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Observational Study of a Peculiar Solar Limb Event Occurred on 11 January 2002  

Hui Li   Submitted: 2009-04-22 18:57

On 11 January 2002, using the Multi-channel Infrared Solar Spectrograph at the Purple Mountain Observatory, we obtained Hα , CaII 8542 and HeI 10830 spectra and slit-jaw Hα images of a peculiar solar limb event. A close resemblance of its intensity to that of a small flare and the GOES X-ray flux indicates that it was an active prominence. However, its morphological evolution and velocity variation were different from general typical active prominences, such as limb flares, post-flare loops, surges and sprays. It started with the ejection of material from the flare site. In the early phase, the ejecta was as bright as a limb flare and kept rising until reaching the height of (8 - 10) imes104 km at an almost constant velocity of 91.7 km s-1 with its lower part always connected to the solar surface. EUV images in 195 Å show similar structure as in the H-apha line, indicating the coexistence of plasmas with temperatures differing more than two orders of magnitude. Later some material started to fall back to another bright area on the solar surface. The falling material did not show the collimated structure of surges or the arc structure of flaring arches. A red-shift velocity of more than 200 km s-1 was detected in a bright point close to the outer edge of the closed loop system formed later, which dispersed in a few minutes and became a part of the newly formed large loop. The ejected material did not leave the sun, indicating that the magnetic reconnection was not sufficient to remove the overlying field lines during the process. The spectral line profiles showed large widths and variable velocities, and therefore the line-pair method is not applicable to this event for the estimation of physical parameters.

Authors: Hui Li and Jianqi You
Projects: None

Publication Status: Solar Physics, in press
Last Modified: 2009-04-23 09:12
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Calibration and initial results of the HeI D3 line flash spectrum obtained during the 2008 total solar eclipse  

Hui Li   Submitted: 2009-04-22 18:52

The flash spectra in the HeI D3 line are obtained during the 2008 total solar eclipse. We describe the instrument and the calibration of the obtained flash spectrum, and present our initial results in this paper. The average integrated intensity is Eave=8.13 imes1013erg cm-1s-1ster-1 at h=1100 km, which confirms that the HeI D3 emission is negatively correlated with the solar activity. The surface brightness reaches a maximum of Fave=8.25 imes105erg cm-2s-1ster-1 at about happrox1290pm75 km and then decreases exponentially with height when h>1800 km with an exponential index eta=1.63 imes10-8cm-1.

Authors: LI Hui, JI HaiSheng, NI HouKun, ZHANG HaiYing, et al.
Projects: None

Publication Status: Science in China, in press
Last Modified: 2009-04-23 09:11
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Subject will be restored when possible  

Hui Li   Submitted: 2007-11-27 20:53

During the JOP178 campaign in August 2006 we observed the disappearance of our target, a large quiescent filament located at S25, after an observation time of three days (August 24 to August 26). Multi-wavelength instruments were operating: THEMIS/MTR (''MulTi-Raies'') vector magnetograph, TRACE (''Transition Region and Coronal Explorer'') at 171 Å and 1600 Å and Hida Domeless Solar telescope. Counter-streaming flows (±10 km s-1) in the filament were detected more than 24 hours before its eruption. A slow rise of the global structure started during this time period with a velocity estimated to be of the order of 1 km s-1. During the hour before the eruption (August 26 around 09:00 UT) the velocity reached 5 km s-1. The filament eruption is suspected to be responsible for a slow CME observed by LASCO around 21:00 UT on August 26. No brightening in Hα or in coronal lines, no new emerging polarities in the filament channel, even with the high polarimetry sensitivity of THEMIS, were detected. We measured a relatively large decrease of the photospheric magnetic field strength of the network (from 400 G to 100 G), whose downward magnetic tension provides stability to the underlying stressed filament magnetic fields. According to some MHD models based on turbulent photospheric diffusion, this gentle decrease of magnetic strength (the tension) could act as the destabilizing mechanism which first leads to the slow filament rise and its fast eruption.

