NOAA AR 12371 is highly dynamic with a complex type magnetic field configuration. During its transit across the visible disc of the Sun from 16 to 29 June 2015, it produced 5 M-class and 41 C-class flares. The observations were conducted on 25 June 2015 between 17:20 UT and 19:00 UT, spanning 100 min. During this period, AR 12371 was positioned at heliographic coordinates N11W45. A mature LB with multiple branching structures developed in the following sunspot of the AR. Simultaneously, long fan-shaped surges with smooth upper edges occurred intermittently and aperiodically on one of the branches, referred to as LB1. The eastern segment of LB1 is filamentary, stemming from a penumbral filament, while its western segment is granular and connects to another branch, LB2 (see Fig. 1).
Observations were conducted using the 1.6-m Goode Solar Telescope (GST; [40]) at BBSO, which offers a field of view approximately 70 arcsecond × 70 arcsecond. Benefiting from the GST's advanced adaptive optics system, the observations achieved spatial resolution close to the telescope's diffraction limit. To observe the dynamic evolution in the photosphere, the Broadband Filter Imager (BFI) with a TiO filter was employed. This filter captures images at a height of 170 km above the photosphere and is centered at 7057 Å with a bandwidth of 1 nm. The TiO images obtained have a cadence of approximately 30 s and a spatial scale of 0.034 arcsecond per pixel. Meanwhile, the surges in the chromosphere were captured using the Visible Imaging Spectrometer (VIS; [41]). It was tuned to five spectral wavelengths around the H line, achieving a cadence of about 33 s and a spatial scale of 0.029 arcsecond per pixel.
The Stokes polarization parameters I, Q, U and V were measured by the Near InfraRed Imaging Spectro-polarimeter (NIRIS; [42]). The near-infrared Fe I line, formed at a depth of around 30 km below the photosphere, was recorded by the instrument. It consists of 60 spectral points, extending from -3.16 Å in the blue wing to +3.10 Å in the red wing, with a center at 15648.5 Å. The spatial resolution was 0.08 arcsecond per pixel, while the temporal resolution was approximately 65 seconds per scan. An inversion from the Stokes profiles to magnetic field vectors is performed using a non-linear least-squares method [43]. This approach minimizes the difference between the observed and theoretical profiles of the Stokes parameters, which are derived from the radiative transfer equation for polarized radiation. To address the 180 • azimuthal ambiguity in the transverse field, an improved version of the Minimum Energy (ME) algorithm [44] was utilized. The ME algorithm globally minimizes a function that includes approximations of the electric current density and the field divergence.
Additionally, the full solar disc continuum images and line of sight magnetic field, as well as the AR vector magnetic field in photosphere, with a spatial resolution of 1 arcsecond, were provided by the Helioseismic and Magnetic Imager (HMI; [45]) on board the Solar Dynamics Observatory (SDO; [46]). We performed a nonlinear force-free field extrapolation (NLFFF, [47]) by using the preprocessed vector magnetogram as boundary input. As a result, a comprehensive and detailed three-dimensional magnetic field structure of AR 12371 and its LB is constructed. In this study, we ensured the co-alignment of all images to the reference continuum image, which was observed by HMI at 18:15:00 UT. To obtain the dynamical properties of moving grains (MGs) within LB from the co-aligned data, we used two complementary methods: visual identification and tracking of MGs frame by frame, and an automatic machine identification technique, following the method by [48]. Research Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x 3 Results
As depicted in Fig. 1, fan-shaped surges occurred along LB1, with their footpoints distributed along LB1's northern flank, featuring a relatively smooth and almost straight upper edge. These significant surges are more prominently visible in H line centre and red wing images compared to blue wing images (see panels c and d). A total of seven significant surges occurred between 17:20:00 UT and 19:00:00 UT (see Table 1) on 25 June 2015. In the movie, the surges on LB1 seem to be an extension of those surges on the penumbral filament which connects the east of LB1. Nonetheless, the former is noted for a larger apparent height and a smoother upper edge than the latter. Each such surge initially appears at the east of LB1's filamentary segment, as seen in the left panels of Fig. 2. As the surge grows in apparent height in the plane of the sky, it also extends westward, halting at the boundary between LB1 and LB2, as illustrated in Fig. 1d. After an indeterminate time interval, another such surge appears with a similar developing process. Panels (c) and (d) show the fifth surge, the most intense among the seven significant surges. Its footpoints spanned nearly the full length of LB1.
