Loop top nonthermal emission sources associated with an over-the-limb flare observed with NoRH and RHESSI

We studied the M3.7 class flare which occurred on 2005 July 27, in the active region NOAA 10792. This flare is an over-the-limb flare, and the footpoints are entirely occulted by the solar disk. The microwave and the hard X-ray images obtained with the Nobeyama Radioheliograph and the RHESSI satellite, respectively, clearly showed emission sources above the post-flare loop system. We examined the emission sources in detail spatially, temporally, and spectroscopically. As a result, one of the hard X-ray emission sources and the microwave emission source are nonthermal.     

Evidence for a solar coronal thick-target hard X-ray source observed by RHESSI

We study a solar flare hard X-ray (HXR) source observed by the Reuven Ramaty high energy solar spectroscopic imager (RHESSI) in which the HXR emission is almost entirely in a coronal loop so dense as to be collisionally thick at electron energies up to ∼45−60 keV. This contrasts with most events previously reported in which the HXR emission is primarily from the loop footpoints in the collisionally dense chromosphere. In particular, we show that the high loop column densities inferred from the GOES and RHESSI soft X-ray emission measure and the volume of the flare loop are consistent with the coronal thick-target interpretation of the HXR images and spectra. The high column densities observed already at the very beginning of the impulsive phase are explained by chromospheric evaporation during a preflare which, as Nobeyama 17 GHz radio images reveal, took place in the same set of nested loops as the main flare.     

日面耀斑环中物质的运动

利用云南天文台1980年7月14日3B级双带耀斑的光学观测资料,以及SMM卫星对同一耀斑的X射线观测结果,讨论日面耀斑环中物质的运动规律。先比较耀斑Hα象和X射线象的日面位置,根据投影效应确定耀斑环的高度;然后从理论上估算由于耀斑环中物质下落,所形成的耀斑活动区视向速度的分布。所得结果与观测资料基本相符。      

Reorganization of the solar corona following a C4.7 flare

Yohkoh X-ray images, multifrequency two-dimentional observations of the Nancay Radioheliograph, Kitt Peak and Mees magnetograms provide a unique set of data with which to study a C4.7 long-duration flare that was observed close to the equator (S07, W11) on 25 Oct. 1994 at 09:49 UT. Linear force-free field extrapolations indicate a very high degree of non-potentiality in the active region. The X-ray flare started with the expansion of spectacular twisted loops. Fifteen minutes after the flare onset sporadic radio (type III) bursts were observed spreading over an area of almost 1/3 of the solar disc and two remote X-ray brightenings appeared over quiet regions of opposite magnetic polarity located in on opposite hemispheres of the Sun. In the close vicinity of these remote brightenings two coronal holes formed. The timing and location of these events combined with the overall magnetic configuration provide evidence for a large-scale magnetic reconnection occurring between the expanding twisted loops and the overlying huge loops which inter-connect quiet solar regions.     

Imaging of reconnection processes in hard X-rays

The Hard X-ray Spectrometer aboard the SMM detected several events of energy release late in the development of two-ribbon flares. One such event, at 21:12 UT on 21 May, 1980 (～ 20 min after the flare onset and 15 min after the peak of the impulsive phase) is studied in detail. The site of new brightening first became visible in hard X-rays (> 22 keV) and only afterwards showed up at lower energies. It was clearly located high in the corona so that one can identify it with energy release at the tops of newly formed post-flare loops. Thus, if the Kopp and Pneuman model of the loop formation is adopted, we may have imaged here a reconnection process in the solar corona. An attempt is made to estimate physical parameters at the reconnection site.     

