Zhenyong Hou

In solar flares, flare ribbons map chromospheric footpoints where flare energy deposition occurs. These locations are associated with field-aligned energy transport from the corona that results from energy liberated during magnetic reconnection. Recent chromospheric observations in the Hα and Hβ bands have revealed fine-scale downflow structures above flare ribbons, referred to as riblets. In this study, we identify similar downflow structures in the extreme-ultraviolet (EUV) wavelength using high-resolution observations from Solar Orbiter/EUI. These fine-scale downflows appear as downward-propagating, bright, and thread-like structures. They exhibit typical velocities of sim100 , lifetimes of sim15 s, and lengths of sim1.6 Mm. Based on their morphological and dynamical properties, we interpret these observed downflows as the EUV counterparts of the riblets that have previously been reported from chromospheric observations. This study presents EUV imaging of ∼ 10^6 K downflows above flare ribbons. We interpret these downflows as a result of (1) the energisation and subsequent compression of pre-existing chromospheric fibrils due to particle beams or (2) adiabatic or shock-driven compression induced by the downward-propagating plasma from the corona. These fine-scale EUV riblets provide a new diagnostic tool for probing the dynamics of magnetic reconnection as well as energy transport and deposition during solar flares. km -1

Using multi-wavelength observations from the Solar Dynamics Observatory (SDO), we investigated the six-month decay process of the solar active region NOAA AR 12738 from April to October 2019. We systematically analyzed the region's evolution by examining extreme ultraviolet (EUV) intensity variations, quantifying magnetic flux diffusion, and investigating thermodynamic changes via Differential Emission Measure (DEM) analysis. This study presents the first long-term tracking of a peripheral dimming region (dark moat), revealing its continuous areal decrease over time. DEM results reveals cooling plasma signatures and thermal restructuring, with the dimming region exhibiting a distinct temperature deficit in range 10 ^5.5 -- 10 ^5.9 K. Potential field extrapolation identify two dominant magnetic configurations: low-lying loops with cool plasma (< 10 ^5.5 K), and high-arching structures connecting to the AR core, contributing to localized emission reduction. We found that the dimming is dominated by high-lying loops extending from the AR core, which are heated to temperatures above the main response of the 171 Å passband (> 10 ^5.8 K), consequently lacking plasma at the typical 10 ^5.8 K formation temperature. The thermal deficit, not just the absence of material, is the key driver of the reduced emission. Our results demonstrate that long-duration dimming provides a valuable diagnostic for understanding active region decay, thermal evolution, and coronal magnetic restructuring.

Jets, as an essential manifestation of the release of the free magnetic energy, are ubiquitous in the solar atmosphere. We present a comprehensive analysis of a multithermal blowout jet that occurred in active region AR 13102 on 2022 September 20, using observations from the Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly, SDO/Helioseismic and Magnetic Imager (HMI), Interface Region Imaging Spectrograph, and Solar Upper Transition Region Imager. The jet is initiated by compact brightenings at its footpoints and exhibits a curtain-like spire with apparent rotational and weak lateral whipping motions. Time–distance analysis reveals a projected axial velocity ∼300 km s ⁻¹ and a rotational speed ∼30 km s ⁻¹ . Differential emission measure analysis shows that the jet plasma spans a broad temperature range, with hot (≳10 MK) plasma concentrated near the flare loops and jet base and cooler components extending along the spire. Using the derived kinetic and thermal properties, we conclude that there is an equipartition between the jet’s kinetic energy (5.4 ± 2.4) × 10 ²⁰ J and thermal energy (7.0 ± 3.6) × 10 ²⁰ J. HMI vector magnetograms and nonlinear force-free field extrapolations reveal that the jet originates from a compact mixed-polarity region at the edge of the active region, where flux emergence and cancellation, a low-lying twisted magnetic flux rope with a maximum twist number of ∼2.1, and a surrounding high- Q quasi-separatrix layer are present. The observed decrease in twist after the eruption, together with the jet’s untwisting, indicates that the jet is driven by the eruption and the reconnection of the twisted flux rope, converting magnetic free energy into plasma heating and bulk motion. Our results highlight the importance of small-scale flux rope eruptions in driving blowout jets and releasing twist through magnetic reconnection and untwisting motions.

