Flare Loops Research Articles

Efficiency of solar microflares in accelerating electrons when rooted in a sunspot

Context. The spectral shape of the X-ray emission in solar flares varies with the event size, with small flares generally exhibiting softer spectra than large events, indicative of a relatively lower number of accelerated electrons at higher energies. Aims. We investigate two microflares of GOES classes A9 and C1 (after background subtraction) observed by STIX onboard Solar Orbiter with exceptionally strong nonthermal emission. We complement the hard X-ray imaging and spectral analysis by STIX with co-temporal observations in the (E)UV and visual range by AIA and HMI to investigate what makes these microflares so efficient in high-energy particle acceleration. Methods. We made a preselection of events in the STIX flare catalog based on the ratio of the thermal to nonthermal quicklook X-ray emission. The STIX spectrogram science data were used to perform spectral fitting to identify the non-thermal and thermal components. The STIX X-ray images were reconstructed to analyze the spatial distribution of the precipitating electrons and the hard X-ray emission they produce. The EUV images from SDO/AIA and SDO/HMI LOS magnetograms were analyzed to better understand the magnetic environment and the chromospheric and coronal response. For the A9 event, EOVSA microwave observations were available, allowing for image reconstruction in the radio domain. Results. We performed case studies of two microflares observed by STIX on October 11, 2021 and November 10, 2022, which showed unusually hard microflare X-ray spectra with power-law indices of the electron flux distributions of δ = (2.98 ± 0.25) and δ = (4.08 ± 0.23), during their non-thermal peaks and photon energies up to 76 keV and 50 keV, respectively. For both events under study, we found that one footpoint is located within a sunspot covering areas with mean magnetic flux densities in excess of 1500 G, suggesting that the hard electron spectra are caused by the strong magnetic fields the flare loops are rooted in. Additionally, we revisited a previously published unusually hard RHESSI microflare and found that in this event, there was also one flare kernel located within a sunspot, which corroborates the result from the two hard STIX microflares under study in this work. Conclusions. The characteristics of the strong photospheric magnetic fields inside the sunspot umbrae and penumbrae where flare loops are rooted play an important role in the generation of exceptionally hard X-ray spectra in these microflares.

Radiation-driven diffusive transport of fast electrons in solar flares

Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modeled. Comparison with Coulomb scattering suggests that stimulated Bremsstrahlung scattering can be dominant in low-density, radiation-driven plasmas, provided that the radiation spectrum has a sufficiently high brightness temperature in the neighborhood of the plasma frequency. While stimulated Bremsstrahlung scattering cannot be easily observed in laboratory plasmas due to their small size, it should operate in large-scale astrophysical plasmas, such as those met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated through a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard x-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops.

Apache Point Observatory (APO)/SMARTS Flare Star Campaign Observations. I. Blue Wing Asymmetries in Chromospheric Lines during Mid-M-Dwarf Flares from Simultaneous Spectroscopic and Photometric Observation Data

We conducted the time-resolved simultaneous optical spectroscopic and photometric observations of mid-M-dwarf flare stars YZ CMi, EV Lac, and AD Leo. Spectroscopic observations were obtained using Apache Point Observatory 3.5 m and Small and Moderate Aperture Research Telescope System 1.5 m telescopes during 31 nights. Among the 41 detected flares, seven flares showed clear blue wing asymmetries in the Hα line, with various correspondences in flare properties. The duration of the blue wing asymmetries range from 20 minutes to 2.5 hr, including a flare showing the shift from blue to red wing asymmetry. Blue wing asymmetries can be observed during both white-light and candidate non-white-light flares. All of the seven flares showed blue wing asymmetries also in the Hβ line, but there are large varieties on which other chromospheric lines showed blue wing asymmetries. One among the 7 flares was also observed with soft X-ray spectroscopy, which enabled us to estimate the flare magnetic field and length of the flare loop. The line-of-sight velocities of the blueshifted components range from –73 to –122 km s−1. Assuming that the blueshifts were caused by prominence eruptions, the mass of upward-moving plasma was estimated to be 1015–1019 g, which are roughly on the relation between flare energy and erupting mass expected from solar coronal mass ejections (CMEs). Although further investigations are necessary for understanding the observed various properties, these possible prominence eruptions on M-dwarfs could evolve into CMEs, assuming the similar acceleration mechanism with solar eruptions.

