High-resolution Observations of Small-scale Flux Emergence by GST
aa r X i v : . [ a s t r o - ph . S R ] S e p Draft version September 16, 2020
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High-resolution Observations of Small-scale Flux Emergence by GST
Jiasheng Wang,
1, 2, 3
Chang Liu,
1, 2, 3
Wenda Cao,
1, 2, 3 and Haimin Wang
1, 2, 3 Institute for Space Weather Sciences, New Jersey Institute of Technology, University Heights,Newark, NJ 07102-1982, USA; [email protected] Big Bear Solar Observatory, New Jersey Institute of Technology, 40386 North Shore Lane, BigBear City, CA 92314-9672, USA Center for Solar-Terrestrial Research, New Jersey Institute of Technology, University Heights,Newark, NJ 07102-1982, USA
ABSTRACTRecent observations demonstrated that emerging flux regions, which constitute theearly stage of solar active regions, consist of emergence of numerous small-scale mag-netic elements. They in turn interact, merge, and form mature sunspots. However,observations of fine magnetic structures on photosphere with sub-arcsecond resolutionare very rare due to limitations of observing facilities. In this work, taking advantageof the high resolution of the 1.6 m Goode Solar Telescope, we jointly analyze vectormagnetic fields, continuum images, and H α observations of NOAA AR 12665 on 2017July 13, with the goal of understanding the signatures of small-scale flux emergence,as well as their atmospheric responses as they emerge through multiple heights inphotosphere and chromosphere. Under such a high resolution of 0 . ′′ . ′′
2, our re-sults confirm two kinds of small-scale flux emergence: magnetic flux sheet emergenceassociated with the newly forming granules, and the traditional magnetic flux loopemergence. With direct imaging in the broadband TiO, we observe that both types offlux emergence are associated with darkening of granular boundaries, while only fluxsheets elongate granules along the direction of emerging magnetic fields and expandlaterally. With a life span of 10 ∼
15 minutes, the total emerged vertical flux is inorder of Mx for both types of emergence. The magnitudes of the vertical andhorizontal fields are comparable in the flux sheets, while the former is stronger influx loops. H α observations reveal transient brightenings in the wings in the events ofmagnetic loop emergence, which are most probably the signatures of Ellerman bombs. Keywords:
Sun: activity – Sun: magnetic fields – Sun: photosphere INTRODUCTIONFlux emergence, through which magnetic fields are transported to the solar atmo-sphere from sub-surface, is considered to be generated by convective motions andaided by magnetic buoyancy in solar interior (Schmidt 1968, 1974). Flux emergencein different scales is important for energy release in different forms, inlcuding small-
Wang et al. scale brightenings and large-scale solar eruptions. The typical scenario of emergenceis that magnetic fields are twisted underneath the photosphere due to flows and riseto form an Ω -loop due to magnetic buoyancy (Parker 1977; Fan 2001). Observationsof emissions in the solar atmosphere such as in UV/EUV provide evidence that energymay be released due to reconnection during the process of emergence. The magneticfields emerging through convection zone are not constrained to rise in an aligned ori-entation with the pre-existing field, so the magnetic reconnection is expected to occurbetween the emerging fluxes and pre-existing fluxes. Overall, on the large scale inthe solar photosphere, the orientation of emerging fields is roughly aligned with thedirection connecting paired polarity spots (Otsuji et al. 2011; Centeno 2012).Taking advantage of high-resolution ( ∼ . ′′
3) observations, De Pontieu (2002) foundthat magnetic concentrations emerge within the granule interior and quickly ( ∼ >
50 G tend toin turn induce the photospheric motions by governing the plasma flows. The authorsalso found that such emergent-flux-related flow fields change velocity distributions aswell as granule elongation.Besides the dynamic magnetic characteristics observed on the photosphere, varia-tions of brightness from continuum images provide clear indications of magnetic fluxemergence. Yurchyshyn et al. (2012) found that small-scale flux emergences have as-sociated bright points on the photosphere, mostly inside solar granulation, in whichthe field emerges at a size scale less than 1–2 Mm (e.g Lites et al. 1996; De Pontieu2002). They suggested that the emergence of relatively strong fields create brightpoints at the footpoints of magnetic loops, which intrude into intergranular lanes.Ellerman bombs (EBs) (Ellerman 1917), the bright signatures essentially observed inH α wings, are found at locations where magnetic elements with opposite polaritiesare close to each other. They are likely linked with the dips of the serpentine mag-netic field through the surface (Pariat et al. 2004; Bello González et al. 2013). Theprevious studies of EBs conclude that such photospheric heating processes are causedby photospheric reconnection of strong opposite-polarity field and are not directlyassociated with chromosphere and transition region dynamics (e.g Watanabe et al.2011; Vissers et al. 2013; Vissers et al. 2015).Since the first observational report of granulation scale emergence events(De Pontieu 2002), high-resolution polarimetric observations focus more frequently FR Observed by BBSO/GST ∼ . ′′
