Leping Li

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.

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.

Coronal mass ejections (CMEs) are associated with eruptions of magnetic flux ropes (MFRs). Recent studies have proposed a new explanation on the nature of the three-part structure of CMEs, in which the front results from plasma pileup along the overlying coronal loops and should exhibit a smooth arc shape. However, it is not uncommon to observe a bulge at the CME front. The bulge can modify the shock geometry of CMEs, which plays a key role in the electron acceleration. Thus it is important to explore the formation of the bulge. In this paper, we investigate two CMEs observed face-on, both of which have a prominent bulge at the front. The first CME originates from the eruption of a north–south oriented prominence. During the eruption, the northern segment of the prominence rises significantly higher than the southern one due to the writhe of prominence/MFR. This produces a bulge at the northern front of the CME and is classified as an internal factor. The second CME originates from the eruption of a hot channel in a complex βγ -type active region. The channel lacks the obvious writhe during the eruption, while its overlying coronal loops have different projected heights prior to and during the eruption. This leads to a bulge at the front and is classified as an external factor. The formation of the bulge can be well understood with the new explanation of the three-part structure, which provides further support to the explanation.

Bipolar light bridges (BLBs) are bright regions located between sunspot umbrae of opposite magnetic polarity. They are typically characterized by strong magnetic fields and intense flows, which are believed to be closely associated with major solar flares. Despite their importance, their fine structure, formation, and evolution remain poorly understood. In this work, we analyze the observations of a well-defined BLB obtained by the Goode Solar Telescope (GST) at the Big Bear Solar Observatory and the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. The high-resolution GST observations reveal that the BLB is composed of fine, penumbral filament-like structures with widths of approximately 100–150 km. The corresponding Doppler velocity maps present a stable pattern of spatially adjacent red- and blueshifted patches within the BLB throughout the 5.5 hr GST observation. HMI observations show that the BLB arises from the converging and shearing motions of sunspots with opposite polarities. Penumbral regions originating from different polarities gradually evolve and interact, eventually forming the BLB. The observed Doppler velocity pattern, characterized by red- and blueshifted patches, can be interpreted as a projection effect of the Evershed flow within the penumbrae. Therefore, we argue that the BLB is formed through the compression and stretching of penumbral structures from oppositely polarized sunspots.

Interaction between two coronal mass ejections (CMEs) affects their kinematics, morphology, space-weather effects, etc. For the first time, we report the interaction of four homologous CMEs in the field of view of Large Angle and Spectrometric Coronagraph (LASCO) on board Solar and Heliospheric Observatory (SOHO). A new active region (NOAA 11303) started to emerge on 2011 September 23 and developed quickly in the first 2 days. Subsequently, four CMEs (labeled as CME1, CME2, CME3, and CME4 sequentially) from this active region were recorded by LASCO/C2 in 10 hr on September 25. Interestingly, CME2 (677 km s ⁻¹ ) and CME4 (921 km s ⁻¹ ) are faster and catch up with CME1 (502 km s ⁻¹ ) and CME3 (516 km s ⁻¹ ) at ∼4.6 and ∼5.0 R _⊙ , respectively. Both CME1 and CME3 are compressed obviously, while the morphology of CME2 and CME4 does not show substantial changes. The aspect ratio of CME3 increases from ∼1.1 to ∼3.2 in 3 hr, indicating that the magnetic field strength inside CME3 increases by a factor of 3 quickly due to the interaction. Very interestingly, CME3 would normally be unable to catch up with CME2. However, the prior interactions accelerated CME3 and decelerated CME2. As a result, the composite of CME3 and CME4 moves faster (624 km s ⁻¹ ) and catches up with the composite of CME1 and CME2 (492 km s ⁻¹ ) at ∼14.3 R _⊙ . The four separately erupted homologous CMEs eventually combined and formed a large composite near the Sun, which can potentially cause intense geomagnetic storms if it reaches the Earth.

Combining the high-quality observations from the Solar Dynamics Observatory, the Global Oscillation Network Group, and the Chinese H α Solar Explorer, we report a solar coronal jet triggered by the sympathetic eruption of two filaments on 2024 January 11. Initially, the western segment of an active region filament erupted. The erupting plasma propagated eastward, approximately along the filament’s axis. This eruption perturbed the magnetic field of a second filament situated near its eastern footpoint, the second filament then erupted sympathetically about 1 hr later. The eruption of the second filament is a failed one, with the majority of the filament material falling back after the initial lifting. Although no GOES flare accompanied these filament eruptions, distinct brightenings were observed following each eruption. The second eruption produced a large coronal jet, which propagated along a bent trajectory with an apparent deflection angle of approximately 90°. No clear evidence of magnetic reconnection was detected at the deflection site, thus we suspect that the jet may have traveled along an S-shaped transequatorial loop and shown a curved trajectory. This event exhibits multiple phenomena: partial filament eruption, failed filament eruption, sympathetic filament eruption, jet initiation by filament eruption, and apparently deflected jet propagation. Collectively, these observations highlight the complexity and diversity of solar activity.

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.

Combining the high-quality observations from the {\it Solar Dynamics Observatory} (SDO), the Global Oscillation Network Group (GONG), and the Chinese H$α$ Solar Explorer (CHASE), we report a solar coronal jet triggered by the sympathetic eruption of two filaments on 2024 January 11. Initially, the western segment of an active region filament erupted. The erupting plasma propagated eastward, approximately along the filament's axis. This eruption perturbed the magnetic field of a second filament situated near its eastern footpoint, the second filament then erupted sympathetically about one hour later. The eruption of the second filament is a failed one, with the majority of the filament material falling back after the initial lifting. Although no GOES flare accompanied these filament eruptions, distinct brightenings were observed following each eruption. The second eruption produced a large coronal jet, which propagated along a bent trajectory with an apparent deflection angle of approximately 90 degrees. No clear evidence of magnetic reconnection was detected at the deflection site, thus we suspect that the jet may have traveled along an S-shaped trans-equatorial loop and shown a curved trajectory. This event exhibits multiple phenomena: partial filament eruption, failed filament eruption, sympathetic filament eruption, jet initiation by filament eruption, and apparently deflected jet propagation. Collectively, these observations highlight the complexity and diversity of solar activity.

A new explanation has been proposed on the nature of the three-part (bright core, dark cavity, and bright front) structure of coronal mass ejections (CMEs). It suggests that the bright front originates from the plasma pileup along the overlying coronal loops rather than the boundary of the magnetic flux rope at the early eruption stage. This has been confirmed with an active-region event through the direct comparison between the EUV loops and the CME front. However, large-scale EUV loops are hardly observed outside of strong active regions; thus, the front nature remains unclear for CMEs from quiet-Sun regions or weak active regions. In this paper, we address this issue through an intermediate filament eruption from a plage region without sunspots on 2024 March 30. Combining the overlapping observations of the Atmospheric Imaging Assembly and K-Coronagraph, we find that the overlying coronal loops appear gradually in EUV images during the filament eruption. The compressed loops and the plasma pileup along the loops (171 Å) correspond to the CME front in white-light images, which is essentially similar to the situation of events from strong active regions. Therefore, we believe that the explanation on the CME front holds true regardless of whether the EUV loops are visible or not prior to the eruption, and it works for events from different regions, including active regions and quiet-Sun regions. The reasons why the density evolution with height does not support or deny the pileup are discussed.

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.