arXiv subjects

Alexey A. Kuznetsov

Publications and source records attributed to Alexey A. Kuznetsov.

16 recordsLinked to original sources

arXiv2025

Magneto-Thermal Coupling and Coronal Heating in Solar Active Regions Inferred from Microwave Observations

The solar corona is much hotter than the photosphere and chromosphere, but the physical mechanism responsible for heating the coronal plasma remains unidentified yet. The thermal microwave emission, which is produced in strong magnetic field above sunspots, is a promising but barely exploited tool for studying the coronal magnetic field and plasma. We analyzed the microwave observations of eight solar active regions obtained with the Siberian Radioheliograph in years 2022-2024 in the frequency range of 6-12 GHz. We produced synthetic microwave images based on various coronal heating models, and determined the model parameters that provided the best agreement with the observations. The observations and simulations strongly favour either a steady-state (continuous) plasma heating process, or high-frequency heating by small energy release events with a short cadence. The average magnetic field strength in a coronal loop was found to decrease with the loop length, following a scaling law with the most probable index of about -0.55. In the majority of cases, the estimated volumetric heating rate was weakly dependent on the magnetic field strength, and decreased with the coronal loop length following a scaling law with the index of about -2.5. Among the known theoretical heating mechanisms, the model based on wave transmission or reflection in coronal loops acting as resonance cavities was found to provide the best agreement with the observations. The obtained results did not demonstrate a significant dependence on the emission frequency in the considered range.

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arXiv2025

Steady-State Heating of Diffuse Coronal Plasma in a Solar Active Region

Solar corona is much hotter than lower layers of the solar atmosphere-photosphere and chromosphere. The coronal temperature is up to 1MK in quiet sun areas, while up to several MK in active regions, which implies a key role of magnetic field in coronal heating. This means that understanding coronal heating requires reliable modeling of the underlying three-dimensional (3D) magnetic structure of an active region validated by observations. Here we employ synergy between 3D modeling, optically thick gyroresonant microwave emission, and optically thin EUV emission to (i) obtain and validate the best magneto-thermal model of the active region and (ii) disentangle various components of the EUV emission known as diffuse component, bright loops, open field regions, and "moss" component produced at the transition region. Surprisingly, the best thermal model corresponds to high-frequency energy release episodes, similar to a steady-state heating. Our analysis did not reveal significant deviations of the elemental abundances from the standard coronal values.

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arXiv2023

Data-Constrained Solar Modeling with GX Simulator

To facilitate the study of solar active regions and flaring loops, we have created a modeling framework, the freely distributed GX Simulator IDL package, that combines 3D magnetic and plasma structures with thermal and non-thermal models of the chromosphere, transition region, and corona. The package has integrated tools to visualize the model data cubes, compute multi-wavelength emission maps from them, and quantitatively compare the resulting maps with observations. Its object-based modular architecture, which runs on Windows, Mac, and Unix/Linux platforms, offers capabilities that include the ability to either import 3D density and temperature distribution models, or to assign numerically defined coronal or chromospheric temperatures and densities, or their distributions to each individual voxel. The application integrates FORTRAN and C++ libraries for fast calculation of radio emission (free-free, gyroresonance, and gyrosynchrotron emission) along with soft and hard X-ray and EUV codes developed in IDL. To facilitate the creation of models, we have developed a fully automatic model production pipeline that downloads the required SDO/HMI vector magnetic field data and (optionally) the contextual SDO/AIA images, performs potential or nonlinear force free field extrapolations, populates the magnetic field skeleton with parameterized heated plasma coronal models that assume either steady-state or impulsive plasma heating, and generates non-LTE density and temperature distribution models of the chromosphere that are constrained by photospheric measurements. The standardized models produced by this pipeline may be further customized through a set of interactive tools provided by the graphical user interface. Here we describe the GX Simulator framework and its applications.