32) observation of small-scale flux emergence reported byCenteno et al. (2007), by using analysis of local thermodynamic equilibrium (LTE)inversion of full Stokes measurements, the author found horizontal field emergenceprior to the appearance of vertical flux elements in the typical granulation time scales(10 minutes). With the advance of observational technology, the existence of flux loopshave been witnessed (e.g., Martínez González & Bellot Rubio 2009; Tian et al. 2010).By implementing magnetohydrodynamic (MHD) simulations of magnetoconvection,Moreno-Insertis et al. (2018) detected two types of flux emergence events: magneticloop emergence and flux sheet emergence. In previous observations of the flux loopemergence with Hinode (e.g., Centeno et al. 2007; Martínez González & Bellot Rubio2009; Smitha et al. 2017), the authors summarized physical characteristics of theemergence: the horizontal field enhances within a well-established granule struc-ture followed by emerged vertical fields drifting in intergranular lanes. The verti-cal field elements are connected by horizontal magnetic patches. Recent studies byCenteno et al. (2017) and Fischer et al. (2019) have reported the flux-sheet emer-gence events, which have different signatures from flux loop emergence. Instead ofevolving within granules, the horizontal field enhances together with the expansion ofa granule. This forms an organized sheet-like mantle that spans both in the emerg-ing direction and to sides. The sheet covers the entire granule, and the emergedlongitudinal flux in footpoints is also in the order of Mx.In this paper, we study the magnetic field structure and evolution during the fluxemergence in the NOAA active region (AR) 12665 on July 13, 2017. Taking advan-tage of the exceptionally high resolution of the 1.6 m off-axis Goode Solar Telescope(GST; Goode & Cao 2012) at Big Bear Solar Observatory (BBSO), we are able toobserve fine magnetic structures on sub-arcsecond level (0 . ′′ . ′′
2) and study themagnetic properties in both flux emergence scenarios as described above. We alsoinvestigate photospheric and chromospheric brightness variation, especially Ellermanbombs, associated with the small-scale flux emergence. The structure of this paperis as follows: We introduce our observations and data processing methods in Sec-tion 2; In Section 3, we present analyses of observational results; Key findings aresummarized and discussions are presented in Section 4. OBSERVATIONS AND DATA PROCESSINGAs the Sun enters the activity minimum, observations of ARs are less often obtained.On the other hand, with the routine operation of GST at BBSO, the quiet Sun andless complicated ARs are more feasible targets. Aided by the high-order adaptiveoptics system with 308 sub-apertures (Shumko et al. 2014) and completion of thesecond generation of spectro-polarimetric instrument – the Near Infra-Red ImagingSpectro-polarimeter (NIRIS) (Cao et al. 2012), BBSO/GST obtained observationsnear the main magnetic polarity inversion line (PIL) of NOAA AR 12665 (31 ◦ W, 6 ◦ S) Wang et al. during ∼ . ′′ . ′′
2. Thedata includes spectro-polarimetric observations of full sets of Stokes measurement atthe Fe I 1564.8 nm line (0.25 Å bandpass) by NIRIS with a round field of view (FOV)of 80 ′′ at 0 . ′′
24 resolution and 56 s cadence, Fabry-Pérot spectroscopic observationsaround H α line center at ± ± ± ′′ circular FOV at 0 . ′′ ′′ circular FOV at 0 . ′′ α observations achieved a diffraction-limited resolution in the order of 0 . ′′ . ′′
24 without speckle reconstruction.Alignment among H α images, TiO images, and magnetograms are processed bymatching the most stable sunspot and plage features in the FOV. After data noise de-duction, the essential vector magnetograms from NIRIS are obtained through Stokesinversion based on Milne-Eddington approximation (see Methods in Wang et al. 2017)and aligned by using interpolation to achieve sub-pixel precision. Vector magne-tograms in the local coordinates were deduced after removing the 180 ◦ azimuthalambiguity with the AUTO-AMBIG code by Leka et al. (2009a,b), which is an opti-mized dis-ambiguation method originally intended for Hinode vector data. It usesthe minimum energy algorithm (Metcalf 1994) to find a minimum of field divergence( ∇· B) and current density (J) in the FOV. To assist in tracking magnetic elementsand quantification of magnetic flux, we applied the Southwest Automatic MagneticIdentification Suite (SWAMIS) (DeForest et al. 2007), which is a demonstrated tech-nique for magnetic identification and tracking. Here we set the threshold of thevertical magnetic field to 100 G. Based on visual inspection, this threshold allows usto include as many detected magnetic elements as possible while maintaining a highS/N ratio. RESULTS AND ANALYSISGST observation was centered at the flare productive NOAA AR 12665 at (432 ′′ ,-164 ′′ ). The AR is classified as the βγ magnetic configuration. Figure 1 and theonline animations show an overview of the AR in magnetograms, TiO images, andH α images at +1.0 and − FR Observed by BBSO/GST α image clearly exhibits brightenings at the footpoints of theemerging fibrils associated with the new flux emergence and growing pores. Thegreen circles outline the locations of small-scale flux emergences labeled 1 to 9. Thediameters of circles correspond to the size of the associated granules in TiO images.The white dashed boxes F1 and F2 indicate the regions of events that we will discussin Sections 3.1 and 3.2. The vertical component of magnetic fields is shown in Figure1(a), which saturates at ±
