The Slipping Magnetic Reconnection and Damped Quasiperiodic Pulsations in a Circular Ribbon Flare

The study of circular ribbon (CR) flares is important to understand the three-dimensional magnetic reconnection in the solar atmosphere. We investigate the slipping brightenings and damped quasiperiodic pulsations in a CR flare by multiwavelength observations. During the flaring process, two extreme ultraviolet brightenings (SP1 and SP2) slip synchronously along the ribbon in a counterclockwise direction. The ribbon and fans between them show synchronous enhancement with the microwave and hard X-ray (HXR) CR source. In the magnetohydrostatic extrapolation results and observations, the dome and outer spine display an evident counterclockwise twisting feature. We propose the slipping reconnection occurs between the fan and outer spine in the null point, which covers the region from SP1 to SP2. The fan of SP1 shows the strongest twist and produces the most efficient reconnection. The ribbon after SP1 becomes weak due to the destruction of the fan configuration. The fan of SP2 is in the front of the slipping motion, which initiates new reconnection and brightens the local ribbon. The twisting of the dome continuously promotes new reconnection in the null point, which brightens the ribbon in sequence to display a counterclockwise slipping feature. Thus, the twist of the dome may trigger and dominate the slipping reconnection, and the rotation of the central positive pole could be one possible cause of the twist. After the peak, the microwave and HXR emission shows damped oscillations at a period of 15 s. The collapse of the fan–spine structure may lead to the standing kink oscillations of the fan to modulate the reconnection and particle acceleration process.


Introduction
Quasiperiodic pulsations (QPPs) are ubiquitous phenomena in the solar atmosphere (Nakariakov & Melnikov 2009).They could be recorded in multiwavelength, including soft X-ray (SXR), hard X-ray (HXR), extreme ultraviolet (EUV), radio, Lyα, gamma rays, and so on (Inglis et al. 2008;Nakariakov et al. 2010;Pugh et al. 2016;Li et al. 2020), which are timely signals of the physics related to magnetic reconnection, energy accumulation and release, or plasma heating (Kupriyanova et al. 2020;Zimovets et al. 2021;Inglis et al. 2023).The oscillating period of QPPs ranges from subseconds to tens of minutes (Kupriyanova et al. 2010;Tan et al. 2010;Kolotkov et al. 2015), and different periods could be identified in different spatial locations (Luo et al. 2022), different stages (Kumar et al. 2017) or different wavelengths (Huang et al. 2016) in the same bursty event.Radio and HXR emission, which is produced by energetic electrons in solar activities (Aschwanden 2004;White et al. 2011), would contain direct information on the dynamic process of electron acceleration/ transportation in a wider space from the lower solar atmosphere to the interplanetary space (Reznikova et al. 2007;Clarke et al. 2021).The spectral structure of radio QPPs in observations always consists of many regularly arranged bright pulsations, which contain the parameters for emission frequency, frequency bandwidth, polarization degree, frequency drifting rate, duration, and so on (Huang et al. 2008;Tan 2008;Huang et al. 2022).These parameters could provide us with valuable diagnostic information on modulation dynamics, which helps us to understand the nature of solar activities (Karlický et al. 2017;Karlický & Rybák 2020;Hong et al. 2021).Based on existing research, radio QPPs are attributed to several proposed mechanisms, such as the modulation of gyrosynchrotron emission by slow magnetoacoustic oscillations (Nakariakov & Melnikov 2006) or fast magnetoacoustic wave (Kupriyanova et al. 2022), the quasiperiodic magnetic reconnection modulated by oscillated current-carrying loop coalescence (Zimovets & Struminsky 2009), flapping oscillations (Zimovets et al. 2021) or intermittent magnetic islands within the current sheet (Kou et al. 2022), the quasiperiodic injection of energetic particles modulated by kink oscillations of coronal loops (Huang et al. 2014), and so on.
The circular-ribbon flare displays a special configuration of a fan-spine structure, including a CR, a central ribbon below the fan, and one remote ribbon connecting the outer spines, which is a typical magnetic structure of three-dimensional coronal null-point topology (Masson et al. 2009;Török et al. 2009).QPPs in circular-ribbon flares present rich features in temporal and spatial observations.In some events, the emission from different structures of the circular-ribbon flare has a similar oscillating period.For example, Altyntsev et al. (2022) reported that the nonthermal emission from the flaring kernel and the remote source shows the same oscillating periods (8 s).They proposed that the oscillations of the current sheet during the loop coalescence modulate the process of the flare energy release, and the electron beams propagating from the flare kernel produced the quasiperiodic microwave emission around the remote ribbon.Zhang et al. (2016) proposed that the 32-42 s period oscillations in both the Si IV line intensity and the SXR derivative are produced by intermittent null-point magnetic reconnections.
In some other events, the emission from different locations may show different oscillating periods during the same flares.For example, Chen et al. (2019) reported that UV QPPs with a period of 4 minutes appeared near the foot point of the inner spine, EUV QPPs with a period of 3 minutes were along the CR, and radio QPPs with a period of 2 minutes were originated from the main flaring region below the dome.Ning et al. (2022) also found that the flaring region, the jets, and the outer loops of a circular flare region presented three different oscillating periods.Kashapova et al. (2020) suggested that the kink oscillation of the outer spine produces the 25 s QPPs in the elongated ribbon, but the dominant 150 s QPPs are closely related to the slipping reconnection in the fan.
Slipping motion of bright emission along flare ribbons has been already extensively observed and studied, which is important for diagnosing three-dimensional magnetic reconnection in solar flares (Aulanier et al. 2007;Janvier et al. 2013;Dudík et al. 2014;Li et al. 2016).The fan-spine structure of the CR flare presents a single ideal three-dimensional topology, where slipping motions are expected to take place along the quasicircular ribbon (Masson et al. 2009;Pontin et al. 2013).Observations show that the CR could brighten sequentially in a clockwise or counterclockwise direction (Wang & Liu 2012;Li et al. 2018).Slipping brightenings along the ribbon are always associated with various activities.Before the slipping reconnection process, sigmoid or minifilament eruption inside the circular region is always observed (Joshi et al. 2015;Liu et al. 2015), which activates the slipping motion.During slipping reconnection, the magnetic structure of the whole system is changed, and jets are always found to take place simultaneously (Shen et al. 2019).Xu et al. (2017) found after the slipping motion, the post-flare loops in the dome display an apparent writhing signature.
In this study, we make an analysis of a CR flare in multiwavelength observations, where EUV slipping brightenings along the CR and microwave and HXR QPPs in the core region are recorded simultaneously.Accompanied by the slipping brightenings, the CR flare presents abundant features, including the rotation of the fan structures, the continuous jet near the slipping region, the expansion and twisting of the out spine, and the straight-line slipping along the remote ribbon.Combining with the magnetohydrostatic magnetic exploration results, we try to explore the relationship between the slipping motion, the primary flaring process, and the oscillating nonthermal emission in the frame of a fan-spine configuration and to understand the nature of slipping reconnection and oscillating particle acceleration.This paper is organized as follows: the observations and overview of this event are presented in Section 2. The analysis and results are included in Section 3. The summary and discussions are shown in Section 4.

