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Predicting Solar Flares Using SDO/HMI Vector Magnetic Data Product and Random Forest Algorithm  

Chang Liu   Submitted: 2017-06-09 14:15

Adverse space weather effects can often be traced to solar flares, prediction of which has drawn significant research interests. The Helioseismic and Magnetic Imager (HMI) produces full-disk vector magnetograms with continuous high cadence, while flare prediction efforts utilizing this unprecedented data source are still limited. Here we report results of flare prediction using physical parameters provided by the Space-weather HMI Active Region Patches (SHARP) and related data products. We survey X-ray flares occurred from 2010 May to 2016 December, and categorize their source regions into four classes (B, C, M, and X) according to the maximum GOES magnitude of flares they generated. We then retrieve SHARP related parameters for each selected region at the beginning of its flare date to build a database. Finally, we train a machine-learning algorithm, called random forest (RF), to predict the occurrence of a certain class of flares in a given active region within 24 hours, evaluate the classifier performance using the 10-fold cross validation scheme, and characterize the results using standard performance metrics. Compared to previous works, our experiments indicate that using the HMI parameters and RF is a valid method for flare forecasting with fairly reasonable prediction performance. To our knowledge, this is the first time that RF is used to make multi-class predictions of solar flares. We also find that the total unsigned quantities of vertical current, current helicity, and flux near polarity inversion line are among the most important parameters for classifying flaring regions into different classes.

Authors: Chang Liu, Na Deng, Jason T. L. Wang, Haimin Wang
Projects: SDO-HMI

Publication Status: Accepted to the Astrophysical Journal
Last Modified: 2017-06-10 19:37
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Flare differentially rotates sunspot on Sun's surface  

Chang Liu   Submitted: 2016-10-10 08:28

Sunspots are concentrations of magnetic field visible on the solar surface (photosphere). It was considered implausible that solar flares, as resulted from magnetic reconnection in the tenuous corona, would cause a direct perturbation of the dense photosphere involving bulk motion. Here we report the sudden flare-induced rotation of a sunspot using the unprecedented spatiotemporal resolution of the 1.6m New Solar Telescope, supplemented by magnetic data from the Solar Dynamics Observatory. It is clearly observed that the rotation is non-uniform over the sunspot: as the flare ribbon sweeps across, its different portions accelerate (up to ~50° h-1) at different times corresponding to peaks of flare hard X-ray emission. The rotation may be driven by the surface Lorentz-force change due to the back reaction of coronal magnetic restructuring and is accompanied by a downward Poynting flux. These results have direct consequences for our understanding of energy and momentum transportation in the flare-related phenomena.

Authors: Chang Liu, Yan Xu, Wenda Cao, Na Deng, Jeongwoo Lee, Hugh S. Hudson, Dale E. Gary, Jiasheng Wang, Ju Jing & Haimin Wang
Projects: BBSO/NST,Fermi/GBM,GOES X-rays,SDO-HMI

Publication Status: Published in Nature Communications
Last Modified: 2016-10-12 15:08
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A Circular-ribbon Solar Flare Following an Asymmetric Filament Eruption  

Chang Liu   Submitted: 2015-09-29 12:57

The dynamic properties of flare ribbons and the often associated filament eruptions can provide crucial information on the flaring coronal magnetic field. This Letter analyzes the GOES-class X1.0 flare on 2014 March 29 (SOL2014-03-29T17:48), in which we found an asymmetric eruption of a sigmoidal filament and an ensuing circular flare ribbon. Initially both EUV images and a preflare nonlinear force-free field model show that the filament is embedded in magnetic fields with a fan-spine-like structure. In the first phase, which is defined by a weak but still increasing X-ray emission, the western portion of the sigmoidal filament arches upward and then remains quasi-static for about five minutes. The western fan-like and the outer spine-like fields display an ascending motion, and several associated ribbons begin to brighten. Also found is a bright EUV flow that streams down along the eastern fan-like field. In the second phase that includes the main peak of hard X-ray (HXR) emission, the filament erupts, leaving behind two major HXR sources formed around its central dip portion and a circular ribbon brightened sequentially. The expanding western fan-like field interacts intensively with the outer spine-like field, as clearly seen in running difference EUV images. We discuss these observations in favor of a scenario where the asymmetric eruption of the sigmoidal filament is initiated due to an MHD instability and further facilitated by reconnection at a quasi-null in corona; the latter is in turn enhanced by the filament eruption and subsequently produces the circular flare ribbon.

