only allow us to detect and characterise these emissions in Stokes I, but would also allow us to investigate their polarisation properties as well. The high angular resolution offered by the SKA will allow a unique association of the detected radio transients with their thermal counterparts. This would enable us to investigate the thermal-nonthermal energy partition even for these rather weak transient emissions, which in turn would allow validation of particle acceleration and magnetic reconnection theories in a regime vastly different from that done previously. In addition, characterization of the nonthermal emissions of these weak coronal transients may also provide a new probe to understand how energy is transferred from the photosphere and dumped into the corona, and thus serve as a new tool to tackle the coronal heating problem.

astro-ph.SR↗

arXiv2026

Radio Wave Propagation as a Probe of the Solar Corona and Solar Wind

Radio waves propagating through an inhomogeneous, turbulent medium such as the solar corona and solar wind become distorted, causing the initially plane wavefronts becomes corrugated and acquire an RMS phase deviation across the wavefront. This leads to observable effects such as angular broadening of radio sources or intensity scintillation. Such waves can be used to probe the solar wind through various techniques, including angular broadening and interplanetary scintillation observations. Such observations enable the study of several key properties, such as the phase structure function, amplitude of turbulence, density modulation index, solar wind heating rates, magnetic field topology, and dissipation scales. These phenomena provide critical insights into the physical processes governing the solar corona and solar wind and its interaction with radio waves, offering valuable constraints on both coronal and solar wind turbulence and coronal magnetic field configurations. Currently, the limited number of radio sources near the ecliptic restricts our observations. However, the SKA-Low and SKA-Mid are expected to detect a significantly larger number of radio sources, thereby providing deeper insights into the solar corona, solar wind, and heliosphere. Long-term observations will be crucial to understanding how the above-mentioned parameters vary with heliocentric distance and over the solar cycle.

astro-ph.SR↗

arXiv2026

Radio Diagnostics of Particle Acceleration in Solar Flares with SKAO Observations

Particle acceleration is a fundamental astrophysical process occurring across diverse systems and scales, producing electromagnetic emission across all wavelengths. Radio bursts from astrophysical systems like active galaxy jets, solar flares, pulsars, etc., provide a probe into the emission mechanism and particle acceleration processes. Among all astrophysical phenomena, magnetically driven solar flares provide unique diagnostics of nonthermal particles due to the advantage of multiple spatial-temporal and spectral measurements. The subsequent emitted radiation spans various sections of the radio spectrum. Based on brightness temperature and spectrum, one can distinguish between bright plasma emission or 'nonthermal' emission. Nonthermal radio bursts in meter and microwave bands arise from suprathermal particles, while the surrounding plasma produces fainter thermal emission. High spatial-temporal studies along with polarimetry will enable tracking of electron beams in evolving magnetic fields and constrain coronal properties such as magnetic strength, density, and temperature. Fine spectral structures from gyrosynchrotron bursts allow mapping of magnetic fields along acceleration tracks. However, emissions from the more numerous, weaker particle populations remain difficult to observe. Particles in solar flares follow a power-law energy distribution, with weaker populations being more common. The Square Kilometre Array Observatory (SKAO) will provide high-fidelity data that will enable detailed characterisation of these populations, statistical studies of particle beams, and insights into their interaction with magnetic topologies. Its high-resolution (especially SKA-Mid) and multi-wavelength synergy (EUV, X-rays) will refine diagnostics of coronal and acceleration-region properties. This chapter reviews particle acceleration models and expected advances from SKAO.

astro-ph.SR↗

arXiv2026

Solar Radio Burst Fine Structures

Solar radio bursts exhibit intricate variability in time, space, and frequency, often displaying a rich variety of fine frequency-time structures such as spikes, drift pairs, striae in Type III bursts, and herringbone patterns in Type II bursts, etc. Historically, limited spatial, spectral, and temporal resolution has hindered detailed investigation of these narrow-band, rapidly evolving features, restricting progress in identifying their physical origins and underlying processes at these scales. Advances in high-time-frequency-resolution solar imaging now offer transformative opportunities. Recent sub-second imaging spectroscopy has revealed that many fine structures challenge existing theoretical models, pointing to the need for new frameworks and a reassessment of current interpretations. The Square Kilometre Array (SKA), with its full-Stokes imaging spectroscopy at sub-second cadences, will provide unprecedented data essential for resolving these long-standing questions. These capabilities promise to significantly deepen our understanding of electron acceleration and transport, magnetic reconnection, and coronal plasma turbulence, thereby advancing our knowledge of solar energetic processes and improving assessments of their space-weather impacts.

