Magnetic Relaxation Seen in a Rapidly Evolving Light Bridge in a Sunspot

We report a magnetic relaxation process inside a sunspot associated with the evolution of a transient light bridge (LB). From high-resolution imaging and spectro-polarimetric data taken by the 1.6 m Goode Solar Telescope installed at Big Bear Solar Observatory, we observe the evolutionary process of a rapidly evolving LB. The LB is formed as a result of the strong intrusion of filamentary structures with relatively horizontal fields into the vertical umbral field region. A strong current density is detected along a localized region where the magnetic field topology changes rapidly in the sunspot, especially in the boundary region between the LB and the umbra, and bright jets are observed intermittently and repeatedly in the chromosphere along this region through magnetic reconnection. In the second half of our observation, the horizontal component of the magnetic field diminishes within the LB, and the typical convection structure within the sunspot, which manifests itself as umbral dots, is restored. Our findings provide a comprehensive perspective not only on the evolution of an LB itself but also on its impacts in the neighboring regions, including the chromospheric activity and the change of magnetic energy of a sunspot.


Introduction
A light bridge (LB) is one of the substructures frequently seen inside a sunspot, appearing relatively bright compared to a surrounding dark umbra (Muller 1979;Sobotka et al. 1994).An LB mainly manifests as an elongated structure that crosses a sunspot umbra, and while it has usually the same magnetic polarity as the host sunspot, its magnetic field structure is significantly different.The typical magnetic field structure above an LB is known as a cusp structure, which has the fieldfree plasma region or weak magnetic fields in the photosphere (Leka 1997;Jurčák et al. 2006;Schüssler & Vögler 2006).Furthermore, the presence of a strong horizontal magnetic field along the LB is frequently observed (Okamoto & Sakurai 2018), which is a feature distinct from the magnetic field structure of the host sunspot umbra.The magnetic field structure of the LB becomes an important factor in determining its internal morphology: a filamentary LB showing bundles of filaments with a relatively enhanced horizontal magnetic field inside the LB (Louis et al. 2008;Lim et al. 2020) or a granular LB composed by convective cells similar to the granules seen in the quiet Sun (Lagg et al. 2014;Song et al. 2017b).
The formation of an LB may cause significant inhomogeneity in the magnetic field structure of a sunspot, which leads to changes in the internal structure and magnetic energy inside a sunspot.Previous high-resolution observational and theoretical studies have suggested that the LB formation can be caused by the intrusion of penumbral filaments (Katsukawa et al. 2007) and the emergence of horizontal magnetic fields inside an umbra (Toriumi et al. 2015).This LB formation process can gradually increase the tangential discontinuity of the magnetic field inside a sunspot (Song et al. 2015), leading to the sunspot activity and changes in the solar atmosphere.Fine-scale plasma ejections (Shimizu et al. 2009;Louis et al. 2014;Song et al. 2017b;Zhang et al. 2017;Tian et al. 2018;Yang et al. 2019a;Lim et al. 2020) and enhanced brightening (Berger & Berdyugina 2003;Song et al. 2017a) are some of the representative solar atmospheric phenomena related to the LB activity.
These chromospheric phenomena are observational evidence for the vigorous interaction that occurs between the different magnetic field structures between a sunspot and an LB, such as magnetic reconnection (Shimizu et al. 2009;Louis et al. 2015;Bharti et al. 2017;Yang et al. 2019b).Bharti et al. (2017) have for the first time reported a manifestation of successive magnetic reconnection seen above a penumbral filament digging into a sunspot umbra and suggested that magnetic reconnection occurs between curved flux rope-like fields in the lower part of the penumbral intrusion and vertical umbral fields.Louis et al. (2015) and Yang et al. (2019b) found observational evidence of a small-scale magnetic flux emergence observed in the strong magnetized environment, such as an LB, and reported that it was associated with the transient brightening and plasma ejections in the chromosphere.These previous results suggest that the LB formation and evolution make a significant contribution to chromospheric activity of a sunspot and lead to internal magnetic field energy changes.
The lifetime of an LB is typically several days.During this period, an LB undergoes a variety of physical changes that directly impact the activity and evolution of a sunspot (Vazquez 1973).Shimizu (2011) reported a long-term evolution study on an LB and demonstrated that the changes of the activity and phenomena appearing in the solar chromosphere are different depending on the internal magnetic structures and plasma flows of the LB.Griñón-Marín et al.
(2021) also reported through their study on the long-term evolution of three different LBs observed on the same sunspot that each LB has its own peculiarities with time and that the atmospheric parameters may be changed accordingly.
Here we report our uninterrupted observations of an LB from birth to decay over the period of several hours.This rapidly evolving LB provided us with a nice chance to comprehensively study the formation and decay of an LB with focus on the change of magnetic field configuration and the associated chromospheric activity.The paper is organized as follows.
Observations and data analysis are described in Section 2. Section 3 presents our results.Section 3.1 provides a full overview of the evolution of the LB in the photosphere.A description of temporal changes in the magnetic fields within the LB according to its evolutionary stage is given in the Section 3.2.Section 3.3 describes the chromospheric activity in sunspots following the LB evolution, i.e., the nature of the bright jets.Finally, in Section 4, our findings are summarized, and we discuss about the entire process of the short-term evolution of the LB and the magnetic relaxation process of the sunspot observed during the LB formation and evolution.

