Using RHESSI data, we have analyzed some 172 hard X-ray peaks during 53 solar flares which exhibited a double-footpoint structure. Fitting both footpoints with power-laws, we find that spectral index differences range mostly between 0 to 0.6, and only rarely go beyond. Asymmetries between footpoints were not observed to be significantly dependent on their mean heliographic position, their relative position with respect to each other, nor their orientation with respect to the solar equator. Assuming a symmetric acceleration process, it is also clear that differences in footpoint spectral indices and footpoint flux ratios can seldom be attributed to a difference in column densities between the two legs of a coronal loop. Our results corroborate better the magnetic mirror trap scenario. Moreover, footpoint asymmetries are more marked during times of peak HXR flux than when averaging over the whole HXR burst, suggesting that the magnetic configuration evolves during individual HXR bursts. We observed also a linear correlation between the peak 50-keV flux and the peak GOES 1-8A channel flux, and that HXR burst duration seem correlated with loop length.
We explore the temporal relationship between microwave/HXR emission and Doppler velocity during the impulsive phase of a solar flare on 2014 October 27 (SOL2014-10-27), which displays a pulse on the light curves in microwave (34 GHz) and hard X-ray (HXR, 25-50 keV) bands before the flare maximum. Imaging observation shows that this pulse mainly comes from one footpoint of a solar flare loop. The slit of Interface Region Imaging Spectrograph (IRIS) stays at this footpoint during this solar flare. The Doppler velocities of Fe XXI 1354.09 A and Si IV 1402.77 A are extracted from the Gaussian fitt…
We report on observations of conversion of bright filament strands into flare loops during 2012 August 31 filament eruption. Prior to the eruption, individual bright strands composing one of the legs of the filament were observed in the 171 A filter channel data of the Atmospheric Imaging Assembly. After the onset of the eruption, one of the hooked ribbons started to propagate and contract, sweeping footpoints of the bright filament strands as well as coronal loops located close by. Later on, hot flare loops appeared in regions swept by the hook, where the filament strands were rooted. Timing …
We study the early evolution of a hot-channel-like magnetic flux rope (MFR) toward eruption. Combining with imaging observation and magnetic field extrapolation, we find that the hot channel possibly originated from a pre-existing seed MFR with a hyperbolic flux tube (HFT). In the precursor phase, three-dimensional tether-cutting reconnection at the HFT is most likely resulting in the heating and buildup of the hot channel. In this process, the forming hot channel was rapidly enlarged at its spatial size and slipped its feet to two remote positions. Afterward, it instantly erupted outwards wit…
The 2014 March 29 X1 solar flare (SOL20140329T17:48) produced bright continuum emission in the far- and near-ultraviolet (NUV) and highly asymmetric chromospheric emission lines, providing long-sought constraints on the heating mechanisms of the lower atmosphere in solar flares. We analyze the continuum and emission line data from the Interface Region Imaging Spectrograph (IRIS) of the brightest flaring magnetic footpoints in this flare. We compare the NUV spectra of the brightest pixels to new radiative-hydrodynamic predictions calculated with the RADYN code using constraints on a nonthermal …
Existing 3D extensions to the standard model show that flux-rope footpoints are surrounded by curved-shaped QSL-footprints that can be related with hook-shaped flare-ribbons. We build upon this finding and further address the joint questions of their time-evolution, and of the formation of flare loops at the ends of flaring PILs of the erupting bipole, which are both relevant for flare understanding in general and for ICME studies in particular. We calculate QSLs and relevant field lines in an MHD simulation of a torus-unstable flux-rope. The evolving QSL footprints are used to define the oute…
What Causes the Asymmetry of Conjugate Hard X-Ray Footpoints in Solar Flares?
