Flare Energy Release Research Articles

Thermal Trigger for Solar Flares III: Effect of the Oblique Layer Fragmentation

We consider the oblique fragmentation of the current layer as a result of the thermal instability described in Ledentsov (Solar. Phys. 296, 74, 2021a). It is shown that the fragmentation transverse to the current is a natural feature of the model. The fragmentation tilt does not exceed a few degrees for realistic preflare parameters of the coronal plasma. As a consequence, oblique fragmentation generally does not have a strong impact on the simulation results, however, extreme changes can reach an order of magnitude. Thus, oblique fragmentation can lead to a decrease in the estimate of the spatial period of the location of elementary energy release in solar flares to 0.1 – 1 Mm instead of 1 – 10 Mm obtained earlier.

Relating the Properties of Chromospheric Condensation to Flare Energy Transported by Thermal Conduction

Abstract Chromospheric condensation is a brief episode of downflow often accompanying energy release and evaporation in a solar flare. While this component of a flare reflects the energy release process only indirectly, it can be observed at high spatial and temporal resolution, even from the ground. It appears in spectroscopic observations of cooler lines, formed below ∼105 K, as a redshift that peaks and decays after less than 1 minute. In order to use this signature to infer characteristics of solar flare energy release, it is important to establish quantitative relationships with properties of the condensation. The initial investigation reported here does so after restricting consideration to energy transport via thermal conduction into a simplified, stratified chromosphere. We develop an analytical expression for the decay of a condensation propagating into a stratified atmosphere. This model provides a relationship between shock velocity and preshock density structure. We also use one-dimensional gasdynamic simulations to explore the dynamics of these shocks as they penetrate into the stratified chromosphere. We find that the peak downflow speed primarily reflects the energy flux into the chromosphere, while the product of this velocity and the redshift duration is proportional to the preshock density scale height as H ≃ 0.6u 0 τ.

Thermal Trigger for Solar Flares I: Fragmentation of the Preflare Current Layer

We consider the effects of the heat balance on the structural stability of a preflare current layer. The problem of small perturbations is solved in the piecewise homogeneous MHD approximation taking into account the viscosity, the electrical and thermal conductivity, and the radiative cooling. Solution of the problem allows the formation of an instability of a thermal nature. There is no external magnetic field inside the current layer in equilibrium state, but it can penetrate inside when the current layer is disturbed. Formation of a magnetic field perturbation inside the layer creates a dedicated frequency in a broadband disturbance subject to thermal instability. In the linear phase, the growth time of the instability is proportional to the characteristic time of radiative cooling of plasma and depends on the logarithmic derivatives of the radiative cooling function with respect to the plasma parameters. The instability results in transverse fragmentation of the current layer with a spatial period of 1-10 Mm along the layer in a wide range of coronal plasma parameters. The role of that instability in the triggering for the primary energy release in solar flares is discussed.

Energetic Electron Distribution of the Coronal Acceleration Region: First Results from Joint Microwave and Hard X-Ray Imaging Spectroscopy

Abstract Nonthermal sources located above bright flare arcades, referred to as the “above-the-loop-top” sources, have been often suggested as the primary electron acceleration site in major solar flares. The X8.2 limb flare on 2017 September 10 features such an above-the-loop-top source, which was observed in both microwaves and hard X-rays (HXRs) by the Expanded Owens Valley Solar Array and the Reuven Ramaty High Energy Solar Spectroscopic Imager, respectively. By combining the microwave and HXR imaging spectroscopy observations with multifilter extreme ultraviolet and soft X-ray imaging data, we derive the coronal magnetic field and energetic electron distribution of the source over a broad energy range from <10 keV up to ∼MeV during the early impulsive phase of the flare. The source has a strong magnetic field of over 800 G. The best-fit electron distribution consists of a thermal “core” from ∼25 MK plasma. A nonthermal power-law “tail” joins the thermal core at ∼16 keV with a spectral index of ∼3.6, which breaks down at above ∼160 keV to >6.0. Temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from >20 to ∼6.0 within 20 s, or less than ∼10 Alfvén crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an ongoing bulk acceleration of energetic electrons is taking place very early in the flare energy release.

