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The temperature reached by solar flares is a key parameter to understanding the physical process that causes the energy release. In this work, we analysed data from a Hinode Observing Programme that focused on high cadence measurement of the flaring plasma. This was carried out when the X-ray imager and spectrometer (STIX) on Solar Orbiter was observing. We analysed 3 small microflares, and determined their evolution and temperature. The temperature of the B2.8 microflare reached 16 MK. There was evidence in the smaller B1.4 flare of Fe xxiv emission, indicating that hot plasma of 15 MK can be reached., Solar Physics, 298 (1), ISSN:0038-0938, ISSN:1573-093X

The Marshall Grazing Incidence X-ray Spectrometer (MaGIXS) sounding rocket experiment launched on July 30, 2021 from the White Sands Missile Range in New Mexico. MaGIXS is a unique solar observing telescope developed to capture X-ray spectral images, in the 6 - 24 Angstrom wavelength range, of coronal active regions. Its novel design takes advantage of recent technological advances related to fabricating and optimizing X-ray optical systems as well as breakthroughs in inversion methodologies necessary to create spectrally pure maps from overlapping spectral images. MaGIXS is the first instrument of its kind to provide spatially resolved soft X-ray spectra across a wide field of view. The plasma diagnostics available in this spectral regime make this instrument a powerful tool for probing solar coronal heating. This paper presents details from the first MaGIXS flight, the captured observations, the data processing and inversion techniques, and the first science results., 20 pages, 18 figures

In this paper, we performed magnetohydrodynamics numerical experiments to look into the dynamic behaviour of the current sheet (CS) between the coronal mass ejection (CME) and the associated solar flare, especially the CS oscillation and plasmoid motions in coronal conditions. During the evolution, the disrupting magnetic configuration becomes asymmetric first in the buffer region at the bottom of the CME bubble. The Rayleigh−Taylor instability in the buffer region and the deflected motion of the plasma driven by the termination shock at the bottom of the CME bubble cause the buffer region to oscillate around the y-axis. The local oscillation propagates downwards through the CS, prompting an overall CS oscillation. As the buffer region grows, the oscillation period becomes longer, increasing from about 30 s to about 16 min. Meanwhile, there is another separated oscillation with a period between 0.25 and 1.5 min in the cusp region of the flare generated by velocity shearing. The tearing mode instability yields formations of plasmoids inside the CS. The motions of all the plasmoids observed in the experiment accelerate, which implies that the large-scale CME/flare CS itself in the true eruptive event is filled with the diffusion region according the the standard theory of magnetic reconnection.

Magnetic reconnection is the key mechanism for energy release in solar eruptions, where the high-temperature emission is the primary diagnostic for investigating the plasma properties during the reconnection process. Non-thermal broadening of high-temperature lines has been observed in both the reconnection current sheet (CS) and flare loop-top regions by UV spectrometers, but its origin remains unclear. In this work, we use a recently developed three-dimensional magnetohydrodynamic (MHD) simulation to model magnetic reconnection in the standard solar flare geometry and reveal highly dynamic plasma flows in the reconnection regions. We calculate the synthetic profiles of the Fe XXI 1354 Å line observed by the Interface Region Imaging Spectrograph (IRIS) spacecraft by using parameters of the MHD model, including plasma density, temperature, and velocity. Our model shows that the turbulent bulk plasma flows in the CS and flare loop-top regions are responsible for the non-thermal broadening of the Fe XXI emission line. The modeled non-thermal velocity ranges from tens of km s−1 to more than two hundred km s−1, which is consistent with the IRIS observations. Simulated 2D spectral line maps around the reconnection region also reveal highly dynamic downwflow structures where the high non-thermal velocity is large, which is consistent with the observations as well.

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 Alfv\'{e}n speeds. 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 outflows. However, they often show a speed much slower than that expected in reconnection theories, 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.

