Reconnection In Flares Research Articles

Double Power-law Formation by Sequential Particle Acceleration

Spectral double power laws are common in solar high-energy phenomena such as flares and interplanetary energetic-electron events. However, the physical mechanism that produces the changes in power-law index within a single spectrum is unclear. We developed a fully analytical method of forming single power-law spectra from sequential acceleration of particles orbiting inside and hopping between simulated large-scale magnetic islands formed by flare reconnection. Here, we extend the analytical method to the formation of double power-law spectra by assuming sequential acceleration in two successive regions with different acceleration and particle-transport rates. The resulting spectral distribution is continuous and smooth, with a flattening at low energies, two power-law regions at mid-energies, and a steep rollover at high energies. The model provides analytical expressions for the spectral indices, all energy breaks, and normalization constants as functions of just three physical parameters of each acceleration region: (1) the energy gain in each accelerator, (2) the percentage of particles transferred between accelerators, and (3) the number of accelerators visited. One of the most salient predictions of our work is that the spectral index at high (low) energies is determined by the parameters of the first “seed” (second) acceleration region. By constructing the spectral distribution through an iterative analytical process, the evolution toward a double power law is easily characterized and explained. Our analytical model provides tools to interpret space- and ground-based observations from RHESSI, FOXSI, NuSTAR, Solar Orbiter/STIX, EOVSA, and future high-energy missions.

Inferring Fundamental Properties of the Flare Current Sheet Using Flare Ribbons: Oscillations in the Reconnection Flux Rates

Magnetic reconnection is understood to be the main physical process that facilitates the transformation of magnetic energy into heat, motion, and particle acceleration during solar eruptions. Yet, observational constraints on reconnection region properties and dynamics are limited due to a lack of high-cadence and high-spatial-resolution observations. By studying the evolution and morphology of postreconnected field-lines footpoints, or flare ribbons and vector photospheric magnetic field, we estimate the magnetic reconnection flux and its rate of change with time to study the flare reconnection process and dynamics of the current sheet above. We compare high-resolution imaging data to study the evolution of the fine structure in flare ribbons as ribbons spread away from the polarity inversion line. Using data from two illustrative events (one M- and X-class flare), we explore the relationship between the ribbon-front fine structure and the temporal development of bursts in the reconnection region. Additionally, we use the RibbonDB database to perform statistical analysis of 73 (C- to X-class) flares and identify quasiperiodic pulsation (QPP) properties using the Wavelet Transform. Our main finding is the discovery of QPP signatures in the derived magnetic reconnection rates in both example events and the large flare sample. We find that the oscillation periods range from 1 to 4 minutes. Furthermore, we find nearly cotemporal bursts in Hard X-ray (HXR) emission profiles. We discuss how dynamical processes in the current sheet involving plasmoids can explain the nearly cotemporal signatures of quasiperiodicity in the reconnection rates and HXR emission.

Enhancing Triangulation of Interplanetary Type III Bursts through Wavevector Correction

Interplanetary Type III bursts, generated by relativistic electron beams at solar flare reconnection sites, are explored through an investigation of 152 instances observed by the Solar Terrestrial Relations Observatory mission. This study reveals that the absolute values of the wavevector deviations from the Sun–spacecraft line are statistically 3.72 and 2.10 larger than predicted by the density model, assuming fundamental and harmonic emission, respectively. Through Monte Carlo simulations, we examine the impact of scattering by density inhomogeneities on the apparent locations of radio emissions in the interplanetary medium. The findings indicate that relative density fluctuations of 0.40 can account for the observed angular shift, a conclusion supported by the multiple flux-tube solar wind model, which confirms the presence of such magnitude of relative perpendicular density fluctuations in the solar wind. We propose a wavevector correction that incorporates this effect to enhance the triangulation of interplanetary Type III bursts, demonstrating that radio triangulation, with this correction, can reliably track electron beams in the interplanetary medium.