The main structure of each surge exhibits a dark characteristic, consisting of a series of closely aligned straight jet-like structures (see Fig. 1d, (e.g., [49,50]). These adjacent jet-like structures share similar apparent heights and brightnesses, forming a fan-shaped surge with a long and relatively smooth upper edge.
Figure 3k shows a space-time diagram at the H + 0.6 Å wavelength, with the slice position in Fig. 1d labelled as 'AB' . The diagram captures the dynamic evolution of chromospheric counterparts along the slit 'AB' over time. We identified seven significant surges, marked by their substantial amplitude and darkening, as indicated by red arrows. Table 1 provides a detailed summary of each surge, including start time, duration, apparent height, mean apparent upward velocity and the time interval between consecutive surges. The durations of such surges range from 235 s to 708 s, averaging 468 s. Surge intervals show significant variability, ranging from 33 s to approximately 19 min, indicating an irregular occurrence without any discernible periodicity. Apparent heights of the surges vary between 1791 km and 3878 km, with an average height of 2460 km. The projected upward velocities in the sky's plane range from 6.9 km s -1 to 33.2 km s -1 , with a mean velocity of 14.4 km s -1 . Since the calculations are based only on images captured in the plane of the sky, the inclination of the surges cannot be measured accurately, which may introduce errors in the height and velocity estimations. Nevertheless, the estimated heights and velocities are generally consistent with those of the light wall phenomena reported by [22], which were approximately 3600 km in height and 15.4 km s -1 in velocity. When compared to the outflows driven by magnetic reconnections in [51,52], the velocities of these surges are similar to chromospheric outflow but are significantly lower than those of the coronal jet.
In addition to the seven significant surges mentioned above, we identified several minor surges, indicated by purple arrows. These minor surges are characterized by their shorter heights (not exceeding 1100 km) and subtler darkening features. They were exclusively detected during the three intervals devoid of significant surges: from 17:29 UT to 17:37 UT, 17:56 UT to 18:14 UT, and 18:42 UT to 18:44 UT, as marked by the red horizontal dashed lines. Notably, within the longest interval of approximately 19 min, five minor surges were observed, exhibiting similar amplitudes and periodicity. Research Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x 3.1.1 Fine photospheric structure in the LB
On 25 June 2015, within the center of the following sunspot of AR 12371, LB1 was identified as a branch of a strong multibranch LB, as shown in Fig. 1a and b. LB1 consists of two segments: the eastern filamentary segment, which links to a penumbral filament, and the western granular segment, which connects to another branch, LB2. The boundary between LB1 and LB2 is marked by purple lines in Fig. 1b-d. LB1 measures approximately 5660 km in length and 870 km in width, with a central dark lane (CDL) running along its axial direction. The width of the CDL is about 60 km, which is narrower than the observation by [13], but similar to the case studied by [16]. At 18:40:14 UT, the width of the northern flank of LB1 was approximately 290 km, and the southern flank was about 520 km at its narrowest cross-section.
In the granular segment of LB1, many granular cells align along both sides of the CDL. Due to the high activity of these grains, the CDL may sometimes deviate from its straight shape and adopt a complex, curved form. Additionally, in some regions with wide cross-sections, granular cells can also create a straight and thick dark inter-granular lane. In such cases, it becomes challenging to distinguish between the CDL and the inter-granular dark lane, particularly in the absence of high-resolution Doppler velocity observations. This study primarily focuses on the morphological evolution of LB convective structures; therefore, we uniformly refer to this relatively thick dark lane as the CDL without further distinction.
