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MHD Simulation of Homologous Eruptions from Solar Active Region 10930 Caused by Sunspot Rotation  

Chaowei Jiang   Submitted: 2022-08-30 01:51

The relationship between solar eruption and sunspot rotation has been widely reported, and the underlying mechanism requires to be studied. Here we performed a full 3D MHD simulation of data-constrained approach to study the mechanism of flare eruptions in active region (AR) NOAA 10930, which is characterized by continuous sunspot rotation and homologous eruptions. We reconstructed the potential magnetic field from the magnetogram of Hinode/SOT as the initial condition and drove the MHD system by applying continuous sunspot rotation at the bottom boundary. The key magnetic structure before the major eruptions and the pre-formed current sheet were derived, which is responsible for the complex MHD evolution with multiple stages. The major eruptions were triggered directly by fast reconnection in the pre-formed current sheet above the main polarity inversion line between the two major magnetic polarities of the AR. Furthermore, our simulation shows the homologous eruption successfully. It has reasonable consistence with observations in relative strength, energy release, X-ray and Hα features and time interval of eruptions. In addition, the rotation angle of the sunspot before the first eruption in the simulation is also close to the observed value. Our simulation offers a scenario different from many previous studies based on ideal instabilities of twisted magnetic flux rope, and shows the importance of sunspot rotation and magnetic reconnection in efficiently producing homologous eruptions by continuous energy injection and impulsive energy release in a recurrent way.

Authors: Xinyi Wang, Chaowei Jiang*, Xueshang Feng, Aiying Duan, Xinkai Bian
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2022-08-31 13:08
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Data-driven modeling of solar coronal magnetic field evolution and eruptions  

Chaowei Jiang   Submitted: 2022-05-16 06:12

Magnetic fields play a fundamental role in the structure and dynamics of the solar corona. As they are driven by their footpoint motions on the solar surface, which transport energy from the interior of the Sun into its atmosphere, the coronal magnetic fields are stressed continuously with buildup of magnetic nonpotentiality in the form of topology complexity (magnetic helicity) and local electric currents (magnetic free energy). The accumulated nonpotentiality is often released explosively by solar eruptions, manifested as solar flares and coronal mass ejections, during which magnetic energy is converted into mainly kinetic, thermal, and nonthermal energy of the plasma, which can cause adverse space weather. To reveal the physical mechanisms underlying solar eruptions, it is vital to know the three-dimensional (3D) structure and evolution of the coronal magnetic fields. Because of a lack of direct measurements, the 3D coronal magnetic fields are commonly studied using numerical modeling, whereas traditional models mostly aim for a static extrapolation of the coronal field from the observable photospheric magnetic field data. Over the last decade, dynamic models that are driven directly by observation magnetograms have been developed and applied successfully to study solar coronal magnetic field evolution as well as its eruption, which offers a novel avenue for understanding their underlying magnetic topology and mechanism. In this paper, we review the basic methodology of the data-driven coronal models, state-of-the-art developments, their typical applications, and new physics that have been derived using these models. Finally, we provide an outlook for future developments and applications of the data-driven models.

Authors: Chaowei Jiang, Xueshang Feng, Yang Guo, Qiang Hu
Projects: None

Publication Status: Published in The Innovation
Last Modified: 2022-05-16 08:37
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Structural evolution of a magnetic flux rope associated with a major flare in the solar active region 12205  

Chaowei Jiang   Submitted: 2022-01-13 01:44

Solar eruptions are often generated as a result of the complex magnetic environment in solar active regions (ARs). Unravelling the relevant structure and evolution is vital to disclosing the underlying mechanisms that initiate such eruptions. In this work, we conduct a comprehensive study of the magnetic field structure and evolution responsible for a major flare eruption in a complex AR: NOAA 12205. The study is based on a detailed analysis of observations from the SDO and a time sequence of coronal magnetic field extrapolations. The AR is characterized by a long sequence of sunspots, harboring two groups of δ type that evolved dynamically via continual rotation, shearing, colliding, and flux cancellation. Our study suggests that the joint effect of the sunspot motions along a large-scale magnetic flux rope (MFR) supporting a filament was gradually built up along the main polarity inversion line. A quantitative analysis of the coronal magnetic evolution strongly indicates that an ideal instability of the MFR finally led to the major eruption of the X1.6 flare, although it was preceded by episodes of localized reconnections. These localized reconnections should play a key role in building up the unstable MFR by, for example, tether-cutting reconnection low near the photosphere, as driven by the shearing and flux cancellation. Through these reconnections, the MF gains a significant amount of twisted flux and is lifted up to a height above the torus unstable threshold, at which the background restraining force decreases fast enough with the height.