Overview of This CR Flare
The event under study is an M1.8 CR flare in NOAA Active Region 12080 on 2014 June 11.It started at 05:30 UT, peaked at 05:34 UT, and ended at 05:36 UT.We use the EUV and UV full-disk observations from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012) to study the evolution and dynamics of this flare.The AIA observations have a temporal cadence of 12 s and an image scale of 0 6 pixel −1 .The full-disk Hα (6562.81Å) images from the Solar Magnetism and Activity Telescope (SMAT) at Huairou Solar Observing Station of National Astronomical Observatories of China have a spatial resolution of 2″ and a cadence of 1 s (Zhang et al. 2007).The Nobeyama radioheliograph (NoRH; Nakajima et al. 1994) provides microwave imaging observations at 17 and 34 GHz with a 1 s cadence.We use the program of Hanaoka to obtain the compact sources, and the pixel size of the synthesized NoRH image is 4 9 at 17 GHz and 2 45 at 34 GHz.At 17 GHz, the right (R) and left (L) circular polarization of sources has been recorded.The total intensity (I component) image is the sum of the signal of R and L (I = R+L).The V-component image is described as the difference value between R and L(V = R-L).The polarization degree can be obtained by P = V/I = (R−L)/ (R+L).We also use the data from Nobeyama Radio Polarimeters (NoRP; Nakajima et al. 1985), which record the microwave total intensity at six fixed frequencies (1.0, 2.0, 3.75,9.4,17,and 35 GHz).The spectrum from the Hiraiso Radio Spectrograph (HiRAS; Kondo et al. 1995) at 15-2500 MHz also recorded this burst in both left and right polarization.We obtain the HXR light curves and images at 3-6, 12-25, and 25-50 keV bands from the observations of the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI; Lin et al. 2002).The temporal resolution of the light curves is 1 s.We use the Fronts (Nos. 1,2,3,4,5,7,8) to produce the RHESSI image, and the best FWHM is 2 63.The integration time step of the images is 5 s, and the pixel size is chosen to be 1″.
Figure 1 presents the light curves of SXR, SXR derivative from GOES observations, total microwave flux at 9.4 and 17 GHz from NoRP data, and HXR light curve of RHESSI observations from 05:32 to 05:38 UT.It is noted that the GOES SXR derivative presents a series of evident oscillations during the whole flaring process (Figure 1(b)).The oscillations start at about 05:32:30 UT and end at 05:36:10 UT.The microwave fluxes at 9.4 (black) and 17 (green) GHz start to increase at 05:32:50 UT and present synchronous oscillations after peak (Figure 1(c)).The HXR light curve at 25-50 keV (orange) also shows oscillations after the maximum, which shares a good temporal correlation with the microwave emission.It is found that the oscillating amplitude of QPPs in SXR derivative, microwave, and HXR light curves displays an evident damped feature.
The configuration of this CR flare in EUV and microwave observations is shown in Figure 2.This flare has a very compact region with a radius of about 8 5 (denoted by CR in Figure 2).A detailed description of the CR region will be shown in Section 2.2.Here we mainly present the global structure of the flare.In Figures 2(d)-(f), the outer spine structure, connecting CR and the remote foot (denoted as RF), is clearly presented at 131 Å band, which has a scale length of more than 100″.In the initial phase at 05:30:46 UT, the spine structure was very weak, and it displayed as a loop-like structure in the 131 Å band (Figure 2(d)).In the 211 Å band, the top of the spine could also be distinguished (denoted by the yellow arrow in Figure 2   appeared near RF (Figures 2(b) and (e)), which was weaker than the CR source.