NuSTAR Observation of Energy Release in 11 Solar Microflares.

Solar flares are explosive releases of magnetic energy. Hard X-ray (HXR) flare emission originates from both hot (millions of Kelvin) plasma and nonthermal accelerated particles, giving insight into flare energy release. The Nuclear Spectroscopic Telescope ARray (NuSTAR) utilizes direct-focusing optics to attain much higher sensitivity in the HXR range than that of previous indirect imagers. This paper presents 11 NuSTAR microflares from two active regions (AR 12671 on 2017 August 21 and AR 12712 on 2018 May 29). The temporal, spatial, and energetic properties of each are discussed in context with previously published HXR brightenings. They are seen to display several "large flare" properties, such as impulsive time profiles and earlier peak times in higher-energy HXRs. For two events where the active region background could be removed, microflare emission did not display spatial complexity; differing NuSTAR energy ranges had equivalent emission centroids. Finally, spectral fitting showed a high-energy excess over a single thermal model in all events. This excess was consistent with additional higher-temperature plasma volumes in 10/11 microflares and only with an accelerated particle distribution in the last. Previous NuSTAR studies focused on one or a few microflares at a time, making this the first to collectively examine a sizable number of events. Additionally, this paper introduces an observed variation in the NuSTAR gain unique to the extremely low livetime (<1%) regime and establishes a correction method to be used in future NuSTAR solar spectral analysis.

Flare energy release and avalanche ionization of plasma by runaway electrons in lower solar atmosphere

Abstract The analysis of the electron acceleration by the quasi-stationary sub-Dreiser electric fields in the lower solar atmosphere has been done. It has been shown that the Dreiser electric field turned out to be several orders of magnitude larger than coronal values due to the inelastic collisions between electrons and hydrogen atoms. The ionization of hydrogen atoms gives rise to the resulting secondary electrons, which become runaway under the action of sub-Dreiser electric fields. This causes an further avalanche-like ionization of the plasma and leads to the acceleration of the large number of fast electrons up to relativistic energies at small (≲ 100 km) distances.

Hot X-ray onsets of solar flares

ABSTRACT The study of the localized plasma conditions before the impulsive phase of a solar flare can help us understand the physical processes that occur leading up to the main flare energy release. Here, we present evidence of a hot X-ray ‘onset’ interval of enhanced isothermal plasma temperatures in the range of 10–15 MK over a period of time prior to the flare’s impulsive phase. This ‘hot onset’ interval occurs during the initial soft X-ray increase and definitely before any detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager, show no signs of gradual increase, and the ‘hot onset’ phenomenon occurs regardless of flare classification or configuration. In a small sample of four representative flare events, we tentatively identify this early hot onset soft X-ray emission to occur within footpoint and low-lying loop regions, rather than in coronal structures, based on images from the Atmospheric Imaging Assembly. We confirm this via limb occultation of a flaring region. These hot X-ray onsets appear before there is evidence of collisional heating by non-thermal electrons, and hence challenge the standard modelling techniques.