Observations of a Failed Solar Filament Eruption Involving External Reconnection

We report a failed solar filament eruption that involves external magnetic reconnection in a quadrupolar magnetic configuration. The evolution exhibits three phases of kinematic evolution: a slow rise, an acceleration, and a deceleration. In the early slow rise, extreme-ultraviolet brightenings appear at the expected null point above the filament and are connected to the outer polarities by the hot loops, indicating the occurrence of a breakout reconnection. Subsequently, the filament is accelerated outward, accompanied by the formation of low-lying high-temperature post-flare loops (>15 MK), complying with the standard flare model. However, after 2–3 minutes, the erupting filament starts to decelerate and is finally confined in the corona. The important finding is that the confinement is closely related to an external reconnection as evidenced by the formation of high-lying large-scale hot loops (>10 MK) with their brightened footpoints at the outer polarities, the fragmentation and subsequent falling of the filament along the newly formed large-scale loops, as well as a hard X-ray source close to one of the outer footpoint brightenings. We propose that, even though the initial breakout reconnection and subsequent flare reconnection commence and accelerate the filament eruption, the following external reconnection between the erupting flux rope and overlying field, as driven by the upward filament eruption, causes the eruption to finally fail, as validated by the numerical simulation of a failed flux rope eruption.

X-Ray, Near-ultraviolet, and Optical Flares Produced by Colliding Magnetospheres in the Young High-eccentricity Binary DQ Tau

DQ Tau is a unique young high-eccentricity binary system that exhibits regular magnetic reconnection flares and pulsed accretion near periastron. We conducted NuSTAR, Swift, and Chandra observations during the 2022 July 30 periastron to characterize X-ray, near-ultraviolet (NUV), and optical flaring emissions. Our findings confirm the presence of X-ray superflares accompanied by substantial NUV and optical flares, consistent with previous discoveries of periastron flares in 2010 and 2021. These observations, supported by new evidence, strongly establish the magnetosphere collision mechanism as the primary driver of magnetic energy release during DQ Tau’s periastron flares. The energetics of the observed X-ray superflares remain consistent across the three periastra, indicating recurring energy sources during each passage, surpassing the capabilities of single stars. The observed flaring across multiple bands supports the Adams et al. model for magnetosphere interaction in eccentric binaries. Evidence from modeling and past and current observations suggests that both the millimeter/X-ray periastron flares and, tentatively, the magnetic-reconnection-related components of the optical/NUV emissions conform to the classical solar/stellar nonthermal thick-target model, except for the distinctive magnetic energy source. However, our NuSTAR observations suffered from high background levels, hindering the detection of anticipated nonthermal hard X-rays. Furthermore, we report the serendipitous discovery of X-ray superflares occurring away from periastron, potentially associated with interacting magnetospheres. The current study is part of a broader multiwavelength campaign, which plans to investigate the influence of DQ Tau’s stellar radiation on gas-phase ion chemistry within its circumbinary disk.

Numerical MHD Simulations of Reconnection in Solar Flares: Effects of the Magnetic-Field Strength in the Current Sheet

Numerical MHD Simulations of Reconnection in Solar Flares: Effects of the Magnetic-Field Strength in the Current Sheet

MHD simulation of rapid change of photospheric magnetic field during solar eruption caused by magnetic reconnection

It has been well observed that the horizontal component of the magnetic field at photosphere changes rapidly and irreversibly after solar eruptions. Specifically, the horizontal magnetic field near the polarity inversion line increases substantially, while that near the center of the magnetic polarity decreases. Such a phenomenon is considered as the dynamic feedback from the corona to the photosphere, but the underlying mechanism remains in debate. Here based on a recent magnetohydrodynamics (MHD) simulation of homologous eruptions initiated by magnetic reconnection, we analyzed the rapid changes of the horizontal magnetic field, the magnetic inclination angle, the Lorentz force and as well as the derivative variation of the horizontal magnetic field. The simulation reproduces a pattern of rapid evolution of the horizontal field during the eruptions in agreement with typical observations. Our analysis suggests the physical reasons for this phenomenon: 1) The magnetic field near the polarity inversion line becomes more horizontal after flares due to the compression of the downward outflow of flare reconnection, and accordingly the magnetic inclination angle decreases and the downward Lorentz force increases; 2) The magnetic field near the center of the magnetic polarities become more vertical mainly due to the expansion effect of the velocity divergence term, and as a result the magnetic inclination angle and the upward Lorentz force increase.