耀斑后环珥系速度场的定量研究

本文研究了在太阳重力、偶极磁压强梯度和气体压力梯度联合作用下环珥系内物质的下落运动,并在等温、定常假设下用数值方法计算了该环系的二维视向速度场。通过将速度场的理论计算跟文[1]中给出的观测速度场拟合,导出了该环系的有关参数及分布。计算表明环系内物质密度和磁场强度对物质下落运动影响较显著。     

双带耀斑磁重联过程的数值研究

本文用多步隐格式求解包含电阻的磁流体力学方程组, 对双带耀斑的主相作数值模拟, 清晰地展示了中性片区由撕裂模线性重联向准稳态重联的过渡以及后随耀斑环的产生和等离子体团的喷发过程.对于在能量方程中计及和忽略焦耳加热两种情况, 分别作了计算.结果表明, 计及焦耳加热时, 电流片中等离子体的温度显著增加(是初始温度的2—3倍), 但等离子体的运动速度却变化不大.两种情况的计算结果均表明:等离子体的运动速度低于声速, 因此不会形成快激波.计及焦耳加热的计算结果显示了两个新的特征:其一是中性片高密度等离子体的受热膨胀, 增大了电流片的有效厚度, 它使重联速率降低, 并逐渐趋于饱和, 其二是同时形成上升和沉降等离子体团, 后者与耀斑环碰撞, 并合并于后随耀斑环内.      

Post-flare thermal waves in the solar corona

While imaging giant post-flare arches in the solar corona, the Hard X-Ray Spectrometer aboard the SMM detected thermal disturbances propagating through the corona after two-ribbon flares. The speed of propagation is close to, or below, 10 km s^?1, and no obvious time-variation of the speed is indicated in the HXIS data. For subsequent two-ribbon flares in the same active region, these thermal disturbances (waves) exhibit highly homologous properties; thus the waves appear to propagate through preexisting arches formed after earlier flares. Temperatures of > 20 × 10⁶ K have been detected in these moving phenomena. We suggest that we see here in X-rays upper products of the consecutive reconnections which create the post-flare loops below. Temperature maps in fine field of view of HXIS offer now a new possibility to detect postflare arches in the corona built during two-ribbon flares.     

Asymmetric precipitation in a coronal loop as explanation of a singular observed spectrum

Almost 10 years of solar submillimeter observations have shown new aspects of solar activity, such as the presence of rapid solar spikes associated with the launch of coronal mass ejections and an increasing submillimeter spectral component in flares. We analyse the singular microwave–submillimeter spectrum of an M class solar flare on 20 December, 2002. Flux density observations measured by Sun patrol telescopes and the Solar Submillimeter Telescope are used to build the radio spectrum, which is fitted using Ramaty’s code. At submillimeter frequencies the spectrum shows a component different from the microwave classical burst. The fitting is achieved proposing two homogeneous sources of emission. This theoretical fitting is in agreement with differential precipitation through a magnetically asymmetric loop or set of loops. From a coronal magnetic field model we infer an asymmetric magnetic structure at the flare location. The model proposed to quantify the differential precipitation rates due to the asymmetry results in a total precipitation ratio Q₂/Q₁≈10⁴–10⁵, where Q₁(Q₂) represents the total precipitation in the loop foot with the high (low) magnetic field intensity. This ratio agrees with the electron total number ratio of the two sources proposed to fit the radio spectrum.     

What can we learn about accelerated electrons in coronal loops from analysis of solar type IV radio bursts?

Specific type IV radio burst with fine structure – quasi-periodic broadband pulsations (BBPs) and zebra pattern (ZP), recorded by OSRA spectrograph (Potsdam) on October 25, 1994, is considered as a source of information about electrons accelerated during solar flare. The type IV bursts are generated in coronal trap-like structures (coronal loops) accumulating accelerated electrons. BBP are considered as a result of periodically repeated injections of fast electrons into the magnetic trap. The ZP is well understood as a result of plasma wave instability at the levels of double plasma resonance. Using the observed features of fine structure, we found the number of electrons and energy of electrons capable to provide the observed structure as well as the physical conditions in the coronal magnetic loop.     

1984年5月21日耀斑环珥的Hα-SSHG观测研究

利用云南天文台太阳光谱分光照相仪(Hα-SSHG),拍摄了1984年5月21日边缘耀斑环珥的Hα光谱资料,结合Hα色球观测资料给出了该事件的发展过程和色球、光谱的形态特征。定性地探讨了该耀斑环内的宏观物质运动.用提出的双环系模型能较好地解释观测结果.      