Observations and Data Analysis
We observed a leading sunspot in active region (AR) NOAA 12080 located in the southern hemisphere (−515″, −198″) using the 1.6 m Goode Solar Telescope (GST) installed at Big Bear Solar Observatory (BBSO) for about 2 hr from 16:34:00 UT on 2014 June 5.Our observations were performed simultaneously by using three different instruments installed in the coudé room of BBSO with support of a high-order adaptive optics (AO; Cao et al. 2010)  The FISS was designed as a dual-channel echelle spectrograph, consisting of a field scanner, a single mirror used as a collimator and an imager, two bandpass filters, an echelle grating with the blaze angle of 63°.4,and two CCD cameras.It can cover spectral windows in a wide range from visible to near-infrared light, but it observes the solar chromosphere generally using two spectral bands simultaneously.Here, we carried out the high-resolution observations of the sunspot using the Hα and the Ca II 8542 Å lines with the aid of a higher-order AO consisting of 308 sub-apertures (Shumko et al. 2014).The wavelength range covers from −5.3 Å to +4.5 Å with a spectral sampling of 19 mÅ in the Hα line and is from −7.0 Å to +6.1 Å with a spectral sampling of 26 mÅ in the Ca II 8542 Å line.The spatial sampling of both lines is 0 16, and the temporal cadence is about 22 s.
We measured linear and circular polarizations of the sunspot in the Fe I 15648.5 Å line using the NIRIS.The NIRIS was designed based on the dual Fabry-Pérot interferometer.Its spectral range is from 15646.9Å to 15651.4Å with a spectral sampling of 75.9 mÅ.The spatial sampling is about 0 16, and the temporal cadence is about 3 minutes.We inverted the Stokes profiles using the Milne-Eddington Stokes inversion code implemented by J. Chae (Landi Degl'Innocenti 1992;Lim et al. 2020) and solved the 180°ambiguity in the azimuth of the observed transverse magnetic fields (Moon et al. 2003).Finally, we obtained photospheric images using the broadband TiO 7057 Å imager with a spatial sampling of about 0 0347 every 15 s.large sunspots with opposite polarities (β-type based on the Mount Wilson classification; Hale et al. 1919) and several small pores in their vicinity.Our observation area, covering 31″ × 44″, is an entire region of a leading sunspot with a positive polarity and is marked with a black box in the SDO/ HMI magnetogram.Figure 1(b) represents a photospheric image taken by the broadband TiO imager at 17:12:00 UT.This is a speckle reconstructed image using the Kiepenheuer-Institut Speckle Interferometry Package code (Wöger et al. 2008).We find from the figure that the sunspot shows an asymmetrical shape and that penumbral structures develop only in one direction of the sunspot umbra.It is divided into four cores by three different LBs: "LB1," "LB2," and "LB3" indicated in Figure 1."LB2" is a main target of this study that lies along the north-south direction and has filamentary structures inside.

