Guiping Zhou

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.

Context. The observational distinction between eruptive and confined solar flares is often based on whether a coronal mass ejection (CME) can be identified in coronagraph data. However, CME detectability depends strongly on viewing geometry, coronal background, and instrumental sensitivity, so some physically eruptive events may appear confined during routine inspection. Aims. We investigate the physical nature of a confined-like X6.4 flare that occurred on 2024 February 22 in NOAA Active Region 13590 and examine why its associated CME is exceptionally faint in coronagraph observations. Methods. We analyzed the event using multiwavelength observations from SDO/AIA, SDO/HMI, CHASE H α , GOES-16, SOHO/LASCO, and complementary STEREO-A data. We furthermore employed differential emission measure (DEM) diagnostics and nonlinear force-free field (NLFFF) extrapolations to characterize the thermal and magnetic structure of the eruption. Results. The event occurred in a magnetically complex active region containing at least three eruptive branches rooted along distinct polarity inversion lines (PILs1–3). Flare brightening first appeared near PIL2, indicating that this branch was involved in triggering the X6.4 flare. However, only the branch associated with PIL1 escaped successfully and produced an extremely faint CME, whereas the branches associated with PIL2 and PIL3 failed and fell back to the solar surface. The successful eruption propagated eastward and was accompanied by pronounced coronal dimming and a fast extreme ultraviolet front, while the failed branches remained confined beneath stronger overlying closed fields. Filamentary structures, hot-channel signatures, DEM results, and NLFFF extrapolations together suggest the presence of three flux-rope-like configurations along the three PILs. The faint CME first becomes identifiable in LASCO only after careful discrimination from repeated backside eruptions using complementary viewpoints. Conclusions. This event shows that a major flare may appear nominally confined during routine coronagraph inspection while still being physically associated with a CME. In this case, the flare-triggering branch and the CME-producing branch are not the same eruptive component, but belong to a sympathetic sequence in a complex active region. Our results highlight how a flare–CME association can be obscured by both magnetic complexity and CME detectability, and they emphasize the need for multiwavelength and multi-viewpoint diagnostics when assessing whether apparently confined major flares are truly noneruptive.

Shearing motion and magnetic flux cancellation around the polarity inversion line (PIL) play significant roles in the build-up of free magnetic energy and magnetic flux rope (MFR) in the source region of major solar flares. Here we investigate the magnetic evolution of a highly sheared PIL in active region 13842, hosting the largest X9.0 flare of Solar Cycle 25. Since 2024 September 29, a positive-polarity pore persistently drifted northward along the western side of the AR’s main negative-polarity sunspot. The main sunspot remained stationary until negative-polarity patches successively emerged to its east and approached. Rear-ended by these same-polarity patches, the sunspot then began moving westward toward the opposite-polarity pore around October 1, forming a collisional PIL. Meanwhile, on the PIL’s other side, the pore was also rear-ended by same-polarity patches sequentially emerging behind it, accelerating the shearing motion around the PIL, where frequent flux cancellations were also observed. Synchronous rapid accumulation of free magnetic energy and formation of MFR were then observed in the PIL, where multiple major flares successively occurred within 2 days. Before these large flares, the area and total free energy of the high-free-energy-density PIL region gradually decreased in the photosphere, which could be caused by the initial ascent of MFR before eruption and serve as a precursor of solar eruptions. These results suggest that persistent flux emergence with cross-separation directions facilitates rapid formation of collisional shearing PIL and frequent flux cancellations, leading to repeated MFR formations and multiple large flares in a relatively short time.

Reliable forecasting of geomagnetic storms (GSTs) remains a major challenge in space weather, particularly at lead times beyond several hours. Most existing models rely on near-Earth solar wind data, which inherently limits the prediction lead time. We extend the forecast horizon by exploiting galactic cosmic ray (GCR) modulation as an upstream precursor. A physics-motivated Long Short-Term Memory (LSTM) model is developed that integrates cosmic-ray flux, solar wind, interplanetary magnetic field, and geomagnetic parameters from a 1995–2020 dataset at 1-hour cadence. The model achieves RMSE values of 5.106–14.788 nT for lead times of 2–48 h, with the inclusion of cosmic-ray inputs improving the 48-hour F1 score by 25.84%. For ICME-driven storms, we further introduce an event-based classification sub-model that predicts whether an approaching ICME will trigger a significant storm 24–48 h in advance; the corresponding F1 scores reach 0.55 (24 h) and 0.48 (48 h). Permutation importance analysis shows that cosmic-ray-related variables contribute substantially to predictive skill, while additional statistical analysis reveals that stronger GCR suppression correlates with more negative minimum Dst, supporting the physical interpretability of GCR modulation as a meaningful precursor.

Unraveling the origins of the quiet Sun’s magnetic fields remains a key challenge. Using observations from Heliospheric Magnetic Imager on board Solar Dynamics Observatory, we identified over 86 million quiet region (QR) structures in daily full-disk magnetograms spanning 2010–2025, a period covering nearly the entire Solar Cycle 24 and half of Cycle 25. By analyzing the cyclic variations, our key findings are as follows. (1) The area coverage and total magnetic flux of QRs closely follow sunspot cycles, indicating that Cycle 25 is stronger than Cycle 24. Throughout Cycle 24, the QR flux surpasses active region (AR) flux for almost the entire cycle. In the first 5.5 yr of Cycle 25, the QR flux remains greater than AR flux except for nine isolated months. (2) From the detection limit to the large-scale end, both the flux and number of QR structures exhibit a sequential pattern of anticorrelation and positive correlation with sunspots. Notably, anticorrelated structures constitute more than 95% of QR structures at any given time, and dominate the QR flux during solar minimum. (3) We develop an empirical model for QR magnetic sources based on long-term flux variations, revealing that the flux contribution of ARs remnants to QRs varies in the solar cycle. During solar maximum, most of the QR flux and positive-correlated QR flux originate from ARs, while roughly 30% of the anticorrelated flux is suppressed by ARs. These findings suggest that the solar cycle should be characterized by the magnetic fields in both ARs and their associated QRs.

