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 Magnetic Flux of Active Regions Determining the Eruptive Character of Large Solar Flares Ting Li   Submitted: 2020-07-16 21:38 We establish the largest eruptive/confined flare database to date and analyze 322 flares of GOES class M1.0 and larger that occurred during 2010-2019, i.e., almost spanning the entire solar cycle 24. We find that the total unsigned magnetic flux (ΦAR) of active regions (ARs) is a key parameter in governing the eruptive character of large flares, with the proportion of eruptive flares exhibiting a strong anti-correlation with ΦAR. This means that an AR containing a large magnetic flux has a lower probability for the large flares it produces to be associated with a coronal mass ejection (CME). This finding is supported by the high positive correlation we obtained between the critical decay index height and ΦAR, implying that ARs with a larger ΦAR have a stronger magnetic confinement. Moreover, the confined flares originating from ARs larger than 1.0×1023 Mx have several characteristics in common: stable filament, slipping magnetic reconnection and strongly sheared post-flare loops. Our findings reveal new relations between the magnetic flux of ARs and the occurrence of CMEs in association with large flares. These relations obtained here provide quantitative criteria for forecasting CMEs and adverse space weather, and have also important implications for "superflares" on solar-type stars and stellar CMEs. Authors: Ting Li, Yijun Hou, Shuhong Yang, Jun Zhang, Lijuan Liu, Astrid M. Veronig Projects: SDO-HMI

 NVST observations of collision-induced apparent fan-shaped jets Ting Li   Submitted: 2020-02-09 20:35 Using high-quality Hα observations from the New Vacuum Solar Telescope, we first report apparent fan-shaped jets (AFJs) generated during the interaction between primary fan-shaped jets (FJs) and nearby facula magnetic structure. The primary FJs were intermittently launched from a sunspot penumbra with negative-polarity magnetic fields in active region 12740 on 2019 May 6, accompanied by impulsive brightenings at the base. While the propagating FJ encountered and collided with the negative-polarity magnetic structure of the west facula, the density of jet material was enhanced to the east of the facula. Meanwhile, the jet structures exhibited a deflection towards the north-west at the jet?facula collision location. Then the primary FJ evolved into two parts, with one part being reflected away from the facula and the other part forming an AFJ. Easily distinguished from the primary FJ, the ejecting AFJ was more ordered and had an apparent end at the facula. The AFJ was impulsively accelerated to speeds of 100 km s-1, and reached lengths of up to 40 Mm. The observed AFJ had a similar morphology to the fan-shaped quasi-separatrix layer (QSL) between the penumbra and facula magnetic systems, implying that the material of the AFJ was mainly guided by the fan plane of the QSL.We suggest that the collision does not cause a change in the field-line connectivity and only leads to the redistribution of jet material. Authors: Ting Li,Yijun Hou,Jun Zhang and Yongyuan Xiang Projects: New Vacuum Solar Telescope (NVST)

 Three-dimensional Magnetic Reconnection Triggering an X-class Confined Flare in Active Region 12192 Ting Li   Submitted: 2018-11-08 19:15 We present an extensive analysis of the X2.0-class confined flare on 2014 October 27 in the great active region AR 12192, observed by the IRIS and the SDO. The slipping motion of the substructures within the negative-polarity flare ribbon (R1) and continual reconnection-induced flows during the confined flare are first presented. The substructures within ribbon R1 were observed to slip in opposite directions at apparent speeds of 10-70 km s-1. The slipping motion exhibited the quasi-periodic pattern with a period of 80-110 s, which can be observed since the flare start and throughout the impulsive phase of the flare. Simultaneously quasi-periodic flows moved along a reverse-S shaped filament, with an average period of about 90 s. The period of reconnection-induced flows is similar to that of the slippage of ribbon substructures, implying the occurrence of quasi-periodic slipping magnetic reconnection. The spectral observations showed that the Si IV line was blueshifted by 50-240 km s-1 at the location of the flows. During the process of the flare, the filament did not show the rise phase and was not associated with any failed eruption. The flare mainly consisted of two sets of magnetic systems, with both of their east ends anchoring in ribbon R1. We suggest that the slipping magnetic reconnection between two magnetic systems triggers the confined flare. Authors: Ting Li, Yijun Hou, Shuhong Yang & Jun Zhang Projects: IRIS