In the TiO images, LB1 exhibits highly dynamic behaviour, with numerous moving grains (MGs; here including the bright points from the filamentary segment and granular cells in the granular segment) displaying different proper motions. Manual tracking was performed by visually identifying individual grains in consecutive frames and recording their positions using the Interactive Data Language (IDL) tracking tool. Automated detecting and tracking employed a 3D region growing method [48], which utilizes intensity thresholds and segments 3D evolutional structures in a space time cube to detect and trace grain trajectories across the LB1 region. By combining manual and automated tracking methods, we effectively identify and monitor the movement of MGs on LB1. We observed three types of proper motions, which are consistent with previous studies [12,13,53]. MGs belonging to the first type (Type I) exhibit unidirectional westwards motion along LB1, while those of the second type (Type II) basically remain stationary near a fixed location. MGs of the third type (Type III) show motion perpendicular to the LB1 axis. We identified a total of 334 MGs, with 97 of them belonging to Type I, 187 to Type II and 50 to Type III. Type I MGs' average displacement and velocity are approximately 561 km and 1.1 km s -1 , respectively. In contrast, type III MGs have an average displacement and velocity of approximately 378 km and 0.6 km s -1 , respectively. The maximum displacement and velocity reach up to 2500 km and 5.0 km s -1 for type I MGs, and 1800 km and 3.4 km s -1 for type III MGs. Type I and type II MGs can originate from any place on LB1, while type III MGs only appear in the granular segment. None of the MGs have been observed to cross the boundary between LB1 and LB2.
The Stokes parameter observations were analyzed using the Milne-Eddington approximation, implemented via the mfit package in SolarSoftWare (SSW) within the IDL environment. The inversion process began with initial estimates for six key atmospheric parameters: magnetic field strength, inclination, azimuth, Doppler velocity, line strength, and damping coefficient. These parameters were iteratively refined through nonlinear least-squares minimization, reducing the residuals between the observed Stokes (I, Q, U, V) profiles (see Fig. 4) and synthetic spectra generated via radiative transfer calculations. Following BBSO's standard pipeline [54], scattered light corrections were omitted. While this method efficiently retrieves photospheric magnetic field properties, systematic errors may still arise in regions with strong heightdependent gradients or unresolved multi-component magnetic structures.
Figure 5 shows the spatial distribution of photospheric Doppler velocity observed at the near-infrared Fe I line with a central wavelength of 15648.5 Å, captured by NIRIS at 18:13:30 UT. The maximum Doppler redshift velocity of LB1 was approximately 1.3 km s -1 , which aligns with the observed proper motion (1.1 km s -1 ) of Type I MGs. The eastern penumbra of the sunspot exhibits a distinct Doppler blueshift, a behaviour also observed in the penumbral filament connected to LB1. Over time, the blueshift 'migrated' into the sunspot umbra from east to west along the bridge, gradually replacing the redshift on LB1.
The absence of a distinct convective Doppler pattern (central upflows flanked by downflows) in our light bridge granules may stem from two observational constraints. First, the active region's position at 45 • W heliographic longitude introduces solar rotation-induced redshifts that potentially mask granular blueshifts, unlike the disk-center locations ( ≤12 • W ) analyzed in previous studies where rotational effects are minimal [6,55]. Second, the 0.16 arcseconds spatial resolution of BBSO/GST approaches the characteristic scales of dark central lanes (approximately 0.2 arcseconds) and intergranular structures, limiting our ability to resolve fine velocity gradients.
