Energy release and transport in arcade flares

This work is based on hard and soft X-ray observations from the YOHKOH satellite. We investigate an example of an arcade flare, for which the arcade channel is seen in soft X-rays as a long bright filament. We have found that:

1. (1) Energy can efficiently flow along the arcade channel from the very beginning of a flare.

2. (2) During flare evolution a few kernels of hard X-ray emission develop along the arcade channel. Clearly, they are new, additional sources of the flare energy release. A probable scheme of formation of such hard X-ray kernels is briefly discussed.

     

非均匀截面耀斑环中硬X射线辐射空间结构和高度分布

本文在非均匀截面耀斑环的模型下,分析了硬X射线辐射的空间结构和高度分布,讨论了A,B和C三类耀斑硬X射线源的形成和特征。结果表明:硬X射线源的结构和高度与耀斑环磁场结构,背景等离子体密度及注入的非热电子能谱等参数有密切关系.      

High-temperature phenomena in flares

High temperature phenomena occurring in solar flares are reviewed based on hard X-ray images and spectral analyses of highly ionized iron lines observed aboard the Hinotori spacecraft.Five basic flare components are proposed, i.e., impulsive (I), gradual hard (GH), thermal (T), quasi thermal (QT) and hot thermal (HT) components. A flare shows some combination of the five components. Energy release and transport for each component would give a lot of variety to the hard X-ray image, spectrum and time history of X-rays.     

Multi-wavelength study of a strong impulsive solar limb flare on 2002 August 3

We made a detailed study of the impulsive solar flare of GOES class X1.0 which occurred near the west limb on 2002 August 3, peak time 19:07 UT. There is particularly good data coverage of this event, with simultaneous observations in EUV, soft and hard X-rays available. We used TRACE 171 Å images to study the morphology and evolution of this event. Soft X-ray spectra in the wavelength range 3.34–6.05 Å measured by the RESIK Bragg crystal spectrometer on CORONAS-F were used for determination of the evolution of the flare plasma temperature. Data from the RHESSI instrument were used to investigate properties of the higher-temperature plasma during the flare.     

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.     

Structural development of the X-ray limb flare of 30 April 1980

We describe the development of the limb flare of 30 April 1980, 20:20 UT, as observed by the Hard X-ray Imaging Spectrometer (HXIS) aboard the Solar Maximum Mission (SMM). It consisted of a short-lived bright nucleus (FWHM < 10,000 km), just inside the Sun's limb; a longer lasting tongue, extending to a height of 30,000 km, and a more complicated feature, approximately situated at the Sun's limb. The tongue was a pre-existing magnetic structure that started emitting X-rays only a few seconds after the bright nucleus, and which had a slightly higher temperature than the nucleus; its X-ray emission may be caused by electrons escaped from the nucleus.     

A question raised from the observation of dynamic cusp formation: When and where does particle acceleration occur?

We present observations of a C9.4 flare on 2002 June 2 in EUV (TRACE) and X-rays (RHESSI). The multiwavelength data reveal: (1) the involvement of a quadrupole magnetic configuration; (2) loop expansion and ribbon motion in the pre-impulsive phase; (3) gradual formation of a new compact loop with a long cusp at the top during the impulsive phase of the flare; (4) appearance of a large, twisted loop above the cusp expanding outward immediately after the hard X-ray peak; and (5) X-ray emission observed only from the new compact loop and the cusp. In particular, the gradual formation of an EUV cusp feature is very clear. The observations also reveal the timing of the cusp formation and particle acceleration: most of the impulsive hard X-rays (>25 keV) were emitted before the cusp was seen. This suggests that fast reconnection occurred during the restructuring of the magnetic configuration, resulting in more efficient particle acceleration, while the reconnection slowed after the cusp was completely formed and the magnetic geometry was stabilized. This observation is consistent with the observations obtained with Yohkoh/Soft X-ray Telescope (SXT) that soft X-ray cusp structures only appear after the major impulsive energy release in solar flares. These observations have important implications for the modeling of magnetic reconnection and particle acceleration.     