 Two Episodes of Magnetic Reconnections During a Confined Circular-ribbon Flare Ting Li   Submitted: 2018-04-18 05:59 We analyze a unique event with an M1.8 confined circular-ribbon flare on 2016 February 13, with successive formations of two circular ribbons at the same location. The flare had two distinct phases of UV and EUV emissions with an interval of about 270 s, of which the second peak was energetically more important. The first episode was accompanied by the eruption of a mini-filament and the fast elongation motion of a thin circular ribbon (CR1) along the counterclockwise direction at a speed of about 220 km s-1. Two elongated spine-related ribbons were also observed, with the inner ribbon co-temporal with CR1 and the remote brightenings forming ~ 20 s later. In the second episode, another mini-filament erupted and formed a blowout jet. The second circular ribbon and two spine-related ribbons showed similar elongation motions with that during the first episode. The extrapolated 3D coronal magnetic fields reveal the existence of a fan-spine topology, together with a quasi-separatrix layer (QSL) halo surrounding the fan plane and another QSL structure outlining the inner spine. We suggest that continuous null-point reconnection between the filament and ambient open field occurs in each episode, leading to the sequential opening of the filament and significant shifts of the fan plane footprint. For the first time, we propose a compound eruption model of circular-ribbon flares consisting of two sets of successively formed ribbons and eruptions of multiple filaments in a fan-spine-type magnetic configuration. Authors: Ting Li, Shuhong Yang, Qingmin Zhang, Yijun Hou & Jun Zhang Projects: IRIS

 Flare Ribbons Approach Observed by the IRIS and the SDO Ting Li   Submitted: 2017-09-12 21:49 We report flare ribbons approach (FRA) during a multiple-ribbon M-class flare on 2015 November 4 in NOAA AR 12443, obtained by the Interface Region Imaging Spectrograph and the Solar Dynamics Observatory. The flare consisted of a pair of main ribbons and two pairs of secondary ribbons. The two pairs of secondary ribbons were formed later than the appearance of main ribbons, with respective time delays of 15 and 19 minutes. The negative-polarity main ribbon spread outward faster than the first secondary ribbon with the same polarity in front of it, and thus the FRA was generated. Just before their encounter, the main ribbon was darkening drastically and its intensity decreased by about 70% in 2 minutes, implying the suppression of main-phase reconnection that produced two main ribbons. The FRA caused the deflection of the main ribbon to the direction of secondary ribbon with a deflection angle of about 60 degree. Post-approach arcade was formed about 2 minutes later and the downflows were detected along the new arcade with velocities of 35-40 km s-1, indicative of the magnetic restructuring during the process of FRA. We suggest that there are three topological domains with footpoints outlined by the three pairs of ribbons. Close proximity of these domains leads to deflection of the ribbons in agreement with the magnetic field topology. Authors: Ting Li, Jun Zhang & Yijun Hou Projects: IRIS

 Slipping Magnetic Reconnection of Flux Rope Structures as a Precursor to an Eruptive X-class Solar Flare Ting Li   Submitted: 2016-08-08 21:02 We present the quasi-periodic slipping motion of flux rope structures prior to the onset of an eruptive X-class flare on 2015 March 11, obtained by the Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory (SDO). The slipping motion occurred at the north part of the flux rope and seemed to successively peel off the flux rope. The speed of the slippage was 30-40 km s-1, with an average period of 130±30 s. The Si IV 1402.77 Å line showed a redshift of 10-30 km s-1 and a line width of 50-120 km s-1 at the west legs of slipping structures, indicative of reconnection downflow. The slipping motion lasted about 40 min and the flux rope started to rise up slowly at the late stage of the slippage. Then an X2.1 flare was initiated and the flux rope was impulsively accelerated. One of the flare ribbons swept across a negative-polarity sunspot and the penumbral segments of the sunspot decayed rapidly after the flare. We studied the magnetic topology at the flaring region and the results showed the existence of a twisted flux rope, together with quasi-separatrix layers (QSLs) structures binding the flux rope. Our observations imply that quasi-periodic slipping magnetic reconnection occurs along the flux-rope-related QSLs in the preflare stage, which drives the later eruption of the flux rope and the associated flare. Authors: Ting Li, Kai Yang, Yijun Hou & Jun Zhang Projects: IRIS