500 G. From the online TiO movie, one can see that themagnetic flux is transported to the photosphere through individual episodes in thescale of granules during flux emergence. Subsequently, the Sun’s pore areas are ex-panded as the same polarity fluxes are merged to them. From H α off-band images,flows in dark fibrils are observed streaming toward or away from the concentratedmagnetic footpoints.During the observation time window, we identified eight good events (see Table 1)of small-scale flux emergence that have high-quality data in all wavelengths obtained.The magnetic topology of event 5 can not be clearly interpreted because the magne-tograms lack the accuracy of azimuthal disambiguation in this event area. For a simi-lar reason, we exclude some emergence events seen in continuum images. Each of themhas an emerged total unsigned flux in the order of Mx and shows prominent mag-netic structure changes on the photosphere. The observed lifetime of these emergenceevents is ∼
10 minutes, which is on the same scale as the lifetime of granulation. Thusthe observed flux emergence events are considered as granular-sized magnetic fluxemergence. Different magnetic characteristics are observed in these small-scale fluxemergence events with high-resolution data. In the case studies of observed emergentevents, we are able to distinguish two different types of flux emergence processes, i.e.,flux sheet emergence and flux loop emergence (e.g Martínez González & Bellot Rubio2009; Centeno et al. 2017; Fischer et al. 2019). In the case studies of the observedemergent events, the two types of flux emergence events are categorized based ongeometric properties of the field evolution and correspondent structure changes.3.1.
Detailed Study of a Flux Sheet Emergence
Since the observed emergence events are visible in granule-sized scale and often ad-jacent to actively evolving granules, the clear event episodes are selected manuallyafter implementing the SWAMIS feature tracking method. In the five identified eventsof flux sheet emergence among all eight selected events, an enhanced horizontal fieldis seen to emerge within small granules as well as in the intergranular dark lane thatlater forms a newly emerged granule cell. The emerging horizontal field expands itsboundaries in the directions both along and across the field lines while the field lineswithin granule cells are aligned between concentrated footpoints of opposite polari-ties. We also found that on average the horizontal magnetic field strength (265 G) is
Wang et al. comparable with the vertical field (272 G) in the emergent area as both are enhancedduring sheet emergence. Despite small variations in individual cases, the emergingflux expands its front at a speed of 1.5 km s − ( ± km s − ). In event 1 we observedthe highest speed of emerged footpoints at 2.1 km s − , and in event 8 we observed thelowest speed at 0.8 km s − . TiO images show that the photospheric granular struc-tures associated with emerged footpoints’ separations undergo expansion during theflux emergence process, then follow the typical life cycle of photospheric granulation.By reviewing the time-lapse movies of event 1 in multi-wavelengths, we identifiedcontinuous flux emergence and evolving granulation structure, which belong to theflux-sheet emergence type. The event 1 lasts ∼
50 minutes, during which the TiOimages and horizontal magnetic field maps clearly show two stages of the emergenceprocess. Figure 2 shows the temporal evolution of magnetic and continuum structuresof this event. Figures 2(a1–a8) present snapshots of image sequence from 21:46 UTto 22:06 UT of vertical field superimposed with horizontal field vectors, whose direc-tions are represented by colors and magnitude is represented by arrow length. Thecutoff value of the horizontal field vectors is 100 G. Figures 2(b1–b8) show TiO im-ages overlaid with the same horizontal field vectors as in Figures 2(a1–a8). FromFigures 2(b3–b4), we clearly observe that the disoriented field vectors overlap en-tirely an expanding granule. Figures 2(c1–c8) present TiO images superimposed withvertical magnetic elements, with the green (red) contours representing negative (pos-itive) magnetic field at a magnitude of 150 G. The concentrated magnetic elementsare seen to be located at the intergranular boundaries as new fluxes emerge to thephotosphere (Jin et al. 2008). In the region where flux emergence occurs (blue circlein Figures 2(a3) and (b3)), concentrated magnetic elements divert along the inter-granular lanes near the western edge of the region and eventually merge with pores ofthe same polarities (as shown throughout Figures 2(a1–a8)). For a very short periodof ∼
