Zhentong Li

Aims. Solar hot channels (HCs) are coherently elongated high-temperature structures observed in the solar corona, and they are believed to represent magnetic flux ropes during solar eruptions. Filaments supported by the dips of helical field lines in flux ropes are expected to locate in the bottom part of HCs. This is the classic scenario regarding the spatial relationship between filaments and HCs. However, in a recently observed eruption of an HC filament system (SOL2023-01-14T20:15), the filament split during the formation of the HC, and one filament branch became wrapped around the HC, which is in contrast to the classic scenario. By investigating this event, we aim to gain a more comprehensive understanding of the formation mechanisms of HCs, the splitting mechanisms of filaments, and the relationships between HCs and filaments. Methods. By utilizing multiwavelength remote-sensing data of the solar corona, we reveal the evolution of plasma structures before and during the eruption. Together with nonlinear force-free field extrapolation, we provide further evidence regarding the magnetic reconfiguration process that causes the observed changes. Results. We find that a portion of the filament-containing flux rope was eroded by an adjacent highly sheared loop via magnetic reconnection. The filament plasma on the newly reconnected field lines was heated and formed the HC. The filament plasma on the other field lines stayed cold and dense, appearing as a split filament with two branches. One of these branches wrapped around the newly formed HC. This study shows that HCs can form through the heating of filament plasma resulting from external magnetic reconnections, under which condition one branch of the split filament may be wrapped around the HC. Moreover, the writhe exhibited by this filament branch represents the twisted field lines of the flux rope and does not necessarily correspond to the writhe of the flux rope axis.

Flare ribbons form as a result of energy deposition associated with particles accelerated in low layers of the solar atmosphere. The fine-scale structures of flare ribbons, also called ribbon kernels, offer a potentially powerful diagnostic of the flare reconnection process, however, to date the dynamic evolution of ribbon kernels has not been fully characterized in statistical studies. Here, we checked the state-of-the-art observations (cadence ≤2.5 s) of solar flares in the ultraviolet from space by Interface Region Imaging Spectrograph over the past 12 yr. Our results showed the first statistical study of multiple spatially resolved flare kernel quasiperiodic pulsation events for 31 flares, with the period of 6–24 s. The ribbon kernels have a spatial scale of 480–1200 km and some kernels exhibit an unprecedented fast “winking” process, i.e., quasiperiodic pulsation-like flashing of individual kernels. The shortest heating time reaches about 2–3 s, implying that the energy is deposited only in a small localized region within flare ribbons, persisting for only a few seconds. Meanwhile, some ribbon kernels were observed to slip along the ribbon at speeds of 20–1800 km s ⁻¹ . These observations strongly imply a joint picture for the dynamics and the bursty nature of ribbon kernels as being due to coupled effects of plasmoid formation and three-dimensional magnetic reconnection in the overlaying coronal current sheet. We suggest that the observed flare behaviors provide strong observational evidences of 3D bursty reconnection.

How flares accelerate electrons efficiently remains a major puzzle in solar and stellar astronomy. Crucial information is carried by microwave bursts that are generated as energetic electrons spiraling around magnetic field lines. Here we report the eruption process during the strongest solar microwave burst (>10,000 solar flux units above 20 GHz, associated with an X1.7 flare) in Solar Cycle 25. We found that the interaction between postflare loops of a preceding M2.6 flare and rising southern loops generates abundant superhot plasma ( n _e ∼ 2.0 × 10 ⁹ cm ⁻³ , T _e ∼ 40 MK), which gets involved (by providing heated seed particles) in the subsequent three successive episodes of reconnection processes of the X1.7 flare. Using the gyrosynchrotron mechanism to fit the obtained spectra for the three subsequent microwave peaks, we found that energetic electrons are progressively abundant, with a hardening spectrum with the index reaching up to ∼2. We highlight the importance of preheated plasma and persistent multiepisode acceleration of energetic electrons in generating such extreme microwave bursts.