The Slipping Brightenings along the CR
In EUV, UV, and Hα observations, CR is able to be clearly outlined before the flare.The emission intensity along CR was very weak and the structure was not uniform.At about 05:28:46 UT, two fan structures (Fan1 and Fan2) were brightened, which can be clearly shown in 94 Å images (Figure 3(a)).Both of them were originated from the central kernel.Fan1 was located in the west part of the CR region, while Fan2 was in the south region.It is interesting to find that Fan1 displays a bright twisted structure, and Fan2 has a weaker smooth fan structure.The ribbons of Fan1 and Fan2 were enhanced correspondingly, which were brighter than the other part (left panel of Figure 3).At 05:30:34 UT, a bright compact point appeared at the northern boundary of Fan1 (denoted as SP1 with the yellow arrow in Figure 3(a)).It moved in a counterclockwise direction along the CR.This slipping motion started at about 05:30:34 UT and ended at about 05:35:58 UT.As shown in the middle panel of Figure 3, SP1 slipped to the southern region along CR, and the emission of SP1 became more intense.When SP1 slipped, there was a bright structure connecting SP1 and the center kernel in the fan, which seemed to rotate around the central kernel in a counterclockwise direction.In the ribbon of Fan2, we cannot find such a compact, bright point as SP1, but we can still distinguish a slipping bright emission in the north end of the ribbon of Fan2 (denoted as SP2 in Figure 3(a)), which also slipped in a counterclockwise direction.The bright structure of Fan2 also displayed a counterclockwise rotary motion.
At 05:33:40 UT, when the emission amplitude of SP1 reached the maximum, another bright ribbon appeared in the north region of CR.It was located near the initial location of SP1 and showed very strong emission.We name it the primary ribbon (denoted as PR by yellow arrows in Figure 3) because the post-flare loops are rooted in it.In the Hα image, we could also find three bright regions: SP1, PR, and kernel (Figure 3(m)).In Figure 3(e), we plot the HXR sources with green (12-25 keV) and orange (25-50 keV) contours at the levels of 85% and 95% of the maximum.It is found that the HXR sources are located around the kernel region.When SP1 moved along the ribbon, many field-like structures rooted in the slipping ribbon were brightened subsequently (Figure 3(e)).Simultaneously, the whole structure of the outer spine was brightened, and the top of the spine grew up with a twisted feature, as shown in Figure 2. Following these bright field-like structures, dark material was continuously ejected upward from the vicinity of the slipping region (Figures 3(b) and (e)).A jet took place after the slipping motion of SP1, which is clearly shown in Figure 3(f).
In the post-flare phase (the right panel of Figure 3), the CR's whole structure could still be distinguished in the 1600 Å images, and the foot of the kernel and PR showed very intense emission.However, the ribbon of the slipping region was incomplete and very weak, which presented irregular and intermittent distribution.In the 94, 335 Å, and Hα images, post-flaring loops were found to connect the kernel and PR in the north part of the circular region.In the 335 Å image, the dark material of the jet moved outward continuously, and it covered the south part of CR (Figure 3(f)).Thus, only part of CR could be distinguished in the 335 Å images.Even so, we can still distinguish the slipping motion of SP1 along the southeast region of CR from the observations at 1600 Å, which helps us to determine the stop of the slipping motion of SP1 at 05:35:58 UT.