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arXiv2022

X-Ray and Ultraviolet Flares on AT Microscopii Observed by AstroSat

We present observations of the active M-dwarf binary AT Mic (dM4.5e+dM4.5e) obtained with the orbital observatory AstroSat. During 20 ks of observations, in the far ultraviolet ($130-180$ nm) and soft X-ray ($0.3-7$ keV) spectral ranges, we detected both quiescent emission and at least five flares on different components of the binary. The X-ray flares were typically longer than and delayed (by $5-6$ min) with respect to their ultraviolet counterparts, in agreement with the Neupert effect. Using X-ray spectral fits, we have estimated the parameters of the emitting plasma. The results indicate the presence of a hot multi-thermal corona with the average temperatures in the range of $\sim 7-15$ MK and the emission measure of $\sim (2.9-4.5)\times 10^{52}$ $\textrm{cm}^{-3}$; both the temperature and the emission measure increased during the flares. The estimated abundance of heavy elements in the corona of AT Mic is considerably lower than at the Sun ($\sim 0.18-0.34$ of the solar photospheric value); the coronal abundance increased during the flares due to chromospheric evaporation. The detected flares had the energies of $\sim 10^{31}-10^{32}$ erg; the energy-duration relations indicate the presence of magnetic fields stronger than in typical solar flares.

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arXiv2022

Modulation of the solar microwave emission by sausage oscillations

The modulation of the microwave emission intensity from a flaring loop by a standing linear sausage fast magnetoacoustic wave is considered in terms of a straight plasma slab with the perpendicular Epstein profile of the plasma density, penetrated by a magnetic field. The emission is of the gyrosynchrotron (GS) nature, and is caused by mildly relativistic electrons which occupy a layer in the oscillating slab, i.e., the emitting and oscillating volumes do not coincide. It is shown that the microwave response to the linear sausage wave is highly non-linear. The degree of the non-linearity, defined as a ratio of the Fourier power of the second harmonic to the Fourier power of the principal harmonic, is found to depend on the combination of the width of the GS source and the viewing angle, and is different in the optically thick and optically thin parts of the microwave spectrum. This effect could be considered as a potential tool for diagnostics of the transverse scales of the regions filled in by the accelerated electrons.

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arXiv2021

Multi-wavelength quasi-periodic pulsations in a stellar superflare

We present the first multi-wavelength simultaneous detection of QPP in a superflare (more than a thousand times stronger than known solar flares) on a cool star, in soft X-rays (SXR, with XMM-Newton) and white light (WL, with Kepler). It allowed for the first-ever analysis of oscillatory processes in a stellar flare simultaneously in thermal and non-thermal emissions, conventionally considered to come from the corona and chromosphere of the star, respectively. The observed QPP have periods $1.5 \pm 0.15$ hours (SXR) and $3 \pm 0.6$ hours (WL), and correlate well with each other. The unique relationship between the observed parameters of QPP in SXR and WL allowed us to link them with oscillations of the electric current in the flare loop, which directly affect the dynamics of non-thermal electrons and indirectly (via Ohmic heating) the thermal plasma. These findings could be considered in favour of the equivalent LCR-contour model of a flare loop, at least in the extreme conditions of a stellar superflare.

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arXiv2021

Ultimate Fast Gyrosynchrotron Codes

The past decade has seen a dramatic increase of practical applications of the microwave gyrosynchrotron emission for plasma diagnostics and three-dimensional modeling of solar flares and other astrophysical objects. This break-through turned out to become possible due to apparently minor, technical development of Fast Gyrosynchrotron Codes, which enormously reduced the computation time needed to calculate a single spectrum, while preserving accuracy of the computation. However, the available fast codes are limited in that they could only be used for a factorized distribution over the energy and pitch-angle, while the distributions of electrons over energy or pitch-angle are limited to a number of predefined analytical functions. In realistic simulations, these assumptions do not hold; thus, the codes free from the mentioned limitations are called for. To remedy this situation, we extended our fast codes to work with an arbitrary input distribution function of radiating electrons. We accomplished this by implementing fast codes for a distribution function described by an arbitrary numerically-defined array. In addition, we removed several other limitations of the available fast codes and improved treatment of the free-free component. The Ultimate Fast Codes presented here allow for an arbitrary combination of the analytically and numerically defined distributions, which offers the most flexible use of the fast codes. We illustrate the code with a few simple examples.

