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 up to tens of seconds prior to the flare's impulsive phase. This `hot onset' interval occurs during the initial soft X-ray increase and prior to the detectable hard X-ray emission. The isothermal temperatures, estimated by the Geostationary Operational Environmental Satellite (GOES) X-ray sensor, and confirmed with data from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), 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 identify this early hot onset soft X-ray emission mainly within footpoint and low-lying loops, rather than with coronal structures, based on images from the Atmospheric Imaging Assembly (AIA). 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 they challenge the standard flare heating modeling techniques.
Flares are a major explosive event in our solar system. They are often followed by coronal mass ejection that has a potential to trigger the geomagnetic storms. There are various studies aiming to predict when and where the flares are likely to occur. Most of these studies mainly discuss the photospheric and chromospheric activity before the flare onset. In this paper we study the coronal features before the famous large flare occurrence on December 13th, 2006. Using the data from Hinode/EUV Imaging Spectrometer (EIS), X-Ray Telescope (XRT), and Solar and Heliospheric Observatory (SOHO) /Extre…
Solar flares are the most powerful explosions occurring in the solar system, which may lead to disastrous space weather events and impact various aspects of our Earth. So far, it is still a big challenge in modern astrophysics to understand the origin of solar flares and predict their onset. Based on the analysis of soft X-ray emission observed by the Geostationary Operational Environmental Satellite (GOES), this work reported a new discovery of very long-periodic pulsations occurred in the preflare phase before the onset of solar flares (preflare-VLPs). These pulsations are typically with per…
It is expected that energetic solar flares releasing large amount of energy at the photosphere may be able to excite the acoustic ($p$-) modes of oscillations. We have determined the characteristic properties of mode parameters by applying the ring diagram technique to 3-D power spectra obtained for solar active region NOAA 10486 during the long duration energetic X17.2/4B flare of October 28, 2003. Strong evidence of substantial increase in mode amplitude and systematic variations in sub-surface flows, i.e., meridional and zonal components of velocity, kinetic helicity, vorticity, is found fr…
Using the observations from the Optical and Near-infrared Solar Eruption Tracer and the Solar Dynamics Observatory, we study an M5.7 flare in AR 11476 on 2012 May 10 and a micro-flare in the quiet Sun on 2017 March 23. Before the onset of each flare, there is a reverse S-shaped filament above the polarity inversion line. Then the filaments become unstable and begin to rise. The rising filaments gain the upper hand over the tension force of the dome-like overlying loops and thus successfully erupt outward. The footpoints of the reconnecting overlying loops successively brighten and are observed…
We studied a flare onset process in terms of stability of a three-dimensional (3D) magnetic field in active region 12371 producing an eruptive M6.5 flare in 2015 June 22. In order to reveal the 3D magnetic structure, we first extrapolated the 3D coronal magnetic fields based on time series of the photospheric vector magnetic fields under a nonlinear force-free field (NLFFF) approximation. The NLFFFs nicely reproduced the observed sigmoidal structure which is widely considered to be preeruptive magnetic configuration. In particular, we found that the sigmoid is composed of two branches of shear…
Plasma Heating and Energization in Hot-Onset Flare Precursor Events
IREAP, University of Maryland（马里兰大学 IREAP）；Dept. of Physics, Institute for Physical Science and Technology, and the Joint Space Science Institute, University of Maryland（马里兰大学物理系、物理科学与技术研究所和联合空间科学研究所）
Very long-periodic pulsations during preflare phases (preflare-VLPs) have been detected in the full-disk solar soft X-Ray (SXR) flux (see Tan et al. 2016). They may be regarded as precursors to solar flares and may help us better understand the trigger mechanism of solar flares. In this letter, we report a preflare-VLP event before the onset of an M1.1 circular-ribbon flare on 2015 October 16. It was simultaneously observed in Halpha, SXR, and extreme ultraviolet (EUV) wavelengths, which were recorded by the NVST, GOES, EVE, and AIA respectively. The preflare-VLP is identified as the repeat an…
