A flux rope is a domain of concentration of the magnetic field $\textbf{B}$. Insofar as $\textbf{B}$ outside such a domain is considered as vanishingly small, a flux rope can be described as the core of a singularity of the outer vector potential $\textbf{A}$, whose topological invariant is the magnetic flux through the rope. By 'topological' it is meant that $\oint_C\textbf{A}\cdot\mathrm d\textbf{s}$ measures along any loop $C$ surrounding the flux rope the same constant flux $Φ$. The electric current intensity is another invariant of the theory, but non-topological. We show that, in this theoretical framework, the linear force-free field (LFFF) Lundquist model and the non-linear (NLFFF) Gold-Hoyle model of a flux rope exhibit stable solutions distributed over quantized strata of increasing energies (an infinite number of strata in the first case, only one stratum in the second case); each stratum is made of a continuous set of stable states. The lowest LFFF stratum and the unique NLFFF stratum come numerically close one to the other, and match with a reasonable accuracy the data collected by spacecrafts travelling across magnetic clouds. The other LFFF strata do not match these data at all. It is not possible at this stage to claim which model fits better the magnetic cloud data. We also analyze in some detail the merging of tubes belonging to the same stratum, with conservation of the magnetic helicity, and the transition of a tube from one stratum to another one, which does not conserve magnetic helicity.
Coronal magnetic flux ropes are generally considered to be the core structure of large-scale solar eruptions. Recent observations found that solar eruptions could be initiated by a sequence of "flux feeding," during which chromospheric fibrils rise upward from below, and merge with a pre-existing prominence. Further theoretical study has confirmed that the flux feeding mechanism is efficient in causing the eruption of flux ropes that are wrapped by bald patch separatrix surfaces. But it is unclear how flux feeding influences coronal flux ropes that are wrapped by hyperbolic flux tubes (HFT), a…
我们首先介绍了2013年5月20-22日在活动区（AR）11745进行的太阳动力学观测。四个通量绳都在AR的中性线以上，端点锚定在同一区域，并且具有大致相似的形态。对于前三个通量绳，它们在出现后以小于30 km s $^{-1}$的速度上升，随后它们在131 ° C处的强度下降，通量绳变得模糊。第四条磁绳最终以约130 km/s的速度$^{-1}$喷发，形成日冕物质抛射。伴随的丝在初始阶段表现出明显的逆时针扭转运动，并且扭转估计为4 $π$。这表明扭结不稳定性可能触发了第四磁通绳的早期上升。激活的细丝材料在空间上处于磁绳内部，它们在早期阶段表现出一致的演化。我们的发现为理解磁绳的特性提供了新的线索。首先，在AR演化过程中，在同一位置连续形成多个磁通绳。其次，缓慢上升的磁通绳不一定导致日冕物质抛射，快速喷发的磁通绳导致日冕物质抛射。
Large-scale solar eruptive activities have a close relationship with coronal magnetic flux ropes. Previous numerical studies have found that the equilibrium of a coronal flux rope system could be disrupted if the axial magnetic flux of the rope exceeds a critical value, so that the catastrophe occurs, initiating the flux rope to erupt. Further studies discovered that the catastrophe does not necessarily exist: the flux rope system with certain photospheric flux distributions could be non-catastrophic. It is noteworthy that most previous numerical studies are under the ideal magnetohydrodynamic…
Combining the Solar Dynamics Observatory and the New Vacuum Solar Telescope observations, we study a confined flare triggered by a rising flux rope within the trailing sunspots of active region 12733. The flux rope lying above the sheared polarity inversion line can be constructed through magnetic extrapolation but could not be detected in multi-wavelength images at the pre-flare stage. The conspicuous shearing motions between the opposite-polarity fields in the photosphere are considered to be responsible for the flux rope formation. The maximum twist of the flux rope is as high as -1.76, and…
