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Dynamics of electron beams in the solar corona plasma with density fluctuations

Propagation of a fast electron cloud in a solar-like plasma of decreasing density

Dynamics of a fast Maxwellian electron cloud in coronal plasma

Propagation of a Maxwellian Electron Cloud in a Plasma

To gasdynamic description of a hot electron cloud in a cold plasma

Propagation of a Monoenergetic Electron Beam in the Solar Corona

The spread of the hot electron cloud in the solar corona

Numerical and analytical study of the propagation of a monoenergetic electron beam in a plasma

Gasdynamic description of electron-beam flying-off in a plasma

In the frame of the theory of weak turbulence the propagation of electron beams is considered. U... more In the frame of the theory of weak turbulence the propagation of electron
beams is considered. Using the smallness of quasilinear time transition
from kinetic equations to gas-dynamic ones has been done. The obtained
gas-dynamic equations are solved for one, two and N monoenergetic
beams. It is shown that each beam generally propagates as a beam-plasma
structure consisting of electrons and Langmuir waves. Interaction of the
structures at their propagation leads to electron exchange and spatial shape
changing. Different situations are discussed in dependence on initial beam
velocities and densities.

Astrophysical radio sources are embedded in turbulent magnetised environments. In the 1 MHz sky, ... more Astrophysical radio sources are embedded in turbulent magnetised environments. In the 1 MHz sky, solar radio bursts are the brightest sources, produced by electrons travelling along magnetic field lines from the Sun through the heliosphere. We demonstrate that the magnetic field not only guides the emitting electrons, but also directs radio waves via anisotropic scattering from density irregularities in the magnetised plasma. Using multi-vantage-point type III solar radio burst observations and anisotropic radio wave propagation simulations, we show that the interplanetary field structure is encoded in the observed radio emission directivity, and that large-scale turbulent channelling of radio waves is present over large distances, even for relatively weak anisotropy in the embedded density fluctuations. Tracing the radio emission at many frequencies (distances), the effects of anisotropic scattering can be disentangled from the electron motion along the interplanetary magnetic field, and the emission source locations are unveiled. Our analysis suggests that magnetic field structures within turbulent media could be reconstructed using radio observations and is found consistent with the Parker field, offering a novel method for remotely diagnosing the large-scale field structure in the heliosphere and other astrophysical plasmas.

Electrons accelerated by solar flares and observed as type III solar radio bursts are not only a ... more Electrons accelerated by solar flares and observed as type III solar radio bursts are not only a crucial diagnostic tool for understanding electron transport in the inner heliosphere but also a possible early indication of potentially hazardous space weather events. The electron beams traveling in the solar corona and heliosphere along magnetic field lines generate Langmuir waves and quasilinearly relax toward a plateau in velocity space. The relaxation of the electron beam over the short distance in contrast to large beam-travel distances observed is often referred to as Sturrok's dilemma. Here, we develop a new electron transport model with quasilinear distance/time selfconsistently changing in space and time. This model results in a nonlinear advection-diffusion equation for the electron beam density with a nonlinear diffusion term that is inversely proportional to the beam density. The solution predicts slow super-diffusive (ballistic) spatial expansion of a fast-propagating electron beam. This model also provides the evolution of the spectral energy density of Langmuir waves, which determines brightness temperature of plasma radiation in solar bursts. The model solution is consistent with the results of numerical simulation using kinetic equations and can explain some characteristics of type III solar radio bursts.

We address the problem of how to test whether an observed solar hard X-ray bremsstrahlung spectru... more We address the problem of how to test whether an observed solar hard X-ray bremsstrahlung spectrum (I ()) is consistent with a purely thermal (locally Maxwellian) distribution of source electrons, and, if so, how to reconstruct the corresponding differential emission measure (ξ (T)). Unlike previous analysis based on the Kramers and Bethe-Heitler approximations to the bremsstrahlung cross-section, here we use an exact (solid-angle-averaged) cross-section. We show that the problem of determining ξ (T) from measurements of I () invOlves two successive inverse problems: the first, to recover the mean source-electron flux spectrum (F(E)) from I () and the second, to recover ξ (T) fromF(E). We discuss the highly pathological numerical properties of this second problem within the framework of the regularization theory for linear inverse problems. In particular, we show that an iterative scheme with a positivity constraint is effective in recovering δ-like forms of ξ (T) while first-order Tikhonov regularization with boundary conditions works well in the case of power-lawlike forms. Therefore, we introduce a restoration approach whereby the low-energy part ofF(E), dominated by the thermal component, is inverted by using the iterative algorithm with positivity, while the high-energy part, dominated by the power-law component, is inverted by using first-order regularization. This approach is first tested by using simulatedF(E) derived from a priori known forms of ξ (T) and then applied to hard X-ray spectral data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI).