Authors: Chang Liu, Na Deng, Rui Liu, Jeongwoo Lee, Etienne Pariat, Thomas Wiegelmann, Yang Liu, Lucia Kleint, Haimin Wang
Projects: IRIS,National Solar Observatory (Sac Peak),RHESSI,SDO-AIA,SDO-HMI

Publication Status: accepted to ApJ Letters
Last Modified: 2015-09-29 13:05
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Three-dimensional Magnetic Restructuring in Two Homologous Solar Flares in the Seismically Active NOAA AR 11283  

Chang Liu   Submitted: 2014-09-23 18:46

We carry out a comprehensive investigation comparing the three-dimensional magnetic field restructuring, flare energy release, and the helioseismic response, of two homologous flares, the 2011 September 6 X2.1 (FL1) and September 7 X1.8 (FL2) flares in NOAA AR 11283. In our analysis, (1) a twisted flux rope (FR) collapses onto the surface at a speed of 1.5 km s-1 after a partial eruption in FL1. The FR then gradually grows to reach a higher altitude and collapses again at 3 km s-1 after a fuller eruption in FL2. Also, FL2 shows a larger decrease of the flux-weighted centroid separation of opposite magnetic polarities and a greater change of the horizontal field on the surface. These imply a more violent coronal implosion with corresponding more intense surface signatures in FL2. (2) The FR is inclined northward, and together with the ambient fields, it undergoes a southward turning after both events. This agrees with the asymmetric decay of the penumbra observed in the peripheral regions. (3) The amounts of free magnetic energy and nonthermal electron energy released during FL1 are comparable to those of FL2 within the uncertainties of the measurements. (4) No sunquake was detected in FL1; in contrast, FL2 produced two seismic emission sources S1 and S2 both lying in the penumbral regions. Interestingly, S1 and S2 are connected by magnetic loops, and the stronger source S2 has weaker vertical magnetic field. We discuss these results in relation to the implosion process in the low corona and the sunquake generation.

Authors: Chang Liu, Na Deng, Jeongwoo Lee, Thomas Wiegelmann, Chaowei Jiang, Brian R. Dennis, Yang Su, Alina Donea, Haimin Wang
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2014-09-24 08:36
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Evidence for Solar Tether-cutting Magnetic Reconnection from Coronal Field Extrapolations  

Chang Liu   Submitted: 2013-10-20 21:28

Magnetic reconnection is one of the primary mechanisms for triggering solar eruptive events, but direct observation of its rapid process has been of challenge. In this Letter we present, using a nonlinear force-free field (NLFFF) extrapolation technique, a visualization of field line connectivity changes resulting from tether-cutting reconnection over about 30 minutes during the 2011 February 13 M6.6 flare in NOAA AR 11158. Evidence for the tether-cutting reconnection was first collected through multiwavelength observations and then by the analysis of the field lines traced from positions of four conspicuous flare 1700 Å footpoints observed at the event onset. Right before the flare, the four footpoints are located very close to the regions of local maxima of magnetic twist index. Especially, the field lines from the inner two footpoints form two strongly twisted flux bundles (up to ~1.2 turns), which shear past each other and reach out close to the outer two footpoints, respectively. Immediately after the flare, the twist index of regions around the footpoints greatly diminish and the above field lines become low lying and less twisted (<~0.6 turns), overarched by loops linking the later formed two flare ribbons. About 10% of the flux (~3x1019 Mx) from the inner footpoints has undergone a footpoint exchange. This portion of flux originates from the edge regions of the inner footpoints that are brightened first. These rapid changes of magnetic field connectivity inferred from the NLFFF extrapolation are consistent with the tether-cutting magnetic reconnection model.

