A Statistical Study of Rhessi Flares

A statistical analysis of RHESSI X-ray flares in the 12–25 keV band during the period from February 2002 to June 2005 is presented. We found that a power-law with an index of 1.80± 0.02 can fit well the frequency distribution of the peak count rates. This power-law does not change significantly with time. However, the frequency distribution of the flare durations cannot be fitted well by a single power-law. There is a weak correlation between the peak count rates and the characteristic times like rise times, decay times, or durations. But the correlation between the rise times and decay times seems to be strong. We discuss the results obtained and compare them with previous works. The frequency distribution of rise times for the sub-group events with a similar magnitude of peak count rates is also shown. In particular, we propose a new parameter R _a , the growth factor of the count rate, defined as the peak count rate divided by the rise time, to reflect the characteristics of the rising phases of flares. The distribution of R _a is shown and discussed.     

X-Ray Polarization of Solar Flares Measured with Rhessi

The degree of linear polarization in solar flares has not yet been precisely determined despite multiple attempts to measure it with different missions. The high energy range, in particular, has very rarely been explored, due to its greater instrumental difficulties. We approached the subject using the Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) satellite to study six X-class and 1 M-class flares in the energy range between 100 and 350 keV. Using RHESSI as a polarimeter requires the application of strict cuts to the event list in order to extract those photons that are Compton scattered between two detectors. Our measurements show polarization values between 2 and 54%, with errors ranging from 10 to 26% in 1σ level. In view of the large uncertainties in both the magnitude and direction of the polarization vector, the results can only reject source models with extreme properties.     

Statistical Moments of Active-Region Images During Solar Flares

We present new temporal-evolution diagnostics of solar flares. The high-order statistical moments (skewness and kurtosis) of the Hα images of active regions during solar flares were computed from their initial phases up to their maxima. The same method was used for quiet active regions for tests and comparison. We found that temporal profiles of the Hα statistical moments during flares roughly correspond to those observed in soft X-rays by the GOES satellite. Maxima of the cross-correlation coefficients between the skewness and the GOES X-rays were found to be 0.82?–?0.98, and the GOES X-rays are delayed 0?–?144 seconds against the skewness. We recognized that these moments are very sensitive to pre-flare activities. Therefore we used them to determine the flare starting-time and to study the pre-flare quasi-periodic processes. We determined the periods of these pre-flare processes in an interval of 20?–?400 seconds by using special convolution filters and Fourier analysis. We propose to use this method to analyze active regions during the very early phases of solar flares, and even in real time.     

Low-Energy Cutoffs in Electron Spectra of Solar Flares: Statistical Survey

The Reuven Ramaty High Energy Spectroscopic Imager (RHESSI) X-ray data base (February 2002 – May 2006) has been searched to find solar flares with weak thermal components and flat photon spectra. Using a regularized inversion technique, we determine the mean electron flux distribution from count spectra for a selection of events with flat photon spectra in the 15 – 20 keV energy range. Such spectral behavior is expected for photon spectra either affected by photospheric albedo or produced by electron spectra with an absence of electrons in a given energy range (e.g., a low-energy cutoff in the mean electron spectra of nonthemal particles). We have found 18 cases that exhibit a statistically significant local minimum (a dip) in the range of 13 – 19 keV. The positions and spectral indices of events with low-energy cutoff indicate that such features are likely to be the result of photospheric albedo. It is shown that if the isotropic albedo correction is applied, all low-energy cutoffs in the mean electron spectrum are removed, and hence the low-energy cutoffs in the mean electron spectrum of solar flares above ∼ 12 keV cannot be viewed as real features. If low-energy cutoffs exist in the mean electron spectra, their energies should be less than ∼ 12 keV.     