In addition to the continuous east-to-west grain motions in the granular segment of LB1 mentioned above, we observed a novel convection pattern. It occurs intermittently and aperiodically in the granular segment of LB1, involving a group of closely packed grains, which we refer to as GG (a group of grains). GG appears intermittently and aperiodically, similar Research Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x
to surge, and moves from east to west along LB1, much like Type I grains. The formation and evolution of these GGs can significantly impact the detailed structure of LB1.
The identification of GGs relies on a combination of fixed brightness thresholds (TiO intensity ≥80% of background) and morphological criteria (e.g., reduced intergranular lane contrast). While some subjectivity exists in defining GG boundaries and lifetimes, we mitigated arbitrariness by cross-validating manual and automated tracking methods. Automated detection utilized machine learning-based clustering algorithms [48] to group grains with correlated trajectories, while manual verification ensured physical coherence.
Figure 6 illustrates the temporal evolution of 10 distinct GGs from 17:20 UT to 19:00 UT. Despite the high spatial resolution of TiO images, clearly determining the boundaries of each grain during its evolution remains challenging, especially in the initial phase when the brightness difference among grains is minimal. The boundary of a GG was primarily determined using the contour of a fixed brightness threshold in TiO images. In the initial phase, several distinct but closed contours may surround one or more grains. Although all these grains are part of the same GG, we selected the grains within the largest contour area to represent the initial state of the GG for simplification of the tracking process.
Throughout their lifecycles, GGs progress through six distinct evolutionary stages, visually delineated by sequentially colored arrows in green, red, yellow, blue, purple, and black. These stages include: (1) Inflation, i.e., the initial expansion of some grains at the onset of a GG; (2) merging, i.e., integration with adjacent grains; (3) brightening, i.e., enhanced brightness of inter-granular dark lanes and some grains within the GG; (4) emergence, i.e., continuous sprouting of new grains within the GG; (5) shrinkage, i.e., contraction of the GG's size; (6) dispersal, i.e., separation or fragmentation of the grains within the GG. The first four stages align with an intensification of convection, leading to lateral expansion of the GG and even occuping the full cross-section of LB1, thus interrupting the CDL/inter-granular lane. As the GG advances westward, its leading edge assumes a distinctive pointed arch configuration. This detailed evolution highlights the dynamic nature of GGs and their significant impact on surrounding structures.
Table 2 summarizes the dynamical properties of the 10 GGs. Their durations range from 180 to 720 s, with an average duration of 439 seconds. The maximum area of each GG varies between 0.17 arcsecond 2 and 0.39 arcsecond 2 , averaging 0.34 arcsecond 2 , which is significantly larger than the size of a single grain of LB. The GG area growth rate during the first four stages ranges from 2.1×10 -4 to 7.7×10 -4 arcsecond 2 s -1 , with an average rate of 4.3×10 -4 arcsecond 2 s -1 . The maximum mean brightness increases from 1.15 times to 1.26 times, with an average increase of 1.20 times throughout Vol.:(0123456789) Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x Research
the GG's evolution. Each GG comprises six to 14 single grains, with an average of 10 grains. Therefore, the GG is smaller in scale than the entire LB but larger than a single grain. Thus, GGs are identified as unique meso-scale convective patterns, bridging the gap between the small scale of an individual grain and the larger scale of the LB as a whole.
Up to now, the highly dynamic grains, GGs, and CDL of varying scales form the fundamental convective structure in the granular segment of LB1. Typically, a distinct and straight CDL can extend throughout the entire length of LB. However, when a GG begins to occupy the entire cross-section of LB1, the CDL is severely disrupted and fragmented (as shown in Figs. 5c, 6, and 9a2). As the GG moves westward, the leading part of the broken CDL becomes severely folded, resulting in a noticeable zigzag shape (see Fig. 7). For instance, at 18:21:33 UT, a CDL segment indicated by a red dashed line had a length of about 750 km. As the GG moved westward (indicated by a blue solid line), the CDL shortened into a zigzag shape resembling the letter 'M' . This situation improves as the GG enters its descending phase. Ultimately, the CDL reconnects, but often with a deviation from its original path (see the blue dashed lines in Fig. 6).