Microwave QPPs at 9.4 and 17 GHz
Figure 4 presents the microwave emission of this CR flare.In Figure 4(c), the microwave fluxes at 1, 2, 3.75, 9.4, and 17 GHz from NoRP observations are plotted in arbitrary units.The background emission before 05:33 UT is subtracted from the total flux for each frequency.It is found that the emission at 9.4 and 17 GHz displays damped oscillations with more than 9 peaks.The light curves at 2 and 3.75 GHz have two main peaks during the peak and a smooth decay.The emission at 1 GHz shows a completely different pattern from the other frequencies.It is increased relatively later than higher frequencies and the light curve shows many irregular pulses.From the spectra of HiRAS data, a group of drifting pulsations (DPS) was recorded from 05:33 UT to 05:35 UT in the right and left polarization spectra.The whole group structure drifted from 1.6 to 0.8 GHz (Figures 4(a) and (b)).Thus, the intensive emission at 1 GHz of NoRP is consistent with the emission of DPS.
The spectral distributions of the microwave emission at five frequencies are plotted in Figure 4(d).We selected five peaks in the light curves of 9.4 and 17 GHz, which are labeled by dashed lines with different colors (Nos.1-5).Figure 4(d) shows that the emission at 17 GHz is slightly smaller than 9.4 GHz, but the emission at 17 and 34 GHz displays a typical negative power law pattern.Although we could not confirm the peak frequency of the microwave spectrum from these five frequencies, the negative power law pattern of 17 and 34 GHz shows a credible feature of optical thin gyrosynchrotron emission.For the emission at 9.4 GHz, we cannot confirm the source condition.Here, we propose it is also produced by optical thin gyrosynchrotron emission because it shows a similar oscillating feature as 17 GHz.In Figure 4(d), we find the emission at 1 GHz is more than 400 sfu, which is much stronger than the other frequencies.As proposed in Karlický et al. (2004), DPS can be interpreted with the plasmoid ejection in the flaring process, which is emitted by plasma emission processes of energetic electrons.Thus, the intensity could be very large from the coherent mechanism.The whole structure of DPS shifts from high to low frequency, which indicates an upward movement of the source.To study the oscillating features of the microwave emission, we make a wavelet analysis of the flux at 9.4 and 17 GHz (Figure 5).By subtracting the background with a smoothed window of 21 s, we obtain the wavelet results of the oscillated amplitudes.It can be seen that the microwave emission at 9.4 and 17 GHz presents the in-phase oscillations at the period of about 15 s, and the amplitude decreases after the first peak, which is shown as a typical pattern of damped oscillations.