The origin of quasi-periodicities during circular ribbon flares

Context. Solar flares with a fan-spine magnetic topology are able to form circular ribbons. A previous study based on Hα line observations of the solar flares on 5 March 2014 revealed a uniform and continuous rotation of the magnetic fan-spine. A preliminary analysis of the flare time profiles revealed quasi-periodic pulsations (QPPs) with similar properties in hard X-rays, Hα, and microwaves. Aims. In this work, we address the question of whether the observed periodicities are related to periodic acceleration of electrons or plasma heating. Methods. We analysed QPPs in the Hα emission from the centre of the fan (inner ribbon R1), a circular ribbon (R2), a remote source (R3), and an elongated ribbon (R4) located between R2 and R3. We used methods of correlation, Fourier, wavelet, and empirical mode decomposition. We compared the QPPs in Hα emission with those in microwave and X-ray emission. Results. We found multi-wavelength QPPs with periods around 150 s, 125 s, and 190 s. The 150 s period is seen to co-exist in Hα, hard X-rays, and microwave emissions, which allowed us to connect it with flare kernels R1 and R2. These kernels spatially coincide with the site of the primary flare energy release. The 125 s period is found in the Hα emission of the elongated ribbon R4 and the microwave emission at 5.7 GHz during the decay phase. The 190 s period is present in the emission during all flare phases in the Hα emission of both the remote source, R3, and the elongated ribbon, R4, in soft X-rays and in microwaves at 4–8 GHz. Conclusions. We connected the dominant 150 s QPPs with the slipping reconnection mechanism occurring in the fan. We suggested that the period of 125 s in the elongated ribbon can be caused by a kink oscillation of the outer spine, connecting the primary reconnection site with the remote footpoint. The period of 190 s is associated with the three-minute sunspot oscillations.

Measurement of magnetic field and relativistic electrons along a solar flare current sheet

In the standard model of solar flares, a large-scale reconnection current sheet is postulated as the central engine for powering the flare energy release and accelerating particles. However, where and how the energy release and particle acceleration occur remain unclear due to the lack of measurements for the magnetic properties of the current sheet. Here we report the measurement of spatially-resolved magnetic field and flare-accelerated relativistic electrons along a current-sheet feature in a solar flare. The measured magnetic field profile shows a local maximum where the reconnecting field lines of opposite polarities closely approach each other, known as the reconnection X point. The measurements also reveal a local minimum near the bottom of the current sheet above the flare loop-top, referred to as a "magnetic bottle". This spatial structure agrees with theoretical predictions and numerical modeling results. A strong reconnection electric field of ~4000 V/m is inferred near the X point. This location, however, shows a local depletion of microwave-emitting relativistic electrons. These electrons concentrate instead at or near the magnetic bottle structure, where more than 99% of them reside at each instant. Our observations suggest that the loop-top magnetic bottle is likely the primary site for accelerating and/or confining the relativistic electrons.

Solar Flare–CME Coupling throughout Two Acceleration Phases of a Fast CME

Abstract Solar flares and coronal mass ejections (CMEs) are closely coupled through magnetic reconnection. CMEs are usually accelerated impulsively within the low solar corona, synchronized with the impulsive flare energy release. We investigate the dynamic evolution of a fast CME and its associated X2.8 flare occurring on 2013 May 13. The CME experiences two distinct phases of enhanced acceleration, an impulsive one with a peak value of ∼5 km s−2, followed by an extended phase with accelerations up to 0.7 km s−2. The two-phase CME dynamics is associated with a two-episode flare energy release. While the first episode is consistent with the “standard” eruption of a magnetic flux rope, the second episode of flare energy release is initiated by the reconnection of a large-scale loop in the aftermath of the eruption and produces stronger nonthermal emission up to γ-rays. In addition, this long-duration flare reveals clear signs of ongoing magnetic reconnection during the decay phase, evidenced by extended hard X-ray bursts with energies up to 100–300 keV and intermittent downflows of reconnected loops for &gt;4 hr. The observations reveal that the two-step flare reconnection substantially contributes to the two-phase CME acceleration, and the impulsive CME acceleration precedes the most intense flare energy release. The implications of this non-standard flare/CME observation are discussed.

Thermodynamical Evolution of Supra-arcade Downflows

Abstract Supra-arcade downflows (SADs) are dark, teardrop-shaped features descending upon flare arcades. They are thought to be the results of magnetic reconnection, but the detailed formation mechanism of SADs and their relationship to flare energy release are still unclear. In this work, we explore the thermodynamical properties of SADs in the 2011 October 22 limb flare using the observations of the Solar Dynamics Observatory/Atmospheric Imaging Assembly and an improved Differential Emission Measure (DEM) technique. Different heating events around SADs are identified and the propagation speeds of plasma heating are calculated. The first heating event starts with the increase of the emission measure at temperatures higher than 5 MK, about 2.8 minutes before the arrival of the first studied SAD. Its propagation speed is about 140 km s−1, a little faster than the speed of the SAD. However, the other two events have fast propagation speeds more than 700 km s−1. We suspect that the first heating event can be explained by adiabatic compression, but the others may have different causes. Besides, we observed that SADs can push away their surrounding spikes. The formation of SADs is further explained on the basis of patchy and bursty magnetic reconnection that reconnection outflows may push away surrounding plasma and leave dark lanes behind them. The reliability of the DEM results, heating and cooling mechanisms, and other SAD explanations are discussed.

