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.     

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.

     

Distinctive spatial configuration of a class ofmicrowave flaring sources

We discuss a class of microwave flares whose source regions exhibit a distinctive spatial configuration; the primaryenergy release in these flares results from the interaction between emerging magnetic flux and an existing overlying region. Such events typically exhibit radio, X-ray and EUV emission at the main flare site (the site of interaction) and in addition radio emission at a remote site up to 1 × 10⁵ km away in another active region. We have identified and studied more than a dozen microwave flares in this class, in order to arrive at some general conclusions on reconnection and energy release in such solar flares. Typically, these flares show a gradual rise showing many subsidiary peaks in both radio and hard X-ray light curves with a quasi-oscillatory nature with periods of 5–6 seconds, a bright compact X-ray & EUV emitting loop in the main flare source, a delay of the radio emission from the remote source relative to the main X-ray-emitting source. The magnetic field in the main flare site changes sharply at the time of the flare, and the remote site appears to be magnetically connected to the main flare site.     

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.     

Relative positions of microwave and hard X-ray burst sources

Simultaneous microwave and hard X-ray imaging observations of 12 bursts show that it is difficult to discern a general pattern between microwave and hard X-ray burst locations. In general, the microwave source is displaced from the hard X-ray source. The commonly believed behavior of the microwave source being located near the top and hard X-ray source near the footpoints of a loop appears to be true in some cases but not all. If the burst source is simple, both may be located near loop tops. Sometimes when the hard X-ray source has two components, one weak and one strong, the microwave source is not located over a neutral line (loop top) but close to a sunspot where the magnetic field is strongest. It appears that more than one loop or arcade may sometimes be involved in the microwave and hard X-ray emission. This is particularly true when several interacting loops trigger the onset of a flare.     

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.      

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.     

X-ray and gamma-ray observations of a white-light flare

HEAO-1 observed hard radiations (X- and gamma-rays) from a major solar flare on 11 July 1978. The observations showed gamma-ray line and continuum emission extending to the highest energy observed. The lines are identified with the 2.2 MeV line of deuterium formation and the 4.4 MeV line of inelastic scattering on ¹²C, both previously observed in the flares of August 1972 [1]. The 11 July flare was identified as a white-light flare by observations at Debrecen [2]. It thus provides the first opportunity for a detailed examination of white-light flare theories that depend upon proton heating of the photosphere. The line strength over a four-minute integration at 2.2 MeV was 1.00 ± 0.29 ph(cm² sec)⁻¹, and the gamma-ray emission (excluding the 2.2 MeV line which was appreciably delayed) lagged by less than 20 sec approximately after the hard X-ray and microwave fluxes. We conclude that the “second-stage” acceleration of high-energy solar particles must commence promptly after the impulsive phase.     

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.      

On the motions of RHESSI flare footpoints

The footpoint motions of flare hard X-ray (HXR) sources are directly related to the reconnection scenario of a solar flare. In this work, we tried to extract the information of footpoint motions for a number of flares observed with RHESSI. We found that the RHESSI flare results of the footpoint motions strongly support the classification proposed from the observations of YOHKOH/HXT. Furthermore, it is found that a flare can consist of two types of footpoint motions. We discussed the connections of the footpoint motions with the two-dimensional reconnection models.     

Solar maximum mission experiment: Early results of the hard X-ray imaging experiment

We have selected four widely different flares from the early period of operations of the Hard X-Ray Imaging Spectrometer (HXIS) on SMM to illustrate the characteristic imaging properties of this experiment. For the small flare of April 4, 1980, we demonstrate the instrument's capability for locating a compact source. In the weak, but extensive, flare of April 6 we show how well the instrument can display spatial structure, and also the low level of the instrument background. In the 1B flare of April 7 we are able to locate positions of the X-ray emission in the soft and hard channels, and estimate the positional variations of the emission patches. Finally, in the IN flare of April 10, which produced the strongest hard X-ray burst we have seen so far, we repeat some of the studies made for the April 7 event, and also demonstrate the capability of the HXIS instrument to study the development, with high time resolution, of individual 8″ × 8″ elements of the flare.     

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.     