Observation of rapid flux coalescence in merging field-reversed configurations

Rapid magnetic reconnection was experimentally observed to occur within a microsecond timescale, leading to the field-reversed configuration (FRC) comprising a single closed magnetic field structure. The compressibility of the plasma within the FRC played a crucial role in accelerating the coalescence process, making it considerably faster than the resistive process. A strong similarity between the observed experimental results and those of the coalescence of solar flare loops indicates that astrophysical phenomena and laboratory plasmas are governed by a common physics.

Multiwavelength Campaign Observations of a Young Solar-type Star, EK Draconis. I. Discovery of Prominence Eruptions Associated with Superflares

Young solar-type stars frequently produce superflares, serving as a unique window into the young Sun-Earth environments. Large solar flares are closely linked to coronal mass ejections (CMEs) associated with filament/prominence eruptions, but observational evidence for stellar superflares remains scarce. Here, we present a 12-day, multiwavelength campaign observation of young solar-type star EK Draconis (G1.5V, 50–120 Myr age) utilizing the Transiting Exoplanet Survey Satellite, the Neutron star Interior Composition ExploreR, and the Seimei telescope. The star has previously exhibited blueshifted Hα absorptions as evidence for a filament eruption associated with a superflare. Our simultaneous optical and X-ray observations identified three superflares of 1.5 × 1033–1.2 × 1034 erg. We report the first discovery of two prominence eruptions on a solar-type star, observed as blueshifted Hα emissions at speeds of 690 and 430 km s−1 and masses of 1.1 × 1019 and 3.2 × 1017 g, respectively. The faster, massive event shows a candidate of post-flare X-ray dimming with the amplitude of up to ∼10%. Several observational aspects consistently point to the occurrence of a fast CME associated with this event. The comparative analysis of the estimated length scales of flare loops, prominences, possible dimming region, and starspots provides the overall picture of the eruptive phenomena. Furthermore, the energy partition of the observed superflares in the optical and X-ray bands is consistent with flares from the Sun, M-dwarfs, and close binaries, yielding the unified empirical relations. These discoveries provide profound implications of the impact of these eruptive events on early Venus, Earth, and Mars and young exoplanets.

Bright ultraviolet knots as possible sources of coherent microwave radiation

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 1011 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

Яркие ультрафиолетовые узлы как возможные источники когерентного микроволнового излучения

A distinctive feature of the September 6, 2012 event was that sources of narrow-band (2–4 GHz) sub-second pulses (SSP) were observed in small areas of flare loops with so-called bright ultraviolet knots with high plasma density up to 10¹¹ cm⁻³. Time profiles of hard X-rays of the flare, although similar to microwave light curves, do not have structures corresponding to SSP. Analysis of microwave, X-ray, and ultraviolet data has shown that the observable pulses of microwave radiation with a narrow spectral band are coherent in nature and are generated by electrons with energies of several tens of kiloelectronvolt in bright knots at a double plasma frequency. The results of the observations suggest that the appearance of bright knots is associated with local processes of energy release due to interaction of flare loops.

Acceleration of Nonthermal Electrons in Coordinated Interaction with Whistler Turbulence in a Flare Loop

Acceleration of Nonthermal Electrons in Coordinated Interaction with Whistler Turbulence in a Flare Loop

A Possible Mechanism for the “Late Phase” in Stellar White-light Flares

M dwarf flares observed by the Transiting Exoplanet Survey Satellite (TESS) sometimes exhibit a peak-bump light-curve morphology, characterized by a secondary, gradual peak well after the main, impulsive peak. A similar late phase is frequently detected in solar flares observed in the extreme ultraviolet from longer hot coronal loops distinct from the impulsive flare structures. White-light emission has also been observed in off-limb solar flare loops. Here, we perform a suite of one-dimensional hydrodynamic loop simulations for M dwarf flares inspired by these solar examples. Our results suggest that coronal plasma condensation following impulsive flare heating can yield high electron number density in the loop, allowing it to contribute significantly to the optical light curves via free-bound and free–free emission mechanisms. Our simulation results qualitatively agree with TESS observations: the longer evolutionary timescale of coronal loops produces a distinct, secondary emission peak; its intensity increases with the injected flare energy. We argue that coronal plasma condensation is a possible mechanism for the TESS late-phase flares.