Large-scale solar eruptions are believed to have a magnetic flux rope as the core structure. However, it remains elusive as to how the flux rope builds up and what triggers its eruption. Recent observations found that a prominence erupted following multiple episodes of "flux feeding". During each episode, a chromospheric fibril rose and merged with the prominence lying above. In this letter, we carried out 2.5-dimensional magnetohydrodynamic (MHD) numerical simulations to investigate whether the flux-feeding mechanism can explain such an eruption. The simulations demonstrate that the discrete …
In past decades, much progress has been achieved on the origin and evolution of coronal mass ejections (CMEs). In-situ observations of the counterparts of CMEs, especially magnetic clouds (MCs) near the Earth, have provided measurements of the structure and total flux of CME flux ropes. However, it has been difficult to measure these properties in the erupting CME flux rope, in particular in the pre-existing flux rope. In this work, we propose a model to estimate the toroidal flux of the pre-existing flux rope by subtracting the flux contributed by magnetic reconnection during the eruption fro…
It is widely believed that magnetic flux ropes are the key structure of solar eruptions; however, their observable counterparts are not clear yet. We study a flare associated with flux rope eruption in a comprehensive radiative magnetohydrodynamic simulation of flare-productive active regions, especially focusing on the thermodynamic properties of the plasma involved in the eruption and their relation to the magnetic flux rope. The pre-existing flux rope, which carries cold and dense plasma, rises quasi-statically before the eruption onsets. During this stage, the flux rope does not show obvio…
Coronal mass ejections (CMEs) are large-scale explosions of the coronal magnetic field. It is believed that magnetic reconnection significantly builds up the core structure of CMEs, a magnetic flux rope, during the eruption. However, the quantitative evolution of the flux rope, particularly its toroidal flux, is still unclear. In this paper, we study the evolution of the toroidal flux of the CME flux rope for four events. The toroidal flux is estimated as the magnetic flux in the footpoint region of the flux rope, which is identified by a method that simultaneously takes the coronal dimming an…
Many prominences are supported by magnetic flux ropes. One important question is how we can determine whether the flux rope is weakly-twisted or strongly-twisted. In this paper, we attempted to check whether prominences supported by weakly-twisted and strongly-twisted flux ropes can manifest different features so that we might distinguish the two types of magnetic structures by their appearance. We performed pseudo three-dimensional simulations of two magnetic flux ropes with different twists. We found that the resulting two prominences differ in many aspects. The prominence supported by a wea…
Context: Many previous studies have shown that the magnetic precursor of a coronal mass ejection (CME) takes the form of a magnetic flux rope, and a subset of them have become known as `hot flux ropes' due to their emission signatures in $\sim$10 MK plasma. Aims: We seek to identify the processes by which these hot flux ropes form, with a view of developing our understanding of CMEs and thereby improving space weather forecasts. Methods: Extreme-ultraviolet observations were used to identify five pre-eruptive hot flux ropes in the solar corona and study how they evolved. Confined flares were o…
It is widely accepted that coronal magnetic flux ropes are the core structures of large-scale solar eruptive activities, which inflict dramatic impacts on the solar-terrestrial system. Previous studies have demonstrated that varying magnetic properties of a coronal flux rope system could result in a catastrophe of the rope, which may trigger solar eruptive activities. Since the total mass of a flux rope also plays an important role in stabilizing the rope, we use 2.5-dimensional magnetohydrodynamic (MHD) numerical simulations in this letter to investigate how a flux rope evolves as its total m…
Flux rope proxies and fan-spine structures in active region NOAA 11897