10 minutes (as seen in first four columns in Figure 2), a granule cell appearsnear the edge (centered at [X,Y] ∼ [5 ′′ ,5 ′′ ]) of a pre-existing granule and grows in thecircled region with the overlying horizontal field emerging in the direction nearly per-pendicular to the predominant direction of ambient fields. The translational motionof negative magnetic elements along the intergranular lane is observed at the westernside of the circled area in Figures 2(a5–a7) and (c5–c7).The background field in the studied region is approximately in the east-west di-rection. At the start of the time sequence in Figure 2, granulation is accompaniedwith the growth of a new granule cell. Along with the disoriented granule expansionoccurrence (Figure 2(b3)), the accompanying horizontal field emerges in an organizeddirection different from the pre-existing field. The newly emerged horizontal fieldextends its boundary as it enhances in 8 minutes. In Figure 3, enhanced horizontalfield patches are observed at ∼ FR Observed by BBSO/GST ∼ [4 ′′ ,5 ′′ ] in Figures 3(b3–b4)). Strong Doppler blue-shifts (red-shifts) with upflow (downflow) velocity up to 1.8 km s − are observedat the positive (negative) footpoints in the intergranular lanes (centered at ∼ [6 ′′ ,6 ′′ ]in Figure 3(b6)). Very weak blue-shifts are seen within the granular cell (centered ∼ [4 ′′ ,6 ′′ ] in Figure 3(b6)), where the average Doppler upflow velocity is ∼ km s − .This is roughly two times smaller than that of emerging flux in the previous studyof Centeno et al. (2017), and is also smaller than the average upflows (downflows) of0.64 (0.49) km s − as found by Oba et al. (2017).To further analyze the magnetic evolution associated with flux emergence, wepresent the time-distance diagrams of horizontal field and TiO image in Figures 4(a)–(d), which display the time-distance evolution of two slits across the flux sheet andalong negative footpoint trail indicated in Figure 4(f) as red and yellow curves, respec-tively. Figure 4(a) clearly shows the enhancement of horizontal field in the expandinggranule, in which the separating bright lanes represent the emerging horizontal fieldwith a magnitude over 150 G. The associated bi-directional extending granule bound-aries are presented in Figure 4(b) based on TiO observations. The observations showthat the emergence in the granulation starts at 21:46 UT, when the horizontal fieldstarts to increase from the background field and fills the granule interior. The on-going emergence lasts ∼
15 minutes before dark intergranular lanes form in place at ∼ ∼ km s − .Associated with the horizontal field emergence in the transverse direction, the frontof the growing granule as indicated by TiO dark lanes (seen in Figure 4(b)) expandsat the same speed. The time-distance diagram (shown in Figure 4(c)) along the yel-low slit indicates that the motion of the negative magnetic element resides in theintergranular lane. Its speed of motion along the slit is 2 km s − . Figure 4(d) showsthe co-spatial TiO evolution in the intergranular lane. Although granular boundariesare observed as dark lanes in TiO images, we find that the concentrated magneticelements are associated with transient TiO bright points. The negative magneticelements and the co-spatial TiO bright points drift together along the intergranularlane. The horizontal field in the flux sheet emergence event 1 increases throughoutthe 20 minutes evolution, reaching up to 450 G. The newly emerged vertical flux atthe negative footpoint is 1.3 × Mx.3.2.
Detailed Study of a Flux Loop Emergence
Wang et al.
On the other hand, in regions where events of emerging granules take place less often,we observed dumbbell-like features in magnetograms representing flux loop emergenceevents, with two ends of loops rooted in opposite magnetic polarities. The emergenceof magnetic concentrations originates in the boundaries of neighboring granules andthen the emerged elements move along the magnetic network. A relatively weak fieldconnects the two emerged footpoints. It is seen that the emerged magnetic footpointsdo not alter the overall evolution of their nearby granules. As shown in the onlinemovies, the passage of flux loop footpoint motions shifts following the nearby granuleemergence and decay, which means that the merged flux loop does not dominate thelocal magnetic field and structure evolutions. By comparing averaged field strengthwe found that the emerged vertical field is 326 G, which is ∼
120 G (60%) higher thanthe emerged horizontal field ( ∼
