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Successive X-class flares and coronal mass ejections driven by shearing motion and sunspot rotation in active region NOAA 12673  

Xiaoli Yan   Submitted: 2018-02-28 18:19

We present a clear case study on the occurrence of two successive X-class flares including a decade-class flare (X9.3) and two coronal mass ejections (CMEs) triggered by shearing motion and sunspot rotation in active region NOAA 12673 on 2017 September 6. A shearing motion between the main sunspots with opposite polarities started on September 5 and even lasted after the second X-class flare on September 6. Moreover, the main sunspot with negative polarity rotated around its umbral center and another main sunspot with positive polarity also exhibited a slow rotation. The sunspot with negative polarity at the northwest of active region also began to rotate counter-clockwise before the onset of the first X-class flare, which is related to the formation of the second S-shaped structure. The successive formation and eruption of two S-shaped structures were closely related to the counter-clockwise rotation of three sunspots. The existence of a flux rope is found prior to the onset of two flares by using non-linear force free field extrapolation based on the vector magnetograms observed by SDO/HMI. The first flux rope corresponds to the first S-shaped structures mentioned above. The second S-shaped structure was formed after the eruption of the first flux rope. These results suggest that shearing motion and sunspot rotation play an important role in the buildup of the free energy and the formation of flux ropes in the corona which produces solar flares and CMEs.

Authors: Yan,X.L., Wang,J.X., Pan, G.M., Kong, D.F., Xue, Z.K., Yang, L.H., Li, Q.L., Feng, X.S.
Projects: New Vacuum Solar Telescope (NVST),SDO-AIA,SDO-HMI

Publication Status: Accepted for publication in ApJ
Last Modified: 2018-03-01 16:29
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Simultaneous observation of a flux rope eruption and magnetic reconnection during an X-class solar flare  

Xiaoli Yan   Submitted: 2018-01-10 18:06

In this letter, we present a spectacular eruptive flare (X8.2) associated with a coronal mass ejection (CME) on 2017 September 10 at the west limb of the Sun. A flux rope eruption is followed by the inflow, the formation of a current sheet and a cusp structure, which were simultaneously observed during the occurrence of this flare. The hierarchical layers of the cusp-shaped structure are well observed in 131 Å observation. The scenario that can be created from these observations is very consistent with the predictions of some eruptive models. Except for the characteristics mentioned above in the process of the flare predicted by classical eruption models, the current sheet separating into several small current sheets is also observed at the final stage of the flux rope eruption. The quantitative calculation of the velocities and accelerations of the inflow, hot cusp structure, and post-flare loops is presented. The width of the current sheet is estimated to be about 3 x 103 km. These observations are very useful to understand the process of solar eruptions.

Authors: X. L. Yan, L. H. Yang, Z. K. Xue, Z. X. Mei, D. F. Kong, J. C. Wang, Q. L. Li
Projects: SDO-AIA

Publication Status: Accepted for publication in ApJL
Last Modified: 2018-01-14 00:01
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The eruption of a small-scale emerging flux rope as the driver of an M-class flare and a coronal mass ejection  

Xiaoli Yan   Submitted: 2017-07-08 10:19

Solar flares and coronal mass ejections (CMEs) are the most powerful explosions in the Sun. They are major sources of potentially destructive space weather conditions. However, the possible causes of their initiation remain controversial. By using high resolution data observed by NST of BBSO, supplemented by Solar Dynamics Observatory (SDO) observations, we present unusual observations of a small-scale emerging flux rope near a large sunspot, whose eruption produced an M-class flare and a coronal mass ejection. The presence of the small-scale flux rope was indicated by static nonlinear force free field (NLFFF) extrapolation as well as data-driven MHD modeling of the dynamic evolution of the coronal 3D magnetic field. During the emergence of the flux rope, rotation of satellite sunspots at the footpoints of the flux rope was observed. Meanwhile, the Lorentz force, magnetic energy, vertical current, and transverse fields were increasing during this phase. The free energy from the magnetic flux emergence and twisting magnetic fields is sufficient to power the M-class flare. These observations for the first time present the complete process from the emergence of the small-scale flux rope to the production of solar eruptions.