Converging motion of conjugate flaring kernels during two large solar flares

In this paper, we analyze the footpoint motion of two large solar flares using observations made by the Transition Region and Coronal Explorer (TRACE) and Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). The two flares are the M5.7 flare of March 14, 2002 and the X10 flare of October 29, 2003. They are both classical two-ribbon flares as observed in TRACE 1600 or 171 Å images and have long-duration conjugate hard X-ray (HXR) footpoint emission. We use the ‘center-of-mass’ method to locate the centroids of the UV/EUV flare ribbons. The results are: (1) The conjugate UV/EUV ribbons and HXR footpoints of the two flares show a converging (inward) motion during the impulsive phase. For the two flares, the converging motion lasts about 3 and 10 min, respectively. The usual separation (outward) motion for the flare ribbons and footpoints take place only after the converging motion. (2) During the inward and the outward motion, the conjugate ribbons and footpoints of the two events exhibit a strong unshear motion. In obtaining above results, TRACE UV/EUV and RHESSI HXR data show an overall agreement. The two events demonstrate that the magnetic reconnection for the flares occurs in highly sheared magnetic field. Furthermore, the results support the magnetic model constructed by Ji et al. [Ji, H., Huang, G., Wang, H. Astrophys. J. 660, 893–900, 2007], who proposed that the contracting motion of flaring loops is the signature of the relaxation of sheared magnetic fields.     

The 1980 April 12 flare and transient: Report on progress in interpretation

A multidisciplinary study of this solar-interplanetary event is summarized by two main points: this flare was an incident in a process that began days before the flare, and continued after the flare; and the chain of events can be interpreted most simply in terms of energy input over scales of time and space that are large compared to the flare seen in the light of Hα. In support of these points, 5 aspects of the flare are described here: (1) hours before the flare, slow changes in coronal structure were associated with radio continuum emission, suggesting large-scale magnetic-field changes and the presence of energetic electrons; (2) long-lived X-ray loops require sustained energy input for at least an hour after the flare start; (3) interplanetary disturbance near earth is probably related to this limb flare, although the (expected) absence of a shock makes identification uncertain; (4) the coronal mass ejection overlay decaying magnetic field; (5) speed derived from frequency drift of the type II radio burst in the low corona, and from the travel time of the disturbance to 1 a.u., are about twice as great as the observed speed of the coronal mass ejection and of the disturbed solar-wind speed.     

Preflare heating of filaments

Disappearances of preflare filaments have been observed on June 22, 1980 (S07,W13) (flare at 13.04 U.T.) and September 3, 1980 (flare at 7.52 U.T.). The analysis of MSDP data (MEUDON) leads to the followings conclusions : - Disappearances begin a few minutes before the Hα impulsive phase. - The filaments can be seen again after the flares. - Upwards motions occur in several points, without disturbing significantly preëxisting downflows.Velocity maps suggest shears or velocity loops.The filament disappearance seems to be due to a heating mechanism beginning before the flare maximum.     

Origin and location of chromospheric evaporation in flares

Observation of two flares obtained with the Solar Maximum Mission spectrometers indicate that at flare onset the emission in soft (3.5 – 8 keV) and hard (16 – 30 keV) X-rays is predominant at the footpoints of the flaring loops. Since, at the same time, blue-shifts are observed in the soft X-ray spectra from the plasma at temperature of 10⁷ K, we infer that material is injected at high velocity into the coronal loops from the footpoints. These areas are also the sites of energy deposition, since their emission in hard X-rays is due to non-thermal electrons penetrating in the denser atmosphere. Hence, chromospheric evaporation occurs where energy is deposited. During the impulsive phase, the configuration of the flare region changes indicating that the flaring loop is progressively filled by hot plasma.     