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arXiv2021

Gyroresonance and free-free radio emissions from multi-thermal multi-component plasma

Thermal plasma of solar atmosphere includes a wide range of temperatures. This plasma is often quantified, both in observations and models, by a differential emission measure (DEM). DEM is a distribution of the thermal electron density square over temperature. In observations, the DEM is computed along a line of sight, while in the modeling -- over an elementary volume element (voxel). This description of the multi-thermal plasma is convenient and widely used in the analysis and modeling of extreme ultraviolet emission (EUV), which has an optically thin character. However, there is no corresponding treatment in the radio domain, where optical depth of emission can be large, more than one emission mechanism are involved, and plasma effects are important. Here, we extend the theory of the thermal gyroresonance and free-free radio emissions in the classical mono-temperature Maxwellian plasma to the case of a multi-temperature plasma. The free-free component is computed using the DEM and temperature-dependent ionization states of coronal ions, contributions from collisions of electrons with neutral atoms, exact Gaunt factor, and the magnetic field effect. For the gyroresonant component, another measure of the multi-temperature plasma is used which describes the distribution of the thermal electron density over temperature. We give representative examples demonstrating important changes in the emission intensity and polarization due to considered effects. The theory is implemented in available computer code.

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arXiv2021

Stellar Superflares Observed Simultaneously with Kepler and XMM-Newton

Solar and stellar flares are powerful events which produce intense radiation across the electromagnetic spectrum. Multiwavelength observations are highly important for understanding the nature of flares, because different flare-related processes reveal themselves in different spectral ranges. To study the correlation between thermal and nonthermal processes in stellar flares, we have searched the databases of Kepler (optical observations) and XMM-Newton (soft X-rays) for the flares observed simultaneously with both instruments; nine distinctive flares (with energies exceeding $10^{33}$ erg) on three stars (of K-M spectral classes) have been found. We have analyzed and compared the flare parameters in the optical and X-ray spectral ranges; we have also compared the obtained results with similar observations of solar flares. Most of the studied stellar flares released more energy in the optical range than in X-rays. In one flare, X-ray emission strongly dominated, which could be caused either by soft spectrum of energetic electrons or by a near-limb position of this flare. The X-ray flares were typically delayed with respect to and shorter than their optical counterparts, which is partially consistent with the Neupert effect. Using the scaling laws based on the magnetic reconnection theory, we have estimated the characteristic magnetic field strengths in the stellar active regions and the sizes of these active regions as about $25-70$ G and $250\,000-500\,000$ km, respectively. The observed stellar superflares appear to be scaled-up versions of solar flares, with a similar underlying mechanism and nearly the same characteristic magnetic field values, but with much larger active region sizes.

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arXiv2021

Coronal Heating Law Constrained by Microwave Gyroresonant Emission

The question why the solar corona is much hotter than the visible solar surface still puzzles solar researchers. Most theories of the coronal heating involve a tight coupling between the coronal magnetic field and the associated thermal structure. This coupling is based on two facts: (i) the magnetic field is the main source of the energy in the corona and (ii) the heat transfer preferentially happens along the magnetic field, while is suppressed across it. However, most of the information about the coronal heating is derived from analysis of EUV or soft X-ray emissions, which are not explicitly sensitive to the magnetic field. This paper employs another electromagnetic channel -- the sunspot-associated microwave gyroresonant emission, which is explicitly sensitive to both the magnetic field and thermal plasma. We use nonlinear force-free field reconstructions of the magnetic skeleton dressed with a thermal structure as prescribed by a field-aligned hydrodynamics to constrain the coronal heating model. We demonstrate that the microwave gyroresonant emission is extraordinarily sensitive to details of the coronal heating. We infer heating model parameters consistent with observations.

