Flux Rope Configuration Research Articles

Characterization of Magnetic Flux Contents for Two Flux Transfer Events From Both In Situ and Ionospheric Observations

AbstractFlux transfer events (FTEs) are a type of magnetospheric phenomena that exhibit distinctive observational signatures from the in situ spacecraft measurements. They are generally believed to possess a magnetic field configuration of a magnetic flux rope and formed through magnetic reconnection at the dayside magnetopause, sometimes accompanied with enhanced plasma convection in the ionosphere. We examine two FTE intervals under the condition of southward interplanetary magnetic field (IMF) with a dawn‐dusk component. We apply the Grad‐Shafranov (GS) reconstruction method to the in situ measurements by the Magnetospheric Multiscale (MMS) spacecraft to derive the magnetic flux contents associated with the FTE flux ropes. In particular, given a cylindrical magnetic flux rope configuration derived from the GS reconstruction, the magnetic flux content can be characterized by both the toroidal (axial) and poloidal fluxes. We then estimate the amount of magnetic flux (i.e., the reconnection flux) encompassed by the area “opened” in the ionosphere, based on the ground‐based Super Dual Auroral Radar Network (SuperDARN) observations. We find that for event 1, the FTE flux rope is oriented in the approximate dawn‐dusk direction, and the amount of its total poloidal magnetic flux falls within the range of the corresponding reconnection flux. For event 2, the FTE flux rope is oriented in the north‐south direction. Both the FTE flux and the reconnection flux have greater uncertainty. We provide a detailed description about a formation scenario of sequential magnetic reconnection between adjacent field lines based on the FTE flux rope configurations from our results.

A Double-decker Flux Rope Model for the Solar Eruption on 2012 March 10

We present a magnetic configuration of a compound solar eruption observed on 2012 March 10, from NOAA AR 11429 near the disk center, which displayed a soft X-ray sigmoid before the eruption. We constructed a series of magnetic field models, including double-decker flux rope configurations, using the flux rope insertion method. This produces three-dimensional coronal magnetic field models constrained by the photospheric magnetogram and observed EUV coronal structures. We used different combinations of flux rope paths. We found that two flux ropes sharing the same path at different heights quickly experience a partial merging in the initial iteration of the magnetofrictional relaxation process. Different paths with less than 30% overlap allowed us to construct stable double-decker structures. The high spatial and temporal resolution of the Solar Dynamics Observatory/Atmospheric Imaging Assembly facilitated the selection of a best-fit model that matches the observations best. Moreover, by varying fluxes in this validated nonlinear force-free field double-decker configuration, we successfully reproduce all three scenarios of eruptions of double-decker configurations: (i) eruption due to the instability of higher flux rope; (ii) eruption due to rising lower flux rope and merging with higher flux rope; and (iii) eruption due to the instability of both flux ropes. This demonstrates that magnetofrictional simulation can capture the large-scale magnetic structure of eruptions for a realistic field configuration at eruption onset.

A magnetic flux rope configuration derived by optimization of two-spacecraft In-situ measurements

Increasingly one interplanetary coronal mass ejection (ICME) structure can propagate across more than one spacecraft in the solar wind. This usually happens when two or more spacecraft are nearly radially aligned with a relatively small longitudinal separation angle from one another. This provides multi-point measurements of the same structure and enables better characterization and validation of modeling results of the structures embedded in these ICMEs. We report such an event during October 13-14, 2019 when the Solar TErrestrial RElations Observatory Ahead (STA) spacecraft and the Parker Solar Probe (PSP) crossed one ICME structure at two different locations with nominal separations in both heliocentric distances and the longitudinal angles. We first perform an optimal fitting to the STA in-situ measurements, based on an analytic quasi-three dimensional (3D) model, yielding a minimum reduced χ2 = 0.468. Then we further apply the optimization approach by combining the magnetic field measurements from both spacecraft along their separate paths across the ICME structure. We find that the output based on the optimization (with the minimum reduced χ2 = 3.15) of the combined two-spacecraft dataset yields a more consistent result, given the much improved agreement of the model output with PSP data. The result demonstrates a magnetic flux rope configuration with clear 3D spatial variations.