200 G). We observe TiO and H α brightenings moreoften in this flux loop type of emergence. In particular, all three events are seen tobe spatially associated with H α brightenings near the emerged magnetic footpoints.Event 2 (indicated by the box F2 in Figure 1) is one of the distinctive magnetic looptype of flux emergence in our observations, in which the emerging magnetic footpointstravel in the network along intergranular dark lanes and are connected by an archedmagnetic field. With the aid of H α off-band images, we also observe Ellerman bombsat the negative polarity footpoint and additional brightenings at the central locationin this event.Figure 5 shows the temporal evolution of the elementary flux emergence that formsa magnetic loop configuration using the magnetic and continuum observations. In thesnapshots of vector magnetic field maps (as shown in Figures 5(a1–a4)), horizontalfield vectors are superimposed on vertical fields and are also overplotted on TiO images(Figures 5(b1–b4)). The direction of the horizontal field is indicated by the directionof the arrow and displayed in different colors for each direction, and the positive(negative) vertical field is indicated by the white (black) background. Figures 5(c1–c4) and Figures 5(d1–d4) show H α images at +1.0 and − ∼ [4 . ′′ . ′′
5] (Fig-ure 5(a1)). The concentrated magnetic elements of opposite polarities continue tostrengthen as they separate (as shown in Figures 5(a1–a3)). It is noticeable fromvector maps that the horizontal field enhances in place with the emerged magneticelements and connects the diverging footpoints. A loop-like magnetic field structure isobserved between the footpoints FP2 and FP3 at 21:46 UT, and the width of the fieldloop reaches ∼ ′′ as observed for its horizontal field component (Figure 8(a3)). Thereis no obvious granular elongation observed to be associated with this horizontal fieldenhancement, while a deformed granule is accompanied by a transient magnetic en- FR Observed by BBSO/GST ∼ km s − . The emerging magnetic footpoints start to cancel withthe preexisting magnetic fields of opposite polarities from 21:46 UT. Such configura-tion of the emerged magnetic arc and the nearby preexisting footpoints in the north ofthe region may indicate the emergence of an undulating field in the emergence on thephotosphere. Adjoining footpoints of opposite polarities in the emergent undulatingfield can easily organize a U-shaped or Ω -shaped bald patch. According to previousstudies (e.g., Pariat et al. 2004; Toriumi et al. 2017), the photospheric locations ofbald patches of serpentine magnetic fields are very likely to be associated with EBs.In the event 2, we witnessed bald patch associated EBs between the footpoints FP1and FP2 (see Figure 5 a4 and c4), where the brightening in H α wing occurs when themagnetic concentrations of opposite polarities approach each other. The separationof emerged magnetic footpoints eventually reaches a maximum distance of 5 Mm at22:02 UT. In Figure 5(c4), H α brightenings at +1.0 Å off-band are observed at themagnetic footpoints ([2 ′′ ,7 ′′ ]) of the emerging flux at 22:02 UT, when the magneticflux cancellation occurs. At the same time, one can clearly observe a brightening inH α − ∼ [4 ′′ ,3 ′′ ] (Figure 5(d4)) between the magnetic footpoints.The time-distance diagrams in Figure 6 display bidirectional motions of the emerg-ing magnetic elements. Similar phenomena were reported by Yang et al. (2016) withTiO broadband filter images. The slit cuts along the extending magnetic loop asshown with the yellow curve in Figure 6(c). Based on the time-distance diagrams, themagnetic footpoints diverge at a speed of 0.6–1.4 km s − , which is much slower thanprevious results (3.8 km s − in Yang et al. 2016). While the vertical fields follow con-fined separating traces, slightly weaker horizontal fields develop between the extendingfront of the horizontal field as seen in Figure 6(b). This is consistent with the obser-vation from vector magnetic field maps that the magnetic footpoints are connectedby diffused horizontal fields (Centeno et al. 2007; Martínez González & Bellot Rubio2009).To understand the relationship between flux emergence and H α brightenings, weplot the temporal evolution of footpoint magnetic flux, H α intensities at − α − Wang et al. emergence, there is no visible H α response, while we observe brightenings in theloop corresponding to the second horizontal field increase starting from 21:46 UT.Comparing the light curves of the horizontal field at different locations, we find thatthe field strength increases at footpoints while decreases in the loop at ∼ α brightenings. Meanwhile, the vertical flux increasesat the negative polarity footpoint. We speculate that H α brightenings in the loopare produced by the magnetic reconnection between the newly emerged magneticloop with the overlying background field. On the other hand, the H α brightenings atfootpoints are likely to be signatures of EBs between FP1 and FP2 (see Figure 5(a3)).The LOS velocity maps of event 2 in Figure 8(b1)-(b4) show that the central loop andmagnetic footpoints of the emerged flux loop is clearly associated with bi-directionalshifts. At 21:13 UT, the velocity of blue-shift corresponding to the emerging loop is0.45 km s − . It increases to 0.98 km s − at 21:28 UT then decreases to 0.37 km s − at 21:46 UT. The separating footpoints are observed to experience red-shifts with amaximum speed of 1.3 km s − at 21:28 UT.3.3. Properties of Other Events