Authors: Aiying Duan, Chaowei Jiang, Yue Guo, Xueshang Feng, and Jun Cui
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted by A&A
Last Modified: 2022-01-14 12:55
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Homologous Coronal Mass Ejections Caused by Recurring Formation and Disruption of Current Sheet within a Sheared Magnetic Arcade  

Chaowei Jiang   Submitted: 2022-01-12 01:14

The Sun often produces coronal mass ejections with similar structure repeatedly from the same source region, and how these homologous eruptions are initiated remains an open question. Here, by using a new magnetohydrodynamic simulation, we show that homologous solar eruptions can be efficiently produced by recurring formation and disruption of coronal current sheet as driven by continuously shearing of the same polarity inversion line within a single bipolar configuration. These eruptions are initiated by the same mechanism, in which an internal current sheet forms slowly in a gradually sheared bipolar field and reconnection of the current sheet triggers and drives the eruption. Each of the eruptions does not release all the free energy but with a large amount left in the post-flare arcade below the erupting flux rope. Thus, a new current sheet can be more easily formed by further shearing of the post-flare arcade than by shearing a potential field arcade, and this is favorable for producing the next eruption. Furthermore, it is found that the new eruption is stronger since the newly formed current sheet has a larger current density and a lower height. In addition, our results also indicate the existence of a magnetic energy threshold for a given flux distribution, and eruption occurs once this threshold is approached.

Authors: Xinkai Bian, Chaowei Jiang, Xueshang Feng, Pingbing Zuo, and Yi Wang
Projects: None

Publication Status: Accepted by ApJL
Last Modified: 2022-01-12 13:27
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Numerical Simulation of a Fundamental Mechanism of Solar Eruption with Different Magnetic Flux Distributions  

Chaowei Jiang   Submitted: 2021-11-16 04:35

Solar eruptions are explosive release of coronal magnetic field energy as manifested in solar flares and coronal mass ejection. Observations have shown that the core of eruption-productive regions are often a sheared magnetic arcade, i.e., a single bipolar configuration, and, particularly, the corresponding magnetic polarities at the photosphere are elongated along a strong-gradient polarity inversion line (PIL). It remains unclear what mechanism triggers the eruption in a single bipolar field and why the one with a strong PIL is eruption-productive. Recently, using high accuracy simulations, we have established a fundamental mechanism of solar eruption initiation that a bipolar field as driven by quasi-static shearing motion at the photosphere can form an internal current sheet, and then fast magnetic reconnection triggers and drives the eruption. Here we investigate the behavior of the fundamental mechanism with different photospheric magnetic flux distributions, i.e., magnetograms, by combining theoretical analysis and numerical simulation. Our study shows that the bipolar fields of different magnetograms, as sheared continually, all exhibit similar evolutions from the slow storage to fast release of magnetic energy in accordance with the fundamental mechanism, which demonstrates the robustness of the mechanism. We further found that the magnetograms with stronger PIL produce larger eruptions, and the key reason is that the sheared bipolar fields with stronger PIL can achieve more non-potentiality, and their internal current sheet can form at a lower height and with a larger current density, by which the reconnection can be more efficient. This also provides a viable trigger mechanism for the observed eruptions in active region with strong PIL.