Sunquakes of Solar Cycle 24

Abstract The paper presents results of a search for helioseismic events (sunquakes) produced by M-X class solar flares during Solar Cycle 24. The search is performed by analyzing photospheric Dopplergrams from the Helioseismic Magnetic Imager. Among the total number of 500 M-X class flares, 94 helioseismic events were detected. Our analysis has shown that many strong sunquakes were produced by solar flares of low M class (M1–M5), while in some powerful X-class flares helioseismic waves were not observed or were weak. Our study has also revealed that only several active regions were characterized by the most efficient generation of helioseismic waves during flares. We found that the sunquake power correlates with the maximum value of the soft X-ray flux time derivative better than with the X-ray class, indicating that the sunquake mechanism is associated with high-energy particles. We also show that the seismically active flares are more impulsive than the flares without helioseismic perturbations. We present a new catalog of helioseismic solar flares, which opens opportunities for performing statistical studies to better understand the physics of sunquakes as well as the flare-energy release and transport.

Flare Energy Release at the Magnetic Field Polarity Inversion Line during the M1.2 Solar Flare of 2015 March 15. II. Investigation of Photospheric Electric Current and Magnetic Field Variations Using HMI 135 s Vector Magnetograms

Abstract This work is a continuation of Paper I and is devoted to the analysis of nonthermal electron dynamics and plasma heating in the confined M1.2 class solar flare SOL2015-03-15T22:43, revealing energy release in the highly sheared interacting magnetic loops in the low corona, above the polarity inversion line (PIL). The aim of the present work is to perform the first extensive quantitative analysis of the photospheric magnetic field and photospheric vertical electric current (PVEC) dynamics in the confined flare region near the PIL using new vector magnetograms obtained with the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory with a high temporal resolution of 135 s. Data analysis reveals sharp changes of the magnetic structure and PVEC associated with flare onset near the PIL. It is found that the strongest plasma heating and electron acceleration are associated with the largest increase in the magnetic reconnection rate, total PVEC, and effective PVEC density in the flare ribbons. Observations and nonlinear force-free field extrapolations show that the magnetic field structure around the PIL is consistent with a tether-cutting magnetic reconnection (TCMR) geometry. We give a qualitative interpretation of the observed dynamics of the flare ribbons, magnetic field, PVEC, and electron acceleration within the TCMR scenario.

Decay of the coronal magnetic field can release sufficient energy to power a solar flare.

Solar flares are powered by a rapid release of energy in the solar corona, thought to be produced by the decay of the coronal magnetic field strength. Direct quantitative measurements of the evolving magnetic field strength are required to test this. We report microwave observations of a solar flare, showing spatial and temporal changes in the coronal magnetic field. The field decays at a rate of ~5 Gauss per second for 2 minutes, as measured within a flare subvolume of ~1028 cubic centimeters. This fast rate of decay implies a sufficiently strong electric field to account for the particle acceleration that produces the microwave emission. The decrease in stored magnetic energy is enough to power the solar flare, including the associated eruption, particle acceleration, and plasma heating.

Flare Energy Release and Electron Acceleration to Relativistic Energies by Quasi-Stationary Electric Fields in the Lower Atmosphere of the Sun

The characteristics of electron acceleration to relativistic energies by sub-Dreicer quasi-stationary electric fields under the conditions of the lower atmosphere of the Sun are considered. The proposed model makes it possible to explain the spatial location of hard X-ray, optical, and gyrosynchrotron sources in the chromosphere/photosphere observed during limb solar flares. Possible mechanisms of electric field generation in the lower atmosphere are discussed.