Coronal mass ejections (CMEs), since their first observation in 1971, have been widely acknowledged as the most significant eruptive phenomena and the primary cause of catastrophic space weather events within our solar system. Whether similar processes involving stellar mass ejections (SMEs) occur on other stars holds immense potential for advancing our understanding of stellar behavior and providing insights into the search for extraterrestrial life. However, detecting SMEs remains challenging, particularly in establishing reliable approaches and diagnostics. Here, we conduct a proof-of-concept Sun-as-a-star experiment using solar CMEs as proxies, analyzing EUV spectral lines from the Extreme Ultraviolet Variability Experiment on board the Solar Dynamics Observatory. By comparing 26 front-side fast CMEs with 14 confined flares of class M 1.0 and above, we find that the Doppler responses in 18.04, 19.51, and 28.42 nm during CME events provide a promising diagnostic to distinguish Sun-as-a-star CME signals from flare features. We further show that CME characteristics can be reasonably reproduced from the Doppler velocities in these three lines, providing a hopeful diagnostic for inferring otherwise unobservable properties of eruptions on remote stars. These findings advance the exploration and understanding of mass ejections in stars. We therefore advocate the resumption of the EUV observations of extrasolar stars.

Geomagnetic storms (GSTs) driven by solar wind-magnetosphere coupling can severely disrupt technological systems, motivating the need for improved prediction accuracy and longer warning times. In this study, we develop a physics-informed Long Short-Term Memory (LSTM) model that incorporates cosmic-ray flux modulation as an additional precursor signal. As coronal mass ejection (CME)-driven disturbances propagate through the heliosphere, enhanced turbulence and magnetic-field compression reduce galactic cosmic-ray (GCR) flux measured by ground-based neutron monitors (Forbush decreases), providing early information that can precede near-Earth solar-wind signatures by 1--3 days. We integrate multi-source space-weather data, spanning 1995-2020, including cosmic-ray observations, solar wind plasma parameters, interplanetary magnetic-field data, and geomagnetic indices. Based on these data, we construct a 19-dimensional feature vector that includes flux background levels, decrease-related indicators, and inter-station correlation measures as model inputs. Employing a 50-unit LSTM architecture, the proposed model achieves root-mean-square errors (RMSE) of 5.106 nT, 8.315 nT, 10.854 nT, 12.883 nT, and 14.788 nT for 2-, 6-, 12-, 24-, and 48-hour predictions, respectively. Incorporating cosmic-ray information further improves 48-hour forecast skill by up to 25.84% (from 0.178 to 0.224). These results demonstrate the value of physics-informed deep learning and cosmic-ray precursors for advancing space-weather forecasting.

Solar prominences (or filaments) are cooler and denser plasma suspended in the much hotter and rarefied solar corona. When viewed on the solar disk, filament barbs or feet protrude laterally from the filament spine. When observed at the limb of the Sun, they reach into the chromosphere or even further down. For a long time, the magnetic field orientation of barbs has remained a mystery due to the paradox that the barbs possess vertical fine structures and flows but are likely to be supported in a horizontal magnetic field. Here we present unambiguous observations of a magnetic dip in a quiescent prominence foot with an upward-curved field. That is indicated by the horizontal bidirectional outflows probably produced by magnetic reconnection between the fields of a tiny erupting filament and those in a prominence foot. The altitude at the bottom of the dip is ∼30 Mm. At the edge of the prominence foot, the angle between the dip field and the local horizontal is ∼4°. Additionally, the curvature radius of the dip bottom is estimated to be around 73 Mm. We also conduct magnetofrictional simulation to self-consistently form a large-scale magnetic flux rope with magnetic dips resembling the spine and feet of the quiescent prominence. The observations shed light on the field structure of prominences, which is crucial for the instability that accounts for the eruption of prominences and coronal mass ejections.

Through three-dimensional MHD simulations, we have uncovered a kind of fast coronal wave originating from both ends of a current sheet (CS) during a solar eruption. These waves are observed to appear near the top and bottom ends of the reconnection-related CS. The simulations demonstrate the presence of termination shock regions above the two ends of the CS. As the reconnection outflows escape from the vertical CS and encounter these termination shocks, they undergo partial reflection, redirecting toward the CS terminal fork walls. The identified waves propagate rapidly at a speed of approximately 1400 km s ⁻¹ with a period of just 2 s. Concurrently, the time evolution of intensity within a small region of the CS terminal fork structures exhibits a similar oscillation period of 2 s. All this evidence supports the notion that these quasiperiodic fast-propagating (QFP) waves were excited by tuning-fork effects within the CS system. Essentially, the rapid reconnection outflows are reflected by the terminal shocks, striking the fork walls at the CS ends. Moreover, parts of the oscillations along the tuning-fork handle are transformed into thermal energy, accumulating in the CS center and elevating the temperature. This is the first time to report such QFP waves resulting from tuning-fork effects within the CS during a solar eruption. These waves are anticipated to manifest closely following the propagation of CMEs and adjacent to the related postflare loops in observations, with partial confirmation in current observations.