Starting from 21:00 UT, with best-seeing quality of the day, we observe other small-scale flux emergence cases in ∼
70 minutes, which demonstrate similar magnetic prop-erties. The derived parameters of magnetic field evolution observed in nine events aregiven in Table 1, including horizontal field, vertical field, vertical flux increments, themaximum distance of emerging bipolar magnetic elements, correspondent separationspeed, LOS Doppler velocities, and associated EB occurrence. The maximum dis-tance and correspondent average speed are measured in the emergence phase, whichstarts from the emergence of opposite polarities till both separation and flux enhance-ment cease. As listed in Table 1, five of the eight selected events in the observationcan be categorized as a flux sheet type of emergence. We find that although the timeinterval between horizontal field emergence and the corresponding expanding gran-ule boundaries is within 10 minutes, which is at the same time scale as summarizedin previous studies (Centeno et al. 2017; Moreno-Insertis et al. 2018), magnetic ele-ments in the granule boundaries continue to enhance as horizontal field increases andthen either merge with adjacent magnetic fields or cancel with elements of oppositepolarities. The flux sheet emergence events 1 and 8 are observed to originate fromintergranular dark lanes and form new expanding granular cells in the emergencelocations. While the other three emergent flux sheets (events 3, 7, and 9) do notshow a direct linkage to pre-existing intergranular dark lanes, they are found to belocated near the newly formed pores. The vertical flux brought into the solar surfacethrough emergence, which is associated with the expanding granules, is in the rangeof 0.9–11.6 × Mx. As the edge of the emerging magnetic field that envelopes thegranule expands at a speed of 1.5 km s − , the granule cells undergoing emergenceare averaged 4 . ′′ .3, which grow by 0.7–1.5 ′′ . Although we observed a close connec- FR Observed by BBSO/GST α brightenings are rarely observed to be associated with flux sheet emergence. H α bright bursts captured in the event 9 region are closely associated with magnetic fluxcancellation starting from 20:16 UT. During its emergence, TiO brightening at thegranular boundary is observed at 21:36 UT.Summarizing the flux loop cases, we find that the vertical flux enhancement in thistype of events is 3.0 ± × Mx.while the separation speed of the emerging loopfootpoints is 1.2 km s − , which is similar to the expanding speed of horizontal field influx sheet emergence, the maximum distance of opposite polarities reaches 5.5 ± ′′ .The difference of maximum separation is consistent with flux sheet and loop topologyas magnetic footpoints of emerging flux loops are expected to extend further in thegranular network. Despite that H α brightenings are observed in event 2 at end ofthe flux emergence, the most prominent H α response occurred 38 minutes later. Inthe other two flux loop emergence events (event 4 and 6), we also observed H α brightenings close to the emerged footpoints of these two events, while time intervalsbetween emergence and H α brightenings do not show a similarity. In event 6 H α brightenings are observed three minutes after loop emergence. Among the studiedevents, five events are spatially associated with H α brightenings, including all threeflux loop emergence and two flux sheet emergence. From Doppler velocity maps ofthe flux emergence events, we find that the active region generally shows an upflowof 0.8 km s − in the background. Three of the listed emergence events (events 3-5)have blue-shifts over 2 km s − , which is comparable to previous observational resultsof photospheric Doppler velocity (Ortiz et al. 2014). Event 5 is excluded from thecategorization of magnetic topology because azimuthal ambiguity is not well resolvedat the event location and Doppler red-shift is observed between opposite polarities.It is interpreted as a U-shaped field. SUMMARY AND DISCUSSIONSIn this paper, we have presented a detailed study of small-scale flux emergence nearthe central PIL of NOAA AR 12665 on 2017 July 13. The study is particularly focusedon magnetic characteristics of two different kinds of flux emergence derived using thenear-infrared polarimetric data obtained by NIRIS at BBSO/GST. In addition, westudied photospheric evolution and chromospheric responses to the flux emergenceusing TiO and H α time-sequence images. Our main results are summarized below.1. In event 1, a typical sheet emergence case, an organized sheet-like structureof enhancing horizontal magnetic flux is seen to span over an entire granule,which expands at a speed of 1.6 km s − . The magnitude of the horizontal fieldin the flux sheet increases for ∼
20 minutes, reaching up to 450 G. The emergedflux at footpoints reaches ∼ × Mx. In a subsequent second stage, thenegative polarity footpoints and the co-spatial TiO bright points move alongthe intergranular lanes at a speed of ∼ km s − .2 Wang et al.