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arXiv2020

Radio Echo in the Turbulent Corona and Simulations of Solar Drift-Pair Radio Bursts

Drift-pair bursts are an unusual type of solar low-frequency radio emission, which appear in the dynamic spectra as two parallel drifting bright stripes separated in time. Recent imaging spectroscopy observations allowed for the quantitative characterization of the drifting pairs in terms of source size, position, and evolution. Here, the drift-pair parameters are qualitatively analyzed and compared with the newly-developed Monte Carlo ray-tracing technique simulating radio-wave propagation in the inhomogeneous anisotropic turbulent solar corona. The results suggest that the drift-pair bursts can be formed due to a combination of the refraction and scattering processes, with the trailing component being the result of turbulent reflection (turbulent radio echo). The formation of drift-pair bursts requires an anisotropic scattering with the level of plasma density fluctuations comparable to that in type III bursts, but with a stronger anisotropy at the inner turbulence scale. The anisotropic radio-wave scattering model can quantitatively reproduce the key properties of drift-pair bursts: the apparent source size and its increase with time at a given frequency, the parallel motion of the source centroid positions, and the delay between the burst components. The trailing component is found to be virtually co-spatial and following the main component. The simulations suggest that the drift-pair bursts are likely to be observed closer to the disk center and below 100 MHz due to the effects of free-free absorption and scattering. The exciter of drift-pairs is consistent with propagating packets of whistlers, allowing for a fascinating way to diagnose the plasma turbulence and the radio emission mechanism.

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arXiv2015

3D Radio and X-Ray Modeling and Data Analysis Software: Revealing Flare Complexity

We have undertaken a major enhancement of our IDL-based simulation tools developed earlier for modeling microwave and X-ray emission. The object-based architecture provides an interactive graphical user interface that allows the user to import photospheric magnetic field maps and perform magnetic field extrapolations to almost instantly generate 3D magnetic field models, to investigate the magnetic topology of these models by interactively creating magnetic field lines and associated magnetic flux tubes, to populate the flux tubes with user-defined nonuniform thermal plasma and anisotropic, nonuniform, nonthermal electron distributions; to investigate the spatial and spectral properties of radio and X-ray emission calculated from the model, and to compare the model-derived images and spectra with observational data. The application integrates shared-object libraries containing fast gyrosynchrotron emission codes developed in FORTRAN and C++, soft and hard X-ray codes developed in IDL, a FORTRAN-based potential-field extrapolation routine and an IDL-based linear force free field extrapolation routine. The interactive interface allows users to add any user-defined radiation code that adheres to our interface standards, as well as user-defined magnetic field extrapolation routines. Here we use this tool to analyze a simple single-loop flare and use the model to constrain the 3D structure of the magnetic flaring loop and 3D spatial distribution of the fast electrons inside this loop. We iteratively compute multi-frequency microwave and multi-energy X-ray images from realistic magnetic fluxtubes obtained from an extrapolation of a magnetogram taken prior to the flare, and compare them with imaging data obtained by SDO, NoRH, and RHESSI instruments. We use this event to illustrate use of the tool for general interpretation of solar flares to address disparate problems in solar physics.

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arXiv2013

Theory of gyroresonance and free-free emissions from non-Maxwellian quasi-steady-state electron distributions

Currently there is a concern about ability of the classical thermal (Maxwellian) distribution to describe quasi-steady-state plasma in solar atmosphere including active regions. In particular, other distributions have been proposed to better fit observations, for example, kappa- and $n$-distributions. If present, these distributions will generate radio emissions with different observable properties compared with the classical gyroresonance (GR) or free-free emission, which implies a way of remote detecting these non-Maxwellian distributions in the radio observations. Here we present analytically derived GR and free-free emissivities and absorption coefficients for the kappa- and $n$-distributions and discuss their properties, which are in fact remarkably different from each other and from the classical Maxwellian plasma. In particular, the radio brightness temperature from a gyrolayer increases with the optical depth $τ$ for kappa-distribution, but decreases with $τ$ for $n$-distribution. This property has a remarkable consequence allowing a straightforward observational test: the gyroresonance radio emission from the non-Maxwellian distributions is supposed to be noticeably polarized even in the optically thick case, where the emission would have strictly zero polarization in the case of Maxwellian plasma. This offers a way of remote probing the plasma distribution in astrophysical sources including solar active regions as a vivid example.