Solar Fast-drifting Radio Bursts in an X1.3 Flare on 2014 April 25

Abstract One of the most important products of solar flares is nonthermal energetic particles, which may carry up to 50% of the energy released in the flaring processes. In radio observations, nonthermal particles generally manifest as spectral fine structures with fast frequency-drifting rates, named as solar fast-drifting radio bursts (FDRBs). This work demonstrated three types of FDRBs, including type III pair bursts, narrowband stochastic spike bursts following the type III bursts, and spike-like bursts superimposed on a type II burst in an X1.3 flare on 2014 April 25. We find that although all of them have fast frequency-drifting rates, they are intrinsically different from each other in frequency bandwidth, drifting rate, and statistical distribution. We suggest that they are possibly generated from different accelerating mechanisms. The type III pair bursts may be triggered by high-energy electron beams accelerated by the flaring magnetic reconnection, spike bursts are produced by the energetic electrons accelerated by a termination shock wave triggered by the fast reconnecting plasma outflows impacting the flaring loop top, and spike-like bursts are possibly generated by nonthermal electrons accelerated by moving magnetic reconnection triggered by interaction between coronal mass ejection and the background magnetized plasma. These results may help us to understand the generation mechanism of nonthermal particles and energy release in solar flares.

MULTI-WAVELENGTH OBSERVATIONS OF A LARGE SOLAR FLARE

We present the results of the multi- wavelength study of the two-ribbon solar flare on July 19, 2000 in the active region NOAA 9087. The evo- lution and morphological properties of the flare pro- ductive active region have been analyzed. The active region was growing rapidly and showed a complex mul- tipolar magnetic field configuration. It was large, pro- ducing many events, including the flare under consid- eration. The 3N/M6.4 two-ribbon flare was a promi- nent, long duration event in the active region evolution. According to Solar Geophysical Data (SGD) the flare lasted 2.5 hours. The flare energy release took place in many sites of the active region. We used combination of data from space and ground based observatories for study. The hard X-ray (HXR) and soft X-ray (SXR) data were obtained at the Yohkoh Telescopes (HXT and SXT) and Geostationary Operational Environmental Satellite (GOES). The full- disk magnetograms and EUV-images were provided by the Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) and Extreme ultra- violet Imaging Telescope (EIT). We used the H α filter- grams from the Meudon spectroheliograph and white light images of Big Bear Solar Observatory (BBSO). All the data show continuously evolving SXR, EUV and H α features during the flare. The HXR and the type III radio bursts were observed at the flare onset. The first H α flare kernels and the surge, connected with the filament eruption, were initiated near a large positive-polarity sunspot. The main bright kernels of the flare occurred at the centre of the active region near magnetic neutral line, after that the flare ribbons appeared along it. It was found that HXR coronal source was located along a magnetic polarity inversion line of the active region. EUV loop structures indicate the observational evidence of a magnetic reconnection during the main phase of the flare.

Особенности развития длительных потоков высокоэнергичного гамма-излучения на разных стадиях солнечных вспышек