The Relationship between the EUV Slipping Brightenings and QPPs
We made the time-distance plots along CR and the remote ribbon at the 335 Å band to display their evolution during the flaring process.As shown in Figure 6(a), the slice A-B-C-D is selected in a counterclockwise direction along the ribbon.The slice A-B-C (red and blue pluses) covers the ribbon where SP1 slips.Point B is the location where SP1 has the strongest emission.The bright field-like structures and the black ejecta cover the ribbon from B to C (blue pluses).The slice from C to D (green pluses) covers the slipping region of SP2.PR is located around the position of A and D. Figure 6(b) presents the region of the remote ribbon at 335 Å, which displays a straight-line distribution.The slice from E to F is plotted with a black dotted line.
Figure 6(c) shows the time-distance plot of slice A-B-C-D.It is found that before 05:30:34 UT, CR had a very weak and smooth structure with bright emission in the south region.Then the bright point SP1 (denoted by the red arrow) appeared near point A and slipped along the ribbon A-B-C from 05:30:34 UT to about 05:35:58 UT.It is shown as a bright shifting structure in the time-distance plot from point A to C. With linear fitting of this bright shifting structure, we obtain a slipping speed of about 40 km s −1 .After slipping, the ribbon A-B-C turned out to be a weak structure as before.After 05:35:58 UT, SP1 did not move anymore.Ribbon B-C was covered by the black ejecta.In the north part of CR, another slipping emission (SP2, denoted by red arrow) is clearly outlined by shifting fronts from C to D. The slipping front SP2 is weaker than SP1, which seems to slip later than SP1 and stop at the location of PR.
To compare the EUV and microwave emission in the flaring region, we plot the light curves of the microwave CR source at 17 GHz (white solid line), the EUV emission of the whole ribbon (yellow dotted line), and the whole CR region, including the ribbon, fans, and kernel (yellow solid line), in Figure 6(c) with arbitrary units.It is noted that before 05:33:46 UT, all the light curves show a smooth increase with two minor in-phase peaks.At about 05:33:46 UT, SP1 presents the strongest emission around point B, and the microwave CR emission is also sharply increased to maximum.The EUV emission of the whole ribbon and the whole CR region also shows a sharp peak.Then, SP1 continues to slip to point C, but its intensity is gradually decreased.The microwave CR light curve starts to show oscillations.Both the total intensity and the oscillated amplitudes display a decreasing pattern.During this period, the EUV emission of the whole ribbon presents two peaks and subsequent smooth decay of several minor peaks, which are synchronously enhanced with microwave pulses.The EUV emission of the whole CR region is also progressively increased with two minor synchronous enhancements.The light curve peaks later at about 05:36:04 UT due to the contribution of the bright post-flaring loops of PR.After that, the emission also presents a gradual decay as the microwave CR source and EUV ribbon.
Figure 6(d) presents the time-distance plot of slice E-F along the RF region.The microwave and EUV emissions of RF are overplotted with white solid (17 GHz) and yellow dotted (335 Å) lines, respectively.It is found that before 05:33:46 UT, the EUV emission of the RF regions is also slightly enhanced, which synchronizes with the minor peaks of the EUV and microwave CR source.At 05:33:46 UT, both EUV and microwave emissions of RF increased significantly.The microwave RF emission has two peaks, which correspond to the peaks of the microwave CR source.After that, it has many weak pulses in the decayed light curve as the CR source.In the time-distance plot of slice E-F at 335 Å, the ribbon of RF also shows a fast shift of bright emission from point E to F. The bright slipping starts at the peak moment (05:33:46 UT) and stops at the end of the slipping motion of SP1.Therefore, the emission of the ribbon, the CR source, and the RF source are closely related to each other.The bright slipping along the RF ribbon is consistent with the slipping motion of SP1.It can be seen that the central region of the CR is the positive magnetic field, which is surrounded by the negative magnetic field.The blue contours are the total intensity (I component) of the microwave source and the yellow contours are the V-component signal.The microwave CR source is above the CR, which shows a negative value of the V component (yellow dotted contours).We calculated the microwave polarization degree of the CR source and obtained a result of about 10%-15%.The RF microwave source displays a positive value of the V component (yellow solid contours), and the polarization degree is up to about 40%.The microwave source at 17 GHz is proposed to be emitted by the gyrosynchrotron emission mechanism, which produces the X-mode electromagnetic wave.The V component of the microwave CR and RF source matches well with their magnetic polarity.

The Magnetogram of the CR Flare
In Figure 7(b), we plot the contour of EUV emission (white thin contour) at the 335 Å band at 05:31:28 UT.The whole CR structure could be clearly displayed in the contour plot.The EUV ribbon perfectly covers the boundary of the central positive magnetic field and the surrounding negative magnetic field.The structure of Fan1 (denoted by a yellow arrow) can be also clearly shown, which originates from the central positive magnetic region and connects to the surrounding negative magnetic field region.The position of Fan2 is denoted by a yellow arrow, which is too weak to be displayed in the contour.The contours of the HXR sources at 12-25 and 25-50 keV from 05:33:25 to 05:34:55 UT are plotted on the magnetogram with green and orange contours, respectively.We choose the contours at the level of 97% of the maxima of each image to display the center of these sources.It is found that all of the HXR sources are located above the central positive magnetic field.They nearly concentrate in one place and slightly deviate from the EUV central kernel of Fan1.