Temporal Evolutions of a Light Bridge
A remarkable finding of this study is that the internal structure of an LB changes rapidly and distinctly in a short time.This is well shown in the sequential images obtained by the combined utilization of the SDO/HMI intensity and the TiO 7057 Å imager.As shown in Figure 2 Our observations started at 16:46:21 UT, about an hour after the LB began to form, at which point the LB had already formed across more than half of the umbral region (Figure 2(c)).After that, we can see that several filamentary structures continuously penetrate into the developing LB (Figure 2(d)).This leads to the formation of an elongated filament bundle in the umbra, approximately 1700 km in width along the LB.At 17:11:55 UT, the LB completely divides the sunspot umbra into two umbral cores, as a filamentary LB (Figure 2(e)).Interestingly, at 17:33:36 UT, the filamentary structures located along the LB suddenly disappear, and the shape of the LB changes into a chain of convective cells (Figures 2(f) and (g)), i.e., it becomes a granular LB.In the second half of our observation, the LB decayed gradually (Figure 2(h)).Our data show in detail an evolutionary process of the LB from its development to decay with the high temporal and spatial resolutions.
Figures 3(a) and (b) show transverse velocity fields determined by tracking 21 consecutive images of the TiO 7057 Å imager taken for 6 minutes from 3.4 and 27.6 minutes after the start of our observation (from 16:39:02 UT), respectively.This was determined by applying a nonlinear affine velocity estimator (NAVE) developed by Chae & Sakurai (2008).We noted in the figure the early stages of the observation, especially the transverse velocity fields observed during the time when the LB is forming.The predominant flow observed along the LB is bidirectional flow, and the pattern of the transverse velocity fields is aligned well with the expansion direction of filamentary structures penetrating into the umbra (Figure 3(a)).The average transverse speed measured inside the LB is about 0.7 km s −1 , and the fast flows reaching up to 1.4 km s −1 are distributed around both the ends of the filaments.In addition, in Figure 3(b), we see filament bundles that are rapidly intruding into the interior of the formed LB.The corresponding transverse velocity fields are marked with red arrows.The measured speed of filament intrusion reaches up to 2.0 km s −1 , and their tips intrude the umbra positioned at both lateral sides of the LB.As the filaments continue to penetrate inside the LB, we can see that the internal shape and magnetic field structure of the LB gradually evolve into a filamentary LB.The growth rate of the LB is approximately 0.48 km s −1 , which is slower than the maximum speed of filament intrusion mentioned above, but we find it to be much faster than that of a typical LB formation previously reported by Katsukawa et al. (2007) and Griñón-Marín et al. (2021).

Evolutions of Magnetic Fields
Figure 4 shows maps of photospheric intensity (I/I QS ), vertical magnetic field (B z ), horizontal magnetic field (B h ), and magnetic field inclination at the specific evolutionary stages of the LB of our interest.We normalized the intensity using the average quiet Sun intensity (I QS ) and corrected the projection effect in the vector magnetogram.We see from the B z map that the vertical magnetic field strength of the LB is lower than that of the host sunspot umbra regardless of its morphology.This is consistent with the results of previous studies (Griñón-Marín et al. 2021).Here we note the change in the inclination of the magnetic field observed along the LB.In a series of inclination maps, we find that the magnetic field structure along the LB gradually becomes horizontal and then returns to vertical fields again.This illustrates that the magnetic field structure within the sunspot undergoes rapid changes.
Figure 5 shows a quantitative representation of the temporal changes in the intensity and magnetic field structure described above.In the figure, the square symbols represent the average values of I/I QS , B z , B h , and inclination, which were calculated using data from all pixels within the selected region outlined in red above the LB, as shown in Figure 4.Each error bar indicates the 1σ value for the spatial distribution of each physical quantity.One notable finding is the rapid increase and decrease in I/I QS by more than 20% depending on the evolutionary stages of the LB.This variation is closely related to the change in the internal structures of the LB.Specifically, the photospheric intensity decreased rapidly during the formation of the LB and then increased again at the same time as the filamentary structures disappeared inside the LB.We see that the photosphere intensity was equal to the average intensity measured in the quiet Sun when the granular pattern appeared inside the LB afterwards, and finally the intensity gradually decreased again as the LB disappeared.
The second thing we noticed is that that maps of inclination and B z exhibit notable and rapid changes in agreement with the evolutionary stages of the LB.In particular, we find from the plots that these changes can be divided into three parts according to the trend, as shown by the two vertical dotted lines in Figure 5(b).The first part corresponds to a time during which inclination undergoes a rapid increase, reaching approximately 60°.Concurrently, the strength of B z shows a rapid decrease, reaching approximately 500 G.These changes occur continuously during the first 25 minutes of the observation, coinciding with the formation of the LB.This indicates that the vertical magnetic fields of the sunspot umbra are rapidly replaced by horizontal magnetic fields during the formation of the LB.The next part is a time at which a rapid decrease in inclination, with a rate of about −1°.7 per minute, accompanied by an increase in B z .We find that it is related to the dramatic changes that were observed in the internal structures of the photospheric LB; specifically, the bundle of filaments seen inside the LB rapidly disappear, and the granular structures began to appear along the LB.Thereafter, the convective motion gradually became predominant inside the LB, and in the second half of the observations, it is restored to a dark umbra as the LB begins to disappear.It can be seen that the temporal changes of inclination and B z observed during this final period proceed relatively slowly compared to the previous two evolutionary stages.
The temporal variations of B h are also observed depending on the LB evolution, while the range of change is relatively smaller than others.Interestingly, we confirm that the strength of B h is higher than that of B z during the LB formation and filamentary LB but is weaker during the granular LB.Our results indicate that the evolution of the LB is closely related to the changes of the magnetic field structures inside the sunspot.