电离层TEC对小耀斑的响应

利用国际GPS观测网(IGS)提供的多个台站的观测数据,分析了M级别以下的小、暗太阳耀斑对向阳面电离层TEC的影响．利用传统分析方法的结果表明,从单条视线(LOS)观测数据得到的电离层TEC及其时间变化率曲线来看,由于它们的波动水平和正常情况下的背景电离层变化相当,使此类小耀斑的信息完全淹没在背景噪声中,不能够显示和分辨出耀斑的发生．利用相干求和的数据处理方法,选用向阳面18个GPS台站的观测数据研究了一次C级SF耀斑引起的电离层TEC增加,结果发现,这种方法能有效地消除背景电离层变化噪声,电离层对耀斑的响应非常清楚和明显,这通常只能在X级别的大耀斑中看到．和GOES卫星X射线数据相比,电离层TEC变化的时间特征和耀斑爆发的开始、最大和结束时间均有很好的符合,其最大平均TEC增量在0．1TECU以下,和X级别的大耀斑相比有一个或多个量级上的差别．     

An Invariable Point in the Energy Spectra of Non-Thermal Electrons of Solar Flares

The power-law energy spectra of non-thermal electrons for each 1.024 second have been drawn together during the flare. For some flares, it is discovered that the energy spectra taken at different times present a roughly fixed crossing point, at which point the flux of non-thermal electrons keeps the same during the flare. The possible significance of this phenomenon is discussed. We conjecture that there may be a flux saturation at the low-energy cutoff in the process of electron acceleration.     

Evolution of Electric Currents Associated with Two M-Class Flares

Using one-minute cadence vector magnetograms from Big Bear Solar Observatory (BBSO), we analyze the temporal behavior of derived longitudinal electric currents associated with two flares on July 26, 2002. One of the events is an M1.0 flare which occurred in active region NOAA 10044, while the other is an M8.7 flare in the adjacent region 10039. Rapid changes of magnetic fields in the form of flux emergence are found to be associated with both of these events. However, the temporal behavior of electric currents are very different. For the M1.0 flare, the longitudinal electric current density drops rapidly near the flaring neutral line; while for the M8.7 flare, the current density rapidly increases, confirming the picture of the current-carrying flux emergence. We offer a possible explanation for such a difference: magnetic reconnection at different heights for the two events, near the photosphere for the M1.0 flare, and higher up for the M8.7 flare.     

Determination of Electron Flux Spectra in a Solar Flare with an Augmented Regularization Method: Application to Rhessi Data

Kontar et al. (2004) have shown how to recover mean source electron spectra in solar flares through a physical constraint regularization analysis of the bremsstrahlung photon spectra I() that they produce. They emphasize the use of non-square inversion techniques, and preconditioning combined with physical properties of the spectra to achieve the most meaningful solution to the problem. Higher-order regularization techniques may be used to generate forms with certain desirable properties (e.g., higher-order derivatives). They further note that such analysis may be used to infer properties of the electron energy spectra at energies well above the maximum photon energy observed. In this paper we apply these techniques to data from a solar flare observed by RHESSI on 26 February, 2002. Results using different orders of regularization are presented and compared for various time intervals. Clear evidence is presented for a change in the value of the high-energy cutoff in the mean source electron spectrum with time. We also show how the construction of the injected electron spectrum F₀(E₀) (assuming that Coulomb collisions in a cold target dominate the electron transport) is facilitated by the use of higher-order regularization methods.     

Automatic Tracking of Active Regions and Detection of Solar Flares in Solar EUV Images

Solar catalogs are frequently handmade by experts using a manual approach or semi-automated approach. The appearance of new tools is very useful because the work is automated. Nowadays it is impossible to produce solar catalogs using these methods, because of the emergence of new spacecraft that provide a huge amount of information. In this article an automated system for detecting and tracking active regions and solar flares throughout their evolution using the Extreme UV Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) spacecraft is presented. The system is quite complex and consists of different phases: i) acquisition and preprocessing; ii) segmentation of regions of interest; iii) clustering of these regions to form candidate active regions which can become active regions; iv) tracking of active regions; v) detection of solar flares. This article describes all phases, but focuses on the phases of tracking and detection of active regions and solar flares. The system relies on consecutive solar images using a rotation law to track the active regions. Also, graphs of the evolution of a region and solar evolution are presented to detect solar flares. The procedure developed has been tested on 3500 full-disk solar images (corresponding to 35 days) taken from the spacecraft. More than 75 % of the active regions are tracked and more than 85 % of the solar flares are detected.     