We report observations of a two-stage coronal dimming in an eruptive event of a two-ribbon flare and a fast coronal mass ejection (CME). Weak gradual dimming persists for more than half an hour before the onset of the two-ribbon flare and the fast rise of the CME. It is followed by abrupt rapid dimming. The two-stage dimming occurs in a pair of conjugate dimming regions adjacent to the two flare ribbons, and the flare onset marks the transition between the two stages of dimming. At the onset of the two-ribbon flare, transient brightenings are also observed inside the dimming regions, before ra…
In the present work we study Hinode/EIS observations of an active region taken before, during and after a small C2.0 flare in order to monitor the evolution of the magnetic field evolution and its relation to the flare event. We find that while the flare left the active region itself unaltered, the event included a large Magnetic Field Enhancement (MFE), which consisted of a large increase of the magnetic field to strengths just short of 500~G in a rather small region where no magnetic field was measured before the flare. This MFE is observed during the impulsive phase of the flare at the foot…
We investigate the formation and eruption of hot channels associated with the M6.5 class flare (SOL2015-06-22T18:23) occurring in NOAA AR 12371 on 2015 June 22. Two flare precursors are observed before the flare main phase. Observations in 94 Å and 131 Å by SDO/AIA have revealed the early morphology of the first hot channel as a group of hot loops, which is termed as seed hot channel. A few seed hot channels are formed above the polarity inversion line (PIL) and the formation is associated with footpoint brightenings' parallel motion along the PIL, which proceeds into the early stage of the fl…
Solar flares are one of the main forces behind space weather events. However the mechanism that drives such energetic phenomena is not fully understood. The standard eruptive flare model predicts that magnetic reconnection occurs high in the corona where hot fast flows are created. Some imaging or spectroscopic observations have indicated the presence of these hot fast flows but there have been no spectroscopic scanning observation to date to measure the two-dimensional structure quantitatively. We analyzed a flare that occurred on the west solar limb on 27 January 2012 observed by the Hinode …
We present the first observational study of the onset and evolution of solar flare turbulence in the lower solar atmosphere on an unprecedented time scale of 1.7 s using the Interface Region Imaging Spectrograph observing plasma at a temperature of 80,000 K. At this time resolution, nonthermal spectral line broadening, indicating turbulent velocity fluctuations, precedes the flare onset at this temperature and is coincident with net blue-shifts. The broadening decreases as the flare brightens and then oscillates with a period of ~10 s. These observations are consistent with turbulence in the l…
We analyze a confined flare that developed a hot cusp-like structure high in the corona (H ~ 66 Mm). A growing cusp-shaped flare arcade is a typical feature in the standard model of eruptive flares, caused by magnetic reconnection at progressively larger coronal heights. In contrast, we observe a static hot cusp during a confined flare. Despite an initial vertical temperature distribution similar to that in eruptive flares, we observe a distinctly different evolution during the late (decay) phase, in the form of prolonged hot emission. The distinct cusp shape, rooted at locations of non-therma…
Multiwavelength Observations of the Precursors Before the Eruptive X4.9 Limb Solar Flare on 25 February, 2014: Current Sheet, Eruptive Filament, Flare and Eruption Onset in the Frame of the Tether-Cutting Magnetic Reconnection Scenario
The forecast method introduced by Korsós et al.(2014) is generalised from the horizontal magnetic gradient (GM), defined between two opposite polarity spots, to all spots within an appropriately defined region close to the magnetic neutral line of an active region. This novel approach is not limited to searching for the largest GM of two single spots as in previous methods. Instead, the pre-flare conditions of the evolution of spot groups is captured by the introduction of the weighted horizontal magnetic gradient, or W_GM. This new proxy enables the potential of forecasting flares stronger th…
Solar flares abruptly release the free energy stored as a non-potential magnetic field in the corona and may be accompanied by eruptions of the coronal plasma. Formation of a non-potential magnetic field and the mechanisms for triggering the onset of flares are still poorly understood. In particular, photospheric dynamics observed near those polarity inversion lines that are sites of major flare production have not been well observed with high spatial resolution spectro-polarimetry. This paper reports on a remarkable high-speed material flow observed along the polarity inversion line located b…
