Solar magnetohydrodynamic (MHD) simulation is an indispensable method for studying solar activities. Modern MHD simulations are increasingly realistic and need to be synthesized with multiwavelength radiation to validate observations. We developed radiation synthesis tools (RST), a Cython-optimized Python program to solve the radiative transfer equation, enabling efficient multiwavelength synthesis of optically thick radiation from arbitrary perspectives. By directly calculating the ray traversal distance ds per cell, RST eliminates interpolation. As a result, the computational complexity in Cartesian coordinates is reduced from O(N) (where N = nx & centerdot; ny & centerdot; nz is the total number of voxels in the simulation volume) to O(k & centerdot;n) (where k is the number of ray footpoints and n is the number of sampling steps along each ray), significantly improving efficiency, especially for localized high-resolution synthesis. For spherical coordinates, RST offers two methods. The first interpolates data into Cartesian coordinates for fast O(k & centerdot;n) integration, sacrificing small r resolution. And the second computes ray-plane intersections directly, with O(k & centerdot;n & centerdot;logn) complexity. RST features a user-friendly interface where users simply specify absorption (kappa) and emissivity (j). It includes physical modules for white-light corona, H alpha, EUV, radio, and pseudo-radiation.

Although interactions between filaments and other atmospheric structures are frequently observed, detailed studies on how these interactions affect the dynamics of the involved structures remain scarce. Here we present an analysis of a filament eruption on 2023 December 31, during which the filament interacted with a nearby coronal loop (L1). Throughout the interaction, the filament continued erupting. At the interaction site, the bright emission appeared within L1 while the filament exhibited a local deceleration here, regaining its velocity only after the bright emission disappeared. Multiwavelength observations and differential emission measure analysis indicated that the bright emission resulted from a marked increase in plasma density rather than in temperature. We propose that the plasma accumulation in L1 was a consequence of condensation induced by the interaction with the filament. The condensation influenced the velocity of the erupting filament in turn. As the condensation drained away, the filament recovered its velocity. Our observations suggest that condensation induced by the interaction can exert dynamic feedback on the erupting filaments.

Magnetic flux ropes are fundamental magnetic structures in solar eruptions, whose formation is generally attributed to: (1) the emergence of subsurface flux tubes; or (2) flux cancellation driven by photospheric horizontal flows and magnetic reconnection. Both mechanisms can operate simultaneously during active region (AR) evolution, making their relative contributions challenging to quantify. Here, we analyze the formation of a flux rope in a MURaM radiative magnetohydrodynamic simulation, which formed and evolved for approximately 3 hr before an M-class flare. The formation process is quantified by the magnetic helicity flux, which drives the nonpotential evolution of the magnetic field, with its advection and shear terms on the photosphere corresponding to the emergence and photospheric horizontal flows, respectively. Examining the helicity injected into the flux rope through the photosphere, we find both terms increase significantly as the eruption approaches, with the shear term prevailing overall. Height-dependent analysis of the helicity flux, together with the magnetic field and velocity distributions, further reveals a gradual transition from the shear to the advection term with an increasing altitude, which is driven by magnetic reconnection above the photosphere. Our results provide quantitative evidence that flux cancellation governs flux rope formation, arising naturally from magnetic field reorganization during AR evolution: as the flux emergence transports the magnetic flux upward, photospheric shearing motions adjust the magnetic field and inject helicity into the solar atmosphere, and magnetic reconnection ultimately assembles the main body of flux ropes.

The evolution of magnetic fields in the tenuous solar corona is predominantly governed by the motions of the underlying dense photosphere. Despite this, coronal magnetic restructuring driven by magnetic reconnection between interacting coronal fields can sometimes react backward to change photospheric magnetic fields. However, the mechanism of reactions remains undetermined. Here, we report the discovery of a back-reaction phenomenon: The untwisting of coronal loops that become twisted during reconnection in an eruption results in enhanced currents at the boundary of their footpoint away from the eruption, manifesting as the growth of a sunspot scar. It is revealed to arise from the Alfvénic reverse transfer of magnetic twist from the corona to the lower atmosphere, through joint space observations and a magnetohydrodynamic simulation. These findings provide a viable and quantitative interpretation for the majority of puzzling photospheric changes associated with coronal mass ejections and/or flares and warn for unexpected magnetic field evolutions in sunspots and starspots.