University of Science and Technology of China（中国科学技术大学）；Samarkand State University（撒马尔罕国立大学）；Institute of Nuclear Technologies, Samarkand State University（撒马尔罕国立大学核技术研究所）；University of Delhi（德里大学）；Polish Academy of Sciences（波兰科学院）
First time delay observation between two mid-infrared channels in solar flare footpoints
School of Physics & Astronomy, University of Glasgow（格拉斯哥大学物理与天文学学院）；School of Physics, Costa Rica Institute of Technology（哥斯达黎加理工学院物理学院）；Rosseland Centre for Solar Physics, University of Oslo（奥斯陆大学罗斯兰太阳物理中心）；Space Sciences Laboratory, University of California, Berkeley（加州大学伯克利分校空间科学实验室）；Center for Radio Astronomy and Astrophysics Mackenzie (CRAAM), Engineering School, Mackenzie Presbyterian University（麦肯锡长老会大学工程学院麦肯锡射电天体物理学中心）
Evidence for Solar Tether-cutting Magnetic Reconnection from Coronal Field Extrapolations
磁重联是引发太阳爆发事件的主要机制之一，但对其快速过程的直接观测一直是一个挑战。在这封信中，我们提出，使用非线性无力场（NLFFF）外推技术，可视化的场线连接的变化，在2011年2月13日在NOAA AR 11158的M6.6耀斑约30分钟的系绳切割重联。首先通过多波长观测收集了系绳切割重连的证据，然后通过分析从事件发生时观察到的四个明显的耀斑1700 A足点的位置追踪到的磁力线。就在耀斑之前，这四个足点非常靠近磁扭曲指数的局部最大值区域。特别地，来自内部两个足点的场线形成两个强扭曲的磁通束（高达~1.2匝），其彼此剪切并分别接近外部两个足点。耀斑发生后，足点周围区域的扭曲指数立即大大减小，上述磁力线变得低洼，扭曲程度降低（约0.6圈），由连接后来形成的两个耀斑带的环所覆盖。来自内部足点的大约10%的通量（~3x10^19 Mx）经历了足点交换。这部分光通量源自首先变亮的内部足点的边缘区域。从NLFFF外推推断的磁场连通性的这些快速变化与系留切割磁重联模型是一致的。
In this study we discuss the excitation of low frequency plasma waves in the lower-middle chromosphere region of loop footpoints for the case when the plasma can be considered in a pre-flare state. It is shown, that among the known semi-empirical models of the solar atmosphere, only the VAL (F) model together with a particular set of basic plasma parameters and amplitudes of the electric and magnetic fields supports generation of low frequency wave instability. Our results show that it is possible to predict the onset of the flare process in the active region by using the interaction of kineti…
Electric currents play a critical role in the triggering of solar flares and their evolution. The aim of the present paper is to test whether the surface electric current has a surface or subsurface fixed source as predicts the circuit approach of flare physics, or is the response of the surface magnetic field to the evolution of the coronal magnetic field as the MHD approach proposes. Out of all 19 X-class flares as observed by SDO from 2011 to 2016 near the disk center, we analyzed the only 9 eruptive flares for which clear ribbon-hooks were identifiable. Flare ribbons with hooks are conside…
Coronal rain is ubiquitous in flare loops, forming shortly after the onset of the solar flare. Rain is thought to be caused by a thermal instability, a localized runaway cooling of material in the corona. The models that demonstrate this require extremely long duration heating on the order of the radiative cooling time, localized near the footpoints of the loops. In flares, electron beams are thought to be the primary energy transport mechanism, driving strong footpoint heating during the impulsive phase that causes evaporation, filling and heating flare loops. Electron beams, however, do not …
We analyze the imaging observations of an M-class eruptive flare of 2015 November, 4. The pre-eruptive H alpha filament was modelled by the non-linear force free field model, which showed that it consisted of two helical systems. Tether-cutting reconnection involving these two systems led to the formation of a hot sigmoidal loop structure rooted in a small hook that formed at the end of the flare ribbon. Subsequently, the hot loops started to slip away form the small hook until it disappeared. The loops continued slipping and the ribbon elongated itself by several tens of arc seconds. A new an…
We study the correlation between abrupt permanent changes of magnetic field during X-class flares observed by the GONG and HMI instruments, and the hard X-ray (HXR) emission observed by RHESSI, to relate the photospheric field changes to the coronal restructuring and investigate the origin of the field changes. We find that spatially the early RHESSI emission corresponds well to locations of the strong field changes. The field changes occur predominantly in the regions of strong magnetic field near the polarity inversion line (PIL). The later RHESSI emission does not correspond to significant …
We present high-resolution and multi-line observations of a C2-class solar flare, occurred in NOAA AR 12740 on May 6, 2019. The rise, peak and decay phases of the flare were recorded continuously and quasi-simultaneously in the Ca II K line with the CHROMIS instrument, the Ca II 8542 and Fe I 6173 Ålines with the CRISP instrument at the SST. The observations in the chromospheric Ca II lines exhibit intense brightening near the flare footpoints. At these locations, a non-LTE inversion code was employed to infer the temperature, magnetic field, line-of-sight (LOS) velocity and microturbulent vel…