Expansion and Compression of a Flare Loop System during the Flare on January 15, 2022 According to Ultraviolet and Microwave Data

Expansion and Compression of a Flare Loop System during the Flare on January 15, 2022 According to Ultraviolet and Microwave Data

Intensity and velocity oscillations in a flaring active region

ABSTRACT Chromospheric oscillations can give us insight into the physical environment in the solar atmosphere, both in quiet Sun and flaring conditions. Many authors have reported increases in the prevalence of 3-minute oscillations which are thought to be excited by events which impact the chromosphere such as flares. In this study, we utilized the Ca ii 8542 Å line to study the oscillatory behaviour of the chromosphere in an active region which underwent two B-class flares. We analysed oscillations in both intensity and velocity, and found different behaviours in both. Intensity oscillations were most prevalent over the umbrae of sunspots and magnetic pores in the active region, and the extent of the area which contained significant oscillations was found to decrease when comparing times after the flares to before. By measuring the evolution of the magnetic field, we found that this could be because the areas surrounding the umbrae were becoming more ‘penumbral’ with an increase to the magnetic field inclination. Velocity oscillations were found across the active region both before and after the flares but were observed clearly in areas which were brightened by the second flare. By comparing to EUV imaging, it was seen that strong chromospheric velocity oscillations with 3–4-minute periods occurred at the same time and location as a flare loop cooling 30 min after the second flare peak. This could be evidence of disturbances in the loop exciting a response from the chromosphere at its acoustic cut-off frequency.

Are Quasi-periodic Pulsations Independent of Loop Oscillations in Solar Flare?

We investigated oscillations in an M8.7 solar flare (SOL2014-10-22), including quasi-periodic pulsations (QPPs) in light curves and Doppler shift oscillations in the flare loops. Using Bayesian-based Markov Chain Monte Carlo, Fast Fourier Transform, and wavelet analysis method, QPPs were identified at microwave and hard X-ray wave bands during the impulsive phase, and the dominant period is 40–50 s. They should be associated with a repetitive energy release process, which accelerates nonthermal electrons periodically. On the other hand, we cannot rule out the possibility of the modulation of external waves because of the lower temporal resolution of spectroscopic observation. However, almost immediately after QPPs, a minority of flare loops display their Doppler velocity oscillations with a significant period of ∼4 minutes, which are observed by the Interface Region Imaging Spectrograph at the coronal line Fe xxi 1354.08 Å (T ∼ 107 K), while its intensity and width show no similar oscillation. Our observations suggest that flare loop oscillations are most likely the fast kink mode waves with a phase speed of ∼840 km s−1. The magnetic field strength in flare loops was estimated to be 54–69 G via the coronal seismology. The QPPs and loop oscillation could be independent of each other in this event.

Three-dimensional Simulation of Thermodynamics on Confined Turbulence in a Large-scale CME-flare Current Sheet

Turbulence plays a key role in forming the complex geometry of the large-scale current sheet (CS) and fast energy release in a solar eruption. In this paper, we present full 3D high-resolution simulations for the process of a moderate coronal mass ejection (CME) and the thermodynamical evolution of the highly confined CS. Copious elongated blobs are generated owing to tearing and plasmoid instabilities, giving rise to a higher reconnection rate, and undergo the splitting, merging, and kinking processes in a more complex way in 3D. A detailed thermodynamical analysis shows that the CS is mainly heated by adiabatic and numerical viscous terms, and thermal conduction is the dominant factor that balances the energy inside the CS. Accordingly, the temperature of the CS reaches to a maximum of about 20 MK, and the range of temperatures is relatively narrow. From the face-on view in the synthetic Atmospheric Imaging Assembly 131 Å, the downflowing structures with similar morphology to supra-arcade downflows are mainly located between the post-flare loops and loop top, while moving blobs can extend spikes higher above the loop top. The downward-moving plasmoids can keep the twisted magnetic field configuration until the annihilation at the flare loop top, indicating that plasmoid reconnection dominates in the lower CS. Meanwhile, the upward-moving ones turn into turbulent structures before arriving at the bottom of the CME, implying that turbulent reconnection dominates in the upper CS. The spatial distributions of the turbulent energy and anisotropy are addressed, which show a significant variation in the spectra with height.