A thorough understanding of the initiation of coronal mass ejections (CMEs), which is manifested as a slow rise of pre-eruptive structures before the impulsive ejection in kinematics, is key for forecasting solar eruptions. In our previous work, we showed that the slow rise of a hot flux rope with coronal mass density is caused by the moderate magnetic reconnection occurring in the hyperbolic flux tube (HFT) combined with the torus instability. However, it remains unclear how the initiation process varies when a filament is present in the pre-eruptive flux rope. In this work, we reveal the complete initiation route of a CME containing filament mass with a state-of-the-art full-magnetohydrodynamics simulation. The comprehensive analyses show that the filament mass has an important impact on the CME initiation through triggering and driving the slow rise of flux rope with its drainage, besides the contributions of HFT reconnection and torus instability. Finally, in combination with our previous work, we propose that the enhanced drainage of filament mass and various features related to the HFT reconnection, such as the split of pre-eruptive structure and the preflare loops and X-ray emissions, can serve as precursors of CME initiation in observations.

Stellar coronae are believed to be the main birthplace of various stellar magnetic activities. However, the structures and properties of stellar coronae remain poorly understood. Using the Space Weather Modeling Framework with the Alfvén Wave Solar Model (SWMF-AWSoM) and dynamo-generated surface magnetic maps, here we model the coronae of four solar-type stars. By incorporating the Sun, our work covers a range of stars with the rotation varying from 1.0 to 23.3Ω _⊙ (periods of 25–1 days). Guided by observations, we scale the magnetic field strength with increasing rotation, covering a range between 6.0 G and 1200 G approximately. In our models, energy release associated with small-scale magnetic flux is a key source of coronal heating and is essential for reproducing realistic coronal structures. Our models capture dense (1–2 orders of magnitude higher than solar values) and ultra-hot (∼10 MK) coronae dominated by closed field structures. Using the CHIANTI atomic database, we also compute synthetic X-ray spectra and derive the corresponding X-ray luminosities ( L _X ), which follow a scaling law to magnetic field L _X ∝ 〈∣ B ∣〉 ^1.75 . Furthermore, the coronal X-ray emission is found to be rotationally modulated by the alternating presence of bright active regions and dark coronal holes. These results provide new insights into the extremely high-energy coronae of rapidly rotating solar-type stars, which differ markedly from the Sun.

Understanding the early evolution of coronal mass ejections (CMEs), in particular their initiation, is the key to forecasting solar eruptions and induced disastrous space weather. Although many initiation mechanisms have been proposed, a full understanding of CME initiation, which is identified as a slow rise of CME progenitors in kinematics before impulsive acceleration, remains elusive. Here, with a state-of-the-art thermal magnetohydrodynamics simulation, we determine a complete CME initiation route in which multiple mainstream mechanisms occur in sequence yet are tightly coupled. The slow rise is first triggered and driven by the developing hyperbolic flux tube (HFT) reconnection. Subsequently, the slow rise continues as driven by the coupling of the HFT reconnection and the early development of torus instability. The end of the slow rise, i.e., the onset of the impulsive acceleration, is induced by the start of the fast magnetic reconnection coupled with the torus instability. These results unveil that CME initiation is a complicated process involving multiple physical mechanisms, thus being hardly resolved by a single initiation mechanism.

Context. The properties of pre-eruptive structures and coronal mass ejections (CMEs) are characterized by those of their footpoints, the latter of which attract a great deal of interest. However, the matter of how to identify the footpoints of pre-eruptive structures and how to do so with the use of ground-based instruments still remains elusive. Aims. In this work, we study an arc-shaped structure intruding in the sunspot umbra. It is located close to the (pre-)eruptive flux rope footpoint and it is expected to help identify the footpoint. Methods. We analyzed this arc-shaped structure, which we call a "sunspot scar", in a CME event on July 12, 2012, and in two CME events from observationally inspired magnetohydrodynamic simulations performed by OHM and MPI-AMRVAC. Results. The sunspot scar displays a more inclined magnetic field with a weaker vertical component and a stronger horizontal component relative to that in the surrounding umbra and is manifested as a light bridge in the white light passband. The hot field lines anchored in the sunspot scar are spatially at the transition between the flux rope and the background coronal loops and temporally in the process of the slipping reconnection which builds up the flux rope. Conclusions. The sunspot scar and its related light bridge mark the edge of the CME flux rope footpoint and particularly indicate the edge of the pre-eruptive flux rope footpoint in the framework of "pre-eruptive structures being flux ropes". Therefore, they provide a new perspective for the identification of pre-eruptive and CME flux rope footpoints, as well as new methods for studying the properties and evolution of pre-eruptive structures and CMEs with photospheric observations only.