Multi-point study of the energy release and transport in the 28 March 2022, M4-flare using STIX, EUI, and AIA during the first Solar Orbiter nominal mission perihelion
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 chromos…
The acceleration and transport of energetic electrons during solar flares is one of the outstanding topics in solar physics. Recent X-ray and radio imaging and spectroscopy observations have provided diagnostics of the distribution of nonthermal electrons and suggested that, in certain flare events, electrons are primarily accelerated in the loop-top and likely experience trapping and/or scattering effects. By combining the focused particle transport equation with magnetohydrodynamic (MHD) simulations of solar flares, we present a macroscopic particle model that naturally incorporates electron…
Quadrupole White-light Sources in an X1.2 Flare Observed by ASO-S/LST/WST and SDO/HMI
Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, CAS（中国科学院紫金山天文台暗物质与空间天文重点实验室）；School of Astronomy and Space Science, University of Science and Technology of China（中国科学技术大学天文与空间科学学院）
Context. Magnetic reconnection during flares takes place in the corona but a substantial part of flare energy is deposited in the chromosphere. However, high-resolution spectro-polarimetric chromospheric observations of flares are very rare. The most used observables are Ca II 8542 Å and He I 10830 Å. Aims. We aim to study the chromosphere during a C3.6 class flare via spectro-polarimetric observations of the He I D3 line. Methods. We present the first SST/CRISP spectro-polarimetric observations of He I D3. We analyze the data using the inversion code HAZEL, and estimate the LOS velocity and t…
We present different signatures of chromospheric evaporation in two solar flares observed by the Interface Region Imaging Spectrograph (IRIS). In the B1.6 flare on 2016 December 6 (SOL2016-12-06T10:40), the transition region Si IV line and the chromospheric C II and Mg II lines show blueshifts with low velocities up to 20 km/s at the flare loop footpoints in the rise phase, indicative of a gentle chromospheric evaporation. While in the C1.6 flare on 2015 December 19 (SOL2015-12-19T10:51), the Si IV, C II, and Mg II lines exhibit redshifts with velocities from several to tens of km/s at the foo…
We present new results from the Interface Region Imaging Spectrograph showing the dynamic evolution of chromospheric evaporation and condensation in a flare ribbon, with the highest temporal and spatial resolution to date. IRIS observed the entire impulsive phase of the X-class flare SOL2014-09-10T17:45 using a 9.4 second cadence `sit-and-stare' mode. As the ribbon brightened successively at new positions along the slit, a unique impulsive phase evolution was observed for many tens of individual pixels in both coronal and chromospheric lines. Each activation of a new footpoint displays the sam…
We explored simultaneous observations of chromospheric evaporation and condensation during the impulsive phase of a C6.7 flare on 9 May 2019. The solar flare was simultaneously observed by multiple instruments, i.e., the New Vacuum Solar Telescope (NVST), the Interface Region Imaging Spectrograph, the Atmospheric Imaging Assembly (AIA), the Fermi, the Mingantu Spectral Radioheliograph, and the Nobeyama Radio Polarimeters. Using the single Gaussian fitting and the moment analysis technique, redshifted velocities at slow speeds of 15-19 km/s are found in the cool lines of C II and Si IV at one f…
We present the first observation of a solar filament formed by magnetic reconnection and associated chromospheric evaporation and subsequent coronal condensation. Driven by shearing motion during flux emergence, a sequential tether-cutting reconnection processes occurred and resulted in an M1.3 confined flare accompany with the formation of a sigmoid structure. It is found that the flare had a conjugate compact footpoint brightenings, which correspond to the footpoints of the sigmoid. Furthermore, observational evidence of explosive evaporation is well diagnosed at the conjugate footpoint brig…
Solar flares signify the sudden release of magnetic energy and are sources of so called space weather. The fine structures (below 500 km) of flares are rarely observed and are accessible to only a few instruments world-wide. Here we present observation of a solar flare using exceptionally high resolution images from the 1.6~m New Solar Telescope (NST) equipped with high order adaptive optics at Big Bear Solar Observatory (BBSO). The observation reveals the process of the flare in unprecedented detail, including the flare ribbon propagating across the sunspots, coronal rain (made of condensing …
