arXiv subjects

Hechao Chen

Publications and source records attributed to Hechao Chen.

At least 19 recordsLinked to original sources

arXiv2026

Solar Soft X-ray Coronal Dimming in a Failed Eruption Associated with Plasma Cooling

Coronal dimmings are observed as sudden and localized reductions in the extreme-ultraviolet and X-ray emission of the solar corona. Traditionally, significant dimmings of spectral lines formed at temperatures of 1-2 MK are regarded as indicators of coronal mass ejections (CMEs), reflecting the density depletion caused by plasma escaping into interplanetary space. In this Letter, we report a peculiar deep coronal dimming event predominantly observed in high-temperature spectral lines following an M8.8-class confined solar flare associated with a failed filament eruption. Sun-as-a-star measurements from the Geostationary Operational Environmental Satellite and the Extreme Ultraviolet Variability Experiment reveal intensity reductions exceeding 30% in soft X-ray (SXR) and measurable decreases in Fe XVIII (6.5 MK) and Fe XX (9.3 MK). Spatially resolved observations from the Atmospheric Imaging Assembly demonstrate that the dimming originates from the active region core, while the Solar Terrestrial Relations Observatory-A shows no evidence for CME-driven mass loss. Differential emission measure analysis reveals plasma at temperatures >5 MK cooling into lower temperatures, supporting plasma cooling as the dominant contributor to the hot-band dimming rather than CME-associated plasma escape. This event demonstrates that deep hot SXR dimmings can occur without substantial CME-driven mass loss, suggesting that alternative physical mechanisms may also account for unresolved stellar dimmings in addition to the commonly inferred CME signatures.

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arXiv2026

Reconnection Jets Detected during the Footpoint Drift of a CME Flux Rope: New Signatures of Ongoing 3D Arcade-Flux Rope Reconnection

Utilizing multi-wavelength data from the Solar Dynamics Observatory, we report the first imaging detection of reconnection jets driven by the footpoint drift of an erupting hot-channel flux rope (HFR) during an X1.6-class solar flare. Our results demonstrate that these jets are inherent byproducts of the HFR eruption, exhibiting strong spatiotemporal coupling with the HFR footpoint drift and the morphological evolution of the associated flare ribbon hooks. Unlike standard coronal jets, these ejections lack an inverted-Y morphology and show no association with underlying photospheric flux cancellation or emergence. They manifest as clusters of small-scale, collimated ejecta (180-200 km s$^{-1}$) featuring co-existing cool and hot emission components visible in both cool (304 and 171 Angstrom) and hot (94 Angstrom) EUV channels. The imaging observations reveal that these jets are triggered by successive 3D arcade-flux rope (ar-rf) reconnection as the expanding HFR interacts with the ambient coronal side arcades. The morphological and dynamic complexity of these observed jets implies that the ongoing ar-rf reconnection in solar eruptions is episodic and bursty in nature. Our estimates show that a decrease of approximately 9 G in localized magnetic field strength is required to power these individual jets. As direct signatures of 3D magnetic reconnection, these observed jets pinpoint the exact site and characterize the nature of the process, providing new observational insight into 3D numerical simulation extensions of the standard flare model. Further observational case studies of these jets are required to fully understand their prevalence across solar eruptions.

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arXiv2026

Successive Coronal Jets as Novel Facilitators for Filament Oscillation and Eruption

Solar filament eruptions are central to coronal mass ejections and space weather, yet their triggering mechanisms remain a fundamental open question. In particular, the early-stage that drives a magnetic flux rope toward instability and its observable signatures are poorly understood. Here, combining multi-instrument observations, we report successive coronal jets impacting a filament, causing its gradual rise and oscillations with growing amplitude and period. When the filament reaches the height where the decay index exceeds the torus instability threshold, the rapid filament eruption commences. This filament eruption is reproduced by magnetohydrodynamic simulations, in which successive thermal jets disturb a stable filament in a magnetic flux rope and excite oscillations together with the eruption of the filament. As the filament rises to erupt, the restoring forces for the oscillation progressively weaken, which naturally leads to an increase of the oscillation amplitude and period. Our results demonstrate the growing oscillations as one of the observable precursors for filament eruptions, enhancing our ability to predict solar eruptions.