Plasmoids, Flows, and Jets during Magnetic Reconnection in a Failed Solar Eruption

We report a detailed analysis of a failed eruption and flare in active region 12018 on 2014 April 3 using multiwavelength observations from the Solar Dynamics Observatory/Atmospheric Imaging Assembly, IRIS, STEREO, and Hinode/Solar Optical Telescope. At least four jets were observed to emanate from the cusp of this small active region (large bright point) with a null-point topology during the 2 hr prior to the slow rise of a filament. During the filament slow rise multiple plasma blobs were seen, most likely formed in a null-point current sheet near the cusp. The subsequent filament eruption, which was outside the IRIS field of view, was accompanied by a flare but remained confined. During the explosive flare reconnection phase, additional blobs appeared repetitively and moved bidirectionally within the flaring region below the erupting filament. The filament kinked, rotated, and underwent leg–leg reconnection as it rose, yet it failed to produce a coronal mass ejection. Tiny jet-like features in the fan loops were detected during the filament slow rise/preflare phase. We interpret them as signatures of reconnection between the ambient magnetic field and the plasmoids leaving the null-point sheet and streaming along the fan loops. We contrast our interpretation of these tiny jets, which occur within the large-scale context of a failed filament eruption, with the local nanoflare-heating scenario proposed by Antolin et al.

Weak Bidirectional Outflows and Flare Current Sheet in a Solar Coronal Jet Driven by the Eruption of a Minifilament

Different from the classical emerging-flux model for solar jets, recent studies proposed that the great majority of solar coronal jets are triggered by minifilament eruptions and two magnetic reconnection processes should take place during the course, named as external reconnection (breakout reconnection) and internal reconnection (flare reconnection). With the excellent data of the Solar Dynamics Observatory, we present the observational signatures of these two magnetic reconnection processes during a solar coronal jet that occurred in a huge coronal hole of northern hemisphere. The jet was triggered by the eruption of a minifilament that located at a coronal bright point in the coronal hole. Weak bidirectional outflows were observed when the erupting minifilament approached the ambient open field, ejecting along the triggered jet spire and the jet base simultaneously. In addition, a flare current sheet occurred after the eruption of the minifilament, connecting the jet spire and the jet bright point (or flare). We suggest the occurrence of the weak bidirectional outflows and the flare current sheet correspond to the external and the internal reconnections, respectively. Prior to the eruption of the minifilament and the jet, photospheric magnetic flux cancellation maintained for more than 7 hr in the source region, and the positive flux decreased for about 28.6% during this period. So, consistent with the recent observations, the trigger mechanism of the minifilament eruption and the following jet in this event may be flux cancellation rather than flux emergence.

Implications of High-density, High-temperature Ridges Observed in Some Two-ribbon Flares

Several two-ribbon solar flares observed on the disk, notably including the Bastille flare of 2000 July 14, show an extended ridge of plasma running along the loop tops of the post-reconnection arcade. In that and two more recent examples, the ridge is visible in emission by Fe xxiv at roughly 17 MK, with a high, steadily increasing emission measure suggesting an expanding column of very dense plasma. We find that ridges are consistent with overhead views of long, vertical plasma sheets, such as seen above certain limb flares. Those vertical features show enhanced temperature and density over their entire lengths, making explanations in terms of termination shocks and evaporation collision seem less plausible. We use observations of several ridge events to argue in favor of compression and heating by slow magnetosonic shocks in the reconnection outflow. In this scenario, the ridge is built up as retracting flux piles hot, compressed plasma atop the post-flare arcade. Thanks to the overhead perspective offered by the ridge observations, we are able to measure the reconnection rate and show it to be consistent with the rate of increase in column emission measure across the ridge. This consistency supports the hypothesis that slow shocks and retraction compress the plasma seen in ridges, vertical plasma sheets, and possibly the high-temperature fans through which post-reconnection downflows are observed. Such a unified picture of these diverse features enhances our understanding of the role played by magnetic reconnection in solar flares.

