Xing Cheng

Superflares are frequently observed on magnetically active stars, particularly on active M dwarfs, thereby influencing the habitability of nearby exoplanets. However, the causes of superflares remain unclear. Here, combining FAST and TESS, we detected a finely structured decimetric radio burst appearing as groups of spikes during a white-light superflare (total bolometric energy ∼ 10 ³³ erg) on the M dwarf binary EQ Peg on 2022 September 8. The most significant ones appeared near the peak time of the optical flare and lasted for about 100 s. The radio spikes present extremely high brightness temperatures (≳10 ¹³ K) and high degrees of circular polarization (∼0.84 ± 0.16), indicating their origin in a coherent emission process, most likely the electron cyclotron maser. Based on Zeeman Doppler imaging measurements of EQ Peg, we further locate the sources of the radio spikes, including their heights and latitudes, under the assumption of a large-scale dipolar magnetic field configuration. The results provide strong evidence for finely structured decimetric radio bursts generated by stellar flares and shed light on the origin of superflares.

Solar flares can release magnetic energy explosively in the corona and produce high-energy particles on short timescales. However, how and where these particles are accelerated remains an open question. Here, we investigate the acceleration and transport of electrons during self-developed three-dimensional turbulent reconnection of solar flares by solving Parker’s transport equation in the framework of high-resolution MHD simulations. We find that thermal electrons at both the flare current sheet and loop top are rapidly accelerated up to ∼90 keV, with energy spectra exhibiting a power-law feature. Although the population of accelerated electrons at the flare loop top is larger than that at the current sheet, their spectral indices are similar, close to the values usually observed. More importantly, the acceleration is achieved by turbulence-driven compression structures of various scales rather than the supposed termination shock, particularly at the flare loop top. A portion of compression structures even forms shocks. These results highlight the critical role of turbulent reconnection in accelerating electrons, thereby shedding new light on the acceleration and transport of particles in other high-energy phenomena.

Observations suggest a connection between steady-wind mass loss and coronal X-ray activity in low-mass main-sequence stars. Interpreting this connection is challenging because the wind is controlled mainly by the large-scale open field, whereas X-ray emission traces heating in small-scale closed fields often unresolved in global wind models. Here we use Space Weather Modeling Framework–Alfvén-Wave Solar Model with global convective dynamo-generated magnetic maps and solar magnetograms. We model Alfvén-wave-heated winds for four solar-type stars plus the Sun, spanning rotation rates of 1.0–23.3 times the solar rate, and magnetic field strengths of 6.0–1200 G. Our models show that faster rotation yields a more tightly wound spiral, a larger Alfvén surface, higher terminal wind speeds, and a harsher wind-pressure environment for orbiting exoplanets, different from that of the present-day Sun. We estimate the mass- and angular-momentum-loss rates and find systematic differences from Zeeman–Doppler imaging based predictions. Building on the successful reproduction of X-ray coronae in our Paper I, we obtain the first self-consistent activity–wind relation in a unified modeling framework: the mass-loss rate scales with the surface X-ray flux following a power law with an index of ∼0.67. We also reexamine magnetic braking via open-flux magnetization, finding that the effective Alfvénic lever arm depends on the magnetization parameter with a power-law index of ∼0.35. Finally, we quantify the stellar wind pressure at the orbits of several super-Earths. Together with our Paper I, the series of results shows the distinct roles of multiscale magnetic fields and provides physically grounded inputs for assessing habitable-zone space weather.

Filaments are cool and dense plasmas suspended in the hot corona of the Sun and other stars. Accurately estimating their masses is of great significance for understanding subsequent eruptions and induced space weather effects, but it remains hindered by their intrinsic geometric uncertainties, particularly in spatially unresolved stellar observations. To test and calibrate the methods for estimating the masses of stellar filaments, we conduct a statistical Sun-as-a-star analysis of solar filaments, utilizing full-disk H α spectroscopic observations from the Chinese H α Solar Explorer (CHASE). A total of 1346 filaments, covering a period from 2024 January to 2025 October, are identified via a machine learning segmentation model. We construct their virtual Sun-as-a-star spectra by spatially integrating the filament regions and then obtain their optical parameters by cloud-model fitting. Upon correcting projection effects, we establish a representative three-dimensional morphological scaling of length, apparent width, and line-of-sight depth ( L : W _app : D _LOS ≈ 4.5:1:1.7), with a median filament depth of about 8000 km. Interestingly, the Sun-as-a-star estimated mass shows high consistency with the resolved intrinsic mass across the full sample, with a log-space regression slope of 1.07. As the first large-sample Sun-as-a-star study of solar filaments, our results provide empirical constraints on filament geometries and masses, offering a critical reference for estimating stellar filament masses based on H α spectroscopy.

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.