Narrowband dm-spikes in the frequency range above 1 GHz and hard X-ray emission

Narrowband dm-spikes observed in nine intervals during five solar flares in the 1–2 GHz range were analyzed together with the RHESSI and HXRS observations. It was found that the over-frequency integrated radio flux during the spike period is closely related with the hard X-ray bursts (the correlation coefficient was 0.7–0.9) and their time delays after X-rays were 2–14 s, with one exception (March 18, 2003) where the time delay was opposite −15 s. Association of spikes with X-ray spectral characteristics enabled us to divide the spikes into two groups: (a) those observed before the soft X-ray flare maximum and, (b) those observed after this maximum. While for the spikes observed after the flare maximum no systematic spectral characteristics were found, the spikes, observed before the flare maximum were at their beginning associated with relatively hard X-ray spectra and their hardness decreased with time. The RHESSI X-ray sources were compact, only in the March 18, 2003 event an additional X-ray source appeared just at the time of the dm-spikes observation. Fourier transformation of the dynamic spectra of spikes was done to compare their dynamics with the X-ray spectral indices. No correlation between power-law spike and X-ray indices were found. It indicates that the MHD turbulence, if it plays a role, does not represent a strong connection between the spectral characteristics of the dm-spikes and associated X-ray bursts. Furthermore, the results were compared with those obtained by (Aschwanden, M.J., Güdel, M. The coevolution of decimetric millisecond spikes and hard X-ray emission during solar flares. Astrophys. J. 401, 736–753, 1992) for spikes observed on lower radio frequencies. Contrary to their results, no monotonic dependence between time delays and X-ray intensities were found. Finally, the results were discussed using the model of the narrowband dm-spikes and model of electron acceleration in the collapsing magnetic trap.     

太阳耀斑硬X射线能谱演变特征

太阳硬X射线是耀斑高能电子束流与太阳大气相互作用产生的韧致辐射,根据简单的太阳耀斑环物理模型,假定具有流量与能谱同步变化的高能电子束流从耀斑环顶部注入,计算了硬X射线辐射在不同的靶物质密度区的能谱演变特征。结果表明:硬X射线辐射在低大气密度靶区呈现软一硬一硬的能谱演变特征,在高密度靶区硬X射线能谱则具有软一硬一软的变化特征。高能电子束流持续时间影响谱型转变区域在耀斑环中的高度。      

Evidence of the radio-quiet hard X-ray precursor of the 13 December 2006 solar flare

We report multi-wavelength investigation of the pre-impulsive phase of the 13 December 2006 X-class solar flare. We use hard X-ray data from the anticoincidence system of spectrometer onboard INTEGRAL (ACS) jointly with soft X-ray data from the GOES-12 and Hinode satellites. Radio data are from Nobeyama and Learmonth solar observatories and from the Culgoora Solar Radio Spectrograph. The main finding of our analysis is a spiky increase of the ACS count rate accompanied by surprisingly gradual and weak growth of microwave emission and without detectable radio emission at meter and decimeter wavelengths about 10 min prior to the impulsive phase of the solar flare. At the time of this pre-flare hard X-ray burst the onset of the GOES soft X-ray event has been reported, positive derivative of the GOES soft X-ray flux started to rise and a bright spot has appeared in the images of the Hinode X-ray telescope (XRT) between the flare ribbons near the magnetic inversion line close to the sources of thermal and non-thermal hard X-ray emission observed by Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) during the flare. These facts we consider as evidences of solar origin of the increased pre-flare ACS count rate. We briefly discuss a possible cause of the pre-flare emission peculiarities.     

Coronal signatures of accelerated electrons

Whereas hard X-rays, microwaves and gamma-rays trace flare accelerated electrons and ions interacting in the lowcorona and the chromosphere, imaging and spectral radio observations in the decimetric-dekametric domain provide signatures of non thermal electrons in the middle and upper corona. These latter radio observations, combined with X-ray, EUV and optical measurements, contain unique information on the various circumstances of electron acceleration whether they are associated with flares or not. In this paper we outline the results of multiwavelength studies which provide: (i) information on the magnetic structure at various spatial scales into which flare accelerated electrons are injected/accelerated and (ii) evidence for various sites of electron acceleration outside flares which are located in the corona at altitudes ranging typically from 0.1 to 1 R above the photosphere.     