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arXiv2011

3D simulations of gyrosynchrotron emission from mildly anisotropic nonuniform electron distributions in symmetric magnetic loops

Microwave emission of solar flares is formed primarily by incoherent gyrosynchrotron radiation generated by accelerated electrons in coronal magnetic loops. The resulting emission depends on many factors, including pitch-angle distribution of the emitting electrons and the source geometry. In this work, we perform systematic simulations of solar microwave emission using recently developed tools (GS Simulator and fast gyrosynchrotron codes) capable of simulating maps of radio brightness and polarization as well as spatially resolved emission spectra. A 3D model of a symmetric dipole magnetic loop is used. We compare the emission from isotropic and anisotropic (of loss-cone type) electron distributions. We also investigate effects caused by inhomogeneous distribution of the emitting particles along the loop. It is found that effect of the adopted moderate electron anisotropy is the most pronounced near the footpoints and it also depends strongly on the loop orientation. Concentration of the emitting particles at the loop top results in a corresponding spatial shift of the radio brightness peak, thus reducing effects of the anisotropy. The high-frequency (around 50 GHz) emission spectral index is specified mainly by the energy spectrum of the emitting electrons; however, at intermediate frequencies (around 10-20 GHz), the spectrum shape is strongly dependent on the electron anisotropy, spatial distribution, and magnetic field nonuniformity. The implications of the obtained results for the diagnostics of the energetic electrons in solar flares are discussed.

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arXiv2010

Kinetic Simulation of the Electron-Cyclotron Maser Instability: Relaxation of Electron Horseshoe Distributions

The electron-cyclotron maser instability (ECMI) is responsible for generation of the planetary auroral radio emissions. Most likely, the same mechanism produces radio bursts from ultracool dwarfs. We investigate amplification of plasma waves by the horseshoe-like electron distribution (similar to those observed in the terrestrial magnetosphere) as well as relaxation of this distribution due to the ECMI. We aim to determine parameters of the generated plasma waves, timescales of the relaxation process, and the conversion efficiency of the particle energy into waves. We have developed a kinetic relativistic quasi-linear 2D code for simulating the coevolution of an electron distribution and the high-frequency plasma waves. The code includes the processes of wave growth and particle diffusion which are assumed to be much faster than other processes (particle injection, etc.). A number of simulations have been performed for different parameter sets which seem to be typical for the magnetospheres of ultracool dwarfs (in particular, the plasma frequency is much less than the cyclotron one). The calculations have shown that the fundamental extraordinary mode dominates strongly. The generated waves have the frequency slightly below the electron cyclotron frequency and propagate across the magnetic field. The final intensities of other modes are negligible. The conversion efficiency of the electron energy into the extraordinary waves is typically around 10%. Complete relaxation of the unstable electron distribution takes much less than a second. Energy efficiency of the ECMI is more than sufficient to provide the observed intensity of radio emission from ultracool dwarfs. On the other hand, the observed light curves of the emission are not related to the properties of this instability and reflect, most likely, dynamics of the electron acceleration process and/or geometry of the radiation source.

astro-ph.SR↗

arXiv2010

Optimized gyrosynchrotron algorithms and fast codes

Gyrosynchrotron (GS) emission of charged particles spiraling in magnetic fields plays an exceptionally important role in astrophysics. In particular, this mechanism makes a dominant contribution to the continuum solar and stellar radio emissions. However, the available exact equations describing the emission process are extremely slow computationally, thus limiting the diagnostic capabilities of radio observations. In this work, we present approximate GS codes capable of fast calculating the emission from anisotropic electron distributions. The computation time is reduced by several orders of magnitude compared with the exact formulae, while the computation error remains within a few percent. The codes are implemented as the executable modules callable from IDL; they are made available for users via web sites.

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