Failed filament eruptions have been reported previously; however, the responsible physical mechanisms remain debated. Using multiwavelength data from Solar Orbiter and Solar Dynamics Observatory, we analyze a failed filament eruption on 2024 March 19. The filament initially manifested as an extreme-ultraviolet (EUV) filament channel and rose without any enhancement in hard X-ray emission. As it subsequently transitioned from acceleration to deceleration, its main axis developed a Y-shaped kink structure, and the hard X-ray flux peaked during the deceleration phase. Brightenings were detected, spatially corresponding to the opposite polarities where the loops overlying the filament channel were rooted. Reconstructed hard X-ray sources revealed the existence of a coronal source at the leading edge of the erupting structure. After the eruption, the overlying loops were heated, and bright materials were observed moving along them. A data-constrained magnetohydrodynamic simulation further demonstrated that the eruptive flux rope interacted with its overlying loops, with its footpoint drifting into the region where the overlying loops were rooted, strongly confirming that external reconnection with the overlying loops eroded the eruptive flux. We also analyzed the decay index above the flux rope before the eruption and found that the flux rope was in the torus stable domain. We propose a possible scenario in which the eruption is initially driven by kink instability, after which external reconnection with the overlying loops leads to its deceleration and ultimately to the failure of the eruption.

Understanding the filament rising process is crucial for unveiling the triggering mechanisms of coronal mass ejections and forecasting space weather. In this paper, we present a detailed study of the filament initial eruption under a fan–spine structure. It was found that the filament underwent two distinct acceleration stages corresponding to class M1.0 and M4.6 flare events. The first acceleration stage commenced with the filament splitting, after which the upper portion was subsequently heated, being a hot channel, and slowly rose at an average speed of 22 km s ⁻¹ . A set of hot reverse C-shaped loops appeared repeatedly during the filament splitting, and a hook structure was recognized at this phase, suggesting ongoing growth of the magnetic flux rope (MFR). When it reached a certain altitude, the hot channel appeared to get into a quasi-static phase with its upper edge seriously decelerated and its lower edge expanding downward. Approximately 30 minutes later, as a distinct annular ribbon appeared outside the hook structure, the hot channel rose again at a velocity of over 50 km s ⁻¹ accompanied by rapidly drifting footpoints and experienced a second acceleration stage with its axial flux increased to 1.1 × 10 ²¹ Mx. It is deduced that the filament initial eruption under a magnetic dome possesses multiple kinetic processes. We suggest that the magnetic reconnection taking place within and beneath the filament continues to trigger the growth of preeruptive MFR and the first acceleration, when the magnetic reconnection above the filament plays a key role in the second acceleration.

Total solar eclipses (TSEs) provide a unique opportunity to observe the large-scale solar corona. The solar wind plays an important role in forming the large-scale coronal structure, and magnetohydrodynamic (MHD) simulations are used to reproduce it for further studying coronal mass ejections (CMEs). We conduct a data-constrained MHD simulation of the global solar corona including solar wind effects of the 2024 April 8 TSE with observed magnetograms using the message-passing interface adaptive mesh refinement versatile advection code (MPI-AMRVAC) within 2.5 R _⊙ . This TSE happened within the solar maximum, hence the global corona was highly structured. Our MHD simulation includes the energy equation with a reduced polytropic index γ = 1.05. We compare the global magnetic field for multiple magnetograms and use synchronic frames from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager to initialize the magnetic field configuration from a magnetofrictionally equilibrium solution, called the outflow field. We detail the initial and boundary conditions employed to time-advance the full set of ideal MHD equations such that the global corona is relaxed to a steady state. The magnetic field, the velocity field, and distributions of the density and thermal pressure are successfully reproduced. We demonstrate direct comparisons with TSE images in white light and Fe XIV emission augmented with quasi-separatrix layers, the integrated current density, and the synthetic white-light radiation, and find a good agreement between simulations and observations. This provides a fundamental background for future simulations to study the triggering and acceleration mechanisms of CMEs under solar wind effects.

In this Letter, taking advantage of microwave data from the Expanded Owens Valley Solar Array and extreme-ultraviolet (EUV) data from the Atmospheric Imaging Assembly, we present the first microwave imaging spectroscopy diagnosis for the slow-rise precursor of a major coronal mass ejection (CME) on 2022 March 30. The EUV images reveal that the CME progenitor, appearing as a hot channel above the polarity inversion line, experiences a slow rise and heating before the eruption. The microwave emissions are found to mainly distribute along the hot channel, with high-frequency sources located at the ends of the hot channel and along precursor bright loops underneath the hot channel. The microwave spectroscopic analysis suggests that microwave emissions in the precursor phase are dominated by thermal emission, largely different from the main phase when a significant nonthermal component is present. These results support the scenario that the precursor reconnection, seeming to be moderate compared with the flare reconnection during the main phase, drives the buildup, heating, and slow rise of CME progenitors toward the explosive eruption.

In this Letter, taking advantage of microwave data from the Expanded Owens Valley Solar Array and extreme-ultraviolet (EUV) data from the Atmospheric Imaging Assembly, we present the first microwave imaging spectroscopy diagnosis for the slow-rise precursor of a major coronal mass ejection (CME) on 2022 March 30. The EUV images reveal that the CME progenitor, appearing as a hot channel above the polarity inversion line, experiences a slow rise and heating before the eruption. The microwave emissions are found to mainly distribute along the hot channel, with high-frequency sources located at the ends of the hot channel and along precursor bright loops underneath the hot channel. The microwave spectroscopic analysis suggests that microwave emissions in the precursor phase are dominated by thermal emission, largely different from the main phase when a significant non-thermal component is present. These results support the scenario that the precursor reconnection, seeming to be moderate compared with the flare reconnection during the main phase, drives the build-up, heating, and slow rise of CME progenitors toward the explosive eruption.