 Subarcsecond Bright Points and Quasi-periodic Upflows Below a Quiescent Filament Observed by the IRIS Ting Li   Submitted: 2016-03-15 05:31 The new Interface Region Imaging Spectrograph (IRIS) mission provides high-resolution observations of UV spectra and slit-jaw images (SJIs). These data has become available for investigating the dynamic features in the transition region (TR) below the on-disk filaments. The driver of counter-streaming" flows along the filament spine is still unknown yet. The magnetic structures and the upflows at the footpoints of the filaments, and their relations with the filament mainbody have not been well understood. We study the dynamic evolution at the footpoints of filaments in order to find some clues for these unsolved questions. Using UV spectra and SJIs from the IRIS, and coronal images and magnetograms from the Solar Dynamics Observatory (SDO), we present the new features in a quiescent filament channel: subarcsecond bright points (BPs) and quasi-periodic upflows. The BPs in the TR have a spatial scale of about 350-580 km and lifetime of more than several tens of minutes. They are located at stronger magnetic structures in the filament channel, with magnetic flux of about 1017-1018 Mx. Quasi-periodic brightenings and upflows are observed in the BPs and the period is about 4-5 min. The BP and the associated jet-like upflow comprise a tadpole-shaped" structure. The upflows move along bright filament threads and their directions are almost parallel to the spine of the filament. The upflows initiated from the BPs with opposite polarity magnetic fields have opposite directions. The velocity of the upflows in plane of sky is about 5-50 km s-1. The emission line of Si IV 1402.77 Å at the locations of upflows exhibits obvious blueshifts of about 5-30 km s-1, and the line profile is broadened with the width of more than 20 km s-1. The BPs seem to be the bases of filament threads and the upflows are able to convey mass for the dynamic balance of the filament. The counter-streaming" flows in previous observations may be caused by the propagation of bi-directional upflows initiated from opposite polarity magnetic fields. We suggest that quasi-periodic brightenings of BPs and quasi-periodic upflows result from small-scale oscillatory magnetic reconnections, which are modulated by solar p-mode waves. Authors: Ting Li & Jun Zhang Projects: IRIS

 High-resolution Observations of a Flux Rope with the Interface Region Imaging Spectrograph Ting Li   Submitted: 2015-08-31 20:10 We report the observations of a flux rope at transition region temperatures with the Interface Region Imaging Spectrograph (IRIS) on 30 August 2014. Initially, magnetic flux cancellation constantly took place and a filament was activated. Then the bright material from the filament moved southward and tracked out several fine structures. These fine structures were twisted and tangled with each other, and appeared as a small flux rope at 1330 Å, with a total twist of about 4π. Afterwards, the flux rope underwent a counter-clockwise (viewed top-down) unwinding motion around its axis. Spectral observations of C II 1335.71 Å at the southern leg of the flux rope showed that Doppler redshifts of 6-24 km s-1 appeared at the western side of the axis, which is consistent with the counter-clockwise rotation motion. We suggest that the magnetic flux cancellation initiates reconnection and some activation of the flux rope. The stored twist and magnetic helicity of the flux rope are transported into the upper atmosphere by the unwinding motion in the late stage. The small-scale flux rope (width of 8.3\prime\prime) had a cylindrical shape with helical field lines, similar to the morphology of the large-scale CME core (width of 1.54 R⊙) on 2 June 1998. This similarity shows the presence of flux ropes of different scales on the Sun. Authors: Ting Li, Jun Zhang Projects: IRIS