astro-ph.SR↗

arXiv2026

Constraining turbulent solar flare acceleration regions by connecting kinetic modeling and X-ray observations

Spatially-resolved X-ray observations are the key to understanding electron acceleration in solar flares. Currently, the underlying processes that efficiently energize solar flare particles are poorly constrained. Abundant flare observations suggest that turbulence plays a crucial role in transferring energy between the magnetic field and energetic electrons. For the first time, we connect inhomogeneous acceleration from turbulence and hard X-ray spectroscopy and imaging observations with kinetic modeling to constrain the properties of flare acceleration. Observing three large flares with RHESSI, or Solar Orbiter/STIX, we extract X-ray imaging and spectroscopy observables. We compare with modeling results, mapping observables to electron acceleration and turbulent properties. We determine that extended regions of turbulence are required to match multiple X-ray observables, suggesting electrons are accelerated over a large fraction (~25%) of the flare loop; a property that is usually unconstrained from X-ray observations alone. Additionally, we determine acceleration timescales that vary between 7 and 22s by using fixed values for the turbulent scattering timescale and the velocity dependence of the acceleration diffusion coefficient. These fixed values are effectively unconstrained, but yield acceleration timescales that will help to restrict possible viable stochastic models.

astro-ph.SR↗

arXiv2024

AtLAST Science Overview Report

Submillimeter and millimeter wavelengths provide a unique view of the Universe, from the gas and dust that fills and surrounds galaxies to the chromosphere of our own Sun. Current single-dish facilities have presented a tantalising view of the brightest (sub-)mm sources, and interferometers have provided the exquisite resolution necessary to analyse the details in small fields, but there are still many open questions that cannot be answered with current facilities. In this report we summarise the science that is guiding the design of the Atacama Large Aperture Submillimeter Telescope (AtLAST). We demonstrate how tranformational advances in topics including star formation in high redshift galaxies, the diffuse circumgalactic medium, Galactic ecology, cometary compositions and solar flares motivate the need for a 50m, single-dish telescope with a 1-2 degree field of view and a new generation of highly multiplexed continuum and spectral cameras. AtLAST will have the resolution to drastically lower the confusion limit compared to current single-dish facilities, whilst also being able to rapidly map large areas of the sky and detect extended, diffuse structures. Its high sensitivity and large field of view will open up the field of submillimeter transient science by increasing the probability of serendipitous detections. Finally, the science cases listed here motivate the need for a highly flexible operations model capable of short observations of individual targets, large surveys, monitoring programmes, target of opportunity observations and coordinated observations with other observatories. AtLAST aims to be a sustainable, upgradeable, multipurpose facility that will deliver orders of magnitude increases in sensitivity and mapping speeds over current and planned submillimeter observatories.

astro-ph.IM↗

arXiv2023

The need for focused, hard X-ray investigations of the Sun

Understanding the nature of energetic particles in the solar atmosphere is one of the most important outstanding problems in heliophysics. Flare-accelerated particles compose a huge fraction of the flare energy budget; they have large influences on how events develop; they are an important source of high-energy particles found in the heliosphere; and they are the single most important corollary to other areas of high-energy astrophysics. Despite the importance of this area of study, this topic has in the past decade received only a small fraction of the resources necessary for a full investigation. For example, NASA has selected no new Explorer-class instrument in the past two decades that is capable of examining this topic. The advances that are currently being made in understanding flare-accelerated electrons are largely undertaken with data from EOVSA (NSF), STIX (ESA), and NuSTAR (NASA Astrophysics). This is despite the inclusion in the previous Heliophysics decadal survey of the FOXSI concept as part of the SEE2020 mission, and also despite NASA's having invested heavily in readying the technology for such an instrument via four flights of the FOXSI sounding rocket experiment. Due to that investment, the instrumentation stands ready to implement a hard X-ray mission to investigate flare-accelerated electrons. This white paper describes the scientific motivation for why this venture should be undertaken soon.