Chromospheric Bright Jets
Temporal variations in magnetic field structures and substructural plasma motion in the photosphere can directly influence the atmospheric activity above the LB. Figure 6 shows consecutive images obtained at varying wavelengths across from the line center to wings of the Ca II 8542 Å and Hα spectral lines, which were taken by the FISS.These raster images demonstrate in detail the temporal variations in the chromospheric structure and activity following the evolutions of the LB.The chromospheric structure that stands out first in the images is fine-scale bright jets indicated by red arrows in Figure 6.These jets, which are approximately 2″ in length, can be readily identified in the Ca II −0.5 Å images.They occur intermittently and recurrently along the boundary between the LB and the host sunspot umbra.
We find from the figure that bright jets show characteristics varying with time.At the beginning of the observation, the bright jets mainly occurred along the leading edge of the LB where the filaments penetrate into the umbra (t obs ∼ 8.95 minutes), and the base of these jets was observed at the boundary between the LB and the host sunspot umbra as the enhanced brightness.The jet's origin coincides with a region where the magnetic field topology is changing rapidly.After the LB is formed, the jet develops into a fan-shaped jet as it expands along the LB (t obs ∼ 39.82 minutes).Afterward, the base of the jet slightly shifts to the right along the LB (t obs ∼ 58.15 minutes), and the jet occurrence primarily concentrates at around the midpoint of the LB (t obs ∼ 69.35 minutes).This evolution of a bright jet is consistent with the evolution of dark plasma ejections inside a filamentary LB previously reported by Lim et al. (2020).On the other hand, these jets are not well observed in the Hα images.Instead, it can be observed that the base of the jets manifests as elongated brightening along the LB (see Hα ± 0.7Å images).Here, we noted that the bright jets are actively occurring for about 78 minutes following the start of our observation, but no further jets are ejected above the LB afterward, and the chromospheric structures remain relatively quiet.Here, we paid attention to the time of t obs ∼ 78 minutes.First, it is the timing when the horizontal flow is no longer observed along the LB.The second thing is that prior to t obs ∼ 78 minutes, the internal structure of the LB seen in the photosphere undergoes a dramatic change, but after this time, it maintains a stable granular shape, which gradually begins to disappear.Of particular importance is the change in the internal magnetic field structure of the LB, which shows a rapid change before this time but not afterwards.This suggests that the generation of the jets is closely related to the evolution of the LB, i.e., the temporal variation of the magnetic field of the LB within the sunspot umbra.
We conducted calculations of the vertical electric current density, J z , around and inside the LB using the formula of , where μ 0 is the magnetic permeability.
Figure 7 shows three examples of J z maps obtained at different times.The background of the figure was selected as the Ca II −0.5 Å image, in which the bright jets can be well identified, and the J z signals were represented by filled contours as reddish and bluish colors in each image.The white contour in the figure presents the boundary of the LB.We can see from the figure that enhanced current patches are primarily observed along the boundary between the LB and the sunspot umbra.The location of strong currents within the LB is consistent with the previous findings reported by Shimizu (2011) and Lim et al. (2020).We find that the evolution of the enhanced currents is closely related to the occurrence of the bright jets.The strong currents were detected only during the period when the jets occurred intermittently but did not appear thereafter.In particular, the location of the enhanced currents corresponded well with the base of the jets.It provides critical observational evidence that the bright jets originate from magnetic reconnection that occurs at the boundary between the LB's new magnetic fields and the preexisting magnetic fields of the ambient umbrae.