Authors: X.L. Yan, C.W. Jiang, Z.K. Xue, J.C. Wang, E.R. Priest, L.H. Yang, D.F. Kong, W.D. Cao, H.S. Ji
Projects: BBSO/NST

Publication Status: accepted for publication in ApJ.
Last Modified: 2017-07-11 11:23
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The formation of an inverse S-shaped active-region filament driven by sunspot motion and magnetic reconnection  

Xiaoli Yan   Submitted: 2016-09-21 04:46

We present a detailed study of the formation of an inverse S-shaped filament prior to its eruption in active region NOAA 11884 from October 31 to November 2, 2013. In the initial stage, clockwise rotation of a small positive sunspot around the main negative trailing sunspot formed a curved filament. Then the small sunspot cancelled with negative magnetic flux to create a longer active-region filament with an inverse S-shape. At the cancellation site a brightening was observed in UV and EUV images and bright material was transferred to the filament. Later the filament erupted after cancellation of two opposite polarities under the upper part of the filament. Nonlinear force-free field (NLFFF) extrapolation of vector photospheric fields suggests that the filament may have a twisted structure, but this cannot be confirmed from the current observations.

Authors: X.L. Yan, E.R. Priest, Q.L. Guo, Z.K. Xue, J.C. Wang, L.H. Yang
Projects: New Vacuum Solar Telescope (NVST)

Publication Status: Accepted for publication in ApJ.
Last Modified: 2016-09-21 11:19
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Observing the release of twist by magnetic reconnection in a solar filament eruption  

Xiaoli Yan   Submitted: 2016-06-16 23:54

Magnetic reconnection is a fundamental process of topology change and energy release, taking place in plasmas on the Sun, in space, in astrophysical objects and in the laboratory. However, observational evidence has been relatively rare and typically only partial. Here we present evidence of fast reconnection in a solar filament eruption using high-resolution Hα images from the New Vacuum Solar Telescope, supplemented by extreme ultraviolet observations. The reconnection is seen to occur between a set of ambient chromospheric fibrils and the filament itself. This allows for the relaxation of magnetic tension in the filament by an untwisting motion, demonstrating a flux rope structure. The topology change and untwisting are also found through nonlinear force-free field modelling of the active region in combination with magnetohydrodynamic simulation. These results demonstrate a new role for reconnection in solar eruptions: the release of magnetic twist.

Authors: Zhike Xue, Xiaoli Yan, Xin Cheng, Liheng Yang, Yingna Su, Bernhard Kliem, Jun Zhang, Zhong Liu, Yi Bi, Yongyuan Xiang, Kai Yang, Li Zhao
Projects: Hinode/XRT,New Vacuum Solar Telescope (NVST),SDO-AIA,SDO-HMI,SXI

Publication Status: Nature Communications, DOI:10.1038/NCOMMS11837
Last Modified: 2016-06-22 09:13
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The Formation and Magnetic Structures of Active-region Filaments Observed by NVST, SDO, and Hinode  

Xiaoli Yan   Submitted: 2015-08-25 17:30

To better understand the properties of solar active-region filaments, we present a detailed study on the formation and magnetic structures of two active-region filaments in active region NOAA 11884 during a period of four days. It is found that the shearing motion of the opposite magnetic polarities and the rotation of the small sunspots with negative polarity play an important role in the formation of two active-region filaments. During the formation of these two active-region filaments, one foot of the filaments was rooted in a small sunspot with negative polarity. The small sunspot rotated not only around another small sunspot with negative polarity, but also around the center of its umbra. By analyzing the nonlinear force-free field extrapolation using the vector magnetic fields in the photosphere, twisted structures were found in the two active-region filaments prior to their eruptions. These results imply that the magnetic fields were dragged by the shearing motion between opposite magnetic polarities and became more horizontal. The sunspot rotation twisted the horizontal magnetic fields and finally formed the twisted active-region filaments.

Authors: X.L. Yan, Z.K. Xue, G.M. Pan, J.C. Wang, Y.Y. Xiang, D.F. Kong, and L.H. Yang
Projects: SDO-AIA,SDO-HMI

Publication Status: Published in the Astrophysical Journal Supplement Series
Last Modified: 2015-08-26 13:13
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Abstracts by Author
Successive X-class flares and coronal mass ejections driven by shearing motion and sunspot rotation in active region NOAA 12673
Simultaneous observation of a flux rope eruption and magnetic reconnection during an X-class solar flare
The eruption of a small-scale emerging flux rope as the driver of an M-class flare and a coronal mass ejection
The formation of an inverse S-shaped active-region filament driven by sunspot motion and magnetic reconnection
Observing the release of twist by magnetic reconnection in a solar filament eruption
The Formation and Magnetic Structures of Active-region Filaments Observed by NVST, SDO, and Hinode

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