astro-ph.IM↗

arXiv2021

Electron Acceleration during Macroscale Magnetic Reconnection

The first self-consistent simulations of electron acceleration during magnetic reconnection in a macroscale system are presented. Consistent with solar flare observations the spectra of energetic electrons take the form of power-laws that extend more than two decades in energy. The drive mechanism for these nonthermal electrons is Fermi reflection in growing and merging magnetic flux ropes. A strong guide field is found to suppress the production of nonthermal electrons by weakening the Fermi drive mechanism. For a weak guide field the total energy content of nonthermal electrons dominates that of the hot thermal electrons even though their number density remains small. Our results are benchmarked with the hard x-ray, radio and extreme ultra-violet (EUV) observations of the X8.2-class solar flare on September 10, 2017.

physics.plasm-ph↗

arXiv2021

Constraints on the acceleration region of type III radio bursts from decimetric radio spikes and faint X-ray bursts

We study the release of energy during the gradual phase of a flare, characterized by faint bursts of non-thermal hard X-ray (HXR) emission associated with decimetric radio spikes and type III radio bursts starting at high frequencies and extending to the heliosphere. We characterize the site of electron acceleration in the corona and study the radial evolution of radio source sizes in the high corona. Imaging and spectroscopy of the HXR emission with Fermi and RHESSI provide a diagnostic of the accelerated electrons in the corona as well as a lower limit on the height of the acceleration region. Radio observations in the decimetric range with the ORFEES spectrograph provide radio diagnostics close to the acceleration region. Radio spectro-imaging with LOFAR in the meter range provide the evolution of the radio source sizes with their distance from the Sun, in the high corona. Non-thermal HXR bursts and radio spikes are well correlated on short timescales. The spectral index of non-thermal HXR emitting electrons is -4 and their number is about $2\times 10^{33}$ electrons/s. The density of the acceleration region is constrained between $1-5 \times 10^9$ cm$^{-3}$. Electrons accelerated upward rapidly become unstable to Langmuir wave production, leading to high starting frequencies of the type III radio bursts, and the elongation of the radio beam at its source is between 0.5 and 11.4 Mm. The radio source sizes and their gradient observed with LOFAR are larger than the expected size and gradient of the size of the electron beam, assuming it follows the expansion of the magnetic flux tubes. These observations support the idea that the fragmentation of the radio emission into spikes is linked to the fragmentation of the acceleration process itself. The combination of HXR and radio diagnostics in the corona provides strong constrains on the site of electron acceleration.

astro-ph.SR↗

arXiv2020

Particle acceleration with anomalous pitch angle scattering in 3D separator reconnection

Understanding how the release of stored magnetic energy contributes to the generation of non-thermal high energy particles during solar flares is an important open problem in solar physics. Magnetic reconnection plays a fundamental role in the energy release and conversion processes taking place during flares. A common approach for investigating particle acceleration is to use test particles in fields derived from magnetohydrodynamic (MHD) simulations of reconnection. These MHD simulations use anomalous resistivities that are much larger than the Spitzer resistivity based on Coulomb collisions. The processes leading to enhanced resistivity should also affect the test particles. We explore the link between resistivity and particle orbits building on a previous study using a 2D MHD simulation of magnetic reconnection. This paper extends the previous investigation to a 3D magnetic reconnection configuration and to study the effect on test particle orbits. We carried out orbit calculations using a 3D MHD simulation of separator reconnection. We use the relativistic guiding centre approximation including stochastic pitch angle scattering. The effects of varying the resistivity and the models for pitch angle scattering on particle orbit trajectories, final positions, energy spectra, final pitch angle distribution, and orbit duration are all studied in detail. Pitch angle scattering widens collimated beams of orbit trajectories, allowing orbits to access previously unaccessible field lines; this causes final positions to spread to topological structures that were previously inaccessible. Scattered orbit energy spectra are found to be predominantly affected by the level of anomalous resistivity, with the pitch angle scattering model only playing a role in isolated cases. Scattering is found to play a crucial role in determining the pitch angle and orbit duration distributions.