Discussion
We have reported on the short-term (a few hours) evolution of a transient LB observed in the AR NOAA 12080, as well as the changes of chromospheric phenomena above the host sunspot associated with it by analyzing the imaging spectroscopic and spectro-polarimetric data obtained from the 1.6 m GST installed at BBSO.Our observations are of significant importance in that the evolution of the LB was observed within a short time of less than 2 hr, with high spatial and temporal resolutions.In particular, we find that the internal structures of the LB changes dramatically with time, which enables a detailed exploration of not only the rapid changes in the internal physical properties of the sunspot and the LB but also the temporal changes in the dynamical activity observed in the chromosphere.Our findings are important in that it provides a comprehensive perspective to understand in detail not only the evolution of an LB itself with time but also its impact on the neighboring regions, particularly changes in the chromospheric activity and magnetic energy of a sunspot.
The first thing that stands out in this study is that unlike the nature of a typical LB previously reported, there is a drastic change in the internal structure of the LB seen in the photosphere.According to the classification scheme of LBs (Korobova 1966;Muller 1979;Sobotka 1997), they can be generally categorized into "filamentary" LBs consisting of filament bundles or "granular" LBs with convective cell patterns, depending on the internal structure of an LB.Various previous LB studies show that most LBs have a single internal structure, such as a filamentary LB (Louis et al. 2008;Yang et al. 2019a;Hou et al. 2020) or a granular LB (Lagg et al. 2014;Song et al. 2017b), and they maintain a consistent morphology in the photosphere for a long time.Furthermore, the entire process of the LB evolution generally occurs over a long time.In particular, it has been reported that these differences in LB shapes mainly originate from differences in the structure of the magnetic field inside an LB and the process of LB formation (Jurčák et al. 2006;Katsukawa et al. 2007;Lagg et al. 2014;Okamoto & Sakurai 2018;Lim et al. 2020) and that LBs with different morphology can undergo different changes in physical properties during the evolution (Shimizu 2011;Griñón-Marín et al. 2021).Meanwhile, our findings demonstrate that these two different morphologies are closely related to the evolutionary stage of the LB, especially the rapid changes in the magnetic field and plasma flow inside the sunspot.At the beginning of the observation, a filamentary LB was formed due to the continuous and strong intrusion of filaments with a horizontal magnetic field into the sunspot umbra, after which a granular LB appeared as the intruding horizontal magnetic fields dissipate through magnetic reconnection with umbral magnetic fields.
The LB of our interest was formed as a result of the strong intrusion of filamentary structures with relatively horizontal fields into the vertical umbral field region.This is in good agreement with the results of previous studies that proposed the LB formation mechanism (Katsukawa et al. 2007;Louis et al. 2020).As the LB gradually formed, the magnetic nonpotentiality inside the sunspot increases, and tangential discontinuity of the magnetic field appears, especially around the local region where the magnetic field topology changes rapidly, namely the boundary region between the LB and umbrae.Indeed, we detected strong current density at this region (Shimizu et al. 2009;Lim et al. 2020) and distributed along the frontal boundary where the LB penetrates into the dark umbra in the early phase of the LB evolution.Afterward, the interior of the LB was gradually filled with filament bundles caused by the successive intrusion of penumbral filaments, and its morphology changes to a filamentary LB.From this point of time, the strong current is mainly detected along the lateral side of the LB.Interestingly, soon after, the filamentous structure disappeared and convective cells were observed to appear inside the LB.At this time, we saw that the magnetic field structure also changed rapidly in the vertical direction.As a result, its morphology changed from a filamentary LB to a granular LB.
The sudden disappearance of the filamentary structures inside a sunspot shows the observational manifestation that the magnetic relaxation process is occurring inside the sunspot.The formation of an LB may lead to the continuous buildup of free magnetic energy inside the sunspot until the fields become unstable.Once the magnetic field reaches a state of instability, it will seek to relax into a lower-energy state that conserves the total magnetic flux and the magnetic helicity (Taylor 1974(Taylor , 1986)).We have observed the conversion of magnetic energy accumulated by the LB formation into kinetic and thermal energy as the magnetic relaxation process of a sunspot.The fine-scale bright jets, which are intermittently and recurrently observed in the chromosphere, are an important manifestation of this energy conversion.Our findings show that these jets occur mainly along the local region of magnetic discontinuity, i.e., strong current formed between the LB and the adjacent umbra, and this supports previous results that the origin of the observed jets above the LB is magnetic reconnection (Toriumi et al. 2015;Bharti et al. 2017;Tian et al. 2018).In particular, we find that the base of the jets appeared as an enhanced brightness along the strong current region, where there is a counterpart region of the magnetic reconnection (Lim et al. 2020), and at the same time, the magnetic field inclination along the LB decreases rapidly.That is, the horizontal component of the magnetic field diminishes within the LB, and the typical convection structure within the sunspot, which manifests itself as umbral dots, is restored.
Note that we observed in detail the magnetic relaxation process within a sunspot.The magnetic system of the sunspot initially in equilibrium became disturbed by the formation of the transient LB of a few hours and then returned to a stable state again.This kind of relaxation process may be occurring during the evolution and activity of the LBs of several-day lifetime as well.Thus our results will contribute to the comprehensive understanding of the evolution of LBs and their relationship with the sunspot system and the associated chromospheric activity in general.
Figure 1(a) shows a line-of-sight magnetogram of AR NOAA 12080 obtained by the Helioseismic and Magnetic Imager (HMI; Schou et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012).This AR contained two