2个二次触发型太阳耀斑的射电时间轮廓及其日冕磁结构

利用国家天文台(北京和昆明)的射电频谱仪(频段为0.65~7.6 GHz)和相关的NoRH/17GHz射电以及TRACE/171 EUV和Yohkoh/SXT的观测资料,分析了2001/04/10和10/19的2个共生精细时间结构的稀有事件,这2个事件的射电爆发时间轮廓和观测特征相似,通过这2个事件的微波(17GHz)偏振观测资料的比较,发现这2个射电爆发均由包含多重(4极)磁结构的复杂活动区引起,特别指出这2个耀斑最后都导致了耀斑后相的分米波射电爆发(第二次触发耀斑),这可能是后环引起的射电爆发。它们都分别对应于双极磁位形,表明这两次触发耀斑是由相似的耀斑模型产生。2个分米波爆发可能是相似(homologous)耀斑的射电表现,可以推测这两次耀斑的驱动器可能皆是磁流浮现或对消(因为源区有新的单或双极出现或消失),而它们的触发器皆是由双极反向Y型位形(具有一个双极拱的单磁流系统)的磁重联,耀斑后环的演化是导致耀斑后相分米波射电爆发的必要条件。我们认为,这双带耀斑对应的宽带射电爆发辐射机制是回旋同步加速辐射过程,而耀斑后相的窄带分米波爆发的辐射机制是等离子体辐射过程。     

Soft X-ray Fluxes of Major Flares Far Behind the Limb as Estimated Using STEREO EUV Images

With increasing solar activity since 2010, many flares from the backside of the Sun have been observed by the Extreme Ultraviolet Imager (EUVI) on either of the twin STEREO spacecraft. Our objective is to estimate their X-ray peak fluxes from EUVI data by finding a relation of the EUVI with GOES X-ray fluxes. Because of the presence of the Fe xxiv line at 192 Å, the response of the EUVI 195 Å channel has a secondary broad peak around 15 MK, and its fluxes closely trace X-ray fluxes during the rise phase of flares. If the flare plasma is isothermal, the EUVI flux should be directly proportional to the GOES flux. In reality, the multithermal nature of the flare and other factors complicate the estimation of the X-ray fluxes from EUVI observations. We discuss the uncertainties, by comparing GOES fluxes with the high cadence EUV data from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). We conclude that the EUVI 195 Å data can provide estimates of the X-ray peak fluxes of intense flares (e.g., above M4 in the GOES scale) to small uncertainties. Lastly we show examples of intense flares from regions far behind the limb, some of which show eruptive signatures in AIA images.     

A Statistical Survey of Hard X-ray Spectral Characteristics of Solar Flares with Two Footpoints

The impulsive phase of the 23 July 2002 2B/X4.8 proton flare with a classical two-ribbon structure was observed with the Irkutsk Large Solar Vacuum Telescope (LSVT) in spectropolarimetric mode, with high spatial, spectral, and time resolution. On the basis of 49 spectrograms and 1200 spectral cuts across the flare ribbons, evidence for Hα line impact polarization has been obtained. A systematic change of the Stokes Q and U parameters has been detected across the ribbons for different flare regions measured with a scanning step of 0.85″. In the eastern side of the ribbons, the degree of polarization is 4 – 8%; its plane is oriented toward the solar disk center (radial direction). In the western side, the polarization degree runs up to 25%, and its plane is perpendicular to the disk center direction (tangential direction). A comparison of these results with hard X-ray data (RHESSI) allows us to conclude that high-energy electron beams reached the chromosphere during this flare. The observed changes of the direction of polarization and the vanishing polarization within the ribbons mean that, at the chromospheric level, the energy of electrons remaining in the beam is about 200 eV. A shift of the peak position of polarization relative to the intensity maximum in the ribbons may result from the inclination of the electron beam axis with respect to the solar surface.     

Evidence of Magnetic Reconnection in Solar Flares and a Unified Model of Flares

The solar X-ray observing satellite Yohkoh has discovered various new dynamic features in solar flares and corona, e.g., cusp-shaped flare loops, above-the-loop-top hard X-ray sources, X-ray plasmoid ejections from impulsive flares, transient brightenings (spatially resolved microflares), X-ray jets, large scale arcade formation associated with filament eruption or coronal mass ejections, and so on. It has soon become clear that many of these features are closely related to magnetic reconnection. We can now say that Yohkoh established (at least phenomenologically) the magnetic reconnection model of flares. In this paper, we review various evidence of magnetic reconnection in solar flares and corona, and present unified model of flares on the basis of these new Yohkoh observations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Rhessi and Trace Observations of the 21 April 2002 x1.5 Flare

Solar Physics - Observations of the X1.5 flare on 21 April 2002 are reviewed using the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) and the Transition Region and Coronal Explorer...     

