Acceleration In Solar Flares Research Articles

Particle acceleration in solar flares with imaging-spectroscopy in soft X-rays

Particle acceleration in solar flares with imaging-spectroscopy in soft X-rays

Dynamical Modulation of Solar Flare Electron Acceleration due to Plasmoid-shock Interactions in the Looptop Region

Abstract A fast-mode shock can form in the front of reconnection outflows and has been suggested as a promising site for particle acceleration in solar flares. Recent developments in the study of magnetic reconnection have shown that numerous plasmoids can be produced in a large-scale current layer. Here we investigate the dynamical modulation of electron acceleration in the looptop region when plasmoids intermittently arrive at the shock by combining magnetohydrodynamics simulations with a particle kinetic model. As plasmoids interact with the shock, the looptop region exhibits various compressible structures that modulate the production of energetic electrons. The energetic electron population varies rapidly in both time and space. The number of 5–10 keV electrons correlates well with the compression area, while that of >50 keV electrons shows good correlation with the strong compression area but only moderate correlation with shock parameters. We further examine the impacts of the first plasmoid, which marks the transition from a quasi-steady shock front to a distorted and dynamical shock. The number of energetic electrons is reduced by ∼20% at 15–25 keV and nearly 40% for 25–50 keV, while the number of 5–10 keV electrons increases. In addition, the electron energy spectrum above 10 keV evolves softer with time. We also find that double or even multiple distinct sources can develop in the looptop region when the plasmoids move across the shock. Our simulations have strong implications to the interpretation of nonthermal looptop sources, as well as the commonly observed fast temporal variations in flare emissions, including the quasi-periodic pulsations.

Thermal-nonthermal energy partition in solar flares derived from X-ray, EUV, and bolometric observations

Context.In solar flares, energy is released impulsively and is partly converted into thermal energy of hot plasmas and kinetic energy of accelerated nonthermal particles. It is crucial to constrain the partition of these two energy components to understand energy release and transport as well as particle acceleration in solar flares. Despite numerous efforts, no consensus on quantifying this energy balance has yet been reached.Aims.We aim to understand the reasons for the contradicting results on energy partition obtained by various recent studies. The overarching question we address is whether there is sufficient energy in nonthermal particles to account for the thermal flare component.Methods.We considered five recent studies that address the thermal-nonthermal energy partition in solar flares. Their results are reviewed, and their methods are compared and discussed in detail.Results.The main uncertainties in deriving the energy partition are identified as (a) the derivation of the differential emission measure distribution and (b) the role of the conductive energy loss for the thermal component, as well as (c) the determination of the low-energy cutoff for the injected electrons. The bolometric radiated energy, as a proxy for the total energy released in the flare, is a useful independent constraint on both thermal and nonthermal energetics. In most of the cases, the derived energetics are consistent with this constraint. There are indications that the thermal-nonthermal energy partition changes with flare strength: in weak flares, there appears to be a deficit of energetic electrons, while the injected nonthermal energy is sufficient to account for the thermal component in strong flares. This behavior is identified as the main cause of the dissimilar results in the studies we considered. The changing partition has two important consequences: (a) an additional direct (i.e. non-beam) heating mechanism has to be present, and (b) considering that the bolometric emission originates mainly from deeper atmospheric layers, conduction or waves are required as additional energy transport mechanisms.

X-ray and gamma-ray emission from solar flares

We present a brief review of the contemporary understanding of and topical problems in solar flare physics that can be clarified by methods of X-ray and gamma-ray astronomy. The review focuses on several issues, including the conditions and mechanisms of electron acceleration in solar flares, the flare energy distribution between thermal and nonthermal components, the gamma-ray emission from solar flares and its dynamics, and the spatial structure of X-ray and gamma-ray sources. Discussed in this context are the latest data obtained by the joint Russia–US experiment Konus-Wind, which in 2019 celebrated the 25th anniversary of continuous operation in space.