Analysis of the Evolution of a Multi-Ribbon Flare and Failed Filament Eruption

How filaments form and erupt are topics about which solar researchers have wondered for more than a century and they are still open to debate. We present observations of a filament formation, its failed eruption, and the associated flare (SOL2019-05-09T05:51) that occurred in active region (AR) 12740 using data from the Solar Dynamics Observatory (SDO), the Solar-Terrestrial Relations Observatory A (STEREO-A), the Interface Region Imaging Spectrograph (IRIS) and the Learmonth Solar Observatory (LSO) of the National Solar Observatory/Global Oscillation Network Group (NSO/GONG). AR 12740 was a decaying region formed by a very disperse following polarity and a strong leading spot, surrounded by a highly dynamic zone where moving magnetic features (MMFs) were seen constantly diverging from the spot. Our analysis indicates that the filament was formed by the convergence of fibrils at a location where magnetic flux cancellation was observed. Furthermore, we conclude that its destabilisation was also related to flux cancellation associated with the constant shuffling of the MMFs. A two-ribbon flare occurred associated with the filament eruption; however, because the large-scale magnetic configuration of the AR was quadrupolar, two additional flare ribbons developed far from the two main ones. We model the magnetic configuration of the AR using a force-free field approach at the AR scale size. This local model is complemented by a global potential-field source-surface one. Based on the local model, we propose a scenario in which the filament failed eruption and the flare are due to two reconnection processes, one occurring below the erupting filament, leading to the two-ribbon flare, and another one above it between the filament flux-rope configuration and the large-scale closed loops. Our computation of the reconnected magnetic flux added to the erupting flux rope, compared to that of the large-scale field overlying it, allows us to conclude that the latter was large enough to prevent the filament eruption. A similar conjecture can be drawn from the computation of the magnetic tension derived from the global field model.

Evidence of a complex structure within the 2013 August 19 coronal mass ejection

Context. Late on 2013 August 19, a coronal mass ejection (CME) erupted from an active region located near the far-side central meridian from Earth’s perspective. The event and its accompanying shock were remotely observed by the STEREO-A, STEREO-B, and SOHO spacecraft. The interplanetary counterpart (ICME) was intercepted by MESSENGER near 0.3 au and by both STEREO-A and STEREO-B near 1 au, which were separated from each other by 78° in heliolongitude. Aims. The main objective of this study is to follow the radial and longitudinal evolution of the ICME throughout the inner heliosphere and to examine possible scenarios for the different magnetic flux-rope configuration observed on the solar disk and measured in situ at the locations of MESSENGER and STEREO-A, separated by 15° in heliolongitude, and at STEREO-B, which detected the ICME flank. Methods. Solar disk observations are used to estimate the “magnetic flux-rope type”, namely, the magnetic helicity, axis orientation, and axial magnetic field direction of the flux rope. The graduated cylindrical shell model is used to reconstruct the CME in the corona. The analysis of in situ data, specifically the plasma and magnetic field, is used to estimate the global interplanetary shock geometry and to derive the magnetic flux-rope type at different in situ locations, which is compared to the type estimated from solar disk observations. The elliptical cylindrical analytical model is used for the in situ magnetic flux-rope reconstruction. Results. Based on the CME geometry and on the spacecraft configuration, we find that the magnetic flux-rope structure detected at STEREO-B belongs to the same ICME detected at MESSENGER and STEREO-A. The opposite helicity deduced at STEREO-B might be due to that fact that it intercepted one of the legs of the structure far from the flux-rope axis, in contrast to STEREO-A and MESSENGER, which were crossing through the core of the magnetic flux rope. The different flux-rope orientations measured at MESSENGER and STEREO-A probably arise because the two spacecraft measure a curved, highly distorted, and rather complex magnetic flux-rope topology. The ICME may have suffered additional distortion in its evolution in the inner heliosphere, such as the west flank propagating faster than the east flank when arriving near 1 au. Conclusions. This work illustrates how a wide, curved, highly distorted, and rather complex CME showed different orientations as observed on the solar disk and measured in situ at 0.3 au and near 1 au. Furthermore, the work shows how the ambient conditions can significantly affect the expansion and propagation of the CME and ICME, introducing additional irregularities to the already asymmetric eruption. The study also manifests how these complex structures cannot be directly reconstructed with the currently available models and that multi-point analysis is of the utmost importance in such complex events.