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arXiv2025

Resolving Interchange Reconnection Dynamics in a Fan-Spine-like Topology Observed by Solar Orbiter

Interchange reconnection is believed to play a significant role in the production of solar jets and solar wind. However, the dynamics of interchange reconnection in the low corona might be more complex than recognized before in higher temporal and spatial resolutions. Using unprecedentedly high-resolution observations from the Extreme Ultraviolet Imager (EUI) onboard the Solar Orbiter, we analyze the dynamics of interchange reconnection in a small-scale fan-spine-like topology. Interchange reconnection that continuously occurs around the multi-null points of the fan-spine-like system exhibits a quasi-periodicity of ~200 s, nearly covering the entire evolution of this system. Continuous evolution and reversal of multiple current sheets are observed over time near the null point. These results reveal that the dynamics of interchange reconnection are likely modulated by the 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, which is similar to the separatrix curtain reported in the pseudo-streamer structure. This study not only demonstrates the complex and variable reconnection dynamics of interchange reconnection within small-scale fan-spine topology but also provides insights into the self-similarity of magnetic field configurations across multiple temporal and spatial scales.

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arXiv2025

The Spontaneous Genesis of Solar Prominence Structures Driven by Supergranulation in Three-Dimensional Simulations

Solar prominences usually have a horizontally elongated body with many feet extending to the solar surface, resembling a multi-arch bridge with many bridge piers. The basic mechanism by which solar prominences acquire these common structures during their evolution, however, remains an unresolved question. For the first time, our three-dimensional magneto-frictional simulation, driven by supergranular motions, self-consistently replicates the commonly observed multi-arch bridge morphology and its characteristic structures of solar quiescent prominences in a magnetic flux rope. In comparison with traditional views, our simulations demonstrate that the spine, feet, and voids (bubbles) are inherent prominence structures spontaneously forming as the flux rope evolves to a mature state. The voids mainly consist of legs of sheared magnetic loops caused by unbalanced supergranular flows, and prominence feet settle at the bottom of helical field lines piled up from the photosphere to the spine. Similarities between the simulated prominences and observed real prominences by the Chinese H$\alpha$ Solar Explorer, the New Vacuum Solar Telescope, and NASA's Solar Dynamics Observatory suggest the high validity of our model. This work corroborates the pivotal role of photospheric supergranulation as a helicity injection source in the formation and shaping of quiescent prominence structures within the solar atmosphere, thereby paving a new avenue for future investigations into their fine dynamics and stability.

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arXiv2025

Witnessing Magnetic Reconnection in Tangled Superpenumbral Fibrils Around a Sunspot

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 H{\alpha} 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.

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arXiv2025

Observational Analysis of Multi-thermal Counter-streaming Flows in a Forming Filament and Their Relationship with Local Heating at Filament Footpoints

Utilizing high-resolution imaging and spectroscopic observations from the New Vacuum Solar Telescope (NVST), the Interface Region Imaging Spectrograph (IRIS), and the Solar Dynamics Observatory (SDO), we investigated the nature and origin of counter-streaming flows within a forming active region filament. The ever-present counter-streaming flows observed within the filament are identified as interleaved unidirectional mass flows in opposing directions occurring in neighboring threads. Multi-wavelength observations corroborate the multi-thermal nature of these counter-streaming flows: the cool H$\alpha$ component flows at about 10--20 km s$^{-1}$, while the warm ultraviolet and extreme ultraviolet components reach 40--70 km s$^{-1}$. The Si~{\sc iv} 1400~{\AA} line reveals significant micro-turbulence in the filament's counter-streaming flows, with a nonthermal velocity width of 40 km s$^{-1}$. These multi-thermal flows emanate from compact brightenings at the filament's ends, manifesting as small-scale, collimated upflows at their nascent phase. They continuously inject both chromospheric and transition region plasma into the filament channel, thereby feeding the counter-streaming flows. At their base, the Si~{\sc iv} and C~{\sc ii} spectral lines show pronounced line broadening and intensity enhancements, indicating significant localized chromospheric heating. Additionally, numerous small-scale photospheric flux emergence and cancellation events, with magnitude of $10^{17}$~Mx, are detected near their base. We suggest such weak magnetic-field activities, possibly associated with unresolved magnetic reconnection events, drive these persistent upflows and localized footpoint heating. This work elucidates the multi-thermal origin of counter-streaming flows within a forming filament and provides evidence of localized chromospheric heating at the filament footpoints.