2. In event 2, a typical loop emergence case, magnetic footpoints at the two ends(the concentrated opposite-polarity flux component) emerge and move in theintergranular lanes with a separation speed of 1.2–1.7 km s − ; meanwhile, ahorizontal field lying in-between enhances, forming elongated, loop-like struc-tures (the central diffused component). The positive vertical flux increases by ∼ × Mx. Later at ∼ ∼
450 G at maximum. While in the flux sheet emer-gence vertical field is comparable with the horizontal field( ∼
270 G), in the loopemergence vertical field is 120 G stronger than the horizontal field. In the fiveflux sheet emergence events, the horizontal field enhances and hovers the emer-gent granule cells as the granules grow. The concentration of field strength inthe granule boundaries at the late phase of the emergence is observed in bothhorizontal and vertical magnetograms. Three out of the eight emergence eventsare observed to have a magnetic loop topology, in which the emergence of mag-netic elements happens in intergranular lanes. The loop-like emergence carries ∼ Mx of flux to the surface.The results of the two types of flux emergence, with one experiencing an enhancedhorizontal field hovering over the granule and the other following the typical Ω –loopconfiguration, have advanced our understandings of small-scale flux emergence andformation of active regions. It is worth noting that observations of flux-sheet emer-gence in both active regions (Centeno et al. 2017) and quiet Sun (Fischer et al. 2019)are rare. The numerical study by Moreno-Insertis et al. (2018) suggested that the oc-currence rate of loop-like emergence (1–3 day − Mm − ) is ∼ − Mm − ) in the quiet Sun. In our study, we foundmore frequent occurrence of flux-sheet emergence events (1.8 ± − Mm − ) thanof loop-like emergence (1.1 ± − Mm − ). We suspect that in the active regionsub-surface magnetic tubes rising up to solar surface can break their original bipolarstructure and emerge sideways due to the active and dynamic transverse motions.Frequent granulation observed in the active region provides higher opportunity thanin quiet Sun to have magnetic tubes emerge with growing granules, which eventu-ally form an emerging flux sheet. In comparison with a previous study, Fischer et al.(2019) observed that the transverse flux density reaches up to 194 Mx cm − , cor-responding to a maximum horizontal field of ∼
300 G. Our results show that thehorizontal field reaches up to 450 G while the total flux is comparable to previousstudies. Based on our results, five out of the eight observed flux emergence episodesin the FOV follow the flux-sheet type of emergence, and the rest follows the looptype emergence. Further, the flux sheets often appear in the emergence sites thatare closely associated with newly evolving granulations. Such a preference leads us
FR Observed by BBSO/GST α brightenings in our observations are found to have a closeconnection with magnetic loop emergence, in which the migrating footpoints collideand cancel with elements of opposite polarity in the intergranular lanes.The magnetic-loop emergences observed by us may evolve in the form of an undu-lating serpentine field. The three confirmed loop type emergences are observed in themagnetic intranetwork. As magnetic footpoints diverge along the intergranular lanes,the emergent horizontal field is observed to enhance the field strength of network inmagnetograms with correspondent dark lanes seen in TiO images. Despite differentemergence topology, the total emerged magnetic flux in the loop emergence events iscomparable with that in the flux-sheet emergence events, and is an order of magnitudehigher than previous studies of granule-sized magnetic loops (Gömöry et al. 2010).As presented in the sample event 2, the magnetic footpoints of opposite polaritiesoriginate within neighboring granules and move apart along the intergranular lanes.Thus as they approach the adjacent footpoints of the emerged field, a U-shaped fieldline can be formed across the surface. Such magnetic field configuration is one typeof bald patches that are found to have a strong connection with EBs (Pariat et al.2004; Jiang et al. 2017). Vissers et al. (2015) found that similar to EBs, flaring archfilaments could also exist in the emerging active region but are often observed asbrightenings at H α core. This phenomenon is believed to be related to the reconnec-tion of curved fields. In comparison, our results in Section 3.2 reveal H α brighteningsat the central loop location (in − α bright-enings are more favorable to the footpoints of the emerging magnetic loops. Also,despite their different locations in the observed AR, both types of emergence bring1–6 × Mx of flux to the solar atmosphere.We thank the BBSO team for providing the data. BBSO operation is supportedby NJIT and US NSF AGS-1821294 grant. GST operation is partly supported bythe Korea Astronomy and Space Science Institute, the Seoul National University,4
Wang et al. and the Key Laboratory of Solar Activities of Chinese Academy of Sciences (CAS)and the Operation, Maintenance and Upgrading Fund of CAS for Astronomical Tele-scopes and Facility Instruments. This work is supported by NSF under grants AGS-1821294, AGS-1954737, and AGS-1927578, by NASA under grants 80NSSC19K0257and 80NSSC20K0025, and by National Science Foundation of China (NSFC) undergrant 11729301. We also thank the referee for the constructive comments and sug-gestions that greatly help improve this paper.
Facilities:
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12 010203040123 45 67 89010203040 ( a r csec ) Figure 1. Overview of the emergence observations.
Multi-wavelength observationsfrom GST at 21:46 UT is displayed in the figure. Panel (a) shows vertical magnetic field map,whose magnitude is represented in gray scale with black (white) meaning negative (positive)polarity. Grayscale of the vertical field map saturates at ±
500 G. Panel (b) and (d) showH α images at +1.0 and − ±
150 G, in which green (red) indicate negative (positive) values.Ananimation of the GST multi-wavelength observations is available in the online Journal. Theanimated Figure, which includes the same panels (a)-(d) shown here, runs from 20:27 to22:35 UT. The animated images are not annotated except for the F1 and F2 FOV.
FR Observed by BBSO/GST ( a r csec ) Figure 2. Temporal evolution of emergence event 1.