Authors: Xinkai Bian, Chaowei Jiang, Xueshang Feng, Pingbing Zuo, Yi Wang, Xinyi Wang
Projects: None

Publication Status: Accepted by A&A
Last Modified: 2021-11-17 13:12
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Investigations of Sizes and Dynamical Motions of Solar Photospheric Granules by a Novel Granular Segmenting Algorithm  

Chaowei Jiang   Submitted: 2021-10-12 03:39

Granules observed in solar photosphere are believed to be convective and turbulent, but the physical picture of granular dynamical process remains unclear. Here we performed an investigation of granular dynamical motions of full length scales based on data obtained by the 1-meter New Vacuum Solar Telescope (NVST) and the 1.6-meter Goode Solar Telescope (GST). We developed a new granule segmenting method, which can detect both small faint and large bright granules. A large number of granules were detected and two critical sizes, 265 km and 1420 km, were found to separate the granules into three length ranges. The granules with sizes above 1420 km follow Gaussian distribution, and demonstrate "flat" in flatness function, which shows that they are non-intermittent and thus are dominated by convective motions. Small granules with sizes between 265 and 1420 km are fitted by a combination of power law function and Gauss function, and exhibit non-linearity in flatness function, which reveals that they are in the mixing motions of convection and turbulence. Mini granules with sizes below 265 km follow power law distribution and demonstrate linearity in flatness function, indicating that they are intermittent and strongly turbulent. These results suggest that a cascade process occurs: large granules break down due to convective instability, which transport energy into small ones; then turbulence is induced and grows, which competes with convection and further causes the small granules to continuously split. Eventually, the motions in even smaller scales enter in a turbulence-dominated regime.

Authors: Liu Yanxiao, Jiang Chaowei, Yuan Ding, Zuo Pingbing, Wang Yi, Cao Wenda
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2021-10-13 13:05
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Formation of Magnetic Flux Rope during Solar Eruption. I. Evolution of Toroidal Flux and Reconnection Flux  

Chaowei Jiang   Submitted: 2021-09-21 02:35

Magnetic flux ropes (MFRs) constitute the core structure of coronal mass ejections (CMEs), but hot debates remain on whether the MFR forms before or during solar eruptions. Furthermore, how flare reconnection shapes the erupting MFR is still elusive in three dimensions. Here we studied a new MHD simulation of CME initiation by tether-cutting magnetic reconnection in a single magnetic arcade. The simulation follows the whole life, including the birth and subsequent evolution, of an MFR during eruption. In the early phase, the MFR is partially separated from its ambient field by a magnetic quasi-separatrix layer (QSL) that has a double-J shaped footprint on the bottom surface. With the ongoing of the reconnection, the arms of the two J-shaped footprints continually separate from each other, and the hooks of the J shaped footprints expand and eventually become closed almost at the eruption peak time, and thereafter the MFR is fully separated from the un-reconnected field by the QSL. We further studied the evolution of the toroidal flux in the MFR and compared it with that of the reconnected flux. Our simulation reproduced an evolution pattern of increase-to-decrease of the toroidal flux, which is reported recently in observations of variations in flare ribbons and transient coronal dimming. The increase of toroidal flux is owing to the flare reconnection in the early phase that transforms the sheared arcade to twisted field lines, while its decrease is a result of reconnection between field lines in the interior of the MFR in the later phase.

Authors: Chaowei Jiang, Jun Chen, Aiying Duan, Xinkai Bian, Xinyi Wang, Jiaying Li, Peng Zou, Xueshang Feng
Projects: None

Publication Status: Accepted by Frontiers in Physics
Last Modified: 2021-09-22 09:43
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A fundamental mechanism of solar eruption initiation  

Chaowei Jiang   Submitted: 2021-07-12 21:13

Solar eruptions are spectacular magnetic explosions in the Sun’s corona, and how they are initiated remains unclear. Prevailing theories often rely on special magnetic topologies that may not generally exist in the pre-eruption source region of corona. Here, using fully three-dimensional magnetohydrodynamic simulations with high accuracy, we show that solar eruptions can be initiated in a single bipolar configuration with no additional special topology. Through photospheric shearing motion alone, an electric current sheet forms in the highly sheared core field of the magnetic arcade during its quasi-static evolution. Once magnetic reconnection sets in, the whole arcade is expelled impulsively, forming a fast-expanding twisted flux rope with a highly turbulent reconnecting region underneath. The simplicity and efficacy of this scenario argue strongly for its fundamental importance in the initiation of solar eruptions.