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arXiv2025

Reconnection nanojets in an erupting solar filament with unprecedented high speeds

Solar nanojets are small-scale jets generated by component magnetic reconnection, characterized by collimated plasma motion perpendicular to the reconnecting magnetic field lines. As an indicator of nanoflare events, they are believed to play a significant role in coronal heating. Using high-resolution extreme-ultraviolet (EUV) imaging observations from the Extreme Ultraviolet Imager (EUI) onboard the Solar Orbiter mission, we identified 27 nanojets in an erupting filament on September 30, 2024. They are potentially associated with the untwisting of magnetic field lines of the filament. Most nanojets exhibit velocities around 450 km s$^{-1}$, with the fastest reaching approximately 800 km s$^{-1}$, significantly higher than previously reported but comparable to the typical coronal Alfv\'en speed. To our knowledge, these are the highest speeds ever reported for small-scale jets (less than ~1 Mm wide) in the solar atmosphere. Our findings suggest that these nanoflare-type phenomena can be more dynamic than previously recognized and may contribute to the energy release process of solar eruptions and the heating of coronal active regions.

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arXiv2025

Radio dimming associated with filament eruptions in the meter and decimeter wavebands

Filament eruptions are considered to be a common phenomenon on the Sun and other stars, yet they are rarely directly imaged in the meter and decimeter wavebands. Using imaging data from the DAocheng solar Radio Telescope (DART) in the 150-450 MHz frequency range, we present two eruptive filaments that manifest as radio dimmings (i.e., emission depressions). Simultaneously, portions of these eruptive filaments are discernible as dark features in the chromospheric images. The sun-as-a-star flux curves of brightness temperature, derived from the DART images, exhibit obvious radio dimmings. The dimming depths range from 1.5% to 8% of the background level and show a negative correlation with radio frequencies and a positive correlation with filament areas. Our investigation suggests that radio dimming is caused by free-free absorption during filament eruptions obscuring the solar corona. This may provide a new method for detecting stellar filament eruptions.

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arXiv2025

Minifilament Eruptions as the Last Straw to Break the Equilibrium of a Giant Solar Filament

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$\alpha$ 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.

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arXiv2025

Science objectives of the Einstein Probe mission

The Einstein Probe (EP) is an interdisciplinary mission of time-domain and X-ray astronomy. Equipped with a wide-field lobster-eye X-ray focusing imager, EP will discover cosmic X-ray transients and monitor the X-ray variability of known sources in 0.5-4 keV, at a combination of detecting sensitivity and cadence that is not accessible to the previous and current wide-field monitoring missions. EP can perform quick characterisation of transients or outbursts with a Wolter-I X-ray telescope onboard. In this paper, the science objectives of the Einstein Probe mission are presented. EP is expected to enlarge the sample of previously known or predicted but rare types of transients with a wide range of timescales. Among them, fast extragalactic transients will be surveyed systematically in soft X-rays, which include {\gamma}-ray bursts and their variants, supernova shock breakouts, and the predicted X-ray transients associated with binary neutron star mergers. EP will detect X-ray tidal disruption events and outbursts from active galactic nuclei, possibly at an early phase of the flares for some. EP will monitor the variability and outbursts of X-rays from white dwarfs, neutron stars and black holes in our and neighbouring galaxies at flux levels fainter than those detectable by the current instruments, and is expected to discover new objects. A large sample of stellar X-ray flares will also be detected and characterised. In the era of multi-messenger astronomy, EP has the potential of detecting the possible X-ray counterparts of gravitational wave events, neutrino sources, and ultra-high energy {\gamma}-ray and cosmic ray sources. EP is expected to help advance the studies of extreme objects/phenomena and their underlying physical processes revealed in the dynamic X-ray universe, as well as studies in other areas of X-ray astronomy.