To facilitate the study of solar flares and active regions, we have created a modeling framework,... more To facilitate the study of solar flares and active regions, we have created a modeling framework, the freely distributed GX Simulator IDL package, that combines 3D magnetic and plasma structures with thermal and nonthermal models of the chromosphere, transition region, and corona. Its object-based modular architecture, which runs on Windows, Mac, and Unix/Linux platforms, offers 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. GX Simulator can apply parametric heating models involving average properties of the magnetic field lines crossing a given voxel, as well as compute and investigate the spatial and spectral properties of radio, (sub)millimeter, EUV, and X-ray emissions calculated from the model, and quantitatively compare them with observations. The package includes a fully automatic model production pipeline that, based on minimal users input, downloads the required SDO/HMI vector magnetic field data, 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 specialized IDL scripts, or a set of interactive tools provided by the graphical user interface. Here, we describe the GX Simulator framework and its applications.

Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, cha... more Solar radio bursts are strongly affected by radio-wave scattering on density inhomogeneities, changing their observed time characteristics, sizes, and positions. The same turbulence causes angular broadening and scintillation of galactic and extragalactic compact radio sources observed through the solar atmosphere. Using large-scale simulations of radio-wave transport, the characteristics of anisotropic density turbulence from 0.1 R e to 1 au are explored. For the first time, a profile of heliospheric density fluctuations is deduced that accounts for the properties of extrasolar radio sources, solar radio bursts, and in situ density fluctuation measurements in the solar wind at 1 au. The radial profile of the spectrum-weighted mean wavenumber of density fluctuations (a quantity proportional to the scattering rate of radio waves) is found to have a broad maximum at around (4-7) R e , where the slow solar wind becomes supersonic. The level of density fluctuations at the inner scale (which is consistent with the proton resonance scale) decreases with heliocentric distance as () () n r r R 2 10 1 i 2 7 3.7   d á ñ´-cm −6. Due to scattering, the apparent positions of solar burst sources observed at frequencies between 0.1 and 300 MHz are computed to be essentially cospatial and to have comparable sizes, for both fundamental and harmonic emission. Anisotropic scattering is found to account for the shortest solar radio burst decay times observed, and the required wavenumber anisotropy is q ∥ /q ⊥ = 0.25-0.4, depending on whether fundamental or harmonic emission is involved. The deduced radio-wave scattering rate paves the way to quantify intrinsic solar radio burst characteristics.

Interplanetary solar radio type III bursts provide the means to remotely study and track energeti... more Interplanetary solar radio type III bursts provide the means to remotely study and track energetic electrons propagating in the interplanetary medium. Due to the lack of direct radio source imaging, several methods have been developed to determine the source positions from space-based observations. Moreover, none of the methods consider the propagation effects of anisotropic radio-wave scattering, which would strongly distort the trajectory of radio waves, delay their arrival times, and affect their apparent characteristics. We investigate the source positions and directivity of an interplanetary type III burst simultaneously observed by Parker Solar Probe, Solar Orbiter, STEREO, and Wind and we compare the results of applying the intensity fit and timing methods with ray-tracing simulations of radio-wave propagation with anisotropic density fluctuations. The simulation calculates the trajectories of the rays, their time profiles at different viewing sites, and the apparent characteristics for various density fluctuation parameters. The results indicate that the observed source positions are displaced away from the locations where emission is produced, and their deduced radial distances are larger than expected from density models. This suggests that the apparent position is affected by anisotropic radio-wave scattering, which leads to an apparent position at a larger heliocentric distance from the Sun. The methods to determine the source positions may underestimate the apparent positions if they do not consider the path of radio-wave propagation and incomplete scattering at a viewing site close to the intrinsic source position.

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

Astrophysical Journal, 2023

Solar flare hard X-ray (HXR) spectroscopy serves as a key diagnostic of the accelerated electron ... more Solar flare hard X-ray (HXR) spectroscopy serves as a key diagnostic of the accelerated electron spectrum. However, the standard approach using the collisional cold thick-target model poorly constrains the lower-energy part of the accelerated electron spectrum, hence the overall energetics of the accelerated electrons are typically constrained only to within one or two orders of magnitude. Here, we develop and apply a physically selfconsistent, warm-target approach that involves the use of both HXR spectroscopy and imaging data. This approach allows an accurate determination of the electron distribution low-energy cutoff, and hence the electron acceleration rate and the contribution of accelerated electrons to the total energy released, by constraining the coronal plasma parameters. Using a solar flare observed in X-rays by RHESSI, we demonstrate that using the standard cold-target methodology, the low-energy cutoff (hence the energy content in electrons) is essentially undetermined. However, the warm-target methodology can determine the low-energy electron cutoff with ∼7% uncertainty at the 3σ level, hence it permits an accurate quantitative study of the importance of accelerated electrons in solar flare energetics.

Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for... more Solar flares are known to be prolific electron accelerators, yet identifying the mechanism(s) for such efficient electron acceleration in solar flare (and similar astrophysical settings) presents a major challenge. This is due in part to a lack of observational constraints related to conditions in the primary acceleration region itself. Accelerated electrons with energies above ∼20 keV are revealed by hard X-ray (HXR) bremsstrahlung emission, while accelerated electrons with even higher energies manifest themselves through radio gyrosynchrotron emission. Here, we show, for a well-observed flare on 2017 September 10, that a combination of RHESSI HXR and and the Solar Dynamics Observatory/Atmospheric Imaging Assembly (SDO/AIA) EUV observations provides a robust estimate of the fraction of the ambient electron population that is accelerated at a given time, with an upper limit of 10 −2 on the number density of nonthermal (20 keV) electrons, expressed as a fraction of the number density of ambient protons in the same volume. This upper limit is about 2 orders of magnitude lower than previously inferred from microwave observations of the same event. Our results strongly indicate that the fraction of accelerated electrons in the coronal region at any given time is relatively small but also that the overall duration of the HXR emission requires a steady resupply of electrons to the acceleration site. Simultaneous measurements of the instantaneous accelerated electron number density and the associated specific electron acceleration rate provide key constraints for a quantitative study of the mechanisms leading to electron acceleration in magnetic reconnection events.

Dynamics of electron beams in the solar corona plasma with density fluctuations

Propagation of a fast electron cloud in a solar-like plasma of decreasing density

Dynamics of a fast Maxwellian electron cloud in coronal plasma

Propagation of a Maxwellian Electron Cloud in a Plasma

To gasdynamic description of a hot electron cloud in a cold plasma

Propagation of a Monoenergetic Electron Beam in the Solar Corona

The spread of the hot electron cloud in the solar corona

Numerical and analytical study of the propagation of a monoenergetic electron beam in a plasma

Gasdynamic description of electron-beam flying-off in a plasma

Astrophysical Journal, 2023

Diffusive transport of energetic electrons in the solar corona: X-ray and radio diagnostics

Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind

Solar type III radio burst time characteristics at LOFAR frequencies and the implications for electron beam transport

Combined Radio and Space-Based Solar Observations: From Techniques to New Results - Preface

Shock location and CME 3D reconstruction of a solar type II radio burst with LOFAR

Energy Deposition by Energetic Electrons in a Diffusive Collisional Transport Model

Electron Distribution and Energy Release in Magnetic Reconnection Outflow Regions during the Pre-impulsive Phase of a Solar Flare

Preface: Solar physics advances from the interior to the heliosphere

A Fokker-Planck Framework for Studying the Diffusion of Radio Burst Waves in the Solar Corona

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

Density Fluctuations in the Solar Wind Based on Type III Radio Bursts Observed by Parker Solar Probe

The Solar Orbiter Science Activity Plan. Translating solar and heliospheric physics questions into action

The Energetic Particle Detector. Energetic particle instrument suite for the Solar Orbiter mission

The Solar Orbiter Radio and Plasma Waves (RPW) instrument

Probing solar flare accelerated electron distributions with prospective X-ray polarimetry missions

Parametric Simulation Studies on the Wave Propagation of Solar Radio Emission: The Source Size, Duration, and Position

Electron Acceleration during Macroscale Magnetic Reconnection

Energy Budget of Plasma Motions, Heating, and Electron Acceleration in a Three-loop Solar Flare

Solar Orbiter's first Venus flyby: Observations from the Radio and Plasma Wave instrument

The Solar Orbiter Radio and Plasma Waves (RPW) instrument (Corrigendum)

High-Energy Aspects of Solar Flares, 2012

High-energy X-rays and γ-rays from solar flares were discovered just over fifty years ago. Since ... more High-energy X-rays and γ-rays from solar flares were discovered just over fifty years ago. Since that time, the standard for the interpretation of spatially integrated flare X-ray spectra at energies above several tens of keV has been the collisional thick-target model. After the launch of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in early 2002, X-ray spectra and images have been of sufficient quality to allow a greater focus on the energetic electrons responsible for the X-ray emission, including their origin and their interactions with the flare plasma and magnetic field. The result has been new insights into the flaring process, as well as more quantitative models for both electron acceleration and propagation, and for the flare environment with which the electrons interact. In this article we review our current understanding of electron acceleration, energy loss, and propagation in flares. Implications of these new results for the collisional thick-target model, for general flare models, and for future flare studies are discussed.

High-Energy Aspects of Solar Flares, 2012

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The ... more X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction , spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion bremsstrahlung, photoelectric absorption and Compton back-scatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons using albedo, improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown 2 E.P. Kontar et al. details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data.

Lecture Notes in Physics, 2007

Hard X-rays and γ-rays are the most direct signatures of energetic electrons and ions in the sun'... more Hard X-rays and γ-rays are the most direct signatures of energetic electrons and ions in the sun's atmosphere which is optically thin at these energies and their radiation involves no coherent processes. Being collisional they are complementary to gyro-radiation in probing atmospheric density as opposed to magnetic field and the electrons are primarily 10-100 keV in energy, complementing the (>100 keV) electrons likely responsible for microwave bursts. The pioneering results of the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) are raising the first new major questions concerning solar energetic particles in many years. Some highlights of these results are discussed-primarily around RHESSI topics on which the authors have had direct research involvement-particularly when they are raising the need for rethinking of entrenched ideas. Results and issues are broadly divided into discoveries in the spatial, temporal and spectral domains, with the main emphasis on flare hard X-rays/fast electrons but touching also on γ-rays/ions, non-flare emissions, and the relationship to radio bursts.

Recent Advances in Understanding Particle Acceleration Processes in Solar Flares