Energetic electrons accelerated at coronal reconnection sites during solar flares precipitate into the lower solar atmosphere, generating nonthermal emissions and regulating energy deposition. However, how their transport and precipitation are jointly governed by the three-dimensional (3D) magnetic topology, turbulent scattering, and Coulomb collisions remains unclear. Here, we aim to disentangle these physical processes by using a data-constrained 3D particle transport model for the 2011 August 4 flare. The simulated distribution of precipitated electrons aligns closely with photospheric quasi-separatrix layers and reproduces the observed two-ribbon morphology in 1700 Å. We reveal a strong polarity asymmetry, with the 10 s precipitation fraction about 6 times higher in the weak positive polarity. This arises primarily from distinct mirror ratios of different polarities under the 3D magnetic configuration and can be understood via a modified escape probability for an asymmetric magnetic bottle. Varying strengths of turbulent scattering lead to a rise-then-fall trend and a pronounced energy dependence in the precipitation fraction. Coulomb collisions globally suppress precipitation, especially at low energies, and further amplify the polarity asymmetry. This integrated modeling framework bridges detailed transport physics to observable flare emissions and advances the development of quantitative models for realistic solar flare events.

Aims. Solar type II radio bursts are commonly attributed to coronal shocks driven by coronal mass ejections (CMEs). However, some metric type II bursts have occasionally been reported to occur in the absence of a CME and to be associated with weak solar activity. The aim of this study is to identify the driver of the coronal shock in this kind of type II event. Methods. We investigated a high-frequency metric type II burst with clear band splitting, observed simultaneously by the Chashan Broadband Solar radio spectrograph and the Nançay Radioheliograph. It is associated with a C3.1-class flare and a small-scale jet, but without a detectable CME in the coronagraphs. Results. The type II burst is preceded by multiple type III bursts, one of which exhibits characteristics of a type N burst. The type II burst source is associated with the jet-induced perturbation front propagating through nearby closed loops at a speed of ∼880 km s ⁻¹ , rather than the much slower jet front. This suggests that the disturbance initiated by the jet can convert to a shock wave within low Alfvénic coronal loops, providing the necessary conditions for electron acceleration and subsequent radio emission. Our findings offer new insights into the formation mechanism of high-frequency type II bursts associated with weak flares and jets.

Context. Circular ribbon flares, a typical kind of multi-ribbon flare, have been the focus of many studies. Along with the flares themselves, many other related phenomena also merit investigation, such as jets, chromosphere condensation, and magnetohydrodynamic (MHD) waves. Aims. We analyzed a circular ribbon flare accompanied by a rotating jet that occurred on 2023 May 25, and we find observational evidence of chromosphere condensation in the flare ribbon and an Alfvén wave along the jet. Methods. We derived the plane-of-sky velocity of the cool component of the jet using the Fourier local correlation tracking method, and we obtained the Doppler velocity of the flare and the cool component of jet by analyzing the spectrum observed by the Chinese H α Solar Explorer (CHASE). To obtain the magnetic topology of the circular ribbon flare, we performed a nonlinear force-free field extrapolation using the photosphere vector magnetogram observed by the Helioseismic and Magnetic Imager (HMI). We additionally fit the hard X-Ray (HXR) energy spectrum obtained by the Hard X-ray Imager (HXI) aboard the Advanced Space-based Solar Observatory (ASO-S), and made a comparison between the Doppler velocity and the HXR sources. Results. We derive the velocity field of the cool component of jet, illustrate the spine-fan magnetic structure of the circular ribbon flare, and by comparing the vector magnetic field and the vector velocity field, we find a strong correlation between their inclination and azimuth angles, indicating that the jet mostly moves along the magnetic field. Through the Doppler velocity of the flare ribbon derived by CHASE observations, the redshift in the H α waveband indicates the existence of chromosphere condensation. The HXI HXR spectra and images demonstrate that nonthermal electrons are the primary source of the chromosphere condensation.

We present a detailed analysis of a C9.3 white-light flare using high-resolution observations from the New Vacuum Solar Telescope. The flare occurred near the eastern solar limb on 2023 September 11, within NOAA AR 13431, and produced beam electrons with energies just below 50 keV as observed by the Hard X-ray Imager onboard the Advanced Space-based Solar Observatory. Two white-light flare kernels were detected in the TiO band, connected by filamentary brightenings aligned with penumbral fibrils, suggesting a photospheric contribution to the white-light emission. Notably, the impact of the flare on the solar photosphere was characterized by sudden vortex flows and significant amplification of the magnetic field in the white-light flare kernel region. We infer that this impact is driven by the propagation of flare-generated Alfvén wave pulses, which deposited energy into the photosphere. These observations support the potential role of the Alfvén wave mechanism in driving energy transport and heating during white-light flares.