Authors: B. Schmieder, V. Bommier, Y. Kitai, T. Matsumoto, T.T. Ishii, M. Hagino, H. Li, L. Golub
Projects: None

Publication Status: Solar Physics, accepted.
Last Modified: 2007-11-28 13:14
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Subject will be restored when possible  

Hui Li   Submitted: 2007-09-16 23:18

In this paper, we use optical and X-ray data from the Hinode satellite to study the response of solar chromosphere and corona to the emerging flux in the photosphere. Totally twelve emerging flux regions (EFRs) are selected to conduct this study. The average separation of the two polarities and lifetime of these EFRs are 3.2'' and 40 min, respectively. All these regions show corresponding enhanced emission in Ca II H line and soft X-rays (SXR) with different filters, indicating that these small-scale and short-lived EFRs heat the chromosphere and the corona to some extent.

Authors: H. Li, T. Sakurai, K. Ichimito, Y. Suematsu, S. Tsuneta, Y. Katsukawa, et al.
Projects: Hinode

Publication Status: PASJ, Vol. 59, Hinode Special Issue (accepted)
Last Modified: 2007-09-17 08:12
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Subject will be restored when possible  

Hui Li   Submitted: 2007-09-07 20:49

The aim of this paper is to understand the magnetic configuration and evolution of an active region, which permits to observe an X1.7 flare during the decaying phase of a long duration X1.5 flare on 13 September 2005. We performed a multi-wavelength analysis using data from space-borne (Solar and Heliospheric Observatory (SOHO), Transition Region and Coronal Explorer (TRACE), Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), GOES) and ground-based (the French-Italian THEMIS telescope and the Huairou Video Vector Magnetograph (HVVM)) instruments. We coaligned all the data in order to study the origin of the flare by comparing the observed magnetic field structures with the emissions detected by different instruments. Reconstructed RHESSI images show three hard X-ray (HXR) sources. In TRACE 195 A images, two loops are seen: a short bright loop and a longer one. Five ribbons are identified in Hα images, two of them being remnant ribbons of the previous flare. We propose the following scenario to explain the X1.7 flare. A reconnection occurs between the short loop system and the longer loops (TRACE 195 A). Two X-ray sources could be the footpoints of the short loop, while the third one between the two others is the site of the reconnection. The Hα ribbons are the footprints in the chromosphere of the reconnected loops. During the reconnection, the released energy is principally non thermal according to the RHESSI energy spectrum analysis (two order of magnitude larger than the maximum thermal energy). The proposed scenario is confirmed by a non linear force-free field (NLFF) extrapolation, which shows the presence of short sheared magnetic field lines before the eruption and less sheared after the econnection, and the connectivity of the field lines involved in the flaring activity is modified after the reconnection process. The evolution of the photospheric magnetic field over few days shows the existence of a continuous emergence of a large-scale magnetic flux tube, the tongue-shape of the two main polarities of the active region being the signature of such an emergence. After the previous X1.5 flare, the emergence of the tube continues and favors new magnetic energy storage and the onset of the X1.7 flare.

Authors: H. Li, B. Schmieder, M.T. Song, and V. Bommier
Projects: None

Publication Status: A&A, accepted.
Last Modified: 2007-09-10 09:05
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Subject will be restored when possible  

Hui Li   Submitted: 2007-09-07 20:49

The aim of this paper is to understand the magnetic configuration and evolution of an active region, which permits to observe an X1.7 flare during the decaying phase of a long duration X1.5 flare on 13 September 2005. We performed a multi-wavelength analysis using data from space-borne (Solar and Heliospheric Observatory (SOHO), Transition Region and Coronal Explorer (TRACE), Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), GOES) and ground-based (the French-Italian THEMIS telescope and the Huairou Video Vector Magnetograph (HVVM)) instruments. We coaligned all the data in order to study the origin of the flare by comparing the observed magnetic field structures with the emissions detected by different instruments. Reconstructed RHESSI images show three hard X-ray (HXR) sources. In TRACE 195 A images, two loops are seen: a short bright loop and a longer one. Five ribbons are identified in Hα images, two of them being remnant ribbons of the previous flare. We propose the following scenario to explain the X1.7 flare. A reconnection occurs between the short loop system and the longer loops (TRACE 195 A). Two X-ray sources could be the footpoints of the short loop, while the third one between the two others is the site of the reconnection. The Hα ribbons are the footprints in the chromosphere of the reconnected loops. During the reconnection, the released energy is principally non thermal according to the RHESSI energy spectrum analysis (two order of magnitude larger than the maximum thermal energy). The proposed scenario is confirmed by a non linear force-free field (NLFF) extrapolation, which shows the presence of short sheared magnetic field lines before the eruption and less sheared after the econnection, and the connectivity of the field lines involved in the flaring activity is modified after the reconnection process. The evolution of the photospheric magnetic field over few days shows the existence of a continuous emergence of a large-scale magnetic flux tube, the tongue-shape of the two main polarities of the active region being the signature of such an emergence. After the previous X1.5 flare, the emergence of the tube continues and favors new magnetic energy storage and the onset of the X1.7 flare.