Two Phases of Solar Flares and a Stochastic Mechanism for Acceleration of Electrons and Protons

The hypothesis of two phases of charged particle acceleration in solar flares is well known, with subrelativistic electrons accelerated in the first phase and relativistic electrons and protons in the second. Both phases are distinguished in the solar proton events and their parent flares of December 26, 2001 (M7.1), November 2, 2003 (X8.3), and August 9, 2011 (X6.9), while in the interplanetary space only electrons were observed from the first phase and electrons and protons from the second. The temporal profiles of the electrons and protons from the second phase are similar; hence, it is concluded that the relativistic electrons and protons observed in the interplanetary space are predominantly accelerated in flares, rather than in the shock front of a coronal mass ejection. Most likely a stochastic acceleration mechanism is realized in flares where protons and electrons gain energy in many elementary events over the entire duration of the flare which lasts much longer than an elementary event. To ensure consistency of a stochastic acceleration process with the existence of two phases in solar flares it is necessary to consider gyrosynchronous radiative electron losses in the second phase which can be neglected in the first phase. The energy of the accelerated protons in the first phase is small for their detection in processes taking place on the sun, but in the second phase it may be sufficient to produce gamma lines, both nuclear and from pion decay.

Relationships between Photospheric Vertical Electric Currents and Hard X-Ray Sources in Solar Flares: Statistical Study

Abstract There are still debates over whether particle acceleration in solar flares may occur due to interruption of electric currents flowing along magnetic loops. To contribute to this debate, we performed the first statistical study of relationships between flare hard X-ray (HXR; 50–100 keV) sources observed by the Ramaty High-Energy Solar Spectroscopic Imager and photospheric vertical electric currents (PVECs, j r ) calculated using vector magnetograms obtained with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory. We analyzed a sample of 48 flares from the C3.0 to X3.1 class and were observed in the central part of the solar disk by both instruments in 2010–2015. We found that ≈70% of all HXR sources overlapped with islands or ribbons of enhanced ( statampere cm−2) PVECs. However, less than ≈40% of the HXR sources overlapped with PVEC maxima, with an accuracy of ±3″. In more than half of the flares, there were HXR sources outside of regions of enhanced PVECs. We found no correlation between the intensity of the HXR sources and PVEC density or total PVEC under them. No systematic dissipation of PVECs under the HXR sources was found during the flares. Collectively, the results do not support the current-interruption flare models. However, the results indicate the importance of the presence of longitudinal currents in flare regions. To understand their specific role in the processes of energy release, plasma heating, and acceleration of particles requires further investigation.

Radio Spectroscopic Imaging of a Solar Flare Termination Shock: Split-band Feature as Evidence for Shock Compression

Abstract Solar flare termination shocks have been suggested as one of the promising drivers for particle acceleration in solar flares, yet observational evidence remains rare. By utilizing radio dynamic spectroscopic imaging of decimetric stochastic spike bursts in an eruptive flare, Chen et al. found that the bursts form a dynamic surface-like feature located at the ending points of fast plasma downflows above the looptop, interpreted as a flare termination shock. One piece of observational evidence that strongly supports the termination shock interpretation is the occasional split of the emission band into two finer lanes in frequency, similar to the split-band feature seen in fast-coronal-shock-driven type II radio bursts. Here, we perform spatially, spectrally, and temporally resolved analysis of the split-band feature of the flare termination shock event. We find that the ensemble of the radio centroids from the two split-band lanes each outlines a nearly co-spatial surface. The high-frequency lane is located slightly below its low-frequency counterpart by ∼0.8 Mm, which strongly supports the shock-upstream–downstream interpretation. Under this scenario, the density compression ratio across the shock front can be inferred from the frequency split, which implies a shock with a Mach number of up to 2.0. Further, the spatiotemporal evolution of the density compression along the shock front agrees favorably with results from magnetohydrodynamics simulations. We conclude that the detailed variations of the shock compression ratio may be due to the impact of dynamic plasma structures in the reconnection outflows, which results in distortion of the shock front.

Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration

Abstract Particle acceleration in solar flares remains an outstanding problem in solar physics. It is currently unclear which of the acceleration mechanisms dominates and how exactly the excessive magnetic energy is transferred to nonthermal and other forms of energy. We emphasize that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected timescales and the highest acceleration efficiency. Here we focus on a detailed multiwavelength analysis of the initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of the extended coronal loop(s). The electrons are promptly accelerated up to several hundreds of keV, with the characteristic acceleration time shorter than 50 ms. We show that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.