Infrared spectropolarimetry of a C-class solar flare footpoint plasma -- I. Spectral features and forward modelling
Astronomical Institute, Slovak Academy of Sciences（斯洛伐克科学院天文研究所）；Evgeni Kharadze Georgian National Astrophysical Observatory（叶夫根尼·哈拉泽格鲁吉亚国家天文台）；Space Research Centre, Ilia State University（伊利亚州立大学太空研究中心）；Instituto de Astrofísica de Canarias (IAC)（加那利群岛天体物理学研究所）；Departamento de Astrofísica, Universidad de La Laguna（拉帕拉纳大学天体物理学系）；AG, Institute of Physics, University of Graz（格拉茨大学物理研究所）；National Solar Observatory（美国国家太阳观测台）
This paper reports on the re-analysis of solar flares in which the hard X-rays (HXRs) come predominantly from the corona rather than from the more usual chromospheric footpoints. All of the 26 previously analyzed event time intervals, over 13 flares, are re-examined for consistency with a flare model in which electrons are accelerated near the top of a magnetic loop that has a sufficiently high density to stop most of the electrons by Coulomb collisions before they can reach the footpoints. Of particular importance in the previous analysis was the finding that the length of the coronal HXR sou…
The Hydrogen Balmer series is a basic radiative loss channel from the flaring solar chromosphere. We report here on the analysis of an extremely rare set of simultaneous observations of a solar flare in the H$α$ and H$β$ lines at high spatial and temporal resolution, which were acquired at the Dunn Solar Telescope. Images of the C3.3 flare (SOL2014-04-22T15:22) made at various wavelengths along the H$α$ line profile by the Interferometric Bidimensional Spectrometer (IBIS) and in the H$β$ with the Rapid Oscillations in the Solar Atmosphere (ROSA) broadband imager are analyzed to obtain the inte…
We report decaying quasi-periodic intensity oscillations in the X-ray (6-12 keV) and extreme ultraviolet (EUV) channels (131, 94, 1600, 304 Å) observed by the Fermi GBM (Gamma-ray Burst Monitor) and SDO/AIA, respectively, during a C-class flare. The estimated period of oscillation and decay time in the X-ray channel (6-12 keV) was about 202 s and 154 s, respectively. A similar oscillation period was detected at the footpoint of the arcade loops in the AIA 1600 and 304 Åchannels. Simultaneously, AIA hot channels (94 and 131 Å) reveal propagating EUV disturbances bouncing back and forth between …
Derivation of Stochastic Acceleration Model Characteristics for Solar Flares From RHESSI Hard X-Ray Observations
湍流对粒子的随机加速模型已成功地解释了太阳耀斑的许多观测特征。本文提出了一种利用RHESSI（RavenRamaty High Energy Solar Spectroscopic Imager）提供的高分辨率硬X射线观测数据获得加速电子能谱和重要加速模型参数的新方法。在我们的模型中，电子加速或非常接近环顶部产生薄的目标韧致辐射，然后向下逃逸产生厚的目标发射在循环的足点。基于RHESSI计数粒子的正则化反演得到的电子通量谱图像，我们导出了加速模型的几个重要参数。我们将此方法应用于2003年11月03日的太阳耀斑，它显示了一个环顶源高达100- 150千电子伏的硬X射线与一个相对平坦的频谱，除了两个脚点源。结果表明，存在强散射和湍流能量的高密度与陡峭的频谱中的加速区域。
Tracing magnetic flux rope footpoints in AR 12975 with coronal dimmings and data-driven magnetofrictional simulations
University of Helsinki（赫尔辛基大学）；University of Graz, Institute of Physics（格拉茨大学物理研究所）；University of Graz, Kanzelhöhe Observatory for Solar and Environmental Research（格拉茨大学坎泽勒霍厄太阳与环境研究天文台）；NorthWest Research Associates（西北研究协会）
We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 km s$^{-1}$ and 200 km s$^{-1}$. Hard X-ray (HXR) footpoints were observed by RHESSI for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon b…
Quasi-periodic pulsations and three-dimensional magnetic reconnection during 2022 March 31 flare observed by IRIS & STIX
Bay Area Environmental Research Institute（湾区环境研究所）；Lockheed Martin Solar & Astrophysics Laboratory（洛马太阳能与天体物理实验室）；University of Applied Sciences and Arts Northwestern Switzerland (FHNW)（西北瑞士应用科学与艺术大学）；ETH Zürich（苏黎世联邦理工学院）；Oregon State University（俄勒冈州立大学）；Dublin Institute for Advanced Studies（都柏林高等研究院）；Southwest Research Institute（西南研究院）；SETI Institute（搜寻地外文明研究所）
Observations of Post-flare Plasma Dynamics during an M1.0 Flare in AR11093 by SDO/AIA
我们使用SDO/AIA 304 μ m和171 μ m滤波器观测了2010年8月7日AR 11093中一个M1.0级双带状耀斑衰变阶段演化的多股耀斑后环系统中冷热等离子体的运动。观察到了多温度下环系统中的运动光强特征及其反射光强特征。所观察到的热等离子体的对应物，可能包围冷约束等离子体，相对较快地移动（$\sim$ 34 km s $^{-1}$）到后者（29 km s $^{-1}$）的形式的扩展强度功能。传播的等离子体和强度从环的另一个足点的区域反射。等离子体运动的亚声速和相关的强度特征很可能是在耀斑后环系统中通过脉冲耀斑加热过程演化出来的。补充我们的意见，移动多温度的强度功能后耀斑环系统及其反射，我们还试图解决二维理想磁流体动力学方程数值模拟使用VAL-IIIC大气作为初始条件来模拟所观察到的等离子体动力学。我们考虑耀斑过程中在环系统的一个足点附近脉冲产生的局部热脉冲，它沿太阳色球层的磁场线发射。脉冲在沿该环系统移动时在较高高度处变陡成缓慢冲击，这触发了密切显示所观察到的等离子体动力学的等离子体扰动。