Spectroscopic analysis of the solar flare event on 2002 August 3 with the use of RHESSI and RESIK data

We use simultaneous observations from RESIK and RHESSI instruments to compare plasma properties of a major solar flare in its rise and gradual phase. This event occurred on 2002 August 3 (peak time at 19:06 UT). The flare had a very good coverage with RESIK data and well-resolved soft and hard X-ray sources were seen in RHESSI images. Spectra of X-ray radiation from RHESSI images are studied and compared with RESIK measurements in different flare phases. Result shows large differences in flare morphology and spectra between flare rise and gradual phase.     

The Neupert effect and new RHESSI measures of thetotal energy in electrons accelerated in solar flares

It is believed that a large fraction of the total energy released in a solar flare goes initially into acceleratedelectrons. These electrons generate the observed hard X-ray bremsstrahlung as they lose most of their energy by coulomb collisions in the lower corona and chromosphere. Results from the Solar Maximum Mission showed that there may be even more energy in accelerated electrons with energies above 25 keV than in the soft X-ray emitting thermal plasma. If this is the case, it is difficult to understand why the Neupert Effect — the empirical result that for many flares the time integral of the hard X-ray emission closely matches the temporal variation of the soft X-ray emission — is not more clearly observed in many flares. From recent studies, it appears that the fraction of the released energy going into accelerated electrons is lower, on average, for smaller flares than for larger flares. Also, from relative timing differences, about 25% of all flares are inconsistent with the Neupert Effect. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) is uniquely capable of investigating the Neupert Effec since it covers soft X-rays down to 3 keV (when both attenuators are out of the field of view) and hard X-rays with keV energy resolution, arcsecond-class angular resolution, and sub-second time resolution. When combined with the anticipated observations from the Soft X-ray Imager on the next GOES satellite, these observations will provide us with the ability to track the Neupert Effect in space and time and learn more about the relation between plasma heating and particle acceleration. The early results from RHESSI show that the electron spectrum extends down to as low as 10 keV in many flares, thus increasing the total energy estimates of the accelerated electrons by an order of magnitude or more compared with the SMM values. This combined with the possible effects of filling factors smaller than unity for the soft X-ray plasma suggest that there is significantly more energy in nonthermal electrons than in the soft X-ray emitting plasma in many flares.     

Hard X-ray and high-frequency decimetric radio observations of the 4 April 2002 solar flare

Hard X-ray and high frequency decimetric type III radio bursts have been observed in association with the soft X-raysolar flare (GOES class M 6.1) on 4 April 2002 (1532 UT). The flare apparently occurred 6 degrees behind the east limb of the Sun in the active region NOAA 9898. Hard X-ray spectra and images were obtained by the X-ray imager on RHESSI during the impulsive phase of the flare. The Brazilian Solar Spectroscope and Ondrejov Radio Telescopes recorded type III bursts in 800–1400 MHz range in association with the flare. The images of the 3–6, 6–12, 12–25, and 25–50 keV X-ray sources, obtained simultaneously by RHESSI during the early impulsive phase of the flare, show that all the four X-ray sources were essentially at the same location well above the limb of the Sun. During the early impulsive phase, the X-ray spectrum over 8–30 keV range was consistent with a power law with a negative exponent of 6. The radio spectra show drifting radio structures with emission in a relatively narrow (Δf ≤ 200 MHz) frequency range indicating injection of energetic electrons into a plasmoid which is slowly drifting upwards in the corona.     

Interpretation of hard X-ray images during the impulsive phase of a limb flare

We show that the observations of a limb flare, in which a hard X-ray (16–30 keV) source is seen at the boundary between two interacting magnetic structures, indicate the presence of hot (T ? 6 × 10⁷ K) plasma within the region. Non thermal bremsstrahlung processes do not agree with these observations. We discuss the possible causes of the heating.     

Flare buildup in X-rays,UV, microwaves and white light

Coordinated observations using space and ground-based instruments were made of active region complex #2522/2530, 24–30 June, 1980. The 10 largest flares from these regions were of importance M1-M6 in X-rays, and all were observed from satellites, except for one observed from a balloon. Several kinds of buildup signature have been found in the tens of minutes before these flares. Among these signatures are the following: 1) Relative faintness in X-ray lines of the pre-flare pixels, 2) X-ray (5–15 keV) “flashes” at points displaced by 1′–2′ from the flare site, 3) Rising filaments seen in Hα and Ultraviolet 4) Microwave intensification, polarization increase and polarization flip 5) Coronal disturbances above limb flares at or before the impulsive phase.     