Summary and Discussion
Using multiwavelength observations, we study the EUV and UV slipping brightenings along the ribbon and the damped QPPs in the microwave, HXR, and SXR derivative light curves in a CR flare.The CR flare displays a typical distribution of the fan-spine configuration, including CR, fan structures, inner central ribbon, kernel, and outer spine.Before the flare, the CR has already been formed, which shows a smooth, weak circular configuration with a brighter structure at the south part.This may suggest that a weak and steady null-point reconnection has already taken place.Then, two slipping brightenings appear and shift in a counterclockwise direction along the ribbon.SP1 is located at the northern edge of Fan1 and SP2 is at the east end of Fan2.As soon as these bright points start to slip, the ribbon and the fans between them and the central kernel are brightened simultaneously.The outer spine structure also becomes bright and begins to expand upward.The DPS recorded at 0.8-1.6GHz with the whole structure shifting from high to low frequency can also suggest the global rise of the spine.Thus, the start of the slipping motion in the ribbon indicates the beginning of the flaring process.
When the emission amplitude of SP1 reaches a maximum at point B, the emission of both microwave and HXR CR source is impulsively increased to the peak.Simultaneously, SP2 slips to point D, and the PR appears in the north region with very intensive emission.The outer spine displays a bright kinked structure at the top.The RF region is also enhanced impulsively at both microwave and EUV bands.At this moment, the flare presents the most efficient energy-release process.After that, SP1 continues the slipping motion to point C with a decreasing intensity.The post-flaring loops gradually emerge to connect the PR and the central kernel.The RF ribbon is also brightened sequentially along the straight-line distribution.The microwave and HXR emission of both CR and RF regions present the in-phase damped oscillations.The EUV intensity of the whole ribbon and the whole CR region also shows corresponding pulses.These suggest that the slipping motion of SP1 and SP2 plays an important role in the whole flaring region, and the slipping magnetic reconnection around the null point may dominate the whole bursty process.In addition, the intensity of SP1 is closely consistent with the microwave and HXR CR source.The fan structure connecting the kernel and SP1 presents the most intensive structure.Hence, the reconnection around the null point may not be homogeneous, i.e., the reconnection process near the region that links SP1 would be significantly more intense than other regions.
To obtain the magnetic field configuration of the fan-spine structure, we performed the magnetohydrostatic (MHS) extrapolation (Zhu & Wiegelmann 2018, 2019) by using the HMI vector magnetogram before the flare as the boundary input (Figure 8).The MHS extrapolation, which takes plasma forces (pressure gradient and gravity) into account, is able to model the three-dimensional magnetic field structure accurately (Zhu et al. 2022).In Figure 8, two groups of fan structures at different heights in the CR region are outlined in the extrapolation result.The underlying fan shows a perfect dome structure, which connects the central positive magnetic field and the surrounding negative one.It is noticed that the whole dome structure is twisted around the central positive pole in a counterclockwise direction.Especially in the northwest part of the dome, the field lines are twisted much more significantly than the other regions.Above the twisted dome, there is a halfround structure at the west of the circular region.Some field lines are found to link with the outer spine.The large outer spine links to the distant positive magnetic field, which also presents an evident twisted feature in a counterclockwise direction.
We deduce the electric current density of the circular region from the extrapolated magnetic field results.The distribution map of the line-of-sight integration of electric current density along the z-axis is plotted in Figure 8.The configuration of electric current presents different features at different heights (Figure 9).In the lower levels from the photosphere up to about 500 km, the electric current presents a circular distribution as the ribbon in EUV and UV images.In the center of CR, we also find a bright point, which could be the ribbon of the central inner spine.From 620 to 1560 km, the electric current displays the bright fan structure.Fan1 is clearly outlined in the west of the circular region.In the higher level from 1680 to 2280 km, the size of Fan1 becomes smaller and the central bright kernel appears.In the level from 1680 to 2280 km, the central bright kernel can still be outlined, which is surrounded by a weak twisted structure.The twisted structure also connects the west ribbon in a counterclockwise direction.
The multiwavelength observations and the magnetohydrostatic extrapolation results allow us to understand well the slipping magnetic reconnection in the fan-spine configuration.The most characteristic feature in the observations is that two bright points (SP1 and SP2) slip synchronously in a counterclockwise direction along the ribbon.The ribbon and fans between them are enhanced consistently with their slipping motion.As Pontin et al. (2013) mentioned, the current sheet would appear where the global orientations of the spine and fan form an angle.In that case, the spiraling of field lines in the current layers is expected.In our extrapolation results, two levels of fan structures are outlined in the slipping region.The underlying dome shows an obvious twist feature, which displays different orientations from the overlying structure.The distribution of the current sheet also presents a similar fan structure as the observations.These configurations may indicate the scene of the three-dimensional null-point reconnection in the quasi-separatrix layers between the dome and the outer spine.The field lines of reconnection cover the region between SP1 and SP2.The particles are accelerated near the null point and produce both HXR and microwave CR sources.