We have studied properties of sustained gamma fluxes having quantum energies of &gt;100 MeV at different stages of flares with 1-min temporal resolution (Fermi/LAT). &#x0D; The most probable process of emergence of high-energy gamma-quanta during the impulsive phase of flares (6 events) has been confirmed. Acceleration of particles, produced by flare energy release (at dissipation of current sheet), occurs when they interact with a shock front of a coronal mass ejection (CME), which develops in the same active region at the same time. Nuclear interactions of accelerated protons (&gt;500 MeV) with plasma ions lead further to the emergence of high-energy gamma-quanta. We have established that the interaction between a flare flux and a high-speed CME during the flare impulsive phase occurs within fairly limited periods — from 2 to 16 min. In the events considered, we have found a direct connection between maximum gamma flux F max (γ &gt; 100 MeV) and CME velocity.&#x0D; High maximum values of gamma fluxes are typical of the flare impulsive phase: 3.5·10⁻⁴ cm⁻²s⁻¹ ≤ F max (γ &gt; 100 MeV) ≤ 1.3·10⁻² cm⁻² s⁻¹. At the same time, the value F max (γ &gt; 100 MeV) = 0.013 cm⁻²s⁻¹ was the highest for the events observed by Fermi/LAT from 2008 to 2017.&#x0D; During the development of CMEs moving with a supersonic speed, shock waves are formed which are the major power source of accelerated particles during the main phase of gradual flares. In some cases, however, the impact of shock waves on particle acceleration is the greatest in the short impulsive phase.&#x0D; To reveal parameters most effectively influencing the generation of high-energy gamma-ray emission, we have compared 17 flare events. The most significant parameter proved to be the time interval of joint action of flare process and CME shocks.&#x0D; We have established that during simultaneous development of flare process and CME attendant on the flare, the most efficient particle acceleration occurs which gives rise to maximum fluxes of high-energy gamma-quanta.

Features of development of sustained fluxes of high-energy gamma-ray emission at different stages of solar flares

We have studied properties of sustained gamma fluxes having quantum energies of &gt;100 MeV at different stages of flares with 1-min temporal resolution (Fermi/LAT). &#x0D; The most probable process of emergence of high-energy gamma-quanta during the impulsive phase of flares (6 events) has been confirmed. Acceleration of particles, produced by flare energy release (at dissipation of current sheet), occurs when they interact with a shock front of a coronal mass ejection (CME), which develops in the same active region at the same time. Nuclear interactions of accelerated protons (&gt;500 MeV) with plasma ions lead further to the emergence of high-energy gamma-quanta. We have established that the interaction between a flare flux and a high-speed CME during the flare impulsive phase occurs within fairly limited periods — from 2 to 16 min. In the events considered, we have found a direct connection between maximum gamma flux F max (γ &gt; 100 MeV) and CME velocity.&#x0D; High maximum values of gamma fluxes are typical of the flare impulsive phase: 3.5·10⁻⁴ cm⁻²s⁻¹ ≤ F max (γ &gt; 100 MeV) ≤ 1.3·10⁻² cm⁻² s⁻¹. At the same time, the value F max (γ &gt; 100 MeV) = 0.013 cm⁻²s⁻¹ was the highest for the events observed by Fermi/LAT from 2008 to 2017.&#x0D; During the development of CMEs moving with a supersonic speed, shock waves are formed which are the major power source of accelerated particles during the main phase of gradual flares. In some cases, however, the impact of shock waves on particle acceleration is the greatest in the short impulsive phase.&#x0D; To reveal parameters most effectively influencing the generation of high-energy gamma-ray emission, we have compared 17 flare events. The most significant parameter proved to be the time interval of joint action of flare process and CME shocks.&#x0D; We have established that during simultaneous development of flare process and CME attendant on the flare, the most efficient particle acceleration occurs which gives rise to maximum fluxes of high-energy gamma-quanta.

Modernized Solar Radio Spectrograph in the L Band Based on Software Defined Radio

The paper presents the concept, implementation, and test operation of a modernized solar radio spectrograph for an investigation of the solar emission and solar bursts in radio frequency bands. Besides having a strong diagnostic significance for studying the flare energy release, the solar radio bursts can also cause strong interference for radio communication and navigation systems. The current spectrograph for the Ondrejov observatory (Astronomical Institute of Czech Academy of Sciences) was modernized by using a direct-conversion receiver connected to a field-programmable gate array (FPGA) for the fast Fourier transform (FFT) spectrum estimation and put into the test operation. The higher time and frequency resolution and lower noise in comparison with the existing analog instrument were reached by the implementation of the latest optimal signal processing methods. To reduce the costs for such modernization, the operating frequency range was divided into four sub-bands of bandwidth 250 MHz, which brings another benefit of greater scalability. The first observations obtained by the new spectrograph and their comparison with the analog device are presented in the paper with future steps to put the spectrograph into the regular operation.