Authors: H. Li, B. Schmieder, M.T. Song, and V. Bommier
Projects:

Publication Status: A&A, accepted.
Last Modified: 2007-09-13 17:41
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Subject will be restored when possible  

Hui Li   Submitted: 2007-09-07 20:49

The aim of this paper is to understand the magnetic configuration and evolution of an active region, which permits to observe an X1.7 flare during the decaying phase of a long duration X1.5 flare on 13 September 2005. We performed a multi-wavelength analysis using data from space-borne (Solar and Heliospheric Observatory (SOHO), Transition Region and Coronal Explorer (TRACE), Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI), GOES) and ground-based (the French-Italian THEMIS telescope and the Huairou Video Vector Magnetograph (HVVM)) instruments. We coaligned all the data in order to study the origin of the flare by comparing the observed magnetic field structures with the emissions detected by different instruments. Reconstructed RHESSI images show three hard X-ray (HXR) sources. In TRACE 195 A images, two loops are seen: a short bright loop and a longer one. Five ribbons are identified in Hα images, two of them being remnant ribbons of the previous flare. We propose the following scenario to explain the X1.7 flare. A reconnection occurs between the short loop system and the longer loops (TRACE 195 A). Two X-ray sources could be the footpoints of the short loop, while the third one between the two others is the site of the reconnection. The Hα ribbons are the footprints in the chromosphere of the reconnected loops. During the reconnection, the released energy is principally non thermal according to the RHESSI energy spectrum analysis (two order of magnitude larger than the maximum thermal energy). The proposed scenario is confirmed by a non linear force-free field (NLFF) extrapolation, which shows the presence of short sheared magnetic field lines before the eruption and less sheared after the econnection, and the connectivity of the field lines involved in the flaring activity is modified after the reconnection process. The evolution of the photospheric magnetic field over few days shows the existence of a continuous emergence of a large-scale magnetic flux tube, the tongue-shape of the two main polarities of the active region being the signature of such an emergence. After the previous X1.5 flare, the emergence of the tube continues and favors new magnetic energy storage and the onset of the X1.7 flare.

Authors: H. Li, B. Schmieder, M.T. Song, and V. Bommier
Projects:

Publication Status: A&A, accepted.
Last Modified: 2007-09-16 23:09
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Spectral Characteristics of Solar Flares in Different Chromospheric Lines and Their Implications  

Hui Li   Submitted: 2007-02-25 22:46

Using the spectral data of representative solar ares observed with the infrared detector system of the solar spectrograph at Purple Mountain Observatory, we study the spectroscopic characteristics of solar ares in the Hα , the CaII 8542 and the HeI 10830 lines in different phases and various locations of ares, and discuss their possible implications in couple with space observations. Our results show that in the initial phase of a are the Hα line displays red-shift only and no wide-wing. Large broadenings of the Hα line are observed in a few minutes after the are onset within small regions of 3'' - 5'' in both disk and limb ares with/without non-thermal processes. Far wings similar to those of damping roadening appear not only in the Hα line but in the HeI 10830 line as well in ares with non-thermal processes. Sometimes we even detect weak far-wing emission in the CaII 8542 line in disk ares. Such large broadenings are observed in both the footpoints and the are loop-top regions, and possibly result from strong turbulence and/or macroscopic motions. Therefore, the so-called 'non-thermal wing' of the Hα line prossile is not a suffcient condition to distinguish whether non-thermal electrons are accelerated or not in a flare. The CaII 8542 line shows lower intensity in the loop-top regions and higher intensity in the parts close to the solar surface. Emissions larger than nearby continuum in the HeI 10830 line are detected only in small regions with strong X-ray emissions and avoid sunspot umbrae.