利用太阳动力学天文台（SDO）和界面区成像光谱仪（IRIS）的高分辨率观测资料，对2013年11月14日至19日期间NOAA AR 11897的通量绳代理进行了统计分析，并显示了该AR中的两个扇脊结构。我们首次在4个不同的位置探测了NOAA 11897的磁绳代理，共30次。这些磁绳代理要么在较低和较高温度的波长跟踪或仅在热通道中检测到。特别是这些弦索代理，没有一个被观测到爆发，而是逐渐消失。除了这些磁绳代理，我们首先检测到一个次级扇脊结构。它被圆顶形的磁场覆盖，属于一个更大的扇脊拓扑结构。这些新的观测结果表明，在AR中可以存在大量的磁通绳，AR磁位形的复杂性远远超出了我们的想象。
We investigate the formation times of eruptive magnetic flux ropes relative to the onset of solar eruptions, which is important for constraining models of coronal mass ejection (CME) initiation. We inspected uninterrupted sequences of 131 Å images that spanned more than eight hours and were obtained by the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO) to identify the formation times of hot flux ropes that erupted in CMEs from locations close to the limb. The appearance of the flux ropes as well as their evolution toward eruptions were determined using morphol…
Zipper reconnection has been proposed as a mechanism for creating most of the twist in the flux tubes that are present prior to eruptive flares and coronal mass ejections. We have conducted a first numerical experiment on this new regime of reconnection, where two initially untwisted parallel flux tubes are sheared and reconnected to form a large flux rope. We describe the properties of this experiment, including the linkage of magnetic flux between concentrated flux sources at the base of the simulation, the twist of the newly formed flux rope and the conversion of mutual magnetic helicity in…
Slipping Magnetic Reconnection of Flux Rope Structures as a Precursor to an Eruptive X-class Solar Flare
我们介绍了在2015年3月11日爆发的X级耀斑爆发之前磁绳结构的准周期滑动运动，由\emph{Interface Region Imaging Spectrograph}（\emph{IRIS}）和\emph{Solar Dynamics Observatory}（\emph{SDO}）获得。滑动运动发生在磁绳的北部，似乎是连续剥离磁绳。滑动速度为30 $-$ 40 km s $^{-1}$，平均周期为130 $\pm$ 30 s。Si { \sc iv} 1402.77 X线在滑动构造的西端表现出10 $-$ 30 km s $^{-1}$的红移和50 $-$ 120 km s $^{-1}$的线宽，表明了向下的重联流。滑动运动持续约40 min，在滑动后期，磁绳开始缓慢上升。然后，一个X2.1耀斑被启动，通量绳被脉冲加速。其中一个耀斑带扫过一个负极性的太阳黑子，在耀斑之后，太阳黑子的半影部分迅速衰减。我们研究了耀斑区的磁拓扑结构，结果表明存在扭曲的磁通绳，以及约束磁通绳的准分界线层（QSL）结构。我们的观测结果表明，在耀斑爆发前，准周期滑动磁重联发生在与磁通绳相关的QSL上，它驱动了后来的磁通绳爆发和耀斑爆发。
Magnetic flux ropes are bundles of twisted magnetic field enveloping a central axis. They harbor free magnetic energy and can be progenitors of coronal mass ejections (CMEs), but identifying flux ropes on the Sun can be challenging. One of the key coronal observables that has been shown to indicate the presence of a flux rope is a peculiar bright coronal structure called a sigmoid. In this work, we show Hinode EUV Imaging Spectrometer (EIS) observations of sigmoidal active region 10977. We analyze the coronal plasma composition in the active region and its evolution as the sigmoid (flux rope) …
Magnetic clouds (MCs) are transient structures containing large-scale magnetic flux ropes from solar eruptions. The twist of magnetic field lines around the rope axis reveals information about flux rope formation processes and geoeffectivity. During propagation, MC flux ropes may erode via reconnection with the ambient solar wind. Any erosion reduces the magnetic flux and helicity of the ropes, and changes their cross-sectional twist profiles. This study relates twist profiles in MC flux ropes observed at 1 AU to the amount of erosion undergone by the MCs in interplanetary space. The twist pro…
We have developed a new automated small-scale magnetic flux rope (SSMFR) detection algorithm based on the Grad-Shafranov (GS) reconstruction technique. We have applied this detection algorithm to the Wind spacecraft in-situ measurements during 1996 - 2016, covering two solar cycles, and successfully detected a total number of 74,241 small-scale magnetic flux rope events with duration from 9 to 361 minutes. This large number of small-scale magnetic flux ropes has not been discovered by any other previous studies through this unique approach. We perform statistical analysis of the small-scale ma…