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

The phenomena observed at the Sun have a variety of unique radio signatures that can be used to diagnose the processes in the solar atmosphere. The insights provided by radio obervations are further enhanced when they are combined with observations from space-based telescopes. This special issue demonstrates the power of combination methodology at work and provides new results on i) type I solar radio bursts and thermal emission to study active regions; ii) type II and IV bursts to better understand the structure of coronal mass ejections; iii)~non-thermal gyro-synchrotron and/or type III bursts to improve characterization of particle acceleration in solar flares. The ongoing improvements in time, frequency, and spatial resolutions of ground-based telescopes reveal new levels of solar phenomena complexity and pose new questions.

Spatially inhomogeneous acceleration of electrons in solar flares

The imaging spectroscopy capabilities of the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) enable the examination of the accelerated electron distribution throughout a solar flare region. In particular, it has been revealed that the energisation of these particles takes place over a region of finite size, sometimes resolved by RHESSI observations. In this paper, we present, for the first time, a spatially distributed acceleration model and investigate the role of inhomogeneous acceleration on the observed X-ray emission properties. We have modelled transport explicitly examining scatter-free and diffusive transport within the acceleration region and compare with the analytic leaky-box solution. The results show the importance of including this spatial variation when modelling electron acceleration in solar flares. The presence of an inhomogeneous, extended acceleration region produces a spectral index that is, in most cases, different from the simple leaky-box prediction. In particular, it results in a generally softer spectral index than predicted by the leaky-box solution, for both scatter-free and diffusive transport, and thus should be taken into account when modelling stochastic acceleration in solar flares.

Electron Acceleration in Collapsing Magnetic Traps during the Solar Flare on July 19, 2012: Observations and Models

AbstractUsing the appropriate kinetic equation, we considered the problem of propagation of accelerated electrons into the solar corona and chromosphere. Its analytical solution was used for modelling the M7.7 class limb flare occurred on July 19, 2012. Coronal above-the-loop-top hard X-Ray source was interpreted in the thin-target approximation, the foot-point source - in the thick-target approximation with account of the reverse-current electric field. For the foot-point source we found a good accordance with the RHESSI observations. For the coronal source we also got very accurate estimate of the power-law spectral index, but significant differences between the modelled and observed hard X-ray intensities were noticed. The last discrepancy was solved by adding the coronal magnetic trap model to the thin target model. The former one implies that the trap collapses in two dimensions, locks and accelerates particles inside itself. In our report, we confirm an existence and high efficiency of the electron acceleration in collapsing magnetic traps during solar flares. Our new results represent (e.g. for RHESSI observations) the theoretical prediction of the double step particle acceleration in solar flares, when the first step is the acceleration in reconnection area and the second one – the acceleration in coronal trap.

ELECTRON ACCELERATION IN CONTRACTING MAGNETIC ISLANDS DURING SOLAR FLARES

Abstract Electron acceleration in solar flares is well known to be efficient at generating energetic particles that produce the observed bremsstrahlung X-ray spectra. One mechanism proposed to explain the observations is electron acceleration within contracting magnetic islands formed by magnetic reconnection in the flare current sheet. In a previous study, a numerical magnetohydrodynamic simulation of an eruptive solar flare was analyzed to estimate the associated electron acceleration due to island contraction. That analysis used a simple analytical model for the island structure and assumed conservation of the adiabatic invariants of particle motion. In this paper, we perform the first-ever rigorous integration of the guiding-center orbits of electrons in a modeled flare. An initially isotropic distribution of particles is seeded in a contracting island from the simulated eruption, and the subsequent evolution of these particles is followed using guiding-center theory. We find that the distribution function becomes increasingly anisotropic over time as the electrons’ energy increases by up to a factor of five, in general agreement with the previous study. In addition, we show that the energized particles are concentrated on the Sunward side of the island, adjacent to the reconnection X-point in the flare current sheet. Furthermore, our analysis demonstrates that the electron energy gain is dominated by betatron acceleration in the compressed, strengthened magnetic field of the contracting island. Fermi acceleration by the shortened field lines of the island also contributes to the energy gain, but it is less effective than the betatron process.