A multiple spacecraft detection of the 2 April 2022 M-class flare and filament eruption during the first close Solar Orbiter perihelion

Context. The Solar Orbiter mission completed its first remote-sensing observation windows in the spring of 2022. On 2 April 2022, an M-class flare followed by a filament eruption was seen both by the instruments on board the mission and from several observatories in Earth’s orbit, providing an unprecedented view of a flaring region with a large range of observations. Aims. We aim to understand the nature of the flaring and filament eruption events via the analysis of the available dataset. The complexity of the observed features is compared with the predictions given by the standard flare model in 3D. Methods. In this paper, we use the observations from a multi-view dataset, which includes extreme ultraviolet (EUV) imaging to spectroscopy and magnetic field measurements. These data come from the Interface Region Imaging Spectrograph, the Solar Dynamics Observatory, Hinode, as well as several instruments on Solar Orbiter. Results. The large temporal coverage of the region allows us to analyse the whole sequence of the filament eruption starting with its pre-eruptive state. Information given by spectropolarimetry from SDO/HMI and Solar Orbiter PHI/HRT shows that a parasitic polarity emerging underneath the filament is responsible for bringing the flux rope to an unstable state. As the flux rope erupts, Hinode EIS captures blue-shifted emission in the transition region and coronal lines in the northern leg of the flux rope prior to the flare peak. This may be revealing the unwinding of one of the flux rope legs. At the same time, Solar Orbiter SPICE captures the whole region, complementing the Doppler diagnostics of the filament eruption. Analyses of the formation and evolution of a complex set of flare ribbons and loops, of the hard and soft X-ray emissions with STIX, show that the parasitic emerging bipole plays an important role in the evolution of the flaring region. Conclusions. The extensive dataset covering this M-class flare event demonstrates how important multiple viewpoints and varied observations are in order to understand the complexity of flaring regions. While the analysed data are overall consistent with the standard flare model, the present particular magnetic configuration shows that surrounding magnetic activity such as nearby emergence needs to be taken into account to fully understand the processes at work. This filament eruption is the first to be covered from different angles by spectroscopic instruments, and provides an unprecedented diagnostic of the multi-thermal structures present before and during the flare. This complete dataset of an eruptive event showcases the capabilities of coordinated observations with the Solar Orbiter mission.

The Role of Magnetic Shear in Reconnection-driven Flare Energy Release

Using observations from the Solar Dynamics Observatory’s Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present novel measurements of the shear of post-reconnection flare loops (PRFLs) in SOL20141218T21:40 and study its evolution with respect to magnetic reconnection and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. We measure the magnetic reconnection rate from the ribbon evolution, and also the shear angle of a large number of PRFLs observed in extreme ultraviolet passbands (≲1 MK). For the first time, the shear angle measurements are conducted using several complementary techniques allowing for cross validation of the results. In this flare, the total reconnection rate is much enhanced before a sharp increase in the hard X-ray emission, and the median shear decreases from 60°–70° to 20°, on a timescale of 10 minutes. We find a correlation between the shear-modulated total reconnection rate and the nonthermal electron flux. These results confirm the strong-to-weak shear evolution suggested in previous observational studies and reproduced in numerical models, and also confirm that, in this flare, reconnection is not an efficient producer of energetic nonthermal electrons during the first 10 minutes when the strongly sheared PRFLs are formed. We conclude that an intermediate shear angle, ≤40°, is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.

Analysis of Solar-like X-class Flare on Wolf 359 Observed Simultaneously with TESS and XMM-Newton

We present an analysis of a flare on the Wolf 359 star based on simultaneous observations of Transiting Exoplanet Survey Satellite and XMM-Newton. A stellar flare with energy comparable to an X-class solar flare is analyzed on this star for the first time. The main goal of the study was to determine whether the same physical processes drive and occur in stellar flares as in solar flares. We tried to estimate the flare class by various direct and indirect methods. Light curves and spectra in different energy ranges were used to determine the parameters and profiles of the flare. From the XMM-Newton EPIC-pn X-ray data, we estimated the temperature and emission measure during the flare. The thermodynamical timescale and the loop semi-length were also determined with two different methods. The RGS spectra enabled us to calculate the differential emission measure (DEM) distributions. The obtained DEM distributions have three components at temperature values of 3, 7, and 16–17 MK. The analysis of the line ratio in helium-like triplets allowed us to determine the plasma electron density. Our results for the flare loop on Wolf 359 were compared to typical parameters for solar flares observed with GOES and RHESSI. This supports our conclusion that the processes taking place in stellar flares are like those in solar flares. The determined geometrical parameters of the phenomenon do not differ from the values of analogs occurring on the Sun.