We investigate the topological evolution of coronal jet containing a sigmoid-like flux rope using a nonlinear force-free field model obtained with the flux-rope insertion method and magnetofrictional relaxation. We examine the topological evolution predicted by the unstable model with respect to observations from Solar Dynamic Observatory’s Atmospheric Imaging Array. We also calculate the squeezing factor, an approximation for sharp discontinuities in the magnetic field, and the coiling rate, an approximation for the amount of twist in the field. We identify at least two topological features where magnetic reconnection is likely taking place: an internal anemone-like region, near the filament, and an external region between the closed dome of the coronal jet and the ambient field. We also find evidence of reconnection below the filament, but it is not clear if the two inner regions are the same. We find that the internal region inflates the jet dome into the external region, which in turn initiates the fast eruption, allowing the inner region to unwind and the filament to escape. Finally, we examine the thermal evolution of the jet and trace the regions of enhanced emission-measure-weighted temperature (T EM) to the location of the expected reconnection regions. We find that magnetic field lines associated with the internal reconnection region are tied to increased T EM and emission in extreme-UV observations, indicative of heating. We identify this eruption as an untwisting jet, where unwinding magnetic field lines impart energy along the magnetic field forming the observed features of the coronal jet.

Elemental composition in the solar wind reflects the fractionation processes at the Sun. In coronal mass ejections (CMEs) measured in the heliosphere, the elemental composition can vary between plasma of high and low ionization states as indicated by the average Fe charge state, 〈QFe〉. It is found that CMEs with higher ionized plasma, 〈QFe〉 greater than 12, are significantly more enriched in low first ionization potential (FIP) elements compared to their less ionized, 〈QFe〉 less than 12, counterparts. In addition, the CME elemental composition has been shown to vary along the solar cycle. However, the processes driving changes in elemental composition in the plasma are not well understood. To gain insight into this variation, this work investigates the effects of gravitational settling in the ejecta to examine how that process can modify signatures of the FIP effect found in CMEs. We examine the absolute abundances of C, N, O, Ne, Mg, Si, S, and Fe in CMEs between 1998 and 2011. Results show that the ejecta exhibits some gravitational settling effects in approximately 33% of all CME periods in plasma where the Fe abundance of the ejecta compared to the solar wind (Fe/HCME:Fe/HSW) is depleted compared to the C abundance (C/HCME:C/HSW). We also find gravitational settling is most prominent in CMEs during solar minimum; however, it occurs throughout the solar cycle. This study indicates that gravitational settling, along with the FIP effect, can become important in governing the compositional makeup of CME source regions.

We have developed a tracking algorithm to determine the speeds of supra-arcade downflows (SADs) and set up a system to automatically track SADs and measure some interesting parameters. By conducting an analysis of six flares observed by the Atmospheric Imaging Assembly on the Solar Dynamics Observatory, we detect more smaller and slower SADs than prior work, due to the higher spatial resolution of our observational data. The inclusion of these events with smaller and slower SADs directly results in lower median velocities and widths than in prior work, but the fitted distributions and evolutions of the parameters still show good consistency with prior work. The observed distributions of the widths, speeds, and lifetimes of SADs are consistent with log-normal distributions, indicating that random and unstable processes are responsible for generating SADs during solar eruptions. Also, we find that the fastest SADs occur at approximately the middle of the height ranges. The number of SADs in each image versus time shows that there are “rest phases” of SADs, when few SADs are seen. These findings support the idea that SADs originate from a fluid instability. We compare our results with a numerical simulation that generates SADs using a mixture of the Rayleigh–Taylor instability and the Richtmyer–Meshkov instability, and find that the simulation generates quantities that are consistent with our observational results.