It is interesting that the dome shows the counterclockwise direction twisting feature in both observations and extrapolation results, which is in the same direction as the EUV slipping brightenings in the ribbon.The fan in the north of the twisted dome, linking SP1 and the central kernel, presents the strongest twist feature and the brightest structure.This may suggest that a stronger twist would produce a more efficient reconnection process.The ribbon after the slipping of SP1 is weakened in sequence, and a jet occurs in the adjacent region of slipping.These indicate that the slipping reconnection changes the connections of the magnetic field lines in the fan of SP1.The fan structure is destructed, and the ejecta move upward along the large-scale structure of the outer spine.On the contrary, the ribbon in the slipping of SP2 is brightened successively when SP2 shifts forward.Especially, the PR appears as soon as SP2 shifts to the north region of the ribbon.Therefore, SP1 and SP2 seem to play different roles in the slipping reconnection process.SP2 is the front of the slipping region, and new reconnection takes place when it shifts forward.SP1 is at the end of the slipping, which may break the link between the central kernel and the ribbon and reduce or terminate the local reconnection.It is proposed that the growing twist of the dome may successively promote new field lines to join in the reconnection process near the null-point region.These reconnecting magnetic fields connect the ribbon following the twisting direction, so the ribbon is brightened sequentially in a counterclockwise direction.Therefore, the twist of the fan structure dominates the direction of the slipping reconnection.
After the peak of the emission of SP1, both the EUV and microwave emission of the CR source show a gradual decrease.Although SP1 still slips forward, its emission turns out to be weaker than before.These observations suggest a gradual decay of the reconnection process.The decrease in the twist of the dome could contribute to the decrease of the angle between the fan and the spine and thus reduce the efficiency of the reconnection process.The microwave CR source and microwave RF source present in-phase damped quasi-periodical pulsations at the period of about 15 s after the peak time.The CR source is produced by the energetic electrons accelerated near the null-point region, while the RF source is emitted by the energetic electrons, transporting from the null point along the outer spine.Thus, they share the same oscillating pattern.When SP1 slips to point B, the kinked feature of the outer spine is destructed, and the field lines in the RF ribbon are brightened successively.The collapse of the fanspine configuration could trigger some kind of oscillations in the dome, which modulates the reconnection process in the null point.The kink oscillations of fans could be one possibility.The scale length of the fan is about 8 5, which could produce oscillations with a period of 15 s.The standing kink of the fan may change the distribution of the fan and spine near the null point, which modulates the reconnection process and also the particle acceleration.Another choice is the tearing mode instability in the current sheet during the process of fan-spine coalescence.
In summary, we report some new features of the slipping reconnection in the three-dimensional null-point configuration by multiwavelength observations and the magnetohydrostatic extrapolation results.The null-point reconnection takes place near the central region, where EUV, UV, microwave, and HXR emission present a bright kernel source, and the particles are accelerated nearby.The magnetic fields twist around the kernel, and the reconnection covers the region between the slipping points (SP1 and SP2), and both the ribbon and fans between them are brightened accordingly.Due to the successive twisting of the dome, the fan of SP1 of the strongest twist goes through the most efficient reconnection and is destroyed continuously.The reconnection there is terminated and the ribbon after it becomes weaker due to the destruction of the fan.The fan of SP2 is twisted much more to trigger a new local reconnection process.Then the ribbon between them displays a counterclockwise slipping feature.The fans of SP1 and SP2 are significantly brightened in sequence, which shows a counterclockwise rotation.Thus, the twisting of the dome may trigger the null-point magnetic reconnection, which can also dominate the slipping direction of the ribbon.The continuous rotation of the central positive pole could be one possible cause to produce the successive counterclockwise twisting of the dome.Because the duration of this flare is very short, we cannot confirm this assumption from the magnetogram observations.However, the twist of the dome can clearly be shown in both observations and the magnetohydrostatic extrapolation results, and the fan with a higher twist produces a more efficient reconnection process.These features could provide new clues for understanding the three-dimensional slipping magnetic reconnection process in the solar atmosphere.
(a)).Then the outer spine started to grow up and presented as a kinked structure at the top at 05:33:58 UT, and the RF region was also brightened synchronously (Figure 2(e)).In the decay phase at 05:38:49 UT, the spine collapsed and had a bright, relaxed structure without any kink or twist feature in 131 Å images (Figure 2(f)).The intensive microwave sources at 17 (blue contours) and 34 (red contours) GHz were located above the CR region, which could be distinguished during the whole flaring process.During the peak, another microwave source