Figure 1 .
Figure 1.(a) SDO/HMI magnetogram of NOAA 12080 AR obtained on 2014 June 5, at 17:12:00 UT.The black rectangle box represents our observation region covering the field of view (FOV) of 31″ × 44″.(b) Photospheric image of a leading sunspot of NOAA AR 12080 taken by the broadband TiO 7057 Å imager.Three LBs seen inside the sunspot are indicated by "LB1," "LB2," and "LB3," respectively.The white box shows the FOV of the region of interest, which is the same as the FOV of the panel in Figure 2.
, the LB is observed in different internal shapes depending on its evolutionary stage.The LB formation was first observed in the photospheric images taken by the SDO/HMI intensity around 15:31:30 UT on 2014 June 5, (see Figures 2(a) and (b)).The LB began to form at one end of the sunspot (red arrow in Figure 2(b)), and it penetrated into an umbral region rapidly.

2.
Temporal evolution of an LB in the photosphere.The FOV corresponds to the white box shown in Figure 1.Images (a) and (b) are obtained from the SDO/ HMI intensity, while images (c) through (h) are generated using data obtained from the broadband TiO 7057 Å imager.Note that the spatial sampling of SDO/HMI is about 0 6, while the spatial sampling of the broadband TiO 7057 Å nm imager is about 0 0347.The red arrow indicates the initial appearance of the LB.An animation of panels (c)-(h) is available.The animation begins on 2014 June 5 at 16:39:02 UT.It ends the same day at 18:20:54 UT.The real-time duration of the animation is 10 s. (An animation of this figure is available.)

Figure 3 .
Figure 3. Transverse velocity fields in the internal structures of the LB determined by the NAVE method.They are determined by tracking 21 consecutive images of the TiO 7057 Å imager taken for 6 minutes from (a) 3.4 and (b) 27.6 minutes after the start of our observation (from 16:39:02 UT), respectively.The length and color of arrows indicate the speed and direction of the flow.

Figure 4 .
Figure 4. I/I QS , B z , B h , and inclination maps (left to right) according to the evolutionary stage of the LB.Here, we set the observation time of 16:39:02 UT to t = 0 minute.The I/I QS are taken from the TiO imager, and other maps are derived from the NIRIS data.The red contour lines shown in the I/I QS maps are the area we used to determine the temporal variations of physical quantities in Figure 5.The white and black contour lines represent the boundary between the LB and umbrae.

Figure 5 .
Figure 5. Temporal variations of (a) I/I QS , (b) B z , (c) B h , and (d) inclination.The evolution of the LB can be divided into three parts based on the time of two vertical dotted lines shown in the B z plot.Here, we set the observation time of 16:39:02 UT to t = 0 minute.The square symbols represent the average values measured using data from all pixels within the selected region outlined in red above the LB, as shown in Figure 4.The error bars represent ±1σ values resulting from the spatial distribution along the LB.The red lines indicate the time of occurrence of bright jets along the evolution, which are identified in the Ca II −0.5 Å images.

Figure 6 .
Figure 6.Consecutive chromospheric images obtained at varying wavelengths across from the line core to wings of the Ca II 854.2 nm (top) and Hα (bottom) spectral lines, which were taken by FISS.The red arrows indicate bright jets occurring in chromosphere.

Figure 7 .
Figure 7. Maps of the vertical electric current density (J z ) obtained at three different times.The white contour lines represent the boundary between the LB and umbrae.White arrows indicate the chromospheric jets seen in the Ca II −0.5 Å images.