The observations indicate that surges in the chromosphere and the appearance of GGs in the photosphere are both intermittent and non-periodic. The temporal extents of the ten GGs are depicted through distinctively colored horizontal lines in Fig. 3k, with detailed durations outlined in Table 2. Among them, GG1 exists between 17:21:05 UT and 17:30:05 UT (540 s); From 17:34:50 UT to 17:50:51 UT (961 s), GG2, GG3 and GG4 appear in sequence, with a time interval of less than 100 s between the two; Similarly, from 18:03:03 UT to 18:37:14 UT (2051 s), GG5, GG6, GG7, and GG8 appear in sequence with less than 30 s between them; The last two GGs, GG9 and GG10, appear from 18:43:15 UT to 18:57:15 UT, with a time interval of 30 s between them. In short, the 10 GGs appear in four groups at random. Similar to GG, the surge activity also appears in four time intervals, also shown in Fig. 3k and Table 1. Among them, the first surge formed between 17:23:46 UT to 17:28:47 UT (301 s); From 17:37:09 UT to 17:56:07 UT (1138 s), the second and the third surges appear in sequence, with a time interval of 35 s between them; The fourth and the fifth surges occurred between 18:14:55 UT and 18:41:48 UT (1613 s); The last two surges of the seven surges spanned from 18:44:35 UT to 18:59:42 UT (907 s). Impressively, the longer GGs appeared, the longer the surge lasted.
Analogously, chromospheric surge activity was also segmented into four distinct periods, as illustrated in Fig. 3k and documented in Table 1: The initial surge developed between 17:23:46 UT and 17:28:47 UT, enduring for 301 s; The second and third surges occurred successively from 17:37:09 UT to 17:56:07 UT, totaling 1138 s, with a mere 35-s interval separating them; The fourth and fifth surges were active from 18:14:55 UT to 18:41:48 UT, spanning 1613 s; Lastly, the final
Table 2 Dynamical properties of GGs: the second to sixth columns present the dynamic information of the ten photospheric GGs
The "Max Mean Brightness" of each GG is normalized by dividing the average background brightness within the field of view indicated by the purple box in Fig. 1b * Normalized by dividing the average background brightness within the field of view indicated by the purple box in Fig. 1b No. Start End Lifetime Max area Area change rate Max mean brightness * Number of grains (UT) (UT) (s) (arcsec 2 ) (10 -4 arcsec 2 s -1 ) 1 17:21:05 17:30:05 540 0.17 3.0 1.15 12 2 17:34:50 17:40:21 331 0.34 6.8 1.17 14 3 17:40:51 17:43:51 180 0.39 5.8 1.19 10 4 17:45:21 17:50:51 330 0.35 4.3 1.17 6 5 18:03:03 18:10:03 420 0.34 4.3 1.21 10 6 18:07:33 18:18:33 660 0.34 3.2 1.18 8 7 18:18:03 18:30:03 720 0.35 2.8 1.20 8 8 18:30:33 18:37:14 401 0.32 3.0 1.26 13 9 18:43:15 18:52:15 540 0.39 2.1 1.17 10 10 18:52:45 18:57:15 270 0.38 7.7 1.24 8 mean --439 0.34 4.3 1.20 10 Vol:.(1234567890) Similarly, the surge activity exhibited three significant gaps during this 100-min observation period: from 17:28:47 UT to 17:37:09 UT (502 s), from 17:56:07 UT to 18:14:55 UT (1128 s), and from 18:41:48 UT to 18:44:35 UT (167 s). Notably, after each gap, there was a significant delay in the onset of the surge relative to the appearance of the GG. Specifically: The second surge appeared 139 s after the second GG; The fourth surge occurred 712 s after the fifth GG; The sixth surge was observed 80 s after the ninth GG. These time relationships suggest that the longer the gap without surges or GGs, the more noticeable the delay became. A recent study by [56] also found a delay of approximately 10 min between the appearance of an abnormal photospheric granule and the onset of a chromospheric surge.