Coronal mass ejections are explosive plasma phenomena prevalently occurring on the Sun and probably on other magnetically active stars. However, how their pre-eruptive configuration evolves toward the main explosion remains elusive. Here, based on comprehensive observations of a long-duration precursor in an event on 2012 March 13, we determine that the heating and slow rise of the pre-eruptive hot magnetic flux rope (MFR) are achieved through a precursor reconnection located above cusp-shaped high-temperature precursor loops. It is observed that the hot MFR threads are built up continually, with their middle initially showing an “M” shape and then being separated from the cusp of precursor loops, causing the slow rise of the entire MFR. The slow rise, in combination with the thermal-dominated hard X-ray source concentrated at the top of the precursor loops, shows that the precursor reconnection is much weaker than the flare reconnection of the main eruption. We also perform a 3D magnetohydrodynamics simulation that reproduces the early evolution of the MFR transiting from the slow to fast rise. It is revealed that the magnetic tension force pertinent to “M”-shaped threads drives the slow rise, which, however, evolves into a magnetic pressure gradient-dominated regime responsible for the rapid acceleration eruption.

Le rôle de la reconnexion magnétique dans l'évolution des tubes de flux des éruptions solaires Les éjections de masse coronale (CME) sont des éruptions impulsives de plasmas dans la couronne solaire. Leur interaction avec la magnétosphère de la Terre peut induire des conditions extrêmes de la météorologie de l’espace, avec un impact important sur les activités humaines liées aux technologies de pointe. Une compréhension approfondie de l'évolution des CMEs et de leurs progéniteurs est extrêmement importante pour prédire les éruptions de CMEs et les phénomènes de météorologie de l'espace qui en découlent. Dans cette thèse, à l'aide de simulations numériques et d'observations spatiales, nous étudions la cinématique, les propriétés thermiques et l'évolution du champ magnétique des tubes de flux dans les progéniteurs de CMEs et les CMEs elles-mêmes, avec en particulier le rôle spécifique de la reconnexion magnétique. Nous avons découvert que l'initiation des CMEs avant leur éruption impulsive est un processus couplé à plusieurs processus physiques. Nous avons montré que l'initiation des CMEs est d'abord déclenchée et pilotée par la reconnexion dans des tubes de flux hyperboliques, puis par le couplage de l'instabilité de tore et de cette même. Nous avons aussi montré que les cœurs chauds avant l'éjection impulsive sont formés par les lignes de champ des tubes de flux torsadés chauds, ces dernières étant progressivement formées et chauffées par la reconnexion glissante, dans des feuillets de courant minces entourant le tube de flux. Nous avons également étudié l'évolution du flux magnétique dans les CMEs, et avons trouvé que le tube torsadé pré-éruptif lui-même, plutôt que la reconnexion magnétique pendant l'éruption, est très probablement le principal contributeur au flux toroïdal de la CME. Plus spécifiquement, la reconnexion magnétique augmente d'abord, puis diminue le flux toroïdal des tubes de flux de la CME pendant l'éruption. En outre, nous avons étudié deux nouveaux phénomènes observationnels liés aux CMEs and flashs des éruptions dans la basse atmosphère solaire, notamment des manifestations de la croissance et de la déformation des tubes de flux des CMEs et de leurs progéniteurs, induites par la reconnexion magnétique. Enfin, nous avons proposé deux méthodes pour l'identification des points d’ancrage des tubes de flux associés aux CMEs, qui seront d'une grande utilité pour des futurs travaux visant à étudier leurs évolutions dans la couronne solaire et dans l'espace interplanétaire.