We present the study of SOL2015-03-15 M1.2 flare, revealing acceleration of electrons and plasma heating in the sheared twisted magnetic structure in the polarity inversion line (PIL). The scope is to make the analysis of nonthermal electrons dynamics and plasma heating in the highly stressed magnetic loops interacting in the PIL by using X-ray, microwave, ultraviolet, and optical observations. It is found that the most probable scenario for the energy release in the PIL is the tether-cutting magnetic reconnection between the low-lying (3 Mm above the photosphere) magnetic loops within a twist…
We investigate the August 9, 2011 solar flare of X-ray class X6.9, the "hottest" flare from 2000 to 2012, with a peak plasma temperature according to GOES data of 32.5 MK. Our goal is to determine the cause of such an anomalously high plasma temperature and to investigate the energy balance in the flare region with allowance made for the presence of a super-hot plasma (>30 MK). We analyze the RHESSI, GOES, AIA/SDO, and EVE/SDO data and discuss the spatial structure of the flare region and the results of our spectral analysis of its X-ray emission. Our analysis of the RHESSI X-ray spectra is pe…
The X9.3 flare of September 6, 2017, was the most powerful flare of Solar Cycle 24. It generated strong white-light emission and multiple helioseismic waves (sunquakes). By using data from Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) as well as hard X-ray data from KONUS instrument onboard WIND spacecraft, and Anti-Coincidence System (ACS) onboard the INTERGRAL space observatory, we investigate spatio-temporal dynamics of photospheric emission sources, identify sources of helioseismic waves and compare the flare photospheric dynamics with the hard X-ray (…
As part of the Mega MUSCLES Hubble Space Telescope (HST) Treasury program, we obtained time-series ultraviolet spectroscopy of the M2.5V star, GJ~674. During the FUV monitoring observations, the target exhibited several small flares and one large flare (E_FUV = 10^{30.75} ergs) that persisted over the entirety of a HST orbit and had an equivalent duration >30,000 sec, comparable to the highest relative amplitude event previously recorded in the FUV. The flare spectrum exhibited enhanced line emission from chromospheric, transition region, and coronal transitions and a blue FUV continuum with a…
It is generally accepted that densities of quiet sun and active region plasma are sufficiently low to justify the optically thin approximation, and it is commonly used in the analysis of line emissions from plasma in the solar corona. However, densities of solar flare loops are substantially higher, compromising the optically thin approximation. This study begins with a radiative transfer model that uses typical solar flare densities and geometries to show that hot coronal emission lines are not generally optically thin. Further, the model demonstrates that the observed line intensity should e…
We use data at 131, 171, and 304 A from the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory (SDO) to search for hot flux ropes in 141 M-class and X-class solar flares that occurred at solar longitudes equal to or larger than 50 degrees. Half of the flares were associated with coronal mass ejections (CMEs). The goal of our survey is to assess the frequency of hot flux ropes in large flares irrespective of their formation time relative to the onset of eruptions. The flux ropes were identified in 131 A images using morphological criteria and their high temperatures were c…
Most solar flares demonstrate a prolonged, hourlong post-flare (or gradual) phase, characterized by arcade-like, post-flare loops (PFLs) visible in many extreme ultraviolet (EUV) passbands. These coronal loops are filled with hot -- $\sim 30 \,\mathrm{MK}$ -- and dense plasma, evaporated from the chromosphere during the impulsive phase of the flare, and they very gradually recover to normal coronal density and temperature conditions. During this gradual cooling down to $\sim 1 \,\mathrm{MK}$ regimes, much cooler -- $\sim 0.01 \,\mathrm{MK}$ -- and denser coronal rain is frequently observed ins…
Several papers provide evidences that the most probable sites of flare onset are the regions of high horizontal magnetic field gradients in solar active regions. Besides the localization of flare producing areas the present work intends to reveal the characteristic temporal variations in these regions prior to flares. This study uses sunspot data instead of magnetograms, it follows the behaviour of a suitable defined proxy measure representing the horizontal magnetic field gradient. The source of the data is the SDD (SOHO/MDI-Debrecen Data) sunspot catalogue. The most promising pre-flare signa…