Authors: Chang Liu, Na Deng, Jeongwoo Lee, Thomas Wiegelmann, Ronald L. Moore, and Haimin Wang
Projects: RHESSI,SDO-AIA,SDO-HMI

Publication Status: accepted to ApJ Letters
Last Modified: 2013-10-21 02:41
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He I D3 Observation of the 1984 May 22 M6.3 Solar Flare  

Chang Liu   Submitted: 2013-06-25 19:17

He I D3 line has a unique response to the flare impact on the low solar atmosphere and can be a powerful diagnostic tool for energy transport processes. Using images obtained from the recently digitized films of Big Bear Solar Observatory, we report D3 observation of the M6.3 flare on 1984 May 22, which occurred in an active region with a circular magnetic polarity inversion line (PIL). The impulsive phase of the flare starts with a main elongated source that darkens in D3, inside of which bright emission kernels appear at the time of the initial small peak in hard X-rays (HXRs). These flare cores subsequently evolve into a sharp emission strand lying within the dark halo simultaneously with the main peak in HXRs, reversing the overall source contrast from -5% to 5%. The radiated energy in D3 during the main peak is estimated to be about 1030 ergs, which is comparable to that carried by nonthermal electrons above 20 keV. Afterwards the flare proceeds along the circular PIL in the counterclockwise direction to form a dark circular ribbon in D3, which apparently mirrors the bright ribbons in Hα and He I 10830 A. All these ribbons last for over one hour in the late gradual phase. We suggest that the present event resembles the so-called black-light flare that is proposed based on continuum images, and that D3 darkening and brightening features herein may be due to, respectively, the thermal conduction heating and the direct precipitation of high-energy electrons.

Authors: Chang Liu, Yan Xu, Na Deng, Jeongwoo Lee, Jifeng Zhang, Debi Prasad Choudhary, and Haimin Wang
Projects: Other

Publication Status: accepted to ApJ
Last Modified: 2013-06-26 11:12
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Rapid Changes of Photospheric Magnetic Field After Tether-cutting Reconnection and Magnetic Implosion  

Chang Liu   Submitted: 2011-12-13 14:20

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Chang Liu, Na Deng, Rui Liu, Jeongwoo Lee, Thomas Wiegelmann, Ju Jing, Yan Xu, Shuo Wang, and Haimin Wang
Projects: Hinode/SOT,RHESSI,SDO-AIA,SDO-HMI

Publication Status: ApJ Letters, accepted
Last Modified: 2011-12-13 14:37
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A Standard-to-Blowout Jet  

Chang Liu   Submitted: 2011-05-18 08:20

This abstract was corrupted following database problems and is being recovered. It will be restored as quickly as possible. Any questions, please send them to Alisdair. Sorry for any incovenience.


Authors: Chang Liu, Na Deng, Rui Liu, Ignacio Ugarte-Urra, Shuo Wang, Haimin Wang
Projects: RHESSI,SDO-AIA,SDO-HMI

Publication Status: accepted to ApJL
Last Modified: 2011-05-18 10:40
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Motions of Hard X-ray Sources During an Asymmetric Eruption  

Chang Liu   Submitted: 2010-08-31 19:41

Filament eruptions and hard X-ray (HXR) source motions are commonly observed in solar flares, which provides critical information on the coronal magnetic reconnection. This Letter reports an event on 2005 January 15, in which we found an asymmetric filament eruption and a subsequent coronal mass ejection together with complicated motions of HXR sources during the GOES-class X2.6 flare. The HXR sources initially converge to the magnetic polarity inversion line (PIL), and then move in directions either parallel or perpendicular to the PIL depending on the local field configuration. We distinguish the evolution of the HXR source motion in four phases and associate each of them with distinct regions of coronal magnetic fields as reconstructed using a non-linear force-free field extrapolation. It is found that the magnetic reconnection proceeds along the PIL toward the regions where the overlying field decreases with height more rapidly. It is also found that not only the perpendicular but the parallel motion of the HXR sources correlates well with the HXR lightcurve. These results are discussed in favor of the torus instability as an important factor in the eruptive process.