On the utility of flux rope models for CME magnetic structure below 30 [formula omitted

We present a comprehensive analysis of the three-dimensional magnetic flux rope structure generated during the Lynch et al. (2019, ApJ 880:97) magnetohydrodynamic (MHD) simulation of a global-scale, 360°-wide streamer blowout coronal mass ejection (CME) eruption. We create both fixed and moving synthetic spacecraft to generate time series of the MHD variables through different regions of the flux rope CME. Our moving spacecraft trajectories are derived from the spatial coordinates of Parker Solar Probe’s past encounters 7 and 9 and future encounter 23. Each synthetic time series through the simulation flux rope ejecta is fit with three different in situ flux rope models commonly used to characterize the large-scale, coherent magnetic field rotations observed in a significant fraction of interplanetary CMEs (ICMEs). We present each of the in situ flux rope model fits to the simulation data and discuss the similarities and differences between the model fits and the MHD simulation’s flux rope spatial orientations, field strengths and rotations, expansion profiles, and magnetic flux content. We compare in situ model properties to those calculated with the MHD data for both classic bipolar and unipolar ICME flux rope configurations as well as more problematic profiles such as those with a significant radial component to the flux rope axis orientation or profiles obtained with large impact parameters. We find general agreement among the in situ flux rope fitting results for the classic profiles and much more variation among results for the problematic profiles. We also examine the force-free assumption for a subset of the flux rope models and quantify properties of the Lorentz force within MHD ejecta intervals. We conclude that the in situ flux rope models are generally a decent approximation to the field structure, but all the caveats associated with in situ flux rope models will still apply (and perhaps moreso) at distances below 30R⊙. We discuss our results in the context of future PSP observations of CMEs in the extended corona.

Coronal Mass Ejections and Dimmings: A Comparative Study Using MHD Simulations and SDO Observations

Abstract Solar coronal dimmings have been observed extensively in recent years. Due to their close association with coronal mass ejections (CMEs), there is a critical need to improve our understanding of the physical processes that cause dimmings as well as their relationship with CMEs. In this study, we investigate coronal dimmings by combining simulation and observational efforts. By utilizing a data-constrained global magnetohydrodynamics model (Alfvén-wave solar model), we simulate coronal dimmings resulting from different CME energetics and flux rope configurations. We synthesize the emissions of different EUV spectral bands/lines and compare with SDO/AIA and EVE observations. A detailed analysis of the simulation and observation data suggests that the transient dimming/brightening are related to plasma heating processes, while the long-lasting core and remote dimmings are caused by mass-loss process induced by the CME. Moreover, the interaction between the erupting flux rope with different orientations and the global solar corona could significantly influence the coronal dimming patterns. Using metrics such as dimming depth and dimming slope, we investigate the relationship between dimmings and CME properties (e.g., CME mass, CME speed) in the simulation. Our result suggests that coronal dimmings encode important information about the associated CMEs, which provides a physical basis for detecting stellar CMEs from distant solar-like stars.

Configuration of a Magnetic Cloud From Solar Orbiter and Wind Spacecraft In-situ Measurements

Coronal mass ejections (CMEs) represent one type of the major eruption from the Sun. Their interplanetary counterparts, the interplanetary CMEs (ICMEs), are the direct manifestations of these structures when they propagate into the heliosphere and encounter one or more observing spacecraft. The ICMEs generally exhibit a set of distinctive signatures from thein-situspacecraft measurements. A particular subset of ICMEs, the so-called Magnetic Clouds (MCs), is more uniquely defined and has been studied for decades, based onin-situmagnetic field and plasma measurements. By utilizing the latest multiple spacecraft measurements and analysis tools, we report a detailed study of the internal magnetic field configuration of an MC event observed by both the Solar Orbiter (SO) and Wind spacecraft in the solar wind near the Sun-Earth line. Both two-dimensional (2D) and three-dimensional (3D) models are applied to reveal the flux rope configurations of the MC. Various geometrical as well as physical parameters are derived and found to be similar within error estimates for the two methods. These results quantitatively characterize the coherent MC flux rope structure crossed by the two spacecraft along different paths. The implication for the radial evolution of this MC event is also discussed.

Investigation of the Interaction Between Magnetosheath Reconnection and Magnetopause Reconnection Driven by Oblique Interplanetary Tangential Discontinuity Using Three‐Dimensional Global Hybrid Simulation