Authors: Chaowei Jiang, Xueshang Feng, Rui Liu, XiaoLi Yan, Qiang Hu, Ronald L. Moore, Aiying Duan, Jun Cui, Pingbing Zuo, Yi Wang & Fengsi Wei
Projects: None

Publication Status: Published in Nature Astronomy
Last Modified: 2021-07-13 05:47
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Structure and Evolution of an Inter–Active Region Large-scale Magnetic Flux Rope  

Chaowei Jiang   Submitted: 2021-05-21 04:07

Magnetic flux rope (MFR) has been recognized as the key magnetic configuration of solar eruptions. While pre-eruption MFRs within the core of solar active regions (ARs) have been widely studied, those existing between two ARs, i.e., the intermediate ones in weak-field regions, were rarely studied. There are also major eruptions that occurred in such intermediate regions and study of the MFR there will help us understand the physics mechanism underlying the eruptions. Here, with a nonlinear force-free field reconstruction of solar coronal magnetic fields, we tracked the five-day evolution covering the full life of a large-scale inter-AR MFR forming between ARs NOAA 11943 and 11944, which is closely cospatial with a long sigmoidal filament channel and an eruptive X1.2 flare occurring on 2014 January 7. Through topological analysis of the reconstructed 3D magnetic field, it is found that the MFR begins to form early on 2014 January 6; then with its magnetic twist degree continuously increasing for over 30 hr, it becomes highly twisted with field lines winding numbers approaching six turns, which might be the highest twisting degree in extrapolated MFRs that have been reported in the literature. The formation and strength of the MFR are attributed to a continuous sunspot rotation of AR 11944 and flux cancellation between the two ARs. The MFR and its associated filaments exhibit no significant change across the flare time, indicating it is not responsible for the flare eruption. After the flare, the MFR slowly disappears, possibly due to the disturbance by the eruption.

Authors: Aiying Duan, Chaowei Jiang, Peng Zou, Xueshang Feng, and Jun Cui
Projects: None

Publication Status: Published in ApJ
Last Modified: 2021-05-23 10:52
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Variation of Magnetic Flux Ropes through Major Solar Flares  

Chaowei Jiang   Submitted: 2021-05-21 04:06

It remains unclear how solar flares are triggered and in what conditions they can be eruptive with coronal mass ejections. Magnetic flux ropes (MFRs) has been suggested as the central magnetic structure of solar eruptions, and their ideal instabilities, including the kink instability (KI) and torus instability (TI), are important candidates for triggering mechanisms. Here, using magnetic field extrapolations from observed photospheric magnetograms, we systematically studied the variation of coronal magnetic fields, focusing on MFRs, through major flares including 29 eruptive and 16 confined events. We found that nearly 90% of events possess MFRs before flares, and 70% have MFRs even after flares. We calculated the controlling parameters of KI and TI, including the MFR's maximum twist number and the decay index of its strapping field. Using the KI and TI thresholds empirically derived solely from the pre-flare MFRs, two distinctly different regimes are shown in the variation of the MFR controlling parameters through flares. For the events with both parameters below their thresholds before flares, we found no systematic change of the parameters after the flares, in either the eruptive or confined events. In contrast, for the events with any of the two parameters exceeding their threshold before flares (most of them are eruptive), there is systematic decrease in the parameters to below their thresholds after flares. These results provide a strong constraint for the values of the instability thresholds and also stress the necessity of exploring other eruption mechanisms in addition to the ideal instabilities.

Authors: Aiying Duan, Chaowei Jiang, Zhenjun Zhou, Xueshang Feng, and Jun Cui
Projects: None

Publication Status: Published in ApJL
Last Modified: 2021-05-23 10:49
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MHD Modeling of Solar Coronal Magnetic Evolution Driven by Photospheric Flow  