astro-ph.SR↗

arXiv2019

First imaging spectroscopy observations of solar drift pair bursts

Drift pairs are an unusual type of fine structure sometimes observed in dynamic spectra of solar radio emission. They appear as two identical short narrowband drifting stripes separated in time; both positive and negative frequency drifts are observed. Using the Low Frequency Array (LOFAR), we report unique observations of a cluster of drift pair bursts in the frequency range of 30-70 MHz made on 12 July 2017. Spectral imaging capabilities of the instrument have allowed us for the first time to resolve the temporal and frequency evolution of the source locations and sizes at a fixed frequency and along the drifting pair components. Sources of two components of a drift pair have been imaged and found to propagate in the same direction along nearly the same trajectories. Motion of the second component source is delayed in time with respect to that of the first one. The source trajectories can be complicated and non-radial; positive and negative frequency drifts correspond to opposite propagation directions. The drift pair bursts with positive and negative frequency drifts, as well as the associated broadband type-III-like bursts, are produced in the same regions. The visible source velocities are variable from zero to a few $10^4$ (up to ${\sim 10^5}$) km/s, which often exceeds the velocities inferred from the drift rate ($\sim 10^4$ km/s). The visible source sizes are of about $10'-18'$; they are more compact than typical type III sources at the same frequencies. The existing models of drift pair bursts cannot adequately explain the observed features. We discuss the key issues that need to be addressed, and in particular the anisotropic scattering of the radio waves. The broadband bursts observed simultaneously with the drift pairs differ in some aspects from common type III bursts and may represent a separate type of emission.

astro-ph.SR↗

arXiv2019

On the Source Position and Duration of a Solar Type III Radio Burst Observed by LOFAR

Solar type III radio bursts are excited by electron beams propagating outward from the Sun. The flux of type III radio burst has a time profile of rising and decay phase at a given frequency, which has been actively studied since 1970s. Several factors that may influence the duration of a type III radio burst has been proposed. However, the major cause of the duration is still an open question. In this work, to study the dominant cause of the duration, we investigate the source positions of the front edge, the peak, and the tail edge in the dynamic spectrum of a single and clear type III radio burst. The duration of this type III burst at a given frequency is about 3 second for decameter wave. The beam-formed observations by the LOw-Frequency ARray (LOFAR) are used, which can provide the radio source positions and the dynamic spectra at the same time. We find that, for this burst, the source positions of the front edge, the peak, and the tail edge split with each other spatially. The radial speed of the front edge, the peak, and the tail edge is 0.42 c, 0.25 c, and 0.16 c, respectively. We estimate the influences of the corona density fluctuation and the electron-velocity dispersion on the duration, and the scattering effect by comparison with a few short-duration bursts from the same region. The analysis yields that, in the frequency range of 30 - 41 MHz, the electron-velocity dispersion is the dominant factor that determines the time duration of type III radio bursts with long duration, while scattering may play important role in the duration of short bursts.

astro-ph.SR↗

arXiv2017

Solar plasma radio emission in the presence of imbalanced turbulence of kinetic-scale Alfvén waves

We study the influence of kinetic-scale Alfvénic turbulence on the generation of plasma radio emission in the solar coronal regions where the plasma/magnetic pressure ratio $β$ is smaller than the electron/ion mass ratio $m_{e}/m_{i}$. The present study is motivated by the phenomenon of solar type I radio storms associated with the strong magnetic field of active regions. The measured brightness temperature of the type I storms can be up to $10^{10}$ K for continuum emission, and can exceed $10^{11}$ K for type I bursts. At present, there is no generally accepted theory explaining such high brightness temperatures and some other properties of the type I storms. We propose the model with the imbalanced turbulence of kinetic-scale Alfvén waves producing an asymmetric quasilinear plateau on the upward half of the electron velocity distribution. The Landau damping of resonant Langmuir waves is suppressed and their amplitudes grow spontaneously above the thermal level. The estimated saturation level of Langmuir waves is high enough to generate observed type I radio emission at the fundamental plasma frequency. Harmonic emission does not appear in our model because the backward-propagating Langmuir waves undergo a strong Landau damping. Our model predicts $100\%$ polarization in the sense of the ordinary (o-) mode of type I emission.