AbstractMagnetosheath reconnection due to the interaction of an interplanetary directional discontinuity with the bow shock and Earth's magnetosphere under an initially northward interplanetary magnetic field (IMF) has been investigated in previous simulations (e.g., Guo et al., 2018, https://doi.org/10.1029/2018ja025679). Under an initially southward IMF, the magnetosheath reconnection could interact with reconnection at the magnetopause. In this study, using three‐dimensional (3D) global‐scale hybrid simulations, we present cases with incoming tangential discontinuities (TDs) in an initially southward IMF, which possess various magnetic field rotation angles (ΔΦ) and half‐width (w), to study the effects of pre‐existing magnetopause reconnection on the formation of magnetosheath flux ropes, as well as the subsequent interaction between the magnetopause reconnection and magnetosheath reconnection, with downstreams of both Quasi‐perpendicular (Q‐) and Quasi‐parallel (Q‐||) shock examined. The initial IMF is assumed to be oblique, with a finite Bx and Bz, similar to that in Guo et al. (2018), https://doi.org/10.1029/2018ja025679 but with a southward Bz < 0. Compared with the cases with an initially northward IMF, magnetopause reconnection weakens the compression processes of the TD and leads to less frequent reconnection in the magnetosheath. The existence and the structure of magnetosheath reconnection are found to strongly depend on the parameters w and of the TD. When interacting with the magnetopause reconnection, the magnetosheath flux ropes can re‐reconnect with the geomagnetic dipole field lines, forming new structures of magnetopause flux ropes. The resulting evolution of flux rope configuration is illustrated.

On the Quasi-Three Dimensional Configuration of Magnetic Clouds.

We develop an optimization approach to model the magnetic field configuration of magnetic clouds, based on a linear force‐free formulation in three dimensions. Such a solution, dubbed the Freidberg solution, is kin to the axisymmetric Lundquist solution, but with more general “helical symmetry.” The merit of our approach is demonstrated via its application to two case studies of in situ measured magnetic clouds. Both yield results of reduced χ 2 ≈ 1. Case 1 shows a winding flux rope configuration with one major polarity. Case 2 exhibits a double‐helix configuration with two flux bundles winding around each other and rooted on regions of mixed polarities. This study demonstrates the three‐dimensional complexity of the magnetic cloud structures.

Small-scale Magnetic Flux Ropes in the First Two Parker Solar Probe Encounters

Abstract Small-scale magnetic flux ropes (SFRs) are a type of structure in the solar wind that possess helical magnetic field lines. In a recent report we presented the radial variations of the properties of SFRs from 0.29 to 8 au using in situ measurements from the Helios, Advanced Composition Explorer/WIND (ACE/Wind), Ulysses, and Voyager spacecrafts. With the launch of the Parker Solar Probe (PSP), we extend our previous investigation further into the inner heliosphere. We apply a Grad–Shafranov-based algorithm to identify SFRs during the first two PSP encounters. We find that the number of SFRs detected near the Sun is much less than at larger radial distances, where magnetohydrodynamic (MHD) turbulence may act as the local source to produce these structures. The prevalence of Alfvénic structures significantly suppresses the detection of SFRs at closer distances. We compare the SFR event list with other event identification methods, yielding a dozen well-matched events. The cross-section maps of two selected events confirm the cylindrical magnetic flux-rope configuration. The power-law relation between the SFR magnetic field and heliocentric distances seems to hold down to 0.16 au.

High-resolution Chromospheric Observations of a Solar Minifilament: Formation and Destabilization

Abstract Using H α line core and off-band imaging data from the New Vacuum Solar Telescope in China, we present a high-resolution observation on the entire life cycle of a solar minifilament from its birth to its final eruption. We find that the minifilament originates from a series of cascade-like reconfigurations of chromospheric fine structures. During which, owing to strong photospheric shearing and converging flows near its polarity inversion line, basic short chromospheric fibrils first slowly coalesce to elongated dark threads, and then further create a longer filament channel in a “head-to-tail” linkage scenario. In this course, obvious magnetic flux cancelation simultaneously proceeds below it, and further facilitates its destabilization. In its onset phase, clear clues indicate that the minifilament first starts to rise without brightening signals; instead, after a slow-to-fast acceleration, obvious runaway reconnection soon takes over its final jet-like eruption. Besides, off-band observations further reveal that the formed minifilament has a possible flux-rope configuration, and chromospheric upflows that detected in its early forming phase persistently supplies cool plasma into its channel. This observation is consistent with earlier observations and supports the view that both miniature and large-scale filaments may share analogous formation and destabilization mechanisms.