Chaowei Jiang   Submitted: 2021-05-21 04:04

It is well-known that magnetic fields dominate the dynamics in the solar corona, and new generation of numerical modeling of the evolution of coronal magnetic fields, as featured with boundary conditions driven directly by observation data, are being developed. This paper describes a new approach of data-driven magnetohydrodynamic (MHD) simulation of solar active region (AR) magnetic field evolution, which is for the first time that a data-driven full-MHD model utilizes directly the photospheric velocity field from DAVE4VM. We constructed a well-established MHD equilibrium based on a single vector magnetogram by employing an MHD-relaxation approach with sufficiently small kinetic viscosity, and used this MHD equilibrium as the initial conditions for subsequent data-driven evolution. Then we derived the photospheric surface flows from a time series of observed magentograms based on the DAVE4VM method. The surface flows are finally inputted in time sequence to the bottom boundary of the MHD model to self-consistently update the magnetic field at every time step by solving directly the magnetic induction equation at the bottom boundary. We applied this data-driven model to study the magnetic field evolution of AR 12158 with SDO/HMI vector magnetograms. Our model reproduced a quasi-static stress of the field lines through mainly the rotational flow of the AR's leading sunspot, which makes the core field lines to form a coherent S shape consistent with the sigmoid structure as seen in the SDO/AIA images. The total magnetic energy obtained in the simulation matches closely the accumulated magnetic energy as calculated directly from the original vector magnetogram with the DAVE4VM derived flow field. Such a data-driven model will be used to study how the coronal field, as driven by the slow photospheric motions, reaches a unstable state and runs into eruptions.

Authors: Chaowei Jiang, Xinkai Bian, Tingting Sun, Xueshang Feng
Projects: None

Publication Status: Published in Frontiers
Last Modified: 2021-05-23 10:48
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The Causes of Peripheral Coronal Loop Contraction and Disappearance Revealed in a Magnetohydrodynamic Simulation of Solar Eruption  

Chaowei Jiang   Submitted: 2021-05-21 04:01

The phenomenon of peripheral coronal loop contraction during solar flares and eruptions, recently discovered in observations, has gradually drawn the attention of solar physicists. However, its underlying physical mechanism is still uncertain. One possible mechanism is Hudson's implosion conjecture, which attributes the contraction of peripheral coronal loops to magnetic pressure reduction in the magnetic energy liberation core, while other researchers proposed alternative explanations. In previous observational studies we also note the disappearance of peripheral shrinking loops in the late phase, of which there is a lack of investigation and interpretation. In this paper, we exploit a full MHD simulation of solar eruption to study the causes of the two phenomena. It is found that the loop motion in the periphery is well correlated with magnetic energy accumulation and dissipation in the core, and the loop shrinkage is caused by a more significant reduction in magnetic pressure gradient force than in magnetic tension force, consistent with the implosion conjecture. The peripheral contracting loops in the late phase act as inflow to reconnect with central erupting structures, which destroys their identities and naturally explains their disappearance. We also propose a positive feedback between the peripheral magnetic reconnection and the central eruption.

Authors: Juntao Wang, Chaowei Jiang*, Ding Yuan, and Peng Zou
Projects: None

Publication Status: Published in ApJ
Last Modified: 2021-05-23 10:47
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Data-driven MHD Simulation of the Formation and Initiation of a Large-scale Pre-flare Magnetic Flux Rope in Solar Active Region 12371  

Chaowei Jiang   Submitted: 2020-02-13 07:14

Solar eruptions are the most powerful drivers of space weather. To understand their cause and nature, it is crucial to know how the coronal magnetic field evolves before eruption. Here we study the formation process of a relatively large-scale magnetic flux rope (MFR) in active region NOAA~12371 that erupts with a major flare and coronal mass ejection on 2015 June 21. A data-driven numerical magnetohydrodynamic model is employed to simulate three-dimensional coronal magnetic field evolution of one-day duration before the eruption. Comparison between the observed features and our modeled magnetic field discloses how the pre-eruption MFR forms. Initially, the magnetic field lines were weakly twisted as being simple sheared arcades. Then a long MFR was formed along the polarity inversion line due to the complex photospheric motion, which is mainly shearing rather than twisting. The presence of the MFR is evidenced by a coherent set of magnetic field lines with twist number above unity. Below the MFR a current sheet is shown in the model, suggesting that tether-cutting reconnection plays a key role in the MFR formation. The MFR's flux grows as more and more field lines are twisted due to continuous injection of magnetic helicity by the photospheric motions. Meanwhile, the height of the MFR's axis increases monotonely from its formation. By an analysis of the decay index of its overlying field, we suggest that it is because the MFR runs into the torus instability regime and becomes unstable that finally triggers the eruption.