Localization of electron acceleration in solar flares based on the spectrum analysis of hard X-ray time delays

Data on hard X-rays of solar flares recorded by the BATSE spectrometer are analyzed. Time delays were determined and their spectra were constructed for X-ray profiles at different energies. The X-ray emission of 82 flares was analyzed, and three types of time delay spectra were identified, i.e., decaying spectra, U-shaped spectra, and spectra that grow with increasing photon energy. In order to interpret delay spectra, the kinetic model of accelerated electrons that propagate in the plasma of the flare loop with a converging magnetic field was considered. Two cases of electron injection were investigated, i.e., isotropic injection and injection in a pitch-angle cone. Of particular note is that delay spectra that decay with increasing energy can be explained only in the case of the spatial diversity of areas of injection and acceleration and a small change in the magnetic field with altitude or if magnetic inhomogeneities is present. Calculations have also shown different types of delay spectra at the top and footpoints of the loop, which are determined by the dynamics of electrons in the loop when selecting different initial conditions, loop geometry, and spatial localization of the areas of acceleration and injection.

NONTHERMALLY DOMINATED ELECTRON ACCELERATION DURING MAGNETIC RECONNECTION IN A LOW- β PLASMA

By means of fully kinetic simulations, we investigate electron acceleration during magnetic reconnection in a nonrelativistic proton--electron plasma with conditions similar to solar corona and flares. We demonstrate that reconnection leads to a nonthermally dominated electron acceleration with a power-law energy distribution in the nonrelativistic low-$\beta$ regime but not in the high-$\beta$ regime, where $\beta$ is the ratio of the plasma thermal pressure and the magnetic pressure. The accelerated electrons contain most of the dissipated magnetic energy in the low-$\beta$ regime. A guiding-center current description is used to reveal the role of electron drift motions during the bulk nonthermal energization. We find that the main acceleration mechanism is a \textit{Fermi}-type acceleration accomplished by the particle curvature drift motion along the electric field induced by the reconnection outflows. Although the acceleration mechanism is similar for different plasma $\beta$, low-$\beta$ reconnection drives fast acceleration on Alfv\'enic timescales and develops power laws out of thermal distribution. The nonthermally dominated acceleration resulting from magnetic reconnection in low-$\beta$ plasma may have strong implications for the highly efficient electron acceleration in solar flares and other astrophysical systems.

Hard X-ray emitting energetic electrons and photospheric electric currents

The energy released during solar flares is believed to be stored in non-potential magnetic fields associated with electric currents flowing in the corona. While no measurements of coronal electric currents are presently available, maps of photospheric electric currents can now be derived from SDO/HMI observations. Photospheric electric currents have been shown to be the tracers of the coronal electric currents. Particle acceleration can result from electric fields associated with coronal electric currents. We revisit here some aspects of the relationship between particle acceleration in solar flares and electric currents in the active region. We study the relation between the energetic electron interaction sites in the solar atmosphere, and the magnitudes and changes of vertical electric current densities measured at the photospheric level, during the X2.2 flare on February 15 2011 in AR NOAA 11158. X-ray images from RHESSI are overlaid on magnetic field and electric current density maps calculated from the spectropolarimetric measurements of SDO/HMI using the UNNOFIT inversion and Metcalf disambiguation codes. X-ray images are also compared with EUV images from SDO/AIA to complement the flare analysis. Part of the elongated X-ray emissions from both thermal and non-thermal electrons overlay the elongated narrow current ribbons observed at the photospheric level. A new X-ray source at 50-100 keV (produced by non-thermal electrons) is observed in the course of the flare and is cospatial with a region in which new vertical photospheric currents appeared during the same period (increase of 15%). These observational results are discussed in the context of the scenarios in which magnetic reconnection (and subsequent plasma heating and particle acceleration) occurs at current-carrying layers in the corona.

Numerical calculation of ion runaway distributions

Ions accelerated by electric fields (so-called runaway ions) in plasmas may explain observations in solar flares and fusion experiments; however, limitations of previous analytic work have prevented definite conclusions. In this work, we describe a numerical solver of the 2D non-relativistic linearized Fokker-Planck equation for ions. It solves the initial value problem in velocity space with a spectral-Eulerian discretization scheme, allowing arbitrary plasma composition and time-varying electric fields and background plasma parameters. The numerical ion distribution function is then used to consider the conditions for runaway ion acceleration in solar flares and tokamak plasmas. Typical time scales and electric fields required for ion acceleration are determined for various plasma compositions, ion species, and temperatures, and the potential for excitation of toroidal Alfvén eigenmodes during tokamak disruptions is considered.