One of the key processes associated with the "standard" flare model is chromospheric evaporation, a process where plasma heated to high temperatures by energy deposition at the flare footpoints is driven upwards into the corona. Despite several decades of study, a number of open questions remain, including the relationship between plasma produced during this process and observations of earlier "superhot" plasma. Hinode/EIS has a wide slot often used as a flare trigger in the He II emission line band. Once the intensity passes a threshold level, the study switches to one focussed on the flaring…
We report a detailed study of the pre-eruption activities that led to the occurrence of an M9.0 flare/CME event on 2012 October 20 in NOAA AR 11598. This includes the study of the preceding confined C2.4 flare that occurred on the same AR ~25 minutes earlier. We observed that the M9.0 flare occurred as a consequence of two distinct triggering events well separated in time. The first triggering episode occurred as early as ~20 minutes before the onset of the M9.0 flare, evidenced by the destabilization and rise of a pre-existing filament to a new position of equilibrium at a higher coronal alti…
Ground level events (GLEs) occupy the high-energy end of gradual solar energetic particle (SEP) events. They are associated with coronal mass ejections (CMEs) and solar flares, but we still do not clearly understand the special conditions that produce these rare events. During Solar Cycle 23, a total of 16 GLEs were registered, using ground-based neutron monitor data. We first ask if these GLEs are clearly distinguishable from other SEP events observed from space. Setting aside possible difficulties in identifying all GLEs consistently, we then try to find observables which may unmistakably is…
X-ray emission provides the most direct diagnostics of the energy-release process in solar flares. Occasionally, a superhot X-ray source is found to be above hot flare loops of ~10 MK temperature. While the origin of the superhot plasma is still elusive, it has conjured up an intriguing image of in-situ plasma heating near the reconnection site high above the flare loops, in contrast to the conventional picture of chromospheric evaporation. Here we investigate an extremely long-duration solar flare, in which EUV images show two distinct flare loop systems that appear successively along a Gamma…
About 22000 Kepler stars and nearly 60000 TESS stars from sectors 1-24 have been classified according to variability type. A large proportion of stars of all spectral types appear to have periods consistent with the expected rotation periods. A previous analysis of A and late B stars strongly suggests that these stars are indeed rotational variables. In this paper we have accumulated sufficient data to show that rotational modulation is present even among the early B stars. A search for flares in TESS A and B stars resulted in the detection of 110 flares in 68 stars. The flare energies exceed …
We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation encompassing a vertical range from the photosphere through the chromosphere and transition region into the low corona. The basis of our proxy measure of activity prediction is the so-called weighted horizontal gradient of magnetic field (WG_M) defined between spots of opposite polarities close to the polarity inversion line of an active region…
Two-ribbon brightenings are one of the most remarkable characteristics of an eruptive solar flare and are often used for predicting the occurrence of coronal mass ejections (CMEs). Nevertheless, it was called in question recently whether all two-ribbon flares are eruptive. In this paper, we investigate a two ribbon-like white-light (WL) flare that is associated with a failed magnetic flux rope (MFR) eruption on 2015 January 13, which has no accompanying CME in the WL coronagraph. Observations by \textit{Optical and Near-infrared Solar Eruption Tracer} and \textit{Solar Dynamics Observatory} re…
In H-alpha most of the solar surface is covered by dense canopies of long opaque fibrils, but predictions for quiet-Sun observations with ALMA have ignored this fact. Comparison with Ly-alpha suggests that the large opacity of H-alpha fibrils is caused by hot precursor events. Application of a recipe that assumes momentary Saha-Boltzmann extinction during their hot onset to millimeter wavelengths suggests that ALMA will observe H-alpha-like fibril canopies, not acoustic shocks underneath, and will yield data more interesting than if these canopies were transparent.