Authors: Hui Li, Jianqi You, Xingfeng Yu and Qiusheng Du
Projects: None

Publication Status: Solar Physics, accepted.
Last Modified: 2007-02-26 11:04
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Results from the Study of an M7.6 Flare and its Associated CME  

Hui Li   Submitted: 2006-12-06 18:29

An M7.6 flare was well observed on 2003 October 24 in active region 10486 by a few instruments and satellites, including GOES, TRACE, SOHO, RHESSI and NoRH. Multi-wavelength study shows that this flare underwent two episodes. During the first episode, only a loop-top source of <40 keV was observed in reconstructed RHESSI images, which showed shrinkage with a velocity of 12 - 14 km s-1 in a period of about 12 minutes. During the second process, in addition to the loop-top source, two footpoint sources were observed in energy channel of as high as ~ 200 keV. One of them showed fast propagation along one of the two TRACE 1600 {AA} flare ribbons and the 195 {AA} loop footpoints, which could be explained by successive magnetic reconnection. The associated CME showed a mass pickup process with decreasing center-of-mass velocity. The decrease of the CME kinetic energy and the increase of its potential energy lead to an almost constant total energy during the CME propagation. Our results reveal that the flare and its associated CME have comparable energy content, and the flare is of non-thermal property.

Authors: Hui Li and Youping Li
Projects: None

Publication Status: Advances in Space Research (accepted)
Last Modified: 2006-12-07 12:02
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IS PRE-ERUPTIVE NULL POINT RECONNECTION REQUIRED FOR TRIGGERING ERUPTIONS?  

Hui Li   Submitted: 2006-06-15 00:35

We study the magnetic field evolution and topology of the active region NOAA 10486 before the 3B/X1.2 flare of 2003 October 26, using observational data from the French-Italian THEMIS telescope, the Michelson Doppler Imager onboard Solar and Heliospheric Observatory, the Solar Magnetic Field Telescope at Huairou Solar Observing Station, and the Transition Region and Coronal Explorer. Three dimensional (3D) extrapolation of photospheric magnetic field, assuming a potential field configuration, reveals the existence of two magnetic null points in the corona above the active region. We look at their role in the triggering of the main flare, by using the bright patches observed in TRACE 1600 {AA} images as tracers at the solar surface of energy release associated with magnetic reconnection at the null points. All the bright patches observed before the flare correspond to the low-altitude null point. They have no direct relationship with the X1.2 flare because the related separatrix is located far from the eruptive site. No bright patch corresponds to the high-altitude null point before the flare. We conclude that eruptions can be triggered without pre-eruptive coronal null point reconnection, and the presence of null points is not a sufficient condition for the occurrence of flares. We propose that this eruptive flare results from the loss of equilibrium due to persistent flux emergence, continuous photospheric motion and strong shear along the magnetic neutral line. The opening of the coronal field lines above the active region should be a byproduct of the large 3B/X1.2 flare rather than its trigger.

Authors: HUI LI, BRIGITTE SCHMIEDER, GUILLAUME AULANIER, ARKADIUSZ BERLICKI
Projects: None

Publication Status: Solar Physics, (accepted)
Last Modified: 2006-06-16 11:07
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Multi-Wavelength Study of the 3B/X1.2 Flare Observed on 2003 October 26  

Hui Li   Submitted: 2005-10-01 23:41

We report the results from the multi-wavelength study of the 3B/X1.2 two-ribbon disk flare (S15E44), which was well observed by both ground-based and space-borne instruments. Two pairs of conjugate kernels - K1 and K4, and K2 and K3 - in Hα images are identified for this flare linked by two different systems of EUV loops. K1 and K4 correspond to the two 17 GHz and 34 GHz microwave sources observed by the Nobeyama Radioheliograph (NoRH), while K2 and K3 have no corresponding microwave source. Optical spectroscopic observations suggest that all the four kernels are possible precipitating sites of non-thermal electrons. Thus the amount of deposited energy in K2 and K3 should be less than 100 keV. Two-dimensional distributions of the full widths at half maximum (FWHM) of the Hα profiles and the line-of-sight (LOS) velocities derived from the CaII 8542 profiles indicate that the largest FWHM and LOS velocity tend to be located near the outer edges of Hα kernels, which is consistent with the scenario of current two-ribbon flare models and previous results. When non-thermal electron bombardment is present, the observed Hα and CaII 8542 profiles are similar to previous observational and theoretical results, while the HeI 10830 profiles are different from the theoretical ones. This puts some constraints on the future theoretical calculation for the HeI 10830 line.