Overexpansion-dominated coronal mass ejection formation and induced radio bursts

Aims. Coronal mass ejections (CMEs) are the most fascinating explosions in the Solar System. Their formation is still not fully understood, however. Methods. We investigated a well-observed CME on 2021 May 7 that showed a typical three-component structure and was continuously observed from 0 to 3 R⊙ by a combination of SDO/AIA (0–1.3 R⊙), PROBA2/SWAP (0–1.7 R⊙), and MLSO/K-Cor (1.05–3 R⊙). Furthermore, we compared the morphological discrepancy between the CME white-light bright core and the extreme-UV (EUV) blob. We finally explored the origin of various radio bursts that are closely related to the interaction of the CME overexpansion with a nearby streamer. Results. An interesting finding is that the height increases of the CME leading front and of the bright core are dominated by the overexpansion during the CME formation. The aspect ratios of the CME bubble and bright core, quantifying the overexpansion, are found to decrease as the SO/STIX 4–10 keV and GOES 1–8 Å soft X-ray flux of the associated flare increases near the peaks. This indicates that the flare reconnection plays an important role in the first overexpansion. The CME bubble even undergoes a second overexpansion, although it is relatively weak, which is closely related to the compression with a nearby streamer and likely arises from an ideal magnetohydrodynamics process. Moreover, the CME EUV blob is found to be relatively lower and wider than the CME white-light bright core, which may correspond to the bottom part of the growing CME flux rope. The interaction between the CME and the streamer leads to two type II radio bursts, one that is drifting normally and another that is stationary, which are speculated to be induced by two different sources of the CME-driven shock front. The bidirectional electrons shown in series of C-shaped type III bursts suggest that the interchange reconnection is also involved during the interaction of the CME and streamer.

A Magnetogram-matching Method for Energizing Magnetic Flux Ropes Toward Eruption

We propose a new “helicity-pumping” method for energizing coronal equilibria that contain a magnetic flux rope (MFR) toward an eruption. We achieve this in a sequence of magnetohydrodynamics relaxations of small line-tied pulses of magnetic helicity, each of which is simulated by a suitable rescaling of the current-carrying part of the field. The whole procedure is “magnetogram-matching” because it involves no changes to the normal component of the field at the photospheric boundary. The method is illustrated by applying it to an observed force-free configuration whose MFR is modeled with our regularized Biot–Savart law method. We find that, in spite of the bipolar character of the external field, the MFR eruption is sustained by two reconnection processes. The first, which we refer to as breakthrough reconnection, is analogous to breakout reconnection in quadrupolar configurations. It occurs at a quasi-separator inside a current layer that wraps around the erupting MFR and is caused by the photospheric line-tying effect. The second process is the classical flare reconnection, which develops at the second quasi-separator inside a vertical current layer that is formed below the erupting MFR. Both reconnection processes work in tandem with the magnetic forces of the unstable MFR to propel it through the overlying ambient field, and their interplay may also be relevant for the thermal processes occurring in the plasma of solar flares. The considered example suggests that our method will be beneficial for both the modeling of observed eruptive events and theoretical studies of eruptions in idealized magnetic configurations.

Sunspot Rotation in High- and Low-Flaring Active Regions

Sunspot rotations are closely linked with flaring activity. They are thought to contribute to the accumulation of helicity in magnetic flux tubes and to triggering magnetic reconnection in large solar flares. This link to solar flares has led to sunspot rotations being used as a parameter in solar flare prediction methods, but analysis for long-period observations of rotations in the literature is scarce. In this study, the rotation profiles of sunspots in a selection of six active regions are studied over time periods of 5 – 10 days to measure how sunspot rotation varies as active regions develop. The active regions are divided into two categories: high-flaring groups, which produced at least one X-class flare, and low-flaring regions that had little flaring activity. Comparison of the rotation profiles in these regions showed that young complex sunspot groups exhibit faster angular velocities and more frequent changes in rotation than older single-spot groups and, although the most rotating groups were also the most flare-productive, sudden changes in rotation were found to not definitively indicate an imminent eruption.