Whether a solar eruption is successful or failed depends on the competition between different components of the Lorentz force exerting on the flux rope that drives the eruption. The present models only consider the strapping force generated by the background magnetic field perpendicular to the flux rope and the tension force generated by the field along the flux rope. Using the observed magnetic field on the photosphere as a time-matching bottom boundary, we perform a data-driven magnetohydrodynamic simulation for the 30 January 2015 confined eruption and successfully reproduce the observed so…
Flux rope formation through flux cancellation of sheared coronal arcades in a 3D convectively-driven MHD simulation
University of Oslo（奥斯陆大学）；Sorbonne Université, Observatoire de Paris-PSL, École Polytechnique, IP Paris, CNRS, Laboratoire de Physique des Plasmas（索邦大学，巴黎天文台-PSL，巴黎综合理工学院，IP巴黎，法国国家科学研究中心，等离子体物理实验室）
Multi-Wavelength Observations of a Flux Rope Failed in the Eruption and Associated M-Class Flare from NOAA AR 11045
利用TRACE、STEREO、SOHO/MDI、Hinode/XRT和BBSO的空间和地面观测资料，对2010年2月12日试图从NOAA AR 11045爆发的通量绳和相关的M级太阳耀斑进行了多波长观测。当磁绳从活动区上升时，在其一个支点附近触发了M1.1/2F级耀斑。我们认为，耀斑触发是由于重新连接的上升通量绳与周围的低洼磁回路。磁绳以约90 km/s的速度达到约0.15 Rs的投影高度，而软X射线通量在其上升过程中逐渐增强。磁通绳被上覆场抑制，充满的等离子体向其激活位点以西的负极性场移动。我们发现的第一个观测证据的初始抑制的通量绳由于可见的残留灯丝在色球和日冕的温度，在活动区的同一位置前几天的演变。SOHO/MDI磁象图显示在耀斑开始前约12小时出现了一个双极。出现的负极性向磁绳激活点移动，在光球极性反转线（PIL）附近触发耀斑。负极性区域朝向PIL的运动有助于在耀斑和磁绳激活部位积聚磁能。这项研究提供了一个独特的观测证据，上升的通量绳未能爆发，由于残留的灯丝和覆盖的磁场，以及相关的触发M级耀斑。
Formation and Eruption of a Vortex-driven Magnetic Flux Rope in the Simulated Quiet Sun
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（南京大学现代天文与天体物理教育部重点实验室）
Oscillatory reconnection is a process that has been suggested to underlie several solar and stellar phenomena, and is likely to play an important role in transient events such as flares. Quasi-periodic pulsations (QPPs) in flare emissions may be a manifestation of oscillatory reconnection, but the underlying mechanisms remain uncertain. In this paper, we present 2D magnetohydrodynamic (MHD) simulations of two current-carrying magnetic flux ropes with an out-of-plane magnetic field undergoing oscillatory reconnection in which the two flux ropes merge into a single flux rope. We find that oscill…
Flux ropes are generally believed to be core structures of solar eruptions that are significant for the space weather, but their formation mechanism remains intensely debated. We report on the formation of a tiny flux rope beneath clusters of active region loops on 2018 August 24. Combining the high-quality multiwavelength observations from multiple instruments, we studied the event in detail in the photosphere, chromosphere, and corona. In the source region, the continual emergence of two positive polarities (P1 and P2) that appeared as two pores (A and B)is unambiguous. Interestingly, P2 and…
Existing 3D extensions to the standard model show that flux-rope footpoints are surrounded by curved-shaped QSL-footprints that can be related with hook-shaped flare-ribbons. We build upon this finding and further address the joint questions of their time-evolution, and of the formation of flare loops at the ends of flaring PILs of the erupting bipole, which are both relevant for flare understanding in general and for ICME studies in particular. We calculate QSLs and relevant field lines in an MHD simulation of a torus-unstable flux-rope. The evolving QSL footprints are used to define the oute…
We report a decaying kink oscillation of a flux rope during a confined eruptive flare, observed off the solar limb by SDO/AIA, that lacked a detectable white-light coronal mass ejection. The erupting flux rope underwent kinking, rotation, and apparent leg-leg interaction during the event. The oscillations were observed simultaneously in multiple AIA channels at 304, 171, and 193 Å, indicating that multithermal plasma was entrained in the rope. After reaching the overlying loops in the active region, the flux rope exhibited large-amplitude, decaying kink oscillations with an apparent initial am…