Authors: Hui Li, Jianping Li, Cheng Fang, Brigitte Schmieder, Arkadiusz Berlicki and Qiusheng Du
Projects: None

Publication Status: Chin. J. Astron. Astrophys., in press
Last Modified: 2005-10-01 23:41
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RHESSI Spectral Analysis of the 1N/M1.9 flare of 20 October 2003  

Hui Li   Submitted: 2004-10-14 17:44

We studied the 1N/M1.9 confined flare of 20 October 2003 observed during a joint observation program (JOP157) with emphasis on the Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) X-ray spectra analysis and synthetic analysis with data from other instruments and in other wavelengths. This flare shows a long impulsive phase and four GOES 1 - 8 A X-ray peaks/plateaus implying multiple magnetic field reconnections during the flare. This results is supported by the change of hard X-ray (HXR) source at different times. Results from the RHESSI spectra analysis reveal that there were super-hot plasma bulks of up to 6.5e7 K during the impulsive phase and HXR emissions of the flare that underwent a `soft - hard - softer - harder - soft' evolution because of the multiple reconnections. Using the fitting parameters of RHESSI spectra analysis and the assumption of a `thin-target' model with a low energy cutoff of 15 keV for the power-law distribution of non-thermal electrons, we calculated the thermal and non-thermal energies stored in the flare, and showed that in most of the impulsive and gradual phase the non-thermal energy represent a small part (<15%) of the total flare energy, suggesting that the HXR emissions are of thermal feature and the energies released during several magnetic field reconnections are transported to the chromosphere mainly by thermal conduction. Factors affecting the energy estimation are discussed.

Authors: Hui Li, Arkadiusz Berlicki, and Brigitte Schmieder
Projects: None

Publication Status: Submitted to A&A
Last Modified: 2004-10-14 17:44
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THE WHITE LIGHT LIMB FLARE OF 16 AUGUST 1989 AND ITS CHROMOSPHERIC COUNTERPART  

Hui Li   Submitted: 2003-06-02 21:56

After carefully comparing the white-light (lambda 5600 ±800 A) and the slit-jaw Hα images (0.5 A passband) of the 2N/X20 white-light flare of 16 August 1989, we found that the Hα counterpart identification of the bright kernels in continuum by Hiei et al. (1992) was incorrect. Now we come to the conclusion that both of those two white-light kernels do not have corresponding bright Hα area. Moreover, the loop shapes in white-light are also different from those in Hα . Hα loops rose more rapidly than white-light loops. However, their height-time variations on the whole are similar. This indicates that the continuum and chromospheric emissions of the flare presumably come from different plasmas, but may be modulated by some mutual factors, such as large-scale magnetic fields. Analysis of the HeI 10830 spectra taken simultaneously with the slit-jaw Hα images shows that the line-center intensity of HeI 10830 doesn't have a good correlation with the intensity of nearby continuum, which supports the above conclusions. In addition, the electron density at the white-light loop top estimated from the continuum around lambda 5600A~ and lambda 10830A is as high as 1012-1013/cm^3.

Authors: Jianqi You, Eijiro Hiei and Hui Li
Projects: None

Publication Status: Solar Physics (submitted)
Last Modified: 2003-06-02 21:56
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Abstracts by Author
Observational Study of a Peculiar Solar Limb Event Occurred on 11 January 2002
Calibration and initial results of the HeI D3 line flash spectrum obtained during the 2008 total solar eclipse
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Subject will be restored when possible
Spectral Characteristics of Solar Flares in Different Chromospheric Lines and Their Implications
Results from the Study of an M7.6 Flare and its Associated CME
IS PRE-ERUPTIVE NULL POINT RECONNECTION REQUIRED FOR TRIGGERING ERUPTIONS?
Multi-Wavelength Study of the 3B/X1.2 Flare Observed on 2003 October 26
RHESSI Spectral Analysis of the 1N/M1.9 flare of 20 October 2003
THE WHITE LIGHT LIMB FLARE OF 16 AUGUST 1989 AND ITS CHROMOSPHERIC COUNTERPART

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