Authors: Chang Liu, Jeongwoo Lee, Ju Jing, Rui Liu, Na Deng, Haimin Wang
Projects: RHESSI,SoHO-EIT,SoHO-MDI

Publication Status: Accepted to ApJL
Last Modified: 2010-09-01 08:06
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Observational Evidence of Back-reaction on the Solar Surface Associated with Coronal Magnetic Restructuring in Solar Eruptions  

Chang Liu   Submitted: 2010-05-25 05:51

Most models of solar eruptions assume that coronal field lines are anchored in the dense photosphere and thus the photospheric magnetic fields would not have rapid, irreversible changes associated with eruptions resulted from the coronal magnetic reconnection. Motivated by the recent work of Hudson, Fisher & Welsch (2008) on quantitatively evaluating the back reaction due to energy release from the coronal fields, in this Letter we synthesize our previous studies and present analysis of new events about flare-related changes of photospheric magnetic fields. For the 11 X-class flares where vector magnetograms are available, we always find an increase of transverse field at the polarity inversion line (PIL) although only 4 events had measurements with 1 minute temporal resolution. We also discuss 18 events with 1 minute cadence line-of-sight magnetogram observation, which all show prominent changes of magnetic flux contained in the flaring Delta spot region. Except in one case, the observed limb-ward flux increases while disk-ward flux decreases rapidly and irreversibly after flares. This observational evidence provides support, either directly or indirectly, for the theory and prediction of Hudson, Fisher & Welsch that the photospheric magnetic fields must respond to coronal field restructuring and turn to a more horizontal state near the PIL after eruptions.

Authors: Haimin Wang, Chang Liu
Projects: None

Publication Status: accepted to ApJ Letters
Last Modified: 2010-05-25 07:48
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Successive Solar Flares and Coronal Mass Ejections on 2005 September 13 from NOAA AR 10808  

Chang Liu   Submitted: 2009-08-04 11:26

We present a multiwavelength study of the 2005 September 13 eruption from NOAA 10808 that produced total four flares and two fast coronal mass ejections (CMEs) within 1.5 hours. Our primary attention is paid to the fact that these eruptions occurred in close succession in time, and that all of them were located along an S-shaped magnetic polarity inversion line (PIL) of the active region. In our analysis, (1) the disturbance created by the first flare propagated southward along the PIL to cause a major filament eruption that led to the first CME and the associated second flare underneath. (2) The first CME partially removed the overlying magnetic fields over the northern Delta spot to allow the third flare and the second CME. (3) The ribbon separation during the fourth flare would indicate reclosing of the overlying field lines opened by the second CME. It is thus concluded that this series of flares and CMEs are interrelated to each other via magnetic reconnections between the expanding magnetic structure and the nearby magnetic fields. These results complement previous works made on this event with the suggested causal relationship among the successive eruptions.

Authors: Chang Liu, Jeongwoo Lee, Marian Karlický, Debi Prasad Choudhary, Na Deng, and Haimin Wang
Projects: RHESSI

Publication Status: Accepted to ApJ
Last Modified: 2009-08-05 14:22
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Reconnection Electric Field and Hardness of X-Ray Emission of Solar Flares  

Chang Liu   Submitted: 2009-03-25 06:14

Magnetic reconnection is believed to be the prime mechanism to trigger solar flares and accelerate electrons up to energies of MeV. In the classical two-dimensional reconnection model, the separation motion of chromospheric ribbons manifests the successive reconnection that takes place higher up in the corona. Meanwhile, downward traveling energetic electrons bombard the dense chromosphere and create hard X-ray (HXR) emissions, which provide a valuable diagnostic of electron acceleration. Analyses of ribbon dynamics and HXR spectrum have been carried out separately. In this Letter, we report a study of the comparison of reconnection electric field measured from ribbon motion and hardness (spectral index) of X-ray emission derived from X-ray spectrum. Our survey of the maximum average reconnection electric field and the minimum overall spectral index for 13 two-ribbon flares show that they are strongly anti-correlated. The former is also strongly correlated with flare magnitude measured using the peak flux of soft X-ray emissions. These provide strong support for electron acceleration models based on the electric field generated at reconnecting current sheet during flares.