The figure shows snapshots ofemergence event 1 from 21:46 UT to 22:06 UT. Panels (a) show vertical field superimposedwith horizontal field vectors, whose directions are represented by vector colors and magnitudeis represented by length. Panels (b) show horizontal field on top of TiO images. Panels (c)show TiO images overlied with vertical magnetic elements, the red (green) contours representpositive (negative) magnetic elements at level of 150 G. The blue circle in Figures 2(a3) and(b3) indicates the location of emergent flux sheet with correspondent expanding granule inthe background. An animation of emergence event 1 is available in the online Journal. Theanimated Figure, which includes the same panels (a)-(c) shown here, runs from 21:32 to22:16 UT. ( a r csec ) Figure 3. Horizontal field and dopplermaps in event 1.
Panels (a) show horizon-tal field map superimposed with vertical field contours at level of 150 G. Panels (b) showupflows (downflows) of Dopplergrams in blue (red) color. The line-of-sight component thecorrespondent velocity is in range of ± km s − . Panels (c) present TiO images superim-posed with horizontal field contours at levels of 200 G and 400 G, indicated by dark andlight blue, respectively. The green (red) contours in (a) and (b) represent magnetic elementsof negative (positive) polarity at level of 150 G. Blue circle in (b3) and (c3) indicate thelocation of expanding granule. intergranular lane is outlined with ellipse in (c4) and (c6).Blue (red) arrows in (b4) and (b6) indicate strong Doppler blue-shift (red-shift) at foot-points. An animation of the horizontal field and dopplermaps in event 1 is available in theonline Journal. The animated Figure, which includes the same panels (a)-(c) shown here,runs from 21:32 to 22:16 UT. Wang et al. ( a r csec ) ( a r csec ) ( a r csec ) ( a r csec ) ( a r csec ) Figure 4. Time-space diagram of event 1.
Panels (a) and (b) show time-space diagramsof horizontal field and TiO along the red slit as shown in (f), which correspond to flux sheetemergence stage. Green lines in (a) and (b) trace the expanding granule. Panels (c) and(d) show time-space diagrams of vertical field and TiO along the yellow slit as shown in (f),which represent negative footpoint motions in the intergranular lane. Red lines in (c) and(d) trace and are used to estimate speed of motion of the magnetic element. Green (red)contours in (e) and (f) outline the concentrated negative (positive) magnetic elements.
FR Observed by BBSO/GST ( a r csec ) Figure 5. Temporal evolution of emergence event 2.
The figure shows snapshots ofemergence event 2 from 21:13 UT to 22:02 UT. Panels (a) show vertical field superimposedwith horizontal field vectors, whose directions are represented by vector directions and mag-nitude is represented by length. Panels (b) show TiO images overlied with horizontal fieldvectors. Panels (c) and (d) show H α images at +1.0 and − Wang et al.
Start Time (13-Jul-17 20:43:57)Start Time (13-Jul-17 20:43:57) ( a r csec ) ( a r csec ) ( a r csec ) Figure 6. Time-space diagrams of event 2.
Panels (a) and (b) show time-spacediagrams of vertical and horizontal field along the slit in the TiO image as shown in (c).Yellow lines in (a) and (b) trace and are used to estimate the speed of separation of theemerged magnetic polarities. Green (red) contours outline the magnetic elements of negative(positive) polarity.
Table 1.
Magnetic Properties of the Observed EventsEvent Horizontal Vertical Flux Maximum Separation Doppler EBs occurrenceNumber Field (G) Field (G) ( × Mx) Distance ( ′′ ) Speed ( km s − ) V ( km s − ) (Y/N)(1) (2) (3) (4) (5) (6) (7) (8)1 ⋆ ⋄ ⋆ ⋄ ⋄ ⋆ ±
40 1.29/0.98 4.3 1.8 1.47 N8 ⋆ ⋆ Note —Flux sheet emergence events are labeled as ( ⋆ ) and flux loop emergence events are labeled as ( ⋄ ) after eventnumbers. Maximum/average field strengths of each event are presented in columns (2) and (3). Positive/negativevertical flux increments of through the emergence are presented in column (4). Maximum distances and speedof oppsite polarity separation in the emergence phase are presented in column (5) and (6), respectively. LOSDoppler upflow speeds are presented in column (7). Emergence event associated with EB observation in H α islabeled as Y and emergence without EB association is labeled as N in column (8). Event 5 is excluded fromdiscussion in Section 3.3. FR Observed by BBSO/GST M a gn e t i c f l u x d e n s i t y [ G ] N o r m a li se d i n t e n s i t y M a gn e t i c f l u x [ M x ] Figure 7. The evolution of magnetic flux, mean brightness, and magnetic fieldsin event 2.
Red and blue light curve in (a) shows averaged vertical flux evolution atfootpoints FP3 and FP2 in Figure 5, respectively, in unit of Mx. Blue (red) light curvein (b) shows normalized intensity of H α − Wang et al. ( a r csec ) Figure 8. Horizontal field and dopplergrams in event 2.
Panels (a) show horizon-tal field map superimposed with vertical field contours at level of 150 G. Panels (b) showupflows (downflows) of Dopplergrams in blue (red) color. The line-of-sight component thecorrespondent velocity is in range of ± km s −1