Turbulence in Sources of Decimetric Flare Continua

Decimetric continua are commonly observed during long-lasting solar flares. Their frequency boundaries vary with time. We studied frequency boundary variations using the power spectrum analysis. Analyzing five decimetric continua, we found that their power spectra have a power-law form with the power-law index close to the Kolmogorov turbulence index −5/3. The same power index was also found in the power spectra of radio flux variations at frequencies in the range of the frequency boundary variations. Moreover, these frequency boundary variations were highly correlated with the radio flux ones. We interpret these results to be due to turbulent density variations in the reconnection plasma outflow to the termination shock formed above flare loops. In three cases of decimetric continua, we estimated the level of the plasma density turbulence to be 7.6 – 11.2% of the mean plasma density. We think that the analysis of variations of decimetric continua can be used in studies of the plasma turbulence in solar flares.

Diagnostics of Flare Loop Parameters in Shrinkage and Ascent Stages Using Radio, X-ray, and UV Emission

We propose the diagnostics of plasma parameters in flare loops using the data of multi-wavelength observations in both shrinkage and expansion phases of the loops. The approach of a flare loop as an equivalent electric circuit is applied. We show that depending on plasma loop parameters, the shrinkage may be accompanied by an increase in the electric current in the loop rather than a decrease. The number density, temperature, electric current, radius, loop-top altitude, and loop volume are determined for the flare events on 16 April 2002 and 24 August 2002.

Lyα Emission Enhancement Associated with Soft X-Ray Microflares

Lyα (Lyα, 1216 Å) is the strongest emission line in the solar ultraviolet spectrum. In the present work, we obtained a Lyα enhancement catalog covering flares larger than B1 class from the GOES/EUVS data during 2010–2016. We focused on the 242 B-class events which are less investigated, however, show non-negligible Lyα emission enhancement. We found that on average the Lyα peak of B-class flares is 0.85% stronger than the background. For the flare energetics, it is found that the weaker the soft X-ray (SXR) flare, the larger the ratio of the radiated energy in Lyα to SXR. Using the RHESSI data and multi-wavelength observations taken by SDO-AIA, we diagnose the thermal and non-thermal properties of several flares. Three case studies show that the coincidence of the Lyα peak with the SXR time-derivative peak is not a sufficient condition of the nonthermal property of a Lyα microflare. The Lyα enhancement in the microflares may be caused by the nonthermal electron beams or/and thermal conduction. However for type III events, we found that the delay of the Lyα peak with respect to the SXR peak can be attributed to either the Lyα emission from a filament erupted or the cooling of the thermal plasma in flare loops. Furthermore, interestingly the Lyα emission from filaments can not only occur in the decay phase of the flare, but also in the pre-flare phase. In this case, the Lyα emission was originated from an erupted filament which probably initiated the flare.

Statistical investigation of the widths of supra-arcade downflows observed during a solar flare

ABSTRACT Supra-arcade downflows (SADs) are dark voids descending towards the post-reconnection flare loops and exhibit obvious variation in properties like width. However, due to the lack of further statistical studies, the mechanism behind such variations hitherto remains elusive. Here, we statistically investigated widths of 81 SADs observed in one flare by the Solar Dynamics Observatory (SDO). For each of SADs, six moments were selected with equal time intervals to measure their widths at different stages of their evolution. It is found that most SADs show a roughly monotonous width decrease during their descents, while some SADs with small initial widths can have complex evolutions. 3D reconstruction results based on SDO and Solar Terrestrial Relations Observatory Ahead (STEREO-A) images and thermal properties analysis reveal that differences in magnetic and plasma environments may result in that SADs in the north are overall wider than those in the south. Additionally, correlation analysis between the width and other parameters of SADs was further conducted and revealed that (1) SADs with different initial widths show no significant differences in their temperature and density evolution characteristics; (2) SADs with small initial widths usually appear in lower heights, where more frequent collisions between SADs could lead to their intermittent acceleration, width increment, and curved trajectories. These results indicate that SADs with different initial widths are produced the same way, while different environments (magnetic field or plasma) could affect their subsequent width evolutions.