Wave Emission of Nonthermal Electron Beams Generated by Magnetic Reconnection

Magnetic reconnection in solar flares can efficiently generate nonthermal electron beams. The energetic electrons can, in turn, cause radio waves through microscopic plasma instabilities as they propagate through the ambient plasma along the magnetic field lines. We aim at investigating the wave emission caused by fast-moving electron beams with characteristic nonthermal electron velocity distribution functions (EVDFs) generated by kinetic magnetic reconnection: two-stream EVDFs along the separatrices and in the diffusion region, and perpendicular crescent-shaped EVDFs closer to the diffusion region. For this purpose, we utilized 2.5D fully kinetic Particle-In-Cell code simulations in this study. We found the following: (1) the two-stream EVDFs plus the background ions are unstable to electron/ion (streaming) instabilities, which cause ion-acoustic waves and Langmuir waves due to the net current. This can lead to multiple-harmonic plasma emission in the diffusion region and the separatrices of reconnection. (2) The perpendicular crescent-shaped EVDFs can cause multiple-harmonic electromagnetic electron cyclotron waves through the electron cyclotron maser instabilities in the diffusion region of reconnection. Our results are applicable to diagnose the plasma parameters, which are associated to magnetic reconnection in solar flares by means of radio wave observations.

Properties and Energetics of Magnetic Reconnection: I. Evolution of Flare Ribbons

In this article, we measure the mean magnetic shear from the morphological evolution of flare ribbons, and examine the evolution of flare thermal and nonthermal X-ray emissions during the progress of flare reconnection. We analyze three eruptive flares and three confined flares ranging from GOES class C8.0 to M7.0. They exhibit two well-defined ribbons along the magnetic polarity-inversion line (PIL), and have been observed by the Atmospheric Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager from the onset of the flare throughout the impulsive phase. The analysis confirms the strong-to-weak shear evolution in the core region of the flare, and the flare hard X-ray emission rises as the shear decreases. In eruptive flares, in this sample, significant nonthermal hard X-ray emission lags the ultraviolet emission from flare ribbons, and rises rapidly when the shear is modest. In all flares, we observe that the plasma temperature rises in the early phase when the flare ribbons rapidly spread along the PIL and the shear is high. We compare these results with prior studies, and discuss their implications, as well as complications, related to the physical mechanisms governing energy partition during flare reconnection.

Variability of the Reconnection Guide Field in Solar Flares

Solar flares may be the best-known examples of the explosive conversion of magnetic energy into bulk motion, plasma heating, and particle acceleration via magnetic reconnection. The energy source for all flares is the highly sheared magnetic field of a filament channel above a polarity inversion line (PIL). During the flare, this shear field becomes the so-called reconnection guide field (i.e., the nonreconnecting component), which has been shown to play a major role in determining key properties of the reconnection, including the efficiency of particle acceleration. We present new high-resolution, three-dimensional, magnetohydrodynamics simulations that reveal the detailed evolution of the magnetic shear/guide field throughout an eruptive flare. The magnetic shear evolves in three distinct phases: shear first builds up in a narrow region about the PIL, then expands outward to form a thin vertical current sheet, and finally is transferred by flare reconnection into an arcade of sheared flare loops and an erupting flux rope. We demonstrate how the guide field may be inferred from observations of the sheared flare loops. Our results indicate that initially the guide field is larger by about a factor of 5 than the reconnecting component, but it weakens by more than an order of magnitude over the course of the flare. Instantaneously, the guide field also varies spatially over a similar range along the three-dimensional current sheet. We discuss the implications of the remarkable variability of the guide field for the timing and localization of efficient particle acceleration in flares.

Eruption of the EUV Hot Channel from the Solar Limb and Associated Moving Type IV Radio Burst

Abstract Using the observations from the Solar Dynamics Observatory, we study an eruption of a hot-channel flux rope (FR) near the solar limb on 2015 February 9. The pre-eruptive structure is visible mainly in EUV 131 Å images, with two highly sheared loop structures. They undergo a slow rising motion and then reconnect to form an eruptive hot channel, as in the tether-cutting reconnection model. The J-shaped flare ribbons trace the footpoint of the FR that is identified as the hot channel. Initially, the hot channel is observed to rise slowly at 40 km s−1, followed by an exponential rise from 22:55 UT at a coronal height of 87 ± 2 Mm. Following the onset of the eruption at 23:00 UT, the flare reconnection then adds to the acceleration process of the coronal mass ejection (CME) within 3 R ⊙. Later on, the CME continues to accelerate at 8 m s−2 during its propagation period. Further, the eruption also launched type II radio bursts, which were followed by type III and type IVm radio bursts. The start and end times of the type IVm burst correspond to the CME’s core height of 1.5 and 6.1 R ⊙, respectively. Also, the spectral index is negative, suggesting that nonthermal electrons are trapped in the closed loop structure. Accompanied by this type IVm burst, this event is unique in the sense that the flare ribbons are very clearly observed together with the erupting hot channel, which strongly suggests that the hooked parts of the J-shaped flare ribbons outline the boundary of the erupting FR.