We carried out detailed analysis of X-ray and radio observations of a simple flare loop that occurred on 12th August 2002, with the impulsive hard X-ray (HXR) light curves dominated by a single pulse. The emission spectra of the early impulsive phase are consistent with an isothermal model in the coronal loop with a temperature reaching several keVs. A power-law high-energy spectral tail is evident near the HXR peak time, in accordance with the appearance of footpoints at high energies, and is well correlated with the radio emission. The energy content of the thermal component keeps increasing…
We investigate the formation and magnetic topology of four flare/CME events with filament-sigmoid systems, in which the sigmoidal hot channels are located above the filaments, and they appear in pairs prior to eruption.} The formation of hot channels usually takes several to dozens of hours during which two J-shape sheared arcades gradually evolve into sigmoidal hot channels, then they keep stable for tens of minutes or hours and erupt. While the low-lying filaments show no significant change. \textbf{We construct a series of magnetic field models and find that the best-fit preflare models con…
Ellerman bombs are transient brightenings of the wings of the Balmer lines that uniquely mark reconnection in the solar photosphere. They are also bright in strong Ca II and ultraviolet lines and in ultraviolet continua, but they are not visible in the optical continuum and the Na I D and Mg I b lines. These discordant visibilities invalidate all published Ellerman bomb modeling. I argue that the assumption of Saha-Boltzmann lower-level populations is informative to estimate bomb-onset opacities for these diverse diagnostics, even and especially for H-alpha, and employ such estimates to gauge …
We present a multi-wavelength analysis of the eruption of a hot coronal channel associated with an X1.0 flare (SOL2013-10-28T02:03) from the active region NOAA 11875 by combining observations from AIA/SDO, HMI/SDO, RHESSI, and HiRAS. EUV images at high coronal temperatures indicated the presence of a hot channel at the core of the active region from the early pre-flare phase evidencing the pre-existence of a quasi-stable magnetic flux rope. The hot channel underwent an activation phase after a localized and prolonged pre-flare event occurring adjacent to one of its footpoints. Subsequently, th…
Multi-Wavelength Diagnostics of Pre-Flare Evolution with Aditya-L1: From the Solar Chromosphere to the Corona
Manipal Academy of Higher Education（马尼帕尔高等教育学院）；Physical Research Laboratory（物理研究所）；U R Rao Satellite Centre（U R Rao卫星中心）；Centre for Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata（印度空间科学卓越中心，加尔各答科学与研究教育研究所）；Inter-University Centre for Astronomy and Astrophysics（大学间天文与天体物理中心）；Sri Dharmasthala Manjunatheshwara College (Autonomous)（斯里达尔马斯塔拉曼朱纳特什瓦拉学院（自治））
In this Letter, we study the kinematic properties of ascending hot blobs associated with confined flares. Taking advantage of high-cadence extreme-ultraviolet images provided by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we find that for the 26 events selected here, the hot blobs are first impulsively accelerated outward, but then quickly slow down to motionlessness. Their velocity evolution is basically synchronous with the temporal variation of the Geostationary Operational Environmental Satellite soft X-ray flux of the associated flares, except that the veloci…
Before the onset of a flare is observed in hard X-rays there is often a prolonged pre-flare or pre-heating phase with no detectable hard X-ray emission but pronounced soft X-ray emission suggesting that energy is being released and deposited into the corona and chromosphere already at this stage. This work analyses the temporal evolution of coronal source heating and the chromospheric response during this pre-heating phase to investigate the origin and nature of early energy release and transport during a solar flare. Simultaneous X-ray, EUV, and microwave observations of a well observed flare…