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arXiv2024

Ubiquitous Small-scale EUV Upflow-Like Events above Network Regions Observed by the Solar Orbiter/Extreme Ultraviolet Imager

Universal small-scale solar activity in quiet region are suggested to be a potential source of solar wind and the upper solar atmosphere. Here, with the high-resoltion 174 \AA~imaging observations from the Solar Orbiter/Extreme Ultraviolet Imager (EUI), we investigate 59 EUV upflow-like events observed in the quiet Sun. Their average apparent (plane-of-sky) velocity, lifetime, and propagation distance are measured as 62 $\speed$, 68.6 s and 3.94 Mm, respectively. These upflow-like events exhibit dynamic characteristics but lack base brightening, featuring a hot front and subsequent cold plasma ejection. 39\% of the EUV upflow-like events exhibit recurrent characteristics. Unprecedented high-resolution 174 \AA~observations reveal that some EUV upflow-like events exhibit blob-like fine structures and multi-strand evolutionary features, and some upflow-like events can cause localized haze-like plasma heating ahead of their spire region during the ejection process. A subset of the EUV upflow-like events covered by the Solar Dynamics Observatory reveals that they appear at the chromospheric networks. Through emission measure analysis, we found that these upflow-like events eject hot plasma of transient region or coronal temperature (an average of $\sim$10$^{5.5}$K). We suggest that EUV upflow-like events may be EUV counterparts of chromospheric spicules and/or transition region network jets, and play a role in heating localized corona above the network regions.

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arXiv2024

Numerous Bidirectionally Propagating Plasma Blobs near the Reconnection Site of a Solar Eruption

Current sheet is a common structure involved in solar eruptions. However, it is observed in minority of the events and the physical properties of its fine structures during a solar eruption are rarely investigated. Here, we report an on-disk observation that displays 108 compact, circular or elliptic bright structures, presumably plasma blobs, propagating bidirectionally along a flare current sheet during a period of $\sim$24 minutes. From extreme ultraviolet images, we have investigated the temporal variation of the blob number around the flare peak time. The current sheet connects the flare loops and the erupting filament. The width, duration, projected velocity, temperature, and density of these blobs are $\sim$1.7$\pm$0.5\,Mm, $\sim$79$\pm$57\,s, $\sim$191$\pm$81\,\kms, $\sim$10$^{6.4\pm0.1}$ K, and $\sim$10$^{10.1\pm0.3}$ cm$^{-3}$, respectively. The reconnection site rises with a velocity of $\leqslant$69\,\kms. The observational results suggest that plasmoid instability plays an important role in the energy release process of solar eruptions.

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arXiv2024

Formation of Polar Crown Filaments Magnetic Fields by Supergranular Helicity Injection

To understand the magnetic fields of the polar crown filaments (PCFs) at high latitudes near polar regions of the Sun, we perform magnetofrictional numerical simulations on the long-term magnetic evolution of bipolar fields with roughly east-west polarity inversion lines (PILs) in a three-dimensional (3D) spherical wedge domain near polar regions. The Coriolis effect induced vortical motions at the boundaries of several supergranular cells inject magnetic helicity from the photospheric boundary into the solar atmosphere. Supergranular-scale helicity injection, transfer, and condensation produce strongly sheared magnetic fields. Magnetic reconnections at footpoints of the sheared fields produce magnetic flux ropes (MFRs) with helicity signs consistent with the observed Hemispheric Helicity Rule (HHR). The cross-sectional area of MFRs exhibits an uneven distribution, resembling a "foot-node-foot" periodic configuration. Experiments with different tilt directions of PILs indicate that the PCFs preferably form along PILs with the western end close to the polar region. The bending of PILs caused by supergranular flows, forming S-shape (Z-shape) PIL segments, promotes the formation of dextral (sinistral) MFRs. The realistic magnetic models we got can serve as starting points for the study of the plasma formation and eruption of PCFs.

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arXiv2024

Two-sided Loop Solar Jet Driven by the Eruption of a Small Filament in a Big Filament Channel

Similar to the cases of anemone jets, two-sided loop solar jets could also be produced by either flux emergence from the solar interior or small scale filament eruptions. Using the high-quality data from the Solar Dynamic Observatory (SDO), we analyzed a two-sided loop solar jet triggered by the eruption of a small filament in this paper. The jet was occurred in a pre-existing big filament channel. The detailed processes involved in the small filament eruption, the interaction between the erupted filament and the big filament channel, and the launch of the two-sided loop jet are presented. The observations further revealed notable asymmetry between the two branches of the jet spire, with the northeastern branch is narrow and short, while the southern branch is wide and long and accompanied by discernible untwisting motions. We explored the unique appearance of the jet by employing the local potential field extrapolation to calculate the coronal magnetic field configuration around the jet. The photospheric magnetic flux below the small filament underwent cancellation for approximately 7 hours before the filament eruption, and the negative flux near the southern foot-point of the filament decreased by about 56 percent during this interval. Therefore, we proposed that the primary photospheric driver of the filament eruption and the associated two-sided loop jet in this event is flux cancellation rather than flux emergence.