LB exhibits a unique dynamic structure in terms of intensity observation and possesses a distinctive magnetic configuration, setting it apart from surrounding sunspot umbra structures. Figure 8 illustrates the magnetic field features of the entire LB1 captured by BBSO/NIRIS, including the adjoining penumbra and umbra regions. Figure 8a presents the changes in total magnetic field strength (B), vertical magnetic field (Bz), and magnetic inclination along the orange curve from east to west in Fig. 8c. The total magnetic field strength generally increases as it sequentially passes through the penumbral filament, filamentary, and granular LB1. The Bz component varies from near zero to its maximum value, consistent with the magnetic inclination changing from approximately 80 • in the penumbral filament, to about 50 • in the filamentary LB1, and to nearly 0 • near the boundary between LB1 and LB2. Figure 8b shows the global coronal magnetic field lines of LB1, extrapolated with the NLFFF model from the photospheric vector magnetogram observed at 17:24 UT by SDO/ Vol.:(0123456789) Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x Research
HMI. These field lines exhibit a more horizontal orientation, almost parallel to the axis in the filamentary segment [32].
The observations and extrapolation results correspond with the established knowledge of the light bridge magnetic field. Figure 8c displays the horizontal magnetic field distribution indicated by arrows, superimposed on a background map of Bz. The dominant positive vertical magnetic field Bz, represented by red and yellow arrows, covers most of the field of view. The direction and length of these arrows illustrate the orientation and strength of the horizontal magnetic field component. However, a small region marked by blue arrows is located near the northern part of the penumbral filament, which connects to the filamentary segment of LB1.
Figure 9 highlights the magnetic field distributions in the granular segment of LB1 at two distinct times: one without GG and the other with the presence of GG7. When GG7 appears, the CDL/thick inter-granular lane of LB1, indicated by the red dotted curve in panels (a1) and (a2), becomes discontinuous. The solid orange curve in the corresponding right panels represents GG7. Notably, the local total magnetic field strength of GG7 increases from approximately 1300 G to about 1450 G, as shown in panels (b1) and (b2). Panels (d1) and (d2) illustrate the distributions and variations of the horizontal field, revealing a significant deflection in LB1. Before and after the surge event, within the region where GG7 is located, the horizontal magnetic field deflects in the range of -24 • to 40 • , while in the part of LB1 shown in panel (d2), the deflection range is -29 • to 47 • . By the equation 0 j = ∇ × B , we reveal that the vertical component of the current density j can be derived from the observed horizontal magnetic field compo- nents B x and B y . Specifically,
y , where 0 represents the magnetic permeability. The vertical current density is mainly distributed on the north side of LB1, as indicated by the thick blue lines with 0.45, 0.30, and Research Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x 0.15 A m -2
, and cyan lines with -0.45, -0.30, and -0.15 A ⋅ m -2 contours in panels 9c1 and c2. The Wilson depression effect induces the emergence of spurious currents intermingled within the calculated current density, leading to errors in the results. However, precisely quantifying these errors remains a challenge due to the complex relationship between geometric height variations and magnetic field measurements.
In this study, we identified seven significant chromospheric fan-shaped surges in LB1 of AR 12371 between 17:20 UT and 19:00 UT on June 25, 2015. These surges exhibited the following characteristics: (1) they recurred intermittently and aperiodically, with an average apparent height of 2460 km and a velocity of 14.4 km/s, accompanied by notable Fig. 9 The first three rows of panels display the photospheric brightness structure of LB1 in TiO, the total magnetic field, and the vertical magnetic field, respectively.