In this study, we investigate motions in the hot plasma above the flare loops during the 2017 September 10 X8.2 flare event. We examine the region to the south of the main flare arcade, where there is data from the Interface Region Imaging Spectrograph (IRIS), and the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. We find that there are initial blue shifts of 20--60 km/s observed in this region in the Fe XXI line in IRIS and the Fe XXIV line in EIS, and that the locations of these blue shifts move southward along the arcade over the course of about 10 min. The cadence of IRIS allows…
In this article, we present the application of the weighted horizontal gradient of magnetic field (WGM) flare prediction method to 3-dimensional (3D) extrapolated magnetic configurations of 13 flaring solar active regions (ARs). The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WGM method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential …
To elucidate the flare trigger mechanism, we have analyzed several flare events which were observed by Hinode/Solar Optical Telescope (SOT), in our previous study. Because of the limitation of SOT field of view, however, only four events in the Hinode data sets have been utilizable. Therefore, increasing the number of events is required for evaluating the flare trigger models. We investigated the applicability of data obtained by the Solar Dynamics Observatory (SDO) to increase the data sample for a statistical analysis of the flare trigger process. SDO regularly observes the full disk of the …
A good observation of preflare activities is important for us to understand the origin and triggering mechanism of solar flares, and to predict the occurrence of solar flares. This work presents the characteristics of microwave spectral fine structures as preflare activities of four solar flares observed by Ondřejov radio spectrograph in the frequency range of 0.8--2.0 GHz. We found that these microwave bursts which occurred 1--4 minutes before the onset of flares have spectral fine structures with relatively weak intensities and very short timescales. They include microwave quasi-periodic pul…
A M-class behind-the-limb solar flare on 1 May 2013 (SOL2013-05-01T02:32), accompanied by a ($\sim$ 400 km/s) CME was observed by several space-based observatories with different viewing angles. We investigated the RHESSI-observed occulted hard X-ray emissions that originated at least 0.1 \solrad{} above the flare site. Emissions below $\sim$10 keV revealed a hot, extended (11 MK, >60 arcsec) thermal source from the escaping CME core, with densities around $10^{9}$ cm$^{-3}$. In such a tenuous hot plasma, ionization times scales are several minutes, consistent with the non-detection of the hot…
We use RHESSI high-resolution imaging and spectroscopy observations from ~6 to 100 keV to determine the statistical relationships between measured parameters (temperature, emission measure, etc.) of hot, thermal plasma in 37 intense (GOES M- and X-class) solar flares. The RHESSI data, most sensitive to the hottest flare plasmas, reveal a strong correlation between the maximum achieved temperature and the flare GOES class, such that "super-hot" temperatures >30 MK are achieved almost exclusively by X-class events; the observed correlation differs significantly from that of GOES-derived temperat…
Using high-resolution images from 1.6 m New Solar Telescope (NST) at Big Bear Solar Observatory (BBSO), we report the direct evidence of chromospheric reconnection at the polarity inversion line (PIL) between two small opposite polarity sunspots. Small jet-like structures (with velocities of ~20-55 km/s) were observed at the reconnection site before the onset of the first M1.0 flare. The slow rise of untwisting jets was followed by the onset of cool plasma inflow (~10 km/s) at the reconnection site, causing the onset of a two-ribbon flare. The reconnection between two sheared J-shaped cool H$α…
Solar flares are generally believed to be powered by free magnetic energy stored in the corona, but the build up of coronal energy alone may be insufficient for the imminent flare occurrence. The flare onset mechanism is a critical but less understood problem, insights into which could be gained from small-scale energy releases known as precursors, which are observed as small pre-flare brightenings in various wavelengths, and also from certain small-scale magnetic configurations such as the opposite polarity fluxes, where magnetic orientation of small bipoles is opposite to that of the ambient…