 Dark Ribbons Propagating and Sweeping Across EUV Structures After Filament Eruptions Ting Li   Submitted: 2015-05-17 21:06 With observations from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we first report that dark ribbons (DRs) moved apart from the filament channel and swept across EUV structures after filament eruptions on 2013 June 23 and 2012 February 10 and 24, respectively. In the first event, the DR with a length of 168Mm appeared at 100Mm to the northwest of the filament channel, where the filament erupted 15 hr previously. The DR moved toward the northwest with the different sections having different velocities, ranging from 0.3 to 1.6 km s-1. When the DR?s middle part swept across a strong EUV structure, the motion of this part was blocked, appearing to deflect the DR. With the DR propagation, the connection of the surrounding EUV structures gradually changed. After one day passed, the DR eventually disappeared. In the other two events, the dynamic evolution of the DRs was similar to that in the first event. Based on the observations, we speculate that the reconnection during the filament eruption changes the configuration of the surrounding magnetic fields systematically. During the reconnection process, magnetic fields are deflecting and the former arbitrarily distributed magnetic fields are rearranged along specific directions. The deflection of magnetic fields results in an instantaneous void region where the magnetic strength is smaller and the plasma density is lower. Consequently, the void region is observed as a DR and propagates outward with the reconnection developing. Authors: Junmin Xiao, Jun Zhang, Ting Li, and Shuhong Yang Projects: SDO-AIA

 Quasi-periodic Slipping Magnetic Reconnection During an X-class Solar Flare Observed by the Solar Dynamics Observatory and Interface Region Imaging Spectrograph Ting Li   Submitted: 2015-04-06 19:42 We firstly report the quasi-periodic slipping motion of flare loops during an eruptive X-class flare on 2014 September 10. The slipping motion was investigated at a specific location along one of the two ribbons and can be observed throughout the impulsive phase of the flare. The apparent slipping velocity was 20-110 km s-1 and the associated period was 3-6 min. The footpoints of flare loops appeared as small-scale bright knots observed in 1400 Å, corresponding to fine structures of the flare ribbon. These bright knots were observed to move along the southern part of the longer ribbon and also exhibited a quasi-periodic pattern. The Si IV 1402.77 Å line was redshifted by 30-50 km s-1 at the locations of moving knots with a ~ 40-60 km s-1 line width, larger than other sites of the flare ribbon. We suggest that the quasi-periodic slipping reconnection is involved in this process and the redshift at the bright knots is probably indicative of reconnection downflow. The emission line of Si IV at the northern part of the longer ribbon also exhibited obvious redshifts of about 10-70 km s-1 in the impulsive phase of the flare, with the redshifts at the outer edges of the ribbon larger than those in the middle. The redshift velocities at post-flare loops reached about 80-100 km s-1 in the transition region. Authors: Ting Li, Jun Zhang Projects: IRIS

 Filament Activation in Response to Magnetic Flux Emergence and Cancellation in Filament Channels Ting Li   Submitted: 2015-04-06 19:40 We make a comparative analysis for two filaments that showed quite different activation in response to the flux emergence within the filament channels. The observations from the Solar Dynamics Observatory (SDO) and Global Oscillation Network Group (GONG) are carried out to analyze the two filaments on 2013 August 17-20 and September 29. The first event showed that the main body of the filament was separated into two parts when an active region (AR) emerged with a maximum magnetic flux of about 6.4*1021 Mx underlying the filament. The close neighborhood and common direction of the bright threads in the filament and the open AR fan loops suggest similar magnetic connectivity of these two flux systems. The equilibrium of the filament was not destroyed within 3 days after the start of the emergence of the AR. To our knowledge, similar observations have never been reported before. In the second event, the emerging flux occurred nearby a barb of the filament with a maximum magnetic flux of 4.2*1020 Mx, about one order of magnitude less than that of the first event. The emerging flux drove the convergence of two patches of parasitic polarity in the vicinity of the barb, and resulted in cancellation between the parasitic polarity and nearby network fields. About 20 hours after the onset of the emergence, the filament was entirely erupted. Our findings imply that the location of emerging flux within the filament channel is probably crucial to filament evolution. If the flux emergence appears nearby the barbs, flux cancellation of emerging flux with the filament magnetic fields is prone to occur, which probably causes the filament eruption. The comparison of the two events shows that the emergence of an entire AR may still not be enough to disrupt the stability of a filament system and the actual eruption does occur only after the flux cancellation sets in. Authors: Ting Li, Jun Zhang, Haisheng Ji Projects: SDO-AIA