Authors: Wen He, Chaowei Jiang*, Peng Zou, Aiying Duan, Xueshang Feng, Pingbing Zuo, Yi Wang
Projects: SDO-AIA,SDO-HMI

Publication Status: Accept by ApJ
Last Modified: 2020-02-13 11:50
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Continuous Null-Point Magnetic Reconnection Builds Up a Torus Unstable Magnetic Flux Rope Triggering the X9.3 Flare in Solar Active Region~12673  

Chaowei Jiang   Submitted: 2020-01-14 22:54

Two X-class solar flares occurred on 2017 September 6 from active region NOAA 12673: the first one is a confined X2.2 flare, and it is followed only ∼3 hours later by the second one, which is the strongest flare in solar cycle 24, reaching X9.3 class and accompanied with a coronal mass ejection. Why these two X-class flares occurred in the same position with similar magnetic configurations, but one is eruptive while the other is not? Here we track the coronal magnetic field evolution via nonlinear force-free field extrapolations from a time sequence of vector magnetograms with high cadence. A detailed analysis of the magnetic field shows that a magnetic flux rope (MFR) forms and grows gradually before the first flare, and shortly afterwards, the MFR's growth is significantly enhanced with a much faster rise in height, from far below the threshold of torus instability to above it, while the magnetic twist only increases mildly. Combining EUV observations and the magnetic field extrapolation, we found that overlying the MFR is a null-point magnetic topology, where recurrent brightening is seen after the first flare. We thus suggest a scenario to interpret the occurrence of the two flares. The first flare occurred since the MFR reached a high enough height to activate the null point, and its continuous expansion forces the null-point reconnection recurrently. Such reconnection weakens the overlying field, allowing the MFR to rise faster, which eventually crosses the threshold of torus instability and triggers the second, eruptive flare.

Authors: Peng Zou, Chaowei Jiang*, Fengsi Wei, Xueshang Feng, Pingbing Zuo, Yi Wang
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2020-01-15 13:04
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A Study of Pre-Flare Solar Coronal Magnetic Fields: Magnetic Flux Ropes  

Chaowei Jiang   Submitted: 2019-08-27 07:12

Magnetic flux ropes (MFRs) are thought to be the central structure of solar eruptions, and their ideal MHD instabilities can trigger the eruption. Here we performed a study of all the MFR configurations that lead to major solar flares, either eruptive or confined, from 2011 to 2017 near the solar disk center. The coronal magnetic field is reconstructed from observed magnetograms, and based on magnetic twist distribution, we identified the MFR, which is defined as a coherent group of magnetic field lines winding an axis with more than one turn. It is found that 90% of the events possess pre-flare MFRs, and their three-dimensional structures are much more complex in details than theoretical MFR models. We further constructed a diagram based on two parameters, the magnetic twist number which controls the kink instability (KI), and the decay index which controls the torus instability (TI). It clearly shows lower limits for TI and KI thresholds, which are n crit = 1.3 and |Tw| crit = 2, respectively, as all the events above n crit and nearly 90% of the events above |Tw| crit erupted. Furthermore, by such criterion, over 70% of the events can be discriminated between eruptive and confined flares, and KI seems to play a nearly equally important role as TI in discriminating between the two types of flare. There are more than half of events with both parameters below the lower limits, and 29% are eruptive. These events might be triggered by magnetic reconnection rather than MHD instabilities.

Authors: Aiying Duan, Chaowei Jiang, Wen He, Xueshang Feng, Peng Zou, Jun Cui
Projects: None

Publication Status: Accept by ApJ
Last Modified: 2019-08-27 10:07
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A Statistical Study of Solar Filament Eruptions That Forms High-Speed Coronal Mass Ejections  