Connecting Chromospheric Condensation Signatures to Reconnection-driven Heating Rates in an Observed Flare

Abstract Observations of solar flare reconnection at very high spatial and temporal resolution can be made indirectly at the footpoints of reconnected loops into which flare energy is deposited. The response of the lower atmosphere to this energy input includes a downward-propagating shock called chromospheric condensation, which can be observed in the UV and visible. In order to characterize reconnection using high-resolution observations of this response, one must develop a quantitative relationship between the two. Such a relation was recently developed, and here we test it on observations of chromospheric condensation in a single footpoint from a flare ribbon of the X1.0 flare on 2014 October 25 (SOL2014-10-25T16:56:36). Measurements taken of Si iv 1402.77 Å emission spectra using the Interface Region Imaging Spectrograph (IRIS) in a single pixel show the redshifted component undergoing characteristic condensation evolution. We apply the technique called the Ultraviolet Footpoint Calorimeter to infer energy deposition into one footpoint. This energy profile, persisting much longer than the observed condensation, is input into a one-dimensional, hydrodynamic simulation to compute the chromospheric response, which contains a very brief condensation episode. From this simulation, we synthesize Si iv spectra and compute the time-evolving Doppler velocity. The synthetic velocity evolution is found to compare reasonably well with the IRIS observation, thus corroborating our reconnection–condensation relationship. The exercise reveals that the chromospheric condensation characterizes a particular portion of the reconnection energy release rather than its entirety, and that the timescale of condensation does not necessarily reflect the timescale of energy input.

The origin of underdense plasma downflows associated with magnetic reconnection in solar flares

Magnetic reconnection is a universal process that powers explosive energy-release events such as solar flares, geomagnetic substorms and some astrophysical jets. A characteristic feature of magnetic reconnection is the production of fast reconnection outflow jets near the plasma Alfvén speeds1,2. In eruptive solar flares, dark finger-shaped plasma downflows moving toward the flare arcade have been commonly regarded as the principal observational evidence for such reconnection-driven outflows3,4. However, they often show a speed much slower than that expected in reconnection theories5,6, challenging the reconnection-driven energy-release scenario in standard flare models. Here we present a three-dimensional magnetohydrodynamics model of solar flares. By comparing the model predictions with the observed plasma downflow features, we conclude that these dark downflows are self-organized structures formed in a turbulent interface region below the flare termination shock where the outflows meet the flare arcade, a phenomenon analogous to the formation of similar structures in supernova remnants. This interface region hosts a myriad of turbulent flows, electron currents and shocks, crucial for flare energy release and particle acceleration. Supra-arcade downflows are dark finger-like structures associated with solar flares. Simulations now suggest that these plasma downflows form within a turbulent region that may control turbulent flows, electron currents and shockwaves.

Connecting theory of plasmoid-modulated reconnection to observations of solar flares

Abstract The short timescale of the solar flare reconnection process has long proved to be a puzzle. Recent studies suggest the importance of the formation of plasmoids in the reconnecting current sheet, with quantifying the aspect ratio of the width to length of the current sheet in terms of a negative power $ \alpha $ of the Lundquist number, that is, $ {S}^{-\alpha } $ , being key to understanding the onset of plasmoids formation. In this paper, we make the first application of theoretical scalings for this aspect ratio to observed flares to evaluate how plasmoid formation may connect with observations. For three different flares that show plasmoids we find a range of $ \alpha $ values of $ \alpha =0.26 $ to $ 0.31 $ . The values in this small range implies that plasmoids may be forming before the theoretically predicted critical aspect ratio ( $ \alpha =1/3 $ ) has been reached, potentially presenting a challenge for the theoretical models.