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arXiv2024

Formation of Fan-spine Magnetic Topology through Flux Emergence and Subsequent Jet Production

Fan-spine magnetic structure, as a fundamental three-dimensional topology in magnetic reconnection theory, plays a crucial role in producing solar jets. However, how fan-spine configurations form in the solar atmosphere remains elusive. Using the Chinese H$α$ Solar Explorer (CHASE) and the Solar Dynamics Observatory (SDO), we present a case study on the complete buildup of fan-spine topology driven by flux emergence and the subsequent jet production. Two fan-spine structures and the two associated null points are present. Variations in null-point heights and locations were tracked over time during flux emergence. The north fan-spine structure is found to be created through magnetic reconnection between the newly emerged flux and the background field. Gentle reconnection persistently occurs after formation of the north fan-spine structure, resulting in weak plasma outflows. Subsequently, as flux emergence and magnetic helicity injection continue, the formation and eruption of mini-filaments after reconnection at the quasi-separatrix layer between the two nulls trigger three homologous jets. The CHASE observations reveal that the circular flare ribbon, inner bright patch, and remote brightening all exhibit redshifted signatures during these jet ejections. This work unveils the key role of flux emergence in the formation of fan-spine topology, and highlights the importance of mini-filaments for subsequent jet production.

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arXiv2023

The triggering process of an X-class solar flare on a small quadrupolar active region

The occurrence of X-class solar flares and their potential impact on the space weather often receive great attention than other flares. But predicting when and where an X-class flare will occur is still a challenge. With the multi-wavelength observation from the Solar Dynamics Observatory and FengYun- 3E satellite, we investigate the triggering of a GOES X1.0 flare occurring in the NOAA active region (AR) 12887. Our results show that this unique X-class flare is bred in a relatively small but complex quadrupolar AR. Before the X-class flare, two filaments (F1 and F2) exist below a null-point topology of the quadrupolar AR. Magnetic field extrapolation and observation reveal that F1 and F2 correspond to two magnetic flux ropes with the same chirality and their adjacent feet rooted at nonconjugated opposite polarities, respectively. Interestingly, these two polarities collide rapidly, accompanied by photospheric magnetic flux emergence, cancellation and shear motion in the AR center. Above this site, F1 and F2 subsequently intersect and merge to a longer filament (F3) via a tether-cutting-like reconnection process. As a result, the F3 rises and erupts, involving the large-scale arcades overlying filament and the quadrupolar magnetic field above the AR, and eventually leads to the eruption of the X-class flare with a quasi-X-shaped flare ribbon and a coronal mass ejection. It suggests that the rapid collision of nonconjugated opposite polarities provides a key condition for the triggering of this X-class flare, and also provides a featured case for flare trigger mechanism and space weather forecasting.

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arXiv2023

Observation of two splitting processes in a partial filament eruption on the sun: the role of breakout reconnection

Partial filament eruptions have often been observed, however, the physical mechanisms that lead to filament splitting are not yet fully understood. In this study, we present a unique event of a partial filament eruption that undergoes two distinct splitting processes. The first process involves vertical splitting and is accompanied by brightenings inside the filament, which may result from internal magentic reconnection within the filament. Following the first splitting process, the filament is separated into an upper part and a lower part. Subsequently, the upper part undergoes a second splitting, which is accompanied by a coronal blowout jet. An extrapolation of the coronal magnetic field reveals a hyperbolic flux tube structure above the filament, indicating the occurrence of breakout reconnection that reduces the constraning field above. Consequently, the filament is lifted up, but at a nonuniform speed. The high-speed part reaches the breakout current sheet to generate the blowout jet, while the low-speed part falls back to the solar surface, resulting in the second splitting. In addition, continuous brightenings are observed along the flare ribbons, suggesting the occurrence of slipping reconnection process. This study presents, for the first time, the unambiguous observation of a two-stage filament splitting process, advancing our understanding of the complex dynamics of solar eruptions.

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