Black dashed curves outline the boundary of LB1, while red and yellow solid lines represent contours of the total magnetic field at 1500 G and 1700 G, respectively. Red dashed lines indicate the chromospheric dynamic lane (CDL) at 17:59:22 UT and 18:25:33 UT on 25 June 2015. The closed orange curve outlines the profile of granulation gradient (GG) 7. In c1 and c2, solid, dotted, and dashed blue and cyan contour lines correspond to current densities of 0.45/-0.45, 0.3/-0.3, 0.15/-0.15 A m -2 , respectively. In the bottom panels, blue and red arrows indicate the strength and direction of the horizontal magnetic field at 17:59:22 UT and 18:25:33 UT on 25 June 2015, respectively Vol.:(0123456789) Discover Applied Sciences (2025) 7:385 | https://doi.org/10.1007/s42452-025-06873-x Research brightening at their footpoints;
(2) they extended over a long distance along LB1, reaching the boundary between LB1 and LB2; (3) they were primarily located along one flank of LB1; and (4) they had relatively smooth upper edges.
In the TiO observations, LB1 demonstrated diverse dynamics and evolution in the photosphere, characterized by: (1) typical proper motions of single grain; (2) a filamentary segment persistently advancing westward during surge events;
(3) multiple grains coalescing into a GG and collectively migrating westward along the LB, which is first observed in LB; and (4) the CDL/thick inter-granular lane of LB1 becdsoming disrupted and compressed as GGs expanded laterally, eventually spanning the full width of LB1.
Similar to the significant surges, the GGs also appear intermittently and aperiodically, moving along LB1 with archshaped fronts. The occurrence of the ten GGs and seven surges can be divided into four distinct clusters, respectively. It was observed that the longer each cluster of GGs persisted, the longer the corresponding cluster of surges lasted. Additionally, both phenomena exhibited three time gaps, and it was noted that the longer the time gap without GGs, the longer the corresponding time gap without surges. Additionally, there is a delay ranging from 80 to 712 s between the emergence of a cluster of GGs and the subsequent cluster of surges eruption. This observation is consistent with previous findings by [34,56], which reported a delay of around 10 min between the appearance of granular-size flux and the surge. These findings suggest a close relationship between the appearance of photospheric GGs and the eruption of surges on LB. Previous studies have proposed that surges can be driven by either slow-mode waves or magnetic reconnection. In our case, the intermittent and aperiodic recurrence of the surges rules out the possibility of a slow-mode wave drive. Therefore, magnetic reconnection is potentially the primary driving force that triggers the surge.
The magnetic field of LB1 exhibits the following three distinct characteristics: Firstly, similar to a magnetic sandwich, the magnetic field of LB1 is weaker and more horizontal than that of the surrounding sunspot umbrae. Secondly, the Bz component of LB1 and the surrounding umbral umbrae had the same (positive) polarity, with a small patch of negative polarity in the eastern penumbra connected to LB1. Thirdly, the stronger vertical current sheets were mainly distributed along the northern flank of LB1, indicating that the north flank had more stored free magnetic energy. Additionally, substantial changes in the local magnetic field were observed during the appearances of GG, indicating interactions between the newly emerging plasma of GG and the pre-existing magnetic field.
Futhermore, to investigate the coronal magnetic structure over LB1 in our study, we performed NLFFF extrapolation by using photospheric vector magnetic field data captured by SDO/HMI. Figure 10 illustrates the presence of a twisted magnetic field component (identified as 'F1') encircling a large-scale horizontal magnetic field (marked as 'F2') emanating from LB1, which is similar to the study by [57] in the same active region AR 12371, but on 22 June 2015, 3 days before our observations.