Chaowei Jiang   Submitted: 2019-08-26 03:05

Coronal mass ejections (CMEs) play a decisive role in driving space weather, especially, the fast ones (e.g., with speeds above 800 km s-1). Understanding the trigger mechanisms of fast CMEs can help us gaining important information in forecasting them. The filament eruptions accompanied with CMEs provide a good tracer in studying the early evolution of CMEs. Here we surveyed 66 filament-accompanied fast CMEs to analyse the correlation between the trigger mechanisms, namely either magnetic reconnection or ideal MHD process, associated flares, and CME speeds. Based on the data gathering from SDO, GONG and STEREO, we find that: (1) Active region (AR) filament and intermediate filaments (IFs) eruptions show a higher probability for producing fast CMEs than quiet Sun (QS) filaments, while the probability of polar crown (PC) filament eruptions is zero in our statistic; (2) AR filament eruptions that produce fast CMEs are more likely triggered by magnetic reconnection, while QS and IFs are more likely triggered by ideal MHD process; (3) For AR filaments and IFs, it seems that the specific trigger mechanism does not have a significant influence on the resulted CME speeds, while for the QS filaments, the ideal MHD mechanism can more likely generate a faster CME; (4) Comparing with previous statistic study, the onset heights of filament eruptions and the decay indexes of the overlying field show some differences: for AR filaments and IFs, the decay indexes are larger and much closer to the theoretical threshold, while for QS filaments, the onset heights are higher than those obtained in previous results.

Authors: Peng Zou, Chaowei Jiang, Fengsi Wei, Pingbing Zuo, Yi Wang
Projects: None

Publication Status: Accept by ApJ
Last Modified: 2019-08-26 10:45
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A Two-Step Magnetic Reconnection in a Confined X-class Flare in Solar Active Region 12673  

Chaowei Jiang   Submitted: 2018-11-29 18:12

Solar flares are often associated with coronal eruptions, but there are confined ones without eruption, even for some X-class flares. How such large flares occurred and why they are confined are still not well understood. Here we studied a confined X2.2 flare in NOAA 12673 on 2017 September 6. It exhibits two episodes of flare brightening with rather complex, atypical ribbons. Based on topology analysis of extrapolated coronal magnetic field, we revealed that there is a two-step magnetic reconnection process during the flare. Prior to the flare, there is a magnetic flux rope (MFR) with one leg rooted in a rotating sunspot. Neighboring to the leg is a magnetic null-point structure. The sunspot drives the MFR to expand, pushing magnetic flux to the null point, and reconnection is first triggered there. The disturbance from the null-point reconnection triggers the second reconnection, i.e., a tether-cutting reconnection below the rope. However, these two reconnections failed to produce an eruption, because the rope is firmly held by its strapping flux. Furthermore, we compared this flare with an eruptive X9.3 flare in the same region with 2 hours later, which has a similar MFR configuration. The key difference between them is that, for the confined flare, the MFR is fully below the threshold of torus instability, while for the eruptive one, the MFR reaches entirely above the threshold. This study provides a good evidence supporting that reconnection alone may not be able to trigger eruption, rather, MHD instability plays a more important role.

Authors: Peng Zou, Chaowei Jiang, Xueshang Feng, Pingbing Zuo, Yi Wang, Fengsi Wei
Projects: SDO-AIA,SDO-HMI

Publication Status: accept by ApJ
Last Modified: 2018-11-30 09:16
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Formation and Eruption of an Active Region Sigmoid. II. Magnetohydrodynamic Simulation of a Multistage Eruption  

Chaowei Jiang   Submitted: 2018-10-30 21:05

Solar eruptions, mainly eruptive flares with coronal mass ejections, represent the most powerful drivers of space weather. Due to the low plasma-β nature of the solar corona, solar eruption has its roots in the evolution of the coronal magnetic field. Although various theoretical models of the eruptive magnetic evolution have been proposed, they still oversimplify the realistic process in observation, which shows a much more complex process due to the invisible complex magnetic environment. In this paper, we continue our study of a complex sigmoid eruption in solar active region 11283, which is characterized by a multipolar configuration embedding a null-point topology and a sigmoidal magnetic flux rope. Based on extreme ultraviolet observations, it has been suggested that a three-stage magnetic reconnection scenario might explain the complex flare process. Here we reproduce the complex magnetic evolution during the eruption using a data-constrained high-resolution magnetohydrodynamic (MHD) simulation. The simulation clearly demonstrates three reconnection episodes, which occurred in sequence in different locations in the corona. Through these reconnections, the initial sigmoidal flux rope breaks one of its legs, and quickly gives birth to a new tornado-like magnetic structure that is highly twisted and has multiple connections to the Sun due to the complex magnetic topology. The simulated magnetic field configuration and evolution are found to be consistent with observations of the corona loops, filaments, and flare ribbons. Our study demonstrates that significant insight into a realistic, complex eruption event can be gained by a numerical MHD simulation that is constrained or driven by observed data.