Identifying Flux Rope Signatures Using a Deep Neural Network

Among the current challenges in Space Weather, one of the main ones is to forecast the internal magnetic configuration within Interplanetary Coronal Mass Ejections (ICMEs). Currently, a monotonic and coherent magnetic configuration observed is associated with the result of a spacecraft crossing a large flux rope with helical magnetic field lines topology. The classification of such an arrangement is essential to predict geomagnetic disturbance. Thus, the classification relies on the assumption that the ICME's internal structure is a well organized magnetic flux rope. This paper applies machine learning and a current physical flux rope analytical model to identify and further understand the internal structures of ICMEs. We trained an image recognition artificial neural network with analytical flux rope data, generated from the range of many possible trajectories within a cylindrical (circular and elliptical cross-section) model. The trained network was then evaluated against the observed ICMEs from WIND during 1995-2015. The methodology developed in this paper can classify 84% of simple real cases correctly and has a 76% success rate when extended to a broader set with 5% noise applied, although it does exhibit a bias in favor of positive flux rope classification. As a first step towards a generalizable classification and parameterization tool, these results show promise. With further tuning and refinement, our model presents a strong potential to evolve into a robust tool for identifying flux rope configurations from in situ data.

Eruptive–Impulsive Homologous M-class Flares Associated with Double-decker Flux Rope Configuration in Minisigmoid of NOAA 12673

Abstract We present a multiwavelength analysis of two homologous, short-lived, impulsive flares of GOES class M1.4 and M7.3 that occurred from a very localized minisigmoid region within the active region NOAA 12673 on 2017 September 7. Both flares were associated with initial jetlike plasma ejection that for a brief amount of time moved toward the east in a collimated manner before drastically changing direction toward the southwest. Nonlinear force-free field extrapolation reveals the presence of a compact double-decker flux rope configuration in the minisigmoid region prior to the flares. A set of open field lines originating near the active region that were most likely responsible for the anomalous dynamics of the erupted plasma gave the earliest indication of an emerging coronal hole near the active region. The horizontal field distribution suggests a rapid decay of the field above the active region, implying high proneness of the flux rope system toward eruption. In view of the low coronal double-decker flux ropes and compact extreme ultraviolet brightening beneath the filament, along with associated photospheric magnetic field changes, our analysis supports the combination of initial tether-cutting reconnection and subsequent torus instability for driving the eruption.

On the Ubiquity of Magnetic Reconnection Inside Flux Transfer Event‐Like Structures at the Earth's Magnetopause

AbstractFlux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced.

Mass Motion in a Prominence Bubble Revealing a Kinked Flux Rope Configuration

Prominence bubbles are cavities rising into quiescent prominences from below. The bubble-prominence interface is often the active location for the formation of plumes, which flow turbulently into quiescent prominences. Not only the origin of prominence bubbles is poorly understood, but most of their physical characteristics are still largely unknown. Here, we investigate the dynamical properties of a bubble, which is observed since its early emergence beneath the spine of a quiescent prominence on 20 October 2017 in the H$\alpha$ line-center and in $\pm$0.4 angstrom line-wing wavelengths by the 1-m New Vacuum Solar Telescope. We report the prominence bubble to be exhibiting a disparate morphology in the H$\alpha$ line-center compared to its line-wings' images, indicating a complex pattern of mass motion along the line-of-sight. Combining Doppler maps with flow maps in the plane of sky derived from a Nonlinear Affine Velocity Estimator, we obtained a comprehensive picture of mass motions revealing a counter-clockwise rotation inside the bubble; with blue-shifted material flowing upward and red-shifted material flowing downward. This sequence of mass motions is interpreted to be either outlining a kinked flux rope configuration of the prominence bubble or providing observational evidence of the internal kink instability in the prominence plasma.

Reconstruction of a Highly Twisted Magnetic Flux Rope for an Inter-active-region X-Class Solar Flare

Solar eruptions are manifestation of explosive release of magnetic energy in the Sun's corona. Large solar eruptions originate mostly within active regions, where strong magnetic fields concentrate on the solar surface. Here we studied the magnetic field structure for an exception, which is a peculiar GOES X1.2 flare accompanied with a very fast coronal mass ejection taking place between two active regions, where the magnetic field is relatively weak. The pre-flare magnetic field is reconstructed from the SDO/HMI vector magnetogram, using a nonlinear force-free field extrapolation method. It is found that prior to the flare, there is a highly twisted magnetic flux rope with magnetic field lines winding over 6 turns, which connects the border of a leading sunspot of one active region and the following polarity of the neighboring active region. The flux rope's configuration is highly consistent with the observed sigmoidal coronal loops and filament channels by SDO/AIA. It resides rather low-lying between the active regions such that the torus instability is not able to be triggered. Thus, it is most likely that, due to the strong magnetic twist, the kink instability of the flux rope triggers the eruption.