Physics of ion acceleration in the solar flare on 2005 September 7 determines γ-ray and neutron production

Relativistic neutrons were observed by the neutron monitors at Mt. Chacaltaya and Mexico City and by the solar neutron telescopes at Chacaltaya and Mt. Sierra Negra in association with an X17.0 flare on 2005 September 7. The neutron signal continued for more than 20 min with high statistical significance. Intense emissions of γ-rays were also registered by INTEGRAL, and during the decay phase by RHESSI. We analyzed these data using the solar-flare magnetic-loop transport and interaction model of Hua et al. [Hua, X.-M., Kozlovsky, B., Lingenfelter, R.E. et al. Angular and energy-dependent neutron emission from solar flare magnetic loops, Astrophys. J. Suppl. Ser. 140, 563–579, 2002], and found that the model could successfully fit the data with intermediate values of loop magnetic convergence and pitch-angle scattering parameters. These results indicate that solar neutrons were produced at the same time as the γ-ray line emission and that ions were continuously accelerated at the emission site.     

1980年11月6日—7日耀斑后冕拱的激活机制

1980年11月6日耀斑后冕拱(Post-flare coronal arch)在母耀斑(AR2779)开始后3小时形成, 并在形成后11小时和25小时两次激活。两次激活均由双带耀斑的增长环系所致。本文提出了激波加热和Petschek重连是该冕拱有效的激活机制。导出并求解了考虑辐射损失、热传导、激波加热和Petschek重连加热的冕拱能量方程。理论计算结果与Svestka根据SMM空间资料所给出的该冕拱的激活曲线基本符合。      

An investigation of small GOES flares with intense hard X-ray bursts

Most solar flare observations show that intense hard X-ray bursts come from large flares that have a large GOES classification (large peak 1 – 8 Å flux). This correlation, known as the “Big Flare Syndrome”, suggests that more intense flares tend to have harder spectra. We have observed 7 flares that are exceptions to this. These flares have small GOES classifications ranging from B1.4 to C5.5 and peak hard X-ray count rates similar to those often observed from M class flares. This paper examines the cause of this anomoly using the Yohkoh Soft X-Ray Telescope, Hard X-Ray Telescope, and Bragg Crystal Spectrometer. Two hypotheses are proposed for the exceptions: (1) flares with multiple magnetic loops and common footpoints, producing multiple hard X-ray emission regions and low density thermal plasma distributed over a large volume, and (2) high densities in the magnetic loops restricting the propagation of the non-thermal electrons in the loop after magnetic reconnection has occurred and suppressing chromospheric evaporation. Two of the flares support the first hypothesis. The other flares either have data missing or are too small to be properly analysed by the Yohkoh instruments.     

Flare build-up study — Homologous flares group — Part I

According to the definition of the homology (optical) kept in reference, the homologous flares (HFl) may be observed wherever flares occur. The supposed supplementary preflaring conditions to have HFl may be either that the preflaring conditions have not been destroyed by the first flare (and then what mechanism has stopped the first and triggered the second ?) or that the preflaring conditions have been destroyed and rebuilt (and then, how ?). The analysis of data related to some selected active regions AR by the members of the working group, and the earlier works on HFl, may be used simultaneously to investigate the differences between one set of HFl and the others, the location of their sites and the evolution of HFl productive AR. This study brings the appearance of new footpoints from one flare to the following, the behaviour of cool arches (surging arches) leading to information on the changes of the magnetic configuration, and to peculiar characteristics of HFl, oif 2nd, 3rd in the time order concerning the chromospheric transition zone or coronal regions. The time delay between two consecutive homologous flares appears very quickly as an essential parameter to study homology. It was found that every set of flares (same type of site) is able to produce “rafales” of homologous flares, i.e. two, three, four, oir more flares with Δt in the range of one hour or less. The observations show no great chantes in macroscopic photospheric patterns (B, V) during this H flaring period. They lead to compare their temporal variation curves of flare brightness. A quantitative brightness parameter of homology relation has been defined. Some scale changes have also been detected in the dynamic spectrum of the site, and it is in good agreement with a very interesting theoretical suggestion made by P. Sturrock to produce such “rafales”. It may be shown that the closely consecutive time-homologous flares (CCHF) or “rafales” represent a good tool to analyse the critical conditions related to the origin and the amount of energy, mechanism of storage and release, necessary and, perhaps, sufficient conditions. New statistical results, applied to the different selected homologous flare active regions are presented and show the existence in homologous flaring areas of a “pivot” of previous filaments interpreted as a signature of an anomaly in the Solar rotation.