Authors: Chaowei Jiang, Xueshang Feng, and Qiang Hu
Projects: SDO-AIA,SDO-HMI

Publication Status: Published in ApJ
Last Modified: 2018-10-31 12:08
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Reconstruction of a Large-scale Pre-flare Coronal Current Sheet Associated with an Homologous X-shaped Flare  

Chaowei Jiang   Submitted: 2017-10-10 19:32

As a fundamental magnetic structure in the solar corona, electric current sheets (CSs) can form either prior to or during solar flare, and they are essential for magnetic energy dissipation in the solar corona by enabling magnetic reconnection. However static reconstruction of CS is rare, possibly due to limitation inherent in available coronal field extrapolation codes. Here we present the reconstruction of a large-scale pre-flare CS in solar active region 11967 using an MHD-relaxation model constrained by SDO/HMI vector magnetogram. The CS is found to be associated with a set of peculiar homologous flares that exhibit unique X-shaped ribbons and loops occurring in a quadrupolar magnetic configuration. This is evidenced by that the field lines traced from the CS to the photosphere form an X shape which nearly precisely reproduces the shape of the observed flare ribbons, suggesting that the flare is a product of the dissipation of the CS through reconnection. The CS forms in a hyperbolic flux tube, which is an intersection of two quasi-separatrix layers. The recurrence of the X-shaped flares might be attributed to the repetitive formation and dissipation of the CS, as driven by the photospheric footpoint motions. These results demonstrate the power of data-constrained MHD model in reproducing CS in the corona as well as providing insight into the magnetic mechanism of solar flares.

Authors: Chaowei Jiang, Xiaoli Yan, Xueshang Feng, Aiying Duan, Qiang Hu, Pingbing Zuo, Yi Wang
Projects: SDO-AIA,SDO-HMI

Publication Status: Accepted by ApJ
Last Modified: 2017-10-11 06:20
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Comparison of Two Coronal Magnetic Field Models for Reconstructing a Sigmoidal Solar Active Region With Coronal Loops  

Chaowei Jiang   Submitted: 2017-10-10 19:30

Magnetic field extrapolation is an important tool to study the three-dimensional (3D) solar coronal magnetic field which is difficult to directly measure. Various analytic models and numerical codes exist but their results often drastically differ. Thus a critical comparison of the modeled magnetic field lines with the observed coronal loops is strongly required to establish the credibility of the model. Here we compare two different non-potential extrapolation codes, a non-linear force-free field code (CESE-MHD-NLFFF) and a non-force-free field (NFFF) code in modeling a solar active region (AR) that has a sigmoidal configuration just before a major flare erupted from the region. A 2D coronal-loop tracing and fitting method is employed to study the 3D misalignment angles between the extrapolated magnetic field lines and the EUV loops as imaged by SDO/AIA. It is found that the CESE-MHD-NLFFF code with preprocessed magnetogram performs the best, outputting a field which matches the coronal loops in the AR core imaged in AIA 94 Å with a misalignment angle of ~10 degree. This suggests that the CESE-MHD-NLFFF code, even without using the information of coronal loops in constraining the magnetic field, performs as good as some coronal-loop forward-fitting models. For the loops as imaged by AIA 171 Å in the outskirts of the AR, all the codes including the potential-field give comparable results of mean misalignment angle (~30 degree). Thus further improvement of the codes is needed for a better reconstruction of the long loops enveloping the core region.

Authors: Aiying Duan, Chaowei Jiang, Qiang Hu, Huai Zhang, G. Allen Gary, S. T. Wu, Jinbin Cao
Projects: SDO-AIA,SDO-HMI

Publication Status: published in ApJ
Last Modified: 2017-10-11 06:20
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