Interchange reconnection is thought to play a significant role in the production of solar jets and the solar wind. The dynamics of interchange reconnection in the low corona might be more complex than recognized before in higher temporal and spatial resolutions, however. Using unprecedentedly high-resolution observations from the Extreme Ultraviolet Imager (EUI) on board the Solar Orbiter, we analyzed the dynamics of interchange reconnection in a small-scale fan-spine-like topology. The interchange reconnection that continuously occurs around the multinull points of the fan-spine-like system exhibits a quasi-periodicity of ∼200 s that nearly covers the entire evolution of this system. Continuous evolution and a reversal of multiple current sheets are observed over time near the null point. These results reveal that the dynamics of interchange reconnection is likely modulated by emerging magnetic structures, such as mini-filaments and emerging arcades. Moreover, a curtain-like feature with a width of 1.7 Mm is also observed near the interchange reconnection region and persistently generates outflows. This is similar to the separatrix curtain reported in the pseudostreamer structure. This study not only demonstrates the complex and variable reconnection dynamics of interchange reconnection within a small-scale fan-spine topology, but also provides insights into the self-similarity of magnetic field configurations across multiple temporal and spatial scales.

Flare ribbons with parallel and circular morphologies are typically associated with different magnetic reconnection models, and the simultaneous observation of both types in a single event remains rare. Using multiwavelength observations from a tandem of instruments, we present an M8.2-class flare that occurred on 2023 September 20, which produced quasi-parallel and semicircular ribbons. The complex evolution of the flare includes two distinct brightening episodes in the quasi-parallel ribbons, corresponding to the two major peaks in the hard X-ray (HXR) light curve. In contrast, the brightening of semicircular ribbons temporally coincides with the local minimum between the two peaks. Using potential field extrapolation, we reconstruct an incomplete dome-like magnetic structure with a negative polarity embedded within the northwestern part of the semicircular positive polarity. Consequently, the magnetic configuration comprises two sets of field lines with distinct magnetic connectivities. We suggest that the standard flare reconnection accounts for the two-stage brightening of quasi-parallel ribbons associated with the two HXR peaks. Between the two stages, this process is constrained by the interaction of eruptive structures with the dome. The interaction drives the quasi-separatrix layer reconnection, leading to the brightening of semicircular ribbons. It also suppresses the standard flare reconnection, resulting in a delayed second HXR peak.

Three-dimensional magnetic reconnection is a fundamental plasma process crucial for heating the solar corona and generating the solar wind, but resolving and characterizing it on the Sun remains challenging. Using high-quality data from the Chinese New Vacuum Solar Telescope, the Solar Dynamics Observatory, and the Interface Region Imaging Spectrograph, this work presents highly suggestive direct imaging evidence of magnetic reconnection during the untangling of braided magnetic structures above a sunspot. These magnetic structures, visible as bright superpenumbral threads in extreme ultraviolet passbands, initially bridge opposite-polarity magnetic fluxes and then gradually tangle in their middle section. Magnetic extrapolation reveals the fibrils to form a small flux rope that is twisted and braided, possibly created by persistent and complex photospheric motions. During untangling, repetitive reconnection events occur inside the flux rope, accompanied by transient plasma heating, bidirectional outflowing blobs, and signatures of nanojets. Emission analysis reveals that the outflowing blobs are multi-thermal structures with temperatures well below 1 MK, undergoing rapid cooling and leaving emission imprints in Hα images. The measured reconnection angles indicate that 16%-22% of the magnetic field along each thread is anti-parallel, with the remaining field acting as a guide field. The estimated energy released during these reconnection events is comparable to nanoflares, which can be powered by up to 6% of the magnetic energy stored in the anti-parallel field. This work presents a textbook example of magnetic flux rope reconnection in the solar atmosphere, providing new insights into fine-scale energy release processes within sunspot superpenumbral fibrils.

We investigate the complex magnetic reconnection process between the filament and surrounding loops during the failed filament eruption that occurred in NOAA active region 13445 on 2023 September 24, using extreme-ultraviolet observations from the Solar Dynamics Observatory and high-resolution Hα imaging from the New Vacuum Solar Telescope. This failed filament eruption is associated with an M4.4 flare (SOL2023-09-24T03:28). At the early phase of the filament eruption, the filament displays a distinct clockwise rotational motion, suggesting an untwisting motion. During the flare precursor phase, the northwest slipping motion of the brightenings at the filament’s left footpoint, along with the brightening and continuous expansion of the nearby loops at the northwest, suggest the occurrence of a slipping magnetic reconnection between the filament and peripheral loops at the quasi-separatrix layer. After the slipping motion begins, significant brightenings of filament materials and multiple bright structures moving toward the two filament footpoints are observed during the two episodes of magnetic reconnection between the filament and the overlying loops, with the most prominent brightenings and the moving structures occurring during the main phase of the M4.4 flare. The observed untwisting motion of the filament and the twist reduction derived from the nonlinear force-free field extrapolation suggest that the magnetic reconnection between the filament and the peripheral and overlying loops plays a significant role in the failure of the filament eruption by greatly reducing its twist.