The Interface Region Imaging Spectrograph (IRIS) has been obtaining near- and far-ultraviolet images and spectra of the solar atmosphere since July 2013. The unique combination of near and far-ultraviolet spectra and images at subarcsecond resolution and high cadence allows the tracing of mass and energy through the critical interface between the solar surface and the corona or solar wind. IRIS has enabled research into the fundamental physical processes thought to play a role in the low solar atmosphere such as ion-neutral interactions, magnetic reconnection, the generation, propagation, and dissipation of various types of waves, the acceleration of non-thermal particles, and various small-scale instabilities. These new findings have helped provide novel insights into a wide range of phenomena including the discovery of non-thermal particles in coronal nanoflares, the formation and impact of spicules and other jets, resonant absorption and dissipation of Alfvenic waves, energy release associated with braiding of magnetic field lines, the thermal instability in the chromosphere-corona mass and energy cycle, the contribution of waves, turbulence, and non-thermal particles in the energy deposition during flares and smaller-scale events such as UV bursts, and the role of flux ropes and other mechanisms in triggering CMEs. IRIS observations have also advanced studies of the connections between solar and stellar physics. Advances in numerical modeling, inversion codes, and machine learning techniques have played a key role in driving these new insights. With the advent of exciting new instrumentation both on the ground (e.g., DKIST, ALMA) and space-based (e.g., Parker Solar Probe, Solar Orbiter), we aim to review new insights based on IRIS observations or related modeling, and highlight some of the outstanding challenges that have been brought to the fore., Comment: 105 pages, 30 figures, accepted for publication in Solar Physics

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 (EOVSA) and the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), respectively. By combining the microwave and HXR imaging spectroscopy observations with multi-filter extreme ultraviolet and soft X-ray imaging data, we derive the energetic electron distribution of this source over a broad energy range from $$6.0. In addition, temporally resolved analysis suggests that the electron distribution above the break energy rapidly hardens with the spectral index decreasing from $>$20 to $\sim$6.0 within 20 s, or less than $\sim$10 Alfv\'{e}n crossing times in the source. These results provide strong support for the above-the-loop-top source as the primary site where an on-going bulk acceleration of energetic electrons is taking place very early in the flare energy release., Comment: 12 pages, 5 figures, accepted for publication in The Astrophysical Journal Letters

Summary As one of the most spectacular energy release events in the solar system, solar flares are generally powered by magnetic reconnection in the solar corona. As a result of the re-arrangement of magnetic field topology after the reconnection process, a series of new loop-like magnetic structures are often formed and are known as flare loops. A hot diffuse region, consisting of around 5–10 MK plasma, is also observed above the loops and is called a supra-arcade fan. Often, dark, tadpole-like structures are seen to descend through the bright supra-arcade fans. It remains unclear what role these so-called supra-arcade downflows (SADs) play in heating the flaring coronal plasma. Here we show a unique flare observation, where many SADs collide with the flare loops and strongly heat the loops to a temperature of 10–20 MK. Several of these interactions generate clear signatures of quasi-periodic enhancement in the full-Sun-integrated soft X-ray emission, providing an alternative interpretation for quasi-periodic pulsations that are commonly observed during solar and stellar flares., Graphical Abstract, Public Summary • Tadpole-like flows known as supra-arcade downflows are observed in a solar flare • Many such flows are found to collide with the flare arcade • The collisions cause strong heating of the local plasma to a temperature of 10–20 MK • These heating events generate quasi-periodic pulsations (QPPs) in solar radiation

We investigate the effects of the evolutionary processes in the internal magnetic structure of two interplanetary coronal mass ejections (ICMEs) detected in situ between 2020 November 29 and December 1 by the Parker Solar Probe (PSP). The sources of the ICMEs were observed remotely at the Sun in EUV and subsequently tracked to their coronal counterparts in white light. This period is of particular interest to the community as it has been identified as the first widespread solar energetic particle event of solar cycle 25. The distribution of various solar and heliospheric-dedicated spacecraft throughout the inner heliosphere during PSP observations of these large-scale magnetic structures enables a comprehensive analysis of the internal evolution and topology of such structures. By assembling different models and techniques, we identify the signatures of interaction between the two consecutive ICMEs and the implications for their internal structure. We use multispacecraft observations in combination with a remote-sensing forward modeling technique, numerical propagation models, and in situ reconstruction techniques. The outcome, from the full reconciliations, demonstrates that the two coronal mass ejections (CMEs) are interacting in the vicinity of the PSP. Thus, we identify the in situ observations based on the physical processes that are associated with the interaction and collision of both CMEs. We also expand the flux rope modeling and in situ reconstruction technique to incorporate the aging and expansion effects in a distorted internal magnetic structure and explore the implications of both effects in the magnetic configuration of the ICMEs.