As per [58], the plasma parameter represents the ratio of plasma pressure to magnetic pressure, calculated as = 16 nkT/B 2 , where = 1/2, n = 1.2×10 17 cm -3 , k = 10 -15.86 erg deg -1 , and T = 6 ×10 3 K in the photospheric active region. With B varying from approximately 900 G to 1700 G (as indicated by Fig. 9c1 and c2), we find values in the range of 3.1 to 0.9. This suggests that plasma dynamics significantly influence magnetic field behaviour (e.g., [32]). Recent observational studies by [59] establish a critical link between photospheric vortices and surge initiation, demonstrating that successive vortices with transverse shear flows at light bridge edges directly drive chromospheric ejections-a process akin to our GG-associated surge events. A recent MHD simulation by [60] further demonstrated that convective motions in light bridges can drive large-scale reconnection through flux tube interactions. Specifically, their model shows that collisions between oppositely twisted flux tubes generate localized vorticity and enhance magnetic shear-a process analogous to our observations where GG emergence perturbs the equilibrium of the verse flux systems. This synergy between photospheric turbulence and magnetic restructuring is reinforced by [61], who showed that vortex-driven magnetic brightening and merger processes create the necessary conditions for energy transfer across atmospheric layers. The arched fronts of moving GGs (Sect. 3.1.4) likely induce vortical flows that amplify horizontal shear, mirroring the simulated vortex-driven dynamics. Crucially, the spatial clustering of GGs suggests an inverse turbulence cascade mechanism, where small-scale convective motions coalesce into coherent structures capable of system-scale magnetic perturbation.
Additionally, the three-dimensional magnetic structure revealed by NLFFF extrapolation (Fig. 10) suggests the existence of twisted systems along LB1. This configuration aligns with recent magnetohydrodynamic (MHD) simulations by [60], where light bridges were modeled as interacting magnetic flux tubes. Their simulations demonstrate that the collision of two flux tubes with opposite twists generates localized vorticity and enhances magnetic shear, driving reconnection events. In our observations, the twisted component and the cusp structure in Fig. 10 may represent analogous flux systems. The intermittent emergence of GGs could perturb the equilibrium between these flux tubes, similar to the simulated vorticity-driven dynamics. This perturbation cascade operates bidirectionallywhile GG motions inject turbulence from below, the resulting magnetic restructuring (via reconnection) conversely enhances photospheric shear flows, creating a positive feedback loop for surge energization. Consequently, accumulated magnetic stress triggers sequential reconnection along LB1's axis, consistent with the fan-shaped surge morphology. This chain of evidence -from photospheric turbulence (GGs) to magnetic topology modulation and finally to reconnection-driven surges-establishes a unified framework bridging multi-scale processes. The delay between GG clusters and surges (Sect. 3.2) reflects both the timescale for energy accumulation via flux tube interactions and the hierarchical merging timescale of turbulent vortices, as predicted by inverse cascade models.
Figure 11 illustrates the formation of a fan-shaped surge characterized by its extensive spread along the axis of LB1, comprising slender, individual jets by a cartoon. The unidirectional horizontal flow transports extra magnetic flux from the penumbra to LB1, augmenting the photospheric magnetic pressure there. This rise in magnetic pressure lifts the twisted magnetic flux in the upper atmosphere, thereby increasing the stored magnetic free energy within LB1's atmosphere. Furthermore, the occurrence of photospheric GGs could deform the magnetic field in LB1's photosphere, elevating the twisted magnetic tube and pushing it towards the canopy field, triggering magnetic reconnection. This scenario aligns with the model proposed by [33], where the twisted magnetic flux tube serves as a conduit for the rapid propagation of GG effects across the entire LB1 region. Fig. 11 A schematic diagram depicts the formation of a fan-shaped surge with a smooth upper edge. The three panels illustrate the flux tube over LB1 and the surrounding umbral magnetic field lines, presented in 3D, YZ plane, and XY plane, respectively. The location of magnetic reconnection between the flux tube and the surrounding umbral field is labeled as 'MR' . The abbreviations 'E' , 'W' , 'N' , and 'S' denote east, west, north, and south, respectively
Vol:.(1234567890)