 Slipping Magnetic Reconnection Triggering a Solar Eruption of a Triangle-flag Flux Rope Ting Li   Submitted: 2014-07-15 20:19 We firstly report the simultaneous activities of a slipping motion of flare loops and a slipping eruption of a flux rope in 131 Å and 94 Å channels on 2014 February 02. The east hook-like flare ribbon propagated slippingly at a speed of about 50 km s-1, which lasted about 40 min and extended by more than 100 Mm, but the west flare ribbon moved in the opposite direction with a speed of 30 km s-1. At the later phase of the flare activity, a bi-fan" system of flare loops was well developed. The east footpoints of the flux rope showed an apparent slipping motion along the hook of the ribbon, simultaneously the fine structures of the flux rope rose up rapidly at a speed of 130 km s-1, much faster the whole flux rope. We infer that the east footpoints of the flux rope are successively heated by a slipping magnetic reconnection during the flare, which results in the apparent slippage of the flux rope. The slipping motion delineates a triangle-flag surface" of the flux rope, implying that the topology of a flux rope is more complex than anticipated. Authors: Ting Li & Jun Zhang Projects: SDO-AIA

 Homologous Flux Ropes Observed by SDO/AIA Ting Li   Submitted: 2013-10-30 03:17 We firstly present the emph{Solar Dynamics Observatory} observations of four homologous flux ropes in active region (AR) 11745 on 2013 May 20-22. The four flux ropes are all above the neutral line of the AR, with endpoints anchoring at the same region, and have the generally similar morphology. For the first three flux ropes, they rose up with a velocity of less than 30 km s-1 after their appearances, and subsequently their intensities at 131 {AA} decreased and the flux ropes became obscure. The fourth flux rope erupted ultimately with a speed of about 130 km s-1 and formed a coronal mass ejection. The associated filament showed an obvious anti-clockwise twist motion at the initial stage, and the twist was estimated at 4pi. This indicates that kink instability possibly triggers the early rise of the fourth flux rope. The activated filament material was spatially within the flux rope and they showed consistent evolution in their early stages. Our findings provide new clues for understanding the characteristics of flux ropes. Firstly, there are multiple flux ropes that are successively formed at the same location during an AR evolution process. Secondly, a slow-rise flux rope does not necessarily result in a CME, and a fast-eruption flux rope results in a CME. Authors: Ting Li & Jun Zhang Projects: SDO-AIA

 Fine-scale Structures of Flux Ropes Tracked by Erupting Material Ting Li   Submitted: 2013-05-12 23:12 We present the Solar Dynamics Observatory observations of two flux ropes respectively tracked out by material from a surge and a failed filament eruption on 2012 July 29 and August 04. For the first event, the interaction between the erupting surge and a loop-shaped filament in the east seems to peel off'' the filament and add bright mass into the flux rope body. The second event is associated with a C-class flare that occurs several minutes before the filament activation. The two flux ropes are respectively composed of 85pm12 and 102pm15 fine-scale structures, with an average width of about 1arcsec.6. Our observations show that two extreme ends of the flux rope are rooted in the opposite polarity fields and each end is composed of multiple footpoints (FPs) of the fine-scale structures. The FPs of the fine-scale structures are located at network magnetic fields, with magnetic fluxes from 5.6 imes1018 Mx to 8.6 imes1019 Mx. Moreover, almost half of the FPs show converging motion of smaller magnetic structures over 10 hr before the appearance of the flux rope. By calculating the magnetic fields of the FPs, we deduce that the two flux ropes occupy at least 4.3 imes1020 Mx and 7.6 imes1020 Mx magnetic fluxes, respectively. Authors: Ting Li & Jun Zhang Projects: SDO-AIA