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., Comment: 16 pages, 8 figures

Solar flares are rapid energy release phenomena that appear as bright ribbons in the chromosphere and high-temperature loops in the corona, respectively. Supra-arcade Downflows (SADs) are plasma voids that first come out above the flare loops and then move quickly towards the flare loop top during the decay phase of the flare. In our work, we study 20 SADs appearing in three flares. By differential emission measure (DEM) analysis, we calculate the DEM weighted average temperature and emission measure (EM) of the front region and the main body of SADs. It is found that the temperatures of the SAD front and body tend to increase during the course of SADs flowing downwards. The relationship between the pressure and temperature fits well with the adiabatic equation for both the SAD front and body, suggesting that the heating of SADs is mainly caused by adiabatic compression. Moreover, we also estimate the velocities of SADs via the Fourier Local Correlation Tracking (FLCT) method and find that increase of the temperature of the SAD front presents a correlation with the decrease of the SAD kinetic energy, while the SAD body does not, implying that the viscous process may also heat the SAD front in spite of a limited role., Comment: 12 pages, 10 figures; published by ApJ

We simulate a coronal mass ejection (CME) using a three-dimensional magnetohydrodynamic (MHD) code that includes coronal heating, thermal conduction, and radiative cooling in the energy equation. The magnetic flux distribution at 1 R$_s$ is produced by a localized subsurface dipole superimposed on a global dipole field, mimicking the presence of an active region within the global corona. Transverse electric fields are applied near the polarity inversion line to introduce a transverse magnetic field, followed by the imposition of a converging flow to form and destabilize a flux rope, producing an eruption. We examine the quantities responsible for plasma heating and cooling during the eruption, including thermal conduction, radiation, adiabatic effects, coronal heating, and ohmic heating. We find that ohmic heating is an important contributor to hot temperatures in the current sheet region early in the eruption, but in the late phase adiabatic compression plays an important role in heating the plasma there. Thermal conduction also plays an important role in the transport of thermal energy away from the current sheet region throughout the reconnection process, producing a ``thermal halo'' and widening the region of high temperatures. We simulate emission from solar telescopes for this eruption and find that there is evidence for emission from heated plasma above the flare loops late in the eruption, when the adiabatic heating is the dominant heating term. These results provide an explanation for hot supra-arcade plasma sheets that are often observed in X-rays and extreme ultraviolet wavelengths during the decay phase of large flares., 22 pages, 12 figures

著者人数: 35名 (所属. 宇宙航空研究開発機構宇宙科学研究所(JAXA)(ISAS): 坂尾, 太郎; 清水, 敏文), Number of authors: 35 (Affiliation. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency(JAXA)(ISAS): Sakao, Taro; Shimizu, Toshifumi), Accepted: 2019-08-01, 資料番号: SA1190166000

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., 16 pages, 10 figures, accepted by The Astrophysical Journal. Version with minor (mostly typo) corrections

In this study, we present the magnetic configuration of an erupting pseudostreamer observed on 2015 April 19, on the southwest limb of the Sun, with a prominence cavity embedded inside. The eruption resulted in a partial halo coronal mass ejection. The prominence eruption begins with a slow rise and then evolves to a fast-rise phase. We analyze this erupting pseudostreamer using the flux-rope insertion method and magnetofrictional relaxation to establish a sequence of plausible out-of-equilibrium magnetic configurations. This approach allows the direct incorporation of observations of structures seen in the corona (filament and cavity) to appropriately model the pseudostreamer based on SDO/HMI line-of-sight photospheric magnetograms. We also perform a topological analysis in order to determine the location of quasiseparatrix layers (QSLs) in the models, producing Q-maps to examine how the QSL locations progress in the higher iterations. We found that the axial flux in our best-fit unstable model was a factor of 20 times higher than we found in our marginally stable case. We computed the average magnetic field strength of the prominence and found that the unstable model exhibits twice the average field strength of the stable model. The eruption height from our modeling matches very well with the prominence eruption height measured from the AIA observation. The Q-maps derived from the model reproduce structures observed in LASCO/C2. Thus, the modeling and topological analysis results are fully consistent with the observed morphological features, implying that we have captured the large magnetic structure of the erupting filament in our magnetofrictional simulation.