We report the possible detection of obscuration dimming, inferred from spectral line asymmetries in Sun-as-a-star extreme ultraviolet spectra. By analyzing asymmetries in the Fe ix 17.1 nm and Fe xii 19.5 nm line profiles obtained from the Solar Dynamics Observatory/Extreme Ultraviolet Variability Experiment during coronal dimming events from 2011 to 2014, we identify two candidate obscuration dimming events associated with M9.3- and X5.4-class flares, respectively. Both events demonstrated prominent irradiance reductions, characterized by greater decreases in blue-wing emission relative to the red wing. These line-profile asymmetries may be attributed to cooler filament material ejected during the flares, moving along the line of sight and partially obscuring hotter coronal emission. The possible obscuration dimming accounted for roughly 18% of the total irradiance loss and persisted for approximately 5–6 hr. Doppler velocity analysis of line profiles enabled us to infer possible motion trends of filament eruptions, which may represent the core regions of coronal mass ejections (CMEs). For event 1, the filament eruption possibly experienced deceleration with partial fallback toward the solar surface or deflection away from the Sun–Earth line, consistent with minimal geomagnetic activity. In contrast, event 2 showed potential acceleration toward Earth, correlating with a significant geomagnetic storm. Our results suggest that the obscuration dimming method presented in this study may serve as a useful diagnostic for constraining CME properties and informing space weather forecasts. In addition, this observational approach may provide a pathway toward identifying stellar CMEs and assessing whether they display the typical three-part structure seen in solar CMEs.

Filament eruptions are magnetically driven violent explosions commonly observed on the Sun and late-type stars, sometimes leading to monster coronal mass ejections that directly affect the nearby planets’ environments. More than a century of research on solar filaments suggests that the slow evolution of photospheric magnetic fields plays a decisive role in initiating filament eruptions, but the underlying mechanism remains unclear. Using high-resolution observations from the Chinese Hα Solar Explorer, the Solar Upper Transition Region Imager, and the Solar Dynamics Observatory, we present direct evidence that a giant solar filament eruption is triggered by a series of minifilament eruptions occurring beneath it. These minifilaments, which are homologous to the giant filament but on a smaller tempo-spatial scale, sequently form and erupt due to extremely weak mutual flux disappearance of opposite-polarity photospheric magnetic fields. Through multifold magnetic interactions, these erupting minifilaments act as the last straw to break the force balance of the overlying giant filament and initiate its ultimate eruption. The results unveil a possible novel pathway for small-scale magnetic activities near the stellar surface to initiate spectacular filament eruptions, and provide new insight into the magnetic coupling of filament eruptions across different tempo-spatial scales.

Filament eruptions are magnetically driven violent explosions commonly observed on the Sun and late-type stars, sometimes leading to monster coronal mass ejections that directly affect the nearby planets' environments. More than a century of research on solar filaments suggests that the slow evolution of photospheric magnetic fields plays a decisive role in initiating filament eruptions, but the underlying mechanism remains unclear. Using high-resolution observations from the \textit{Chinese H$α$ Solar Explorer}, the \textit{Solar Upper Transition Region Imager}, and the \textit{Solar Dynamics Observatory}, we present direct evidence that a giant solar filament eruption is triggered by a series of minifilament eruptions occurring beneath it. These minifilaments, which are homologous to the giant filament but on a smaller tempo-spatial scale, sequently form and erupt due to extremely weak mutual flux disappearance of opposite-polarity photospheric magnetic fields. Through multi-fold magnetic interactions, these erupting minifilaments act as the last straw to break the force balance of the overlying giant filament and initiate its ultimate eruption. The results unveil a possible novel pathway for small-scale magnetic activities near the stellar surface to initiate spectacular filament eruptions, and provide new insight into the magnetic coupling of filament eruptions across different tempo-spatial scales.