Authors: Chang Liu and Haimin Wang
Projects: RHESSI

Publication Status: accepted to ApJ Letters
Last Modified: 2009-03-25 10:04
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Subject will be restored when possible  

Chang Liu   Submitted: 2007-11-08 15:55

The rare phenomenon of ribbon-like hard X-ray (HXR) sources up to 100 keV found in the 2005 May 13 M8.0 flare observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager provides detailed information on the spatial distribution of flare HXR emission. In this Letter, we further investigate the characteristics of HXR emission in this event using imaging spectroscopy, from which we obtain spatially resolved HXR spectral maps during the flare impulsive phase. As a result we found, along a flare ribbon, an anticorrelation relationship between the local HXR flux and the local HXR spectral index. We suggest that this can be regarded as a spatial analog of the well-known temporal soft-hard-soft spectral evolution pattern of the integrated HXR flux. We also found an anticorrelation between HXR spectral index and local electric field along the ribbon, which suggests the electron acceleration by the electric field during flares.

Authors: Chang Liu, Jeongwoo Lee, Ju Jing, Dale E. Gary, Haimin Wang
Projects: RHESSI

Publication Status: Accepted to ApJL
Last Modified: 2007-11-09 07:53
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The Eruption from a Sigmoidal Solar Active Region on 2005 May 13  

Chang Liu   Submitted: 2007-07-15 12:57

This paper presents a multiwavelength study of the M8.0 flare and its associated fast halo CME that originated from a bipolar active region NOAA 10759 on 2005 May 13. The source active region has a conspicuous sigmoid structure at TRACE 171 Å channel as well as in the SXI soft X-ray images, and we mainly concern ourselves with the detailed process of the sigmoid eruption as evidenced by the multiwavelength data ranging from Hα , WL, EUV/UV, radio, and hard X-rays (HXRs). The most important finding is that the flare brightening starts in the core of the active region earlier than that of the rising motion of the flux rope. This timing clearly addresses one of the main issues in the magnetic eruption onset of sigmoid, namely, whether the eruption is initiated by an internal tether-cutting to allow the flux rope to rise upward or a flux rope rises due to a loss of equilibrium to later induce tether cutting below it. Our high time cadence SXI and Hα data shows that the first scenario is relevant to this eruption. As other major findings, we have the RHESSI HXR images showing a change of the HXR source from a confined footpoint structure to an elongated ribbon-like structure after the flare maximum, which we relate to the sigmoid-to-arcade evolution. Radio dynamic spectrum shows a type II precursor that occurred at the time of expansion of the sigmoid and a drifting pulsating structure in the flare rising phase in HXR. Finally type II and III bursts are seen at the time of maximum HXR emission, simultaneous with the maximum reconnection rate derived from the flare ribbon motion in UV. We interpret these various observed properties with the runaway tether-cutting model proposed by Moore et al. in 2001.

Authors: Chang Liu, Jeongwoo Lee, Vasyl Yurchyshyn, Na Deng, Kyung-Suk Cho, Marian Karlický, and Haimin Wang
Projects: RHESSI,SoHO-EIT,SoHO-LASCO,TRACE

Publication Status: The Astrophysical Journal, accepted July, 2007
Last Modified: 2007-07-15 15:48
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The Ribbon-Like Hard X-Ray Emission in a Sigmoidal Solar Active Region  

Chang Liu   Submitted: 2007-02-14 12:07

Solar flare emissions at Hα and EUV/UV wavelengths often appear in the form of two ribbons, which has been regarded as evidence for a typical configuration of solar magnetic reconnection. However, such a ribbon structure has rarely been observed in hard X-rays (HXRs), although it is expected as well. In this letter, we report a ribbon-like HXR source observed with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) at energies as high as 25-100 keV during the 2005 May 13 flare. For a qualitative understanding of this unusual HXR morphology, we also note that the source active region appeared in a conspicuous sigmoid shape before the eruption and changed to an arcade structure afterward as observed with the Transition Region and Coronal Explorer (TRACE) at 171 A. We suggest that the ribbon-like HXR structure is associated with the sigmoid-to-arcade transformation during this type of reconnection.