Magnetic reconnection is a key mechanism involved in solar eruptions and is also a prime possibility to heat the low corona to millions of degrees. Here, we present ultra-high-resolution extreme ultraviolet observations of persistent null-point reconnection in the corona at a scale of about 390 km over one hour observations of the Extreme-Ultraviolet Imager on board Solar Orbiter spacecraft. The observations show formation of a null-point configuration above a minor positive polarity embedded within a region of dominant negative polarity near a sunspot. The gentle phase of the persistent null-point reconnection is evidenced by sustained point-like high-temperature plasma (about 10 MK) near the null-point and constant outflow blobs not only along the outer spine but also along the fan surface. The blobs appear at a higher frequency than previously observed with an average velocity of about 80 km/s and life-times of about 40 s. The null-point reconnection also occurs explosively but only for 4 minutes, its coupling with a mini-filament eruption generates a spiral jet. These results suggest that magnetic reconnection, at previously unresolved scales, proceeds continually in a gentle and/or explosive way to persistently transfer mass and energy to the overlying corona.

We investigate the initiation and early evolution of 12 solar eruptions, including six active region hot channel and six quiescent filament eruptions, which were well observed by the \textsl{Solar Dynamics Observatory}, as well as by the \textsl{Solar TErrestrial RElations Observatory} for the latter. The sample includes one failed eruption and 11 coronal mass ejections, with velocities ranging from 493 to 2140~km~s$^{-1}$. A detailed analysis of the eruption kinematics yields the following main results. (1) The early evolution of all events consists of a slow-rise phase followed by a main-acceleration phase, the height-time profiles of which differ markedly and can be best fit, respectively, by a linear and an exponential function. This indicates that different physical processes dominate in these phases, which is at variance with models that involve a single process. (2) The kinematic evolution of the eruptions tends to be synchronized with the flare light curve in both phases. The synchronization is often but not always close. A delayed onset of the impulsive flare phase is found in the majority of the filament eruptions (5 out of 6). This delay, and its trend to be larger for slower eruptions, favor ideal MHD instability models. (3) The average decay index at the onset heights of the main acceleration is close to the threshold of the torus instability for both groups of events (although based on a tentative coronal field model for the hot channels), suggesting that this instability initiates and possibly drives the main acceleration.

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 and the hook of the flare ribbon into account. We find that the toroidal flux of the CME flux rope for all four events shows a two-phase evolution: a rapid increasing phase followed by a decreasing phase. We further compare the evolution of the toroidal flux with that of the Geostationary Operational Environmental Satellites soft X-ray flux and find that they are basically synchronous in time, except that the peak of the former is somewhat delayed. The results suggest that the toroidal flux of the CME flux rope may be first quickly built up by the reconnection mainly taking place in the sheared overlying field and then reduced by the reconnection among the twisted field lines within the flux rope, as enlightened by a recent 3D magnetohydrodynamic simulation of CMEs.

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 from the flux measured in the MC. The flux by the reconnection is derived from geometric properties of two-ribbon flares based on a quasi-2D reconnection model. We then apply the model to four CME/flare events and find that the ratio of toroidal flux in the pre-existing flux rope to that of the associated MC lies in the range of 0.40--0.88. It indicates that the toroidal flux of the pre-existing flux rope has an important contribution to that of the CME flux rope and is usually at least as large as the flux arising from the eruption process for the selected events.

In this Letter, we investigate the long-duration quasi-static evolution of 12 pre-eruptive filaments (four active region (AR) and eight quiescent filaments), mainly focusing on the evolution of the filament height in 3D and the decay index of the background magnetic field. The filament height in 3D is derived through two-perspective observations of Solar Dynamics Observatory (SDO) and Solar TErrestrial RElations Observatory (STEREO). The coronal magnetic field is reconstructed using the potential field source surface model. A new finding is that the filaments we studied show two types of long-duration evolution: one type comprises a long-duration static phase and a short, slow rise phase with a duration of less than 12 hr and a speed of 0.1-0.7 km s(-1), while the other one only presents a slow rise phase but with an extremely long duration of more than 60 hr and a smaller speed of 0.01-0.2 km s(-1). At the moment approaching the eruption, the decay index of the background magnetic field at the filament height is similar for both AR and quiescent filaments. The average value and upper limit are similar to 0.9 and similar to 1.4, close to the critical index of torus instability. Moreover, the filament height and background magnetic field strength are also found to be linearly and exponentially related with the filament length, respectively.