 SDO/AIA Observations of Large-Amplitude Longitudinal Oscillations in a Solar Filament Ting Li   Submitted: 2012-10-18 05:24 We present the first Solar Dynamics Observatory/Atmospheric Imaging Assembly observations of the large-amplitude longitudinal (LAL) oscillations in the south and north parts (SP and NP) of a solar filament on 2012 April 7. Both oscillations are triggered by flare activities close to the filament. The period varies with filamentary threads, ranging from 44 to 67 min. The oscillations of different threads are out of phase, and their velocity amplitudes vary from 30 to 60 km s-1, with a maximum displacement of about 25 Mm. The oscillations of the SP repeat for about 4 cycles without any significant damping and then a nearby C2.4 flare causes the transition from the LAL oscillations of the filament to its later eruption. The filament eruption is also associated with a coronal mass ejection and a B6.8 flare. However, the oscillations of the NP damp with time and die out at last. Our observations show that the activated part of the SP repeatedly shows a helical motion. This indicates that the magnetic structure of the filament is possibly modified during this process. We suggest that the restoring force is the coupling of the magnetic tension and gravity. Authors: Ting Li and Jun Zhang Projects: SDO-AIA

 SDO/AIA Observations of Secondary Waves Generated by Interaction of the 2011 June 7 Global EUV Wave With Solar Coronal Structures Ting Li   Submitted: 2011-11-08 08:38 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: Ting Li, Jun Zhang, Shuhong Yang, Wei Liu Projects: SDO-AIA,STEREO

 3D Reconstructions of an Erupting Filament with SDO and STEREO Observations Ting Li   Submitted: 2011-07-06 02:58 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: Ting Li, Jun Zhang, Yuzong Zhang, Shuhong Yang Projects: SDO-AIA,SDO-HMI,SoHO-MDI,STEREO

 3D Shape and Evolution of Two Eruptive Filaments Ting Li   Submitted: 2010-07-03 04:29 On 2009 Sep 26, a dramatic and large filament (LF) and a small filament (SF) eruptions were observed in the He II 304 Å ?A line by the two EUVI telescopes aboard the STEREO A and B spacecrafts. The LF heads out into space and becomes the bright core of a gradual CME, while the eruption of the SF is characterized by motions of filament materials. Using stereoscopic analysis of EUVI data, we reconstruct the 3D shape and evolution of two eruptive filaments. For the first time, we investigate the true velocities and accelerations of 12 points along the axis of the LF, and find that the velocity and acceleration vary with the measured location. The highest points among the 12 points are the fastest in the first half hour, and then the points at the low-latitude leg of the LF become the fastest. For the SF, it is an asymmetric whip-like filament eruption, and the downward material motions lead to the disappearance of the former highlatitude endpoint and the formation of a new low-latitude endpoint. Based on the temporal evolution of the two filaments, we infer that the two filaments lie in the same filament channel. By combining EUVI, COR1 and COR2 data of STEREO A together, we find that there is no impulsive or fast acceleration in this event. It displays a weak and persistent acceleration for more than 17 hours. The average velocity and acceleration of the LF are 101.8 km s-1 and 2.9 m s-2, respectively. The filament eruptions are associated with a slow CME with the average velocity of 177.4 km s-1. The velocity of the CME is nearly 1.6 times as large as that of the filament material. This event is one example of a gradual filament eruption associated with a gradual CME. In addition, the moving direction of the LF changes from a non-radial to a nearly radial direction with a variation of inclination angle of nearly 38.2◦. Authors: Ting Li, Jun Zhang, Hui Zhao, Shuhong Yang Projects: STEREO