Authors: Chang Liu, Jeongwoo Lee, Dale E. Gary, Haimin Wang
Projects: None

Publication Status: ApJL (accepted)
Last Modified: 2007-02-14 12:11
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Large-Scale Activities Associated with the 2003 October 29 X10 Flare  

Chang Liu   Submitted: 2006-01-25 11:53

We present a multiwavelength study focusing on the large-scale activities associated with the 2003 October 29 X10 flare and a halo coronal mass ejection (CME). This event was so strong to clearly show several large-scale activities, such as, remote brightenings detected at Hα and soft X-rays, Moreton waves at Hα off-bands, and type II radio bursts, which offers an excellent opportunity to clarify the relationship among them. The remote brightenings were found near two coronal holes more than 2 times 105 km away from the main flare in eastern and southern directions, respectively. Coronal dimmings were seen at the locus of the remote brightenings right after the flare at both extreme-ultraviolet (EUV) and soft X-ray wavelengths. The Moreton waves propagated both northeastward and southward, toward the afore-mentioned remote regions, at speeds of approximately 1100 km s-1 and 1900 km s-1, respectively. The type II radio bursts, while location and directivity are unidentified, are also observed to propagate at two different speeds. Our timing analyses show that the Moreton waves, the type II radio bursts, and the CME started almost simultaneously, but were not co-temporal with the remote brightenings. The remote brightenings are rather consistent, in timing and kinematics, with the flare hard X-ray emissions within the active region although they have much smaller scales. Based on these results, it is concluded that the two remote brightening regions were magnetically connected to the flaring active region, and that the remote brightenings as well as all other activities were due to the interaction of an erupting flux rope at the core of the flare with magnetic field overlying the region. The inferred scenario requires that the large overlying loops should open to allow the flare activity underneath them. In this sense, our results point to a picture similar to the magnetic break-out process in such a large scale.

Authors: C. Liu, J. Lee, N. Deng, D. E. Gary and H. Wang
Projects: None

Publication Status: ApJ accepted (10 May 2006, v642 2 issue)
Last Modified: 2006-05-09 11:08
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Abstracts by Author
Predicting Solar Flares Using SDO/HMI Vector Magnetic Data Product and Random Forest Algorithm
Flare differentially rotates sunspot on Sun's surface
A Circular-ribbon Solar Flare Following an Asymmetric Filament Eruption
Three-dimensional Magnetic Restructuring in Two Homologous Solar Flares in the Seismically Active NOAA AR 11283
Evidence for Solar Tether-cutting Magnetic Reconnection from Coronal Field Extrapolations
He I D3 Observation of the 1984 May 22 M6.3 Solar Flare
Rapid Changes of Photospheric Magnetic Field After Tether-cutting Reconnection and Magnetic Implosion
A Standard-to-Blowout Jet
Motions of Hard X-ray Sources During an Asymmetric Eruption
Observational Evidence of Back-reaction on the Solar Surface Associated with Coronal Magnetic Restructuring in Solar Eruptions
Successive Solar Flares and Coronal Mass Ejections on 2005 September 13 from NOAA AR 10808
Reconnection Electric Field and Hardness of X-Ray Emission of Solar Flares
Subject will be restored when possible
The Eruption from a Sigmoidal Solar Active Region on 2005 May 13
The Ribbon-Like Hard X-Ray Emission in a Sigmoidal Solar Active Region
Large-Scale Activities Associated with the 2003 October 29 X10 Flare

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