Flares and changing magnetic fields

An observational study of maps of the longitudinal component of the photospheric fields in flaring active regions leads to the following conclusions:

1. The broad-wing Hα kernels characteristic of the impulsive phase of flares occur within 10″ of neutral lines encircling features of isolated magnetic polarity (‘satellite sunspots’).
2. Photospheric field changes intimately associated with several importance 1 flares and one importance 2B flare are confined to satellite sunspots, which are small (10″ diam). They often correspond to spot pores in white-light photographs.
3. The field at these features appears to strengthen in the half hour just before the flares. During the flares the growth is reversed, the field drops and then recovers to its previous level.
4. The magnetic flux through flare-associated features changes by about 4 × 10¹⁹ Mx in a day. The features are the same as the ‘Structures Magnétiques Evolutives’ of Martres et al. (1968a).
5. An upper limit of 10²¹ Mx is set for the total flux change through McMath Regions 10381 and 10385 as the result of the 2B flare of 24 October, 1969.
6. Large spots in the regions investigated did not evince flux changes or large proper motions at flare time.
7. The results are taken to imply that the initial instability of a flare occurs at a neutral point, but the magnetic energy lost cannot yet be related to the total energy of the subsequent flare.
8. No unusual velocities are observed in the photosphere at flare time.

     

Flares of spotless regions

An analysis of 20 flares of spotless regions observed at Yunnan Observatory during the peak years of Cycle 21 shows 1) the fraction of flares produced in spotless regions is about 3%, 2) their Carrington longitudes show a tendency to drift eastward, 3) the majority of spotless flares are low-energy flares, 4) the background conditions for producing spotless flares are the same as for flares in general, namely, there must be local magnetic structures of opposite polarities. The spotless flares occur on the sides or in the vicinity of the local neutral line.The quiescent dark filaments floating on the neutral line are activated a few hours and one or two days before the flare, the filament nearest to the flare position first enlarges, accompanied by brightening of plages. A few minutes before the flare, or during the flare, this filament rapidly weakens, even vanishes. Meanwhile, visible fibrils become less inclined to the main filament showing pressure force is transformed into shear force.     

两类长γ暴的傅里叶功率谱

分析了Balastegui等（2001）应用神经网络划分出的两类长γ暴的平均傅里叶功率谱（PDS），两类暴的平均PDS都具有明显的幂律谱结构，对两类暴分别按亮度和能谱硬度分组，计算各组的平均功率谱，两类暴都具有平均功率谱随亮度和能谱硬度的增加而变平的趋势。     

Modeling Images and Spectra of a Solar Flare Observed by RHESSI on 20 February 2002

We have analyzed a C7.5 limb flare observed by RHESSI on 20 February 2002. The RHESSI images appear to show two footpoints and a loop-top source. Our goal was to determine if the data are consistent with a simple steady-state model in which high-energy electrons are continuously injected at the top of a semicircular flare loop. A comparison of the RHESSI images with simulated images from the model has made it possible for us to identify spurious sources and fluxes in the RHESSI images. We find that the RHESSI results are in many aspects consistent with the model if a thermal source is included between the loop footpoints, but there is a problem with the spectral index of the loop-top source. The thermal source between the footpoints is likely to be a low-lying loop interacting with the northern footpoint of a higher loop containing the loop-top source.     

Flares and separatrices between magnetic loops

Time sequences of vector magnetograms of Hauirou and Big Bear Solar Observatories have provided us the opportunity to identify the individual magnetic loops and athe separatrices between them.

Based on the continuous observatin of vector magnetic field of NOAA 7469 from 4 to 12 April 1993, for the first time, the authors have identified the magnetic loop systems and relevant separatrices for such an active region. The observational signature ofthe cross-section of separatrices on the photosphere is as follows:

1. (1) High degree of magnetic shear at or close to the separatrices;

2. (2) Steep gradient of line-of-sight magnetic field ( 0.1 G/km) crossing the neutral line.

3. (3) Flux cancellation from both sides of the separatrices. At this point the transverse field partly changes its alignment.

During the observed period, flare activity took place repeatedly in the vicinity of the separatrices.