Small interplanetary magnetic flux rope

Small interplanetary magnetic flux ropes (SIMFRs) are often detected by space satellites in the interplanetary space near 1 AU. These ropes can be fitted by a cylindrically symmetric magnetic model. The durations of SIMFRs are usually and the diameters of SIMFRs are 0.20 AU and show power law distribution. Most SIMFRs are observed in the typically slow solar wind ( and only several events are observed with high speed (>700 km/s). Some SIMFRs demonstrate abnormal heavy ion compositions, such as abnormally high He abundance, abnormally high average iron ionization, and enhanced O7+ abundance. These SIMFRs originate from remarkably hot coronal origins. Approximately 74.5% SIMFRs exhibit counterstreaming suprathermal electron signatures. Given their flux rope configuration, SIMFRs are potentially more effective for substorms. SIMFRs and magnetic clouds have many similar observational properties but also show some different observations. These similar properties may indicate that SIMFRs are the interplanetary counterparts of small coronal mass ejections. Some direct bodies of evidence have confirmed that several SIMFRs are interplanetary counterparts of CMEs. However, their different properties may imply that some SIMFRs have interplanetary origins. Therefore, one of the main aims of future research on SIMFRs is to determine whether SIMFRs originate from two different sources, that is, some events are formed in the solar coronal atmosphere, whereas others originate from the interplanetary space. Finally, in this paper, we offer some prospects that should be addressed in the future.

Heliospheric Evolution of Magnetic Clouds

Abstract The interplanetary evolution of 11 magnetic clouds (MCs) recorded by at least two radially aligned spacecraft is studied. The in situ magnetic field measurements are fitted to a cylindrically symmetric Gold–Hoyle force-free uniform-twist flux-rope configuration. The analysis reveals that in a statistical sense, the expansion of the studied MCs is compatible with self-similar behavior. However, individual events expose a large scatter of expansion rates, ranging from very weak to very strong expansion. Individually, only four events show an expansion rate compatible with isotropic self-similar expansion. The results indicate that the expansion has to be much stronger when the MCs are still close to the Sun than in the studied 0.47–4.8 au distance range. The evolution of the magnetic field strength shows a large deviation from the behavior expected for the case of isotropic self-similar expansion. In the statistical sense, as well as in most of the individual events, the inferred magnetic field decreases much slower than expected. Only three events show behavior compatible with self-similar expansion. There is also a discrepancy between the magnetic field decrease and the increase of the MC size, indicating that magnetic reconnection and geometrical deformations play a significant role in the MC evolution. About half of the events show a decay of the electric current as expected for self-similar expansion. Statistically, the inferred axial magnetic flux is broadly consistent with remaining constant. However, events characterized by a large magnetic flux show a clear tendency toward decreasing flux.

Eruption of a multi-flux-rope system in solar active region 12673 leading to the two largest flares in Solar Cycle 24

Context. Solar active region (AR) 12673 in 2017 September produced the two largest flares in Solar Cycle 24: the X9.3 flare on September 6 and the X8.2 flare on September 10. Aims. We attempt to investigate the evolutions of the two large flares and their associated complex magnetic system in detail. Methods. Combining observations from the Solar Dynamics Observatory and results of nonlinear force-free field (NLFFF) modeling, we identify various magnetic structures in the AR core region and examine the evolution of these structures during the flares. Results. Aided by the NLFFF modeling, we identify a double-decker flux rope configuration above the polarity inversion line (PIL) in the AR core region. The north ends of these two flux ropes were rooted in a negative- polarity magnetic patch, which began to move along the PIL and rotate anticlockwise before the X9.3 flare on September 6. The strong shearing motion and rotation contributed to the destabilization of the two magnetic flux ropes, of which the upper one subsequently erupted upward due to the kink-instability. Then another two sets of twisted loop bundles beside these ropes were disturbed and successively erupted within five minutes like a chain reaction. Similarly, multiple ejecta components were detected as consecutively erupting during the X8.2 flare occurring in the same AR on September 10. We examine the evolution of the AR magnetic fields from September 3 to 6 and find that five dipoles emerged successively at the east of the main sunspot. The interactions between these dipoles took place continuously, accompanied by magnetic flux cancellations and strong shearing motions. Conclusions. In AR 12673, significant flux emergence and successive interactions between the different emerging dipoles resulted in a complex magnetic system, accompanied by the formations of multiple flux ropes and twisted loop bundles. We propose that the eruptions of a multi-flux-rope system resulted in the two largest flares in Solar Cycle 24.