Publications

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 ∼ 10³³ erg) on the M-dwarf binary EQ Peg on September 8th, 2022. 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 (≳ 10¹³ K) and high degrees of circular polarization (∼ 0.84 ± 0.16), indicating the origin of 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.

Context. Spectropolarimetric inversions are usually employed to obtain the physical parameters (e.g., magnetic field, velocity, temperature) of the solar atmosphere that contribute to the observed Stokes spectra. However, this process involves optimization in a multi-dimensional parameter space, which not only demands substantial computational time, but also frequently encounters local minima during the search. Aims. Our aim is to develop a spectropolarimetric inversion code leveraging the physics-informed neural networks (PINNs) and a novel gradient-bias annealing (GBA) algorithm with global minimum search capabilities to enable computationally efficient convergence toward near-optimal solutions. Methods. Inversions were trained on synthetic data generated by a forward model. The training process incorporated both data regression and cyclic physics-informed constraints to ensure physical consistency. Additionally, we developed a GBA algorithm to enable preferential annealing during optimization. The entire framework was implemented with CUDA for parallel acceleration. Results. We evaluated the effectiveness of the GBA algorithm, PINN, and their combined approach using the Helioseismic and Magnetic Imager (HMI) Stokes spectrum observations. When employed independently, the PINN achieved inversion results comparable to HMI’s quick products, via Very Fast Inversion of the Stokes Vector (VFISV), within significantly shorter timescales. The GBA algorithm alone demonstrated superior spectral recovery, despite requiring several hours to process millions of pixels in a single field of view. By initializing the annealing process with the PINN results, we obtained quality-enhanced results on minute-level timescales.

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 = n_x · n_y · n_z is the total number of voxels in the simulation volume) to O(k · 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 · n) integration, sacrificing small r resolution. And the second computes ray-plane intersections directly, with O(k · n · log n) complexity. RST features a user-friendly interface where users simply specify absorption (κ) and emissivity (j). It includes physical modules for white-light corona, Hα, EUV, radio, and pseudo-radiation.

The magnetic field in the solar corona cannot be observed routinely at present. Nonlinear force-free field (NLFFF) extrapolation serves as a crucial method for modeling the coronal magnetic configuration. We present a novel physics-reinforced generative adversarial network (PRO-GAN), an integrated deep learning framework designed to efficiently transform solar potential fields into corresponding NLFFFs while enforcing physical constraints to improve the result. This dual-phase framework synthesizes multifaceted integration, including the data-driven mapping relation acquisition, the constraint of physics equations, and supervision by a numerical solver. On the other hand, by incorporating a gradient penalty reinforced by a numerical method during the physics reinforcement training process, this approach resolves the inherent gradient conflicts in physics-informed neural networks from a novel perspective. The validity and efficiency of PRO-GAN have also been verified on the dataset and two analytic models.

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.

We observed three recurrent blowout jets in an active region with the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory. Using Helioseismic Magnetic Imager data we found that the magnetic flux of an emerging negative pole increases steadily before declining just as the jets erupt. Certain physical quantities, such as the total unsigned vertical current, align with the periodicity of the jets. The differential affine velocity of vector magnetograms reveals strong shear around the negative pole. The Doppler velocity map, calculated from the Hα spectra observed by the Chinese Hα Solar Explorer, shows upflows with large initial velocity before they can be observed by AIA. The magnetic field derived from the nonlinear force-free field (NLFFF) model suggests a topology akin to a fan–spine structure, consistent with AIA images. We calculated the evolution of the volumetric helicity ratio using the NLFFF model and found its phase aligns with the jet flux in AIA 171 Å. These results suggest that recurrent jets may be triggered by the accumulation and release of energy and helicity, driven by emergence, shearing, and cancellation of photospheric magnetic field.

Jet precession has previously been proposed to explain the apparently repeating features in the light curves of a few gamma-ray bursts (GRBs). In this paper, we further apply the precession model to a bright GRB 220408B by examining both its temporal and spectral consistency with the predictions of the model. As one of the recently confirmed GRBs observed by our GRID CubeSat mission, GRB 220408B is noteworthy as it exhibits three apparently similar emission episodes. Furthermore, the similarities are reinforced by their strong temporal correlations and similar features in terms of spectral evolution and spectral lags. Our analysis demonstrates that these features can be well explained by the modulated emission of a Fast-Rise-Exponential-Decay (FRED) shape light curve intrinsically produced by a precessing jet with a precession period of 18.4 ± 0.2 s, a nutation period of 11.1 ± 0.2 s and viewed off-axis. This study provides a straightforward explanation for the complex yet similar multi-episode GRB light curves.