1980年11月5日耀斑的硬、软X射线及微波的象与Hα等光度象的比较

本文对1980年11月5日22点25分开始的1B/M1-M4的Hα耀斑进行了图象处理,绘制了等光度图;与硬、软X射线象,微波象进行了比较.结果表明:1.耀斑的第一次极大,高能电子没有穿透到色球.Hα耀斑主要是由T=107—108K(产生软硬X射线的热区)等离子体向下传导到色球而形成.2.Hα耀斑的第二次极大,是由高能电子轰击色球而形成,Hα耀斑滞后数秒(小于5秒).3.耀斑闪光相,Hα面积与Hα强度同步增长.4.从耀斑前后的横向磁场变化(Hα短纤维的变化),估计磁能释放～1031尔格.      

RHESSI observation of the M4.0 flare on 17 March 2002

The M4.0/SF flare on 17 March 2002 is a good example of the early observations with RHESSI. We presenthard X-ray images, light curves and energy spectra of individual hard X-ray sources, the spatial relationship between the hard X-ray sources and the H emission regions, and comparisons of light curves observed by RHESSI and GOES. We found that the picture exhibited by RHESSI is consistent with the general cartoon of a solar flare. In particular, we showed that the hard X-ray image spectra could be explained by a power-law electron beam with a lower energy cutoff E_c. The derived E_c could be as high as 40 keV, larger than the usually value of 20 keV.     

On the effects of electron-cyclotron masers during flares

First recognized by Wu and Lee (Ap. J. 230, 621, 1979), electron-cyclotron masers can be activated under very mild conditions. Large growth rates can occur even for relatively mild anisotropies in the electron velocity distribution, e.g., the one-sided loss cones that commonly occur when electrons with small pitch angles precipitate into high density regions at the footpoints of flaring loops while others are reflected in the converging field in the corona. Maser action can plausibly occur at the second harmonic of the local gyrofrequency and so explain certain very bright (? 10¹⁰ K) microwave bursts from the sun and other stars. However, the preponderance of the energy is at the first harmonic.We suggest that masers operating at the local gyrofrequency in a flaring loop generate radiation at decimeter wavelengths that is a significant fraction of the total energy of the flare, in fact (and not coincidentally) comparable with the energy in electrons associated with hard X-ray bursts. Essentially all of the radio energy is trapped in the corona and serves to produce localized heating in a volume large compared with the energy release region. Thus it can transfer energy by radiation from one magnetic loop to another, possibly inducing further instabilities, and spreading the course of the flare. Eventually the energy probably escapes the corona as soft X-rays. The electron-cyclotron maser saturates by extracting the perpendicular energy of the electrons, thereby diffusing them into the loss cone at the maximum possible rate; the enhanced precipitation into the footpoints can produce bright emission in hard X-rays, EUV and Hα and remove any necessity for directive acceleration in the energy release region.Details of the proposed mechanism and effects are contained in two papers by Melrose and Dulk (Ap. J. 259, 1982).This work was sponsored by NASA under grants NAGW-91 and NSG-7287 to the University of Colorado.     

太阳耀斑显著的热和非热事件的统计特征

本文利用GOES卫星和SMM卫星软、硬X射线耀斑观测资料,分析耀斑中软、硬X射线辐射流量的分布,发现太阳耀斑存在着显著的热事件(PT事件)和显著的非热事件(PNT事件),它们主要特征是:(1)PT事件为缓变型耀斑,PNT事件为脉冲型耀斑;(2)PT事件的硬X射线谱较软,PNT事件能谱较硬;(3)PNT事件非热能量释放速率比PT事件快3—10倍;(4)耀斑发展趋缓慢,PT事件中软X射线峰值流量越大;(5)耀斑中PNT事件约占60％,PT事件约占40％.最后定性讨论了产生PT和PNT事件的可能机制.