physics.space-ph↗

arXiv2017

Polarisation of microwave emission from reconnecting twisted coronal loops

Magnetic reconnection and particle acceleration due to the kink instability in twisted coronal loops can be a viable scenario for confined solar flares. Detailed investigation of this phenomenon requires reliable methods for observational detection of magnetic twist in solar flares, which may not be possible solely through extreme UV and soft X-ray thermal emission. Polarisation of microwave emission in flaring loops can be used as one of the detection criteria. The aim of this study is to investigate the effect of magnetic twist in flaring coronal loops on the polarisation of gyro-synchrotron microwave (GSMW) emission, and determine whether it could provide a means for magnetic twist detection. We use time-dependent magnetohydrodynamic and test-particle models developed using LARE3D and GCA codes to investigate twisted coronal loops relaxing following the kink-instability. Synthetic GSMW emission maps (I and V Stokes components) are calculated using GX simulator. It is found that flaring twisted coronal loops produce GSMW radiation with a gradient of circular polarisation across the loop. However, these patterns may be visible only for a relatively short period of time due to fast magnetic reconfiguration after the instability. Their visibility also depends on the orientation and position of the loop on solar disk. Typically, it would be difficult to see these characteristic polarisation pattern in a twisted loop seen from the top (close to the centre of the solar disk), but easier in a twisted loop seen from the side (i.e. observed very close to the limb).

astro-ph.SR↗

arXiv2017

Exploring impulsive solar magnetic energy release and particle acceleration with focused hard X-ray imaging spectroscopy

How impulsive magnetic energy release leads to solar eruptions and how those eruptions are energized and evolve are vital unsolved problems in Heliophysics. The standard model for solar eruptions summarizes our current understanding of these events. Magnetic energy in the corona is released through drastic restructuring of the magnetic field via reconnection. Electrons and ions are then accelerated by poorly understood processes. Theories include contracting loops, merging magnetic islands, stochastic acceleration, and turbulence at shocks, among others. Although this basic model is well established, the fundamental physics is poorly understood. HXR observations using grazing-incidence focusing optics can now probe all of the key regions of the standard model. These include two above-the-looptop (ALT) sources which bookend the reconnection region and are likely the sites of particle acceleration and direct heating. The science achievable by a direct HXR imaging instrument can be summarized by the following science questions and objectives which are some of the most outstanding issues in solar physics (1) How are particles accelerated at the Sun? (1a) Where are electrons accelerated and on what time scales? (1b) What fraction of electrons is accelerated out of the ambient medium? (2) How does magnetic energy release on the Sun lead to flares and eruptions? A Focusing Optics X-ray Solar Imager (FOXSI) instrument, which can be built now using proven technology and at modest cost, would enable revolutionary advancements in our understanding of impulsive magnetic energy release and particle acceleration, a process which is known to occur at the Sun but also throughout the Universe.

astro-ph.SR↗

arXiv2016

Global Energetics of Solar Flares: III. Non thermal Energies

This study entails the third part of a global flare energetics project, in which Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) data of 191 M and X-class flare events from the first 3.5 yrs of the Solar Dynamics Observatory (SDO) mission are analyzed. We fit a thermal and a nonthermal component to RHESSI spectra, yielding the temperature of the differential emission measure (DEM) tail, the nonthermal power law slope and flux, and the thermal/nonthermal cross-over energy $e_{\mathrm{co}}$. From these parameters we calculate the total nonthermal energy $E_{\mathrm{nt}}$ in electrons with two different methods: (i) using the observed cross-over energy $e_{\mathrm{co}}$ as low-energy cutoff, and (ii) using the low-energy cutoff $e_{\mathrm{wt}}$ predicted by the warm thick-target bremsstrahlung model of Kontar et al. {\bf Based on a mean temperature of $T_e=8.6$ MK in active regions we find low-energy cutoff energies of $e_{\mathrm{wt}} =6.2\pm 1.6$ keV for the warm-target model, which is significantly lower than the cross-over energies $e_{\mathrm{co}}=21 \pm 6$ keV. Comparing with the statistics of magnetically dissipated energies $E_{\mathrm{mag}}$ and thermal energies $E_{\mathrm{th}}$ from the two previous studies, we find the following mean (logarithmic) energy ratios with the warm-target model: $E_{\mathrm{nt}} = 0.41 \ E_{\mathrm{mag}}$, $E_{\mathrm{th}} = 0.08 \ E_{\mathrm{mag}}$, and $E_{\mathrm{th}} = 0.15 \ E_{\mathrm{nt}}$. The total dissipated magnetic energy exceeds the thermal energy in 95% and the nonthermal energy in 71% of the flare events, which confirms that magnetic reconnection processes are sufficient to explain flare energies. The nonthermal energy exceeds the thermal energy in 85\% of the events, which largely confirms the warm thick-target model.

astro-ph.SR↗