Context. The method of spatial seismology can be applied to the amplitude profile of transverse coronal loop oscillations to constrain the distributions of physical parameters, such as the loop density, magnitude of the magnetic field, and so on. Aims. We intend to develop and apply a practical spatial seismology technique to detect physical parameters of plasma and validate its effectiveness by comparing it with other methods. Methods. A spatial seismology inversion was conducted by numerically optimizing a parametric dynamic model of the loop’s density stratification and magnetic field variation to best fit the measured amplitude profile of the loop. Results. The spatial seismology inversion technique developed here was applied to a transverse coronal loop oscillation that occurred on 2013 April 11, whose oscillation amplitude profile of both the fundamental mode and first overtone was reported in previous work. The consistency between the time domain analysis and spatial seismology has been verified. Meanwhile, we accounted for the asymmetric profile of the fundamental mode by forward modeling and we derived the magnetic field distribution by inverse modeling, which is coincident with that of the extrapolated one. In addition, spatial seismology inversion was applied to the transverse oscillation event on 2022 March 30 to obtain the distribution of the loop’s density and magnetic field, which are compared with the results derived from the differential emission measure (DEM) diagnostics and the direct potential field extrapolation. Conclusions. Spatial seismology inversion can be used as an effective method to independently measure various physical parameters, for example the density and magnetic field of coronal loops, which are consistent with the results obtained by DEM diagnostics and potential field extrapolation.

Supernovae and their remnants provide energetic feedback to the ambient interstellar medium (ISM), which is often distributed in multiple gas phases. Among them, warm molecular hydrogen (H₂) often dominates the cooling of the shocked molecular ISM, which has been observed with the H₂ emission lines at near-infrared wavelengths. Such studies, however, were either limited in narrow filter imaging or sparsely sampled mid-infrared spectroscopic observations with relatively poor angular resolutions. Here we present near-infrared (H and K bands) spectroscopic mosaic observations towards the A, B, C, and G regions of the supernova remnant (SNR) IC 443, with the K-band Multi-Object Spectrograph (KMOS) onboard the Very Large Telescope (VLT). We detected 20 rotational–vibrational transitions of H₂, one H line (Brγ), and two [Fe ii] lines, which dominate broad-band images at both H and K bands. The spatial distribution of H₂ lines at all regions is clumpy on scales from ∼0.1 down to ∼0.008 pc. The fitted excitation temperature of H₂ is between 1500 and 2500 K, indicating warm shocked gas in these regions. The multigas phase comparison shows stratified shock structures in all regions, which explains the coexistence of multiple types of shocks in the same regions. Lastly, we verify the candidates of young stellar objects previously identified in these regions with our spectroscopic data, and find none of them are associated with young stars. This sets challenges to the previously proposed scenario of triggered star formation by SNR shocks in IC 443.

Context. Coronal loop oscillations can be triggered by solar eruptions, for example, and are observed frequently by the Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board SDO offers us the opportunity to measure the photospheric vector magnetic field and carry out solar magneto-seismology (SMS). Aims. By applying SMS, we aim to verify the consistency between the observed period and the one derived from the information of coronal density, magnetic field, and loop geometry, that is, the shape of the loop axis. Methods. We analysed the data of three coronal loop oscillation events detected by SDO/AIA and SDO/HMI. First, we obtained oscillation parameters by fitting the observational data. Second, we used a differential emission measure (DEM) analysis to diagnose the temperature and density distribution along the coronal loop. Subsequently, we applied magnetic field extrapolation to reconstruct the three-dimensional magnetic field and then, finally, used the shooting method to compute the oscillation periods from the governing equation. Results. The average magnetic field determined by magnetic field extrapolation is consistent with that derived by SMS. A new analytical solution is found under the assumption of exponential density profile and uniform magnetic field. The periods estimated by combining the coronal density and magnetic field distribution and the associated loop geometry are closest to the observed ones, and are more realistic than when the loop geometry is regarded as being semi-circular or having a linear shape. Conclusions. The period of a coronal loop is sensitive to not only the density and magnetic field distribution but also the loop geometry.

The Chinese CubeSat Mission, Gamma Ray Integrated Detectors (GRID), recently detected its first gamma-ray burst, GRB 210121A, which was jointly observed by the Gravitational wave high-energy Electromagnetic Counterpart All-sky Monitor (GECAM). This burst is confirmed by several other missions, including Fermi and Insight-HXMT. We combined multimission observational data and performed a comprehensive analysis of the burst’s temporal and spectral properties. Our results show that the burst is relatively special in its high peak energy, thermal-like low-energy indices, and large fluence. By putting it to the E_p–E_γ,iso relation diagram with assumed distance, we found that this burst can be constrained at the redshift range of [0.3, 3.0]. The thermal spectral component is also confirmed by the direct fit of the physical models to the observed spectra. Interestingly, the physical photosphere model also constrained a redshift of z ∼ 0.3 for this burst, which helps us to identify a host galaxy candidate at such a distance within the location error box. Assuming that the host galaxy is real, we found that the burst can be best explained by the photosphere emission of a typical fireball with an initial radius of r₀ ∼ 3.2 × 10⁷ cm.