Collaborative VLA,SOHO and RHESSI observations of evolving sources of energy release in the corona above active regions

Very Large Array (VLA) observations at 20 and 91 cm wavelength are compared with data from the SOHO (EIT and MDI) and RHESSI solar missions to investigate the evolution of decimetric Type I noise storms and Type III bursts and related magnetic activity in the photosphere and corona. The combined data sets provide clues about the mechanisms that initiate and sustain the decimetric bursts and about interactions between thermal and nonthermal plasmas at different locations in the solar atmosphere. On one day, frequent, low-level hard X-ray flaring observed by RHESSI appears to have had no clear affect on the evolution of two closely-spaced Type I noise storm sources lying above the target active region. EIT images however, indicate nearly continuous restructuring of the underlying EUV loops which, through accompanying low-level magnetic reconnection, might give rise to nonthermal particles and plasma turbulence that sustain the long-lasting Type I burst emission. On another day, the onset of an impulsive hard X-ray burst and subsequent decimetric burst emission followed the gradual displacement and coalescence of a small patch of magnetic magnetic polarity with a pre-existing area of mixed magnetic polarity. The time delay of the impulsive 20 and 91 cm bursts by up to 20 min suggests that these events were unlikely to represent the main sites of flare electron acceleration, but instead are related to the rearrangement of the coronal magnetic field after the main flare at lower altitude. Although the X-ray flare is associated with the decimetric burst, the brightness and structure of a long-lasting Type I noise storm from the same region was not affected by the flare. This suggests that the reconfiguration of the coronal magnetic fields and the subsequent energy release that gave rise to the impulsive burst emission did not significantly perturb that part of the corona where the noise storm emission was located.     

Hard X-ray footpoint motion and acceleration mechanism

An intense (X9.4/2B) flare, which occurred on 6 November 1997, was observed with the hard X-ray telescope on board Yohkoh. In the M2- (33 – 53 keV) and H-band (53 – 93 keV), This flare clearly show double footpoint sources during its impulsive phase. We have analyzed the locations and motions of these sources in detail. It is found that, at 11:53:06 UT, one of the footpoint sources in the M2-band moved to a new position earlier than the corresponding source in the H-band. The time-lag is about one second and the separation between the old and new positions is 5 arcsec. This happened between two major spikes in the time profile of hard X-ray intensity. This apparent motion might indicate that an epoch of energy release finished somewhere high in the corona and the next epoch started in another magnetic field system. This observation clearly shows that higher energy electrons precipitate into the footpoint region later. We try to interpret this particular phenomenon under the two assumptions, which are the direct precipitation model and the trap-and-precipitation model.     

硬X射线暂现源的偶发性星风增强模型

本文讨论了硬X射线暂现源的偶发性星风增强模型,计算了爆发期间光学光度的变化,解释了光学光度和X射线光度变化的不同步现象,结果表明这种模型可能是比较合理的。      

Structure of the Program of Short-term Prediction of Powerful Solar Flares

Input data of the system are two-dimensional images and one-dimensional distributions of total and polarized solar emission at 5.2 cm wavelength obtained with SSRT. Together with photoheliograms, magnetograms, Hα-filtergrams and characteristics of active regions received from other sources, they form the initial database. The first stage includes superimposing the images, identifying microwave sources with active regions, assigning NOAA numbers to the sources, and determining for each active region the heliolatitude, extent, and inclination angle of the group's axis to the equator. These data are used to calculate the boundaries of longitude zones for each active region. A next stage involves determining the brightness temperatures of microwave sources less than the polarization distribution, the degree of polarization, and microwave emission flux, as well as calculating the parameters of microwave sources. Each parameter is assigned its own value of the weight factor, and the sum of values is used to draw the conclusion about the flare occurrence probability in each active region and on the Sun in general.      

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.     

Physics of the impulsive phase of solar flares

The physics of the impulsive phase of solar flares is discussed in relation to high resolution microwave, hard X-ray and ultraviolet observations. High spatial resolution observations of the structure of microwave flaring loops and their interpretation in terms of arcades of loops as the sites of primary energy release are presented. Theoretical interpretation of the confinement of microwave producing energetic electrons in the coronal part of loops is discussed. High temporal and spatial resolution measurements in hard X-rays, as well as observations of the spectral evolution of the hard X-ray emission are presented. Observations of the relative locations of microwave and hard X-ray emitting regions are presented and their significance with respect to the energy release site and electron acceleration is discussed. The relative timing of the peaks of impulsive hard X-ray and microwave burst is discussed. The significance of ultraviolet measurements in obtaining the density of flaring regions is discussed. Possible diagnostics of impulsive phase onsets from cm-λ polarization data are presented, and the role of the emergence of new flux and of the current sheet formed between closed loops in producing impulsive energy release at centimeter wavelengths are analyzed. Decimeter and meter wave manifestations of preflash phase and millisecond pulsations at centimeter and decimeter wavelengths and the relevant physical processes involved are discussed.     

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.     

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.