Figure 2 .
Figure 2. Distribution of the CR flare in the initial (left panel), peak (middle panel), and decay (right panel) phase.The EUV images at 211 (top panel) and 131 (bottom panel) Å are plotted with the microwave sources at 17 (blue contour) and 34 (red contour) GHz.The circular-ribbon region and remote foot are denoted as CR and RF, respectively.The whole outer spine structure connects CR and RF, denoted by yellow solid arrows in 131 Å.The top of the outer spine is denoted by a yellow solid arrow in (a)-(b) at 211 Å.

Figure 3 .
Figure 3. Evolution of the CR region in the 94, 335, and 1600 Å bands from AIA and the Hα band from SMAT.Left panel: initial phase of the flare.Two fan structures (Fan1 and Fan2), two slipping points (SP1 and SP2), and the central kernel are denoted by yellow arrows.Middle panel: peak phase of the flare.SP1 moves along the ribbon in a counterclockwise direction.The primary flaring ribbon (denoted by PR with yellow arrows) appears in the north region of CR.The green and orange contours in (e) are the HXR sources at 12-25 and 25-50 keV, respectively.Right panel: decay phase of the flare.The PR and the foot of the kernel are shown in the 1600 Å band, and the post-flare loops are displayed in the 94, 335 Å, and Hα images.The jet is denoted by a yellow arrow in the 335 Å band.

Figure 4 .
Figure 4. (a) and (b) HiRAS spectra at 0.8-1.8GHz in left and right polarization from 05:33 UT to 05:37 UT.A group of drifting pulsations (denoted by DPS) is recorded in the left and right polarization spectrum.(c) Light curves of microwave emission at 1, 2, 3.75, 9.4, and 17 GHz of NoRH data.Five peaks labeled by Nos.1-5 are noted by dashed lines with different colors.(d) The spectrum of the microwave emission during the selected five peaks with corresponding colors.The background emission before 05:33 UT is subtracted from the total flux.

Figure 5 .
Figure 5. Left panel: oscillating amplitudes after subtracting the smoothed gradual background from the total intensity at 9.4 and 17 GHz of NoRP data.Right panel: wavelet analysis results of the oscillating amplitudes at 9.4 and 17 GHz of the left panel.The microwave emission at 9.4 and 17 GHz shows the damped oscillations at a period of about 15 s.

Figure 6 .
Figure 6.(a) Slice A-B-C-D along CR in a counterclockwise direction at the 335 Å band.The slice A-B-C denotes the slipping region of SP1.The green slice from C to D denotes the slipping region of SP2.(b) Slice E-F along the remote ribbon at the 335 Å band.(c) Time-distance plot of slice A-B-C-D of CR.SP1 and SP2 are denoted by red arrows.The light curves are the emission of the microwave CR source at 17 GHz (white solid line), the sum of the EUV emission along the whole ribbon (yellow dotted line), and the EUV emission of the whole CR region, including ribbon, fans, and kernel (yellow solid line).(d) Time-distance plot of slice E-F along the remote ribbon.The white solid line is the microwave emission of the RF source at 17 GHz.The yellow dotted line is the flux of EUV emission at the 335 Å band of the whole remote ribbon region.The light curves of microwave and EUV emission are in arbitrary units.

Figure 7
Figure 7(a) presents the Helioseismic and Magnetic Imager (HMI) magnetogram and the contours of microwave sources at 17 GHz of this flare.The zoomed-in box covers the CR region (Figure 7(b)).It can be seen that the central region of

Figure 7 .
Figure 7. (a) Magnetogram of the CR flare (gray image) at 05:33:45 UT from HMI/SDO and contours of the microwave I-component (blue) and V-component (yellow) signals of 17 GHz at 05:33:50 UT.The dotted and solid yellow lines denote the negative and positive values of the V component, respectively.The thin blue contour in the top right corner is the half-power beam width.(b) Zoomed-in box of the CR region.The white contour is the EUV emission of the 335 Å band at 05:31:28 UT.The locations of Fan1 and Fan2 are denoted by yellow arrows.The HXR sources at 12-25 (green) and 25-50 (orange) keV from 05:33:25 to 05:34:55 UT are overplotted.The contours are at the level of 97% of the maximum of each image to display the center of the sources.

Figure 8 .
Figure 8. Magnetohydrostatic extrapolation results of this CR flare using the HMI vector magnetogram at 05:24 UT.Two levels of structures at different heights are outlined.The underlying dome is twisted around the kernel in a counterclockwise direction.The overlying half-round structure is noted to link the twisted outer spine.

Figure 9 .
Figure 9. Distribution map of the line-of-sight integration of electric current density along the z-axis at different heights deduced from the extrapolated magnetic field results.(a) The integrated electric current density from 0 to 500 km displays the circular distribution and central bright structure.(b) The integrated electric current density from 620 to 1560 km shows the bright fan structure of Fan1.(c) The integrated electric current density from 1680 to 2280 km has the structure of Fan1 and the bright central kernel.(d) The integrated electric current density from 2400 to 3480 km presents the central kernel and the surrounding weak twisted structure.