HIGH-ENERGY GAMMA-RAY EMISSION FROM SOLAR FLARES: SUMMARY OFFERMILARGE AREA TELESCOPE DETECTIONS AND ANALYSIS OF TWO M-CLASS FLARES

We present the detections of 19 solar flares detected in high-energy gamma rays (above 100 MeV) with the Fermi Large Area Telescope (LAT) during its first four years of operation. Interestingly, all flares are associated with fairly fast Coronal Mass Ejections (CMEs) and are not all powerful X-ray flares. We then describe the detailed temporal, spatial and spectral characteristics of the first two long-lasting events: the 2011 March 7 flare, a moderate (M3.7) impulsive flare followed by slowly varying gamma-ray emission over 13 hours, and the 2011 June 7 M2.5 flare, which was followed by gamma-ray emission lasting for 2 hours. We compare the Fermi-LAT data with X-ray and proton data measurements from GOES and RHESSI. We argue that a hadronic origin of the gamma rays is more likely than a leptonic origin and find that the energy spectrum of the proton distribution softens after the 2011 March 7 flare, favoring a scenario with continuous acceleration at the flare site. This work suggests that proton acceleration in solar flares is more common than previously thought, occurring for even modest X-ray flares, and for longer durations.

THE EFFECTS OF WAVE ESCAPE ON FAST MAGNETOSONIC WAVE TURBULENCE IN SOLAR FLARES

One of the leading models for electron acceleration in solar flares is stochastic acceleration by weakly turbulent fast magnetosonic waves ({sup f}ast waves{sup )}. In this model, large-scale flows triggered by magnetic reconnection excite large-wavelength fast waves, and fast-wave energy then cascades from large wavelengths to small wavelengths. Electron acceleration by large-wavelength fast waves is weak, and so the model relies on the small-wavelength waves produced by the turbulent cascade. In order for the model to work, the energy cascade time for large-wavelength fast waves must be shorter than the time required for the waves to propagate out of the solar-flare acceleration region. To investigate the effects of wave escape, we solve the wave kinetic equation for fast waves in weak turbulence theory, supplemented with a homogeneous wave-loss term. We find that the amplitude of large-wavelength fast waves must exceed a minimum threshold in order for a significant fraction of the wave energy to cascade to small wavelengths before the waves leave the acceleration region. We evaluate this threshold as a function of the dominant wavelength of the fast waves that are initially excited by reconnection outflows.

Time-dependent particle acceleration in a Fermi reservoir

Context. A steady model was presented by Burn, in which energy conservation is used to constrain the parameters of stochastic Fermi acceleration. A steady model, however, is unlikely to be adequate for particle acceleration in impulsive solar flares. Aims. This paper describes a time-dependent model for particle acceleration in a Fermi reservoir Methods. The calculation is based on the original formulation of stochastic acceleration by Fermi, with additional physically motivated assumptions about the turbulent and particle energy densities within the reservoir, that are similar to those of the steady analysis. The problem is reduced to an integro-differential equation that possesses an analytical solution. Results. The model predicts the formation of a power-law differential energy spectrum N(E) ∼ E−2, that is observable outside the reservoir. The predicted spectral index is independent of the parameters of the model. The results may help in understanding particle acceleration in solar flares and other astrophysical applications.

Solar flares and energetic particles

Solar flares are now observed at all wavelengths from γ-rays to decametre radio waves. They are commonly associated with efficient production of energetic particles at all energies. These particles play a major role in the active Sun because they contain a large amount of the energy released during flares. Energetic electrons and ions interact with the solar atmosphere and produce high-energy X-rays and γ-rays. Energetic particles can also escape to the corona and interplanetary medium, produce radio emissions (electrons) and may eventually reach the Earth's orbit. I shall review here the available information on energetic particles provided by X-ray/γ-ray observations, with particular emphasis on the results obtained recently by the mission Reuven Ramaty High-Energy Solar Spectroscopic Imager. I shall also illustrate how radio observations contribute to our understanding of the electron acceleration sites and to our knowledge on the origin and propagation of energetic particles in the interplanetary medium. I shall finally briefly review some recent progress in the theories of particle acceleration in solar flares and comment on the still challenging issue of connecting particle acceleration processes to the topology of the complex magnetic structures present in the corona.