arXiv:2106.12833v1 [astro-ph.SR] 24 Jun 2021 MNRAS rdea h sie,telne de nprle ihthe ⋆ with parallel in edges longer the “spine”, the as bridge egyAnfinogentov Sergey oei rvdn otrgst h ih rde( bridge light the to gas 2014 hotter providing importan in very a role plays probably that suggested S is quiet the in ex- granules and the ( to dynamics similar along cells photospheric fila- convection filaments reveal hibit a thin usually as many bridges termed ular has is ( it spine it if i its or bridge, bridge, it; light light within photospheric) mentary restored (or granular is a convection as classified is © ioFu Libo ih rde a upesteFraino ooa Loops Coronal of Formation the Suppress Can Bridges Light 5Jn 2021 June 25 uso rpntaeit t hyaecmol on in found commonly ( are decaying a They or spans it. nascent that into a structure penetrate bright elongated or an sunspot is bridge light A INTRODUCTION 1 3 2 1 4 6 5 China 100049, opevndrVote l 2010 al. et Voort der van Rouppe uuMiao Yuhu olg fCetv ein hnhnTcnlg Universit Technology Shenzhen Astronomica Design, National Creative Activity, of I Solar College Harbin of Technology, Laboratory Applied Key and CAS Science Space of Institute colo srnm n pc cecs nvriyo Chine of University Sciences, Space and Astronomy of School nttt fSlrTretilPyis 603 Irkutsk, 664033, Physics, Kunmin Solar-Terrestrial Sciences, of of Institute Academy Chinese Observatories, Yunnan E-mail:[email protected] 01TeAuthors The 2021 .I h olwn et ecl h kltno light a of skeleton the call we text, following the In ). 000 ook 1997 Sobotka , 1 1 inDu Xian , – ?? 22)Pern 5Jn 01Cmie sn NA L MNRAS using Compiled 2021 June 25 Preprint (2021) 1 , 2 , ; askw ta.2007 al. et Katsukawa 3 igYuan Ding , 6 ulr1979 Muller efre.Amdlo ih rdei rpsdt xli h magnetic explain words: the could Key model explain bridges. This to light loops. proposed with coronal is associated the and bridge processes bridge light light of a model between extend polarities that A opposite loops formed. coronal these be therefore, to and connect loops, could minor-polarities. magnetic line opposite range field accompanying magnetic an the have that usually bridge light a euba h ooa opsat ofr gi.Tevco mag vector The again. d onboard form Imager become Magnetic to bridge Helioseismic light starts the a by loop the coronal If coro point. the the anchoring boundary, penumbra, the umbra-penumbra around the formed at be anchors bridge light a sdtedt fteAmshrcIaigAsml nor the onboard Assembly a Imaging rooted Atmospheric loops lett the servatory coronal of the th and data In with bridge the interacts processes. light used dynamical it a of sunspot, between variety a connectivity a into netic excites intrusion and magnetic field a netic is bridge light A ABSTRACT antcfields magnetic ; ook ta.2013 al. et Sobotka ( .Algtbridge light A ). SDO ciiy–snpt anthdoyais–mgei reconnec magnetic – magnetohydrodynamics – – activity .Tegran- The ). 1 age al. et Lagg osuytefaue fsnpt ihlgtbigs ti on t found is It bridges. light with sunspots of features the study to ) ⋆ hoe Jiang Chaowei , Russia .It ). ,Gagog581,China 518118, Guangdong y, un siueo ehooy hnhn undn 105 China 518055, Guangdong Shenzhen, Technology, of nstitute t f bevtre,Biig101,China 100012, Beijing Observatories, l eAaeyo cecs 9AYqa od hjnsa Distri Shijingshan Road, Yuquan A 19 Sciences, of Academy se 501 China 650011, g ol ekrta h urudn mr ftesunspot the of umbra surrounding ( the than weaker monly are anchors shorter two “ends”. whereas as “flanks”, named called are spine ovcinpoesa eosrtdwt aitv MHD radiative magneto- with have by by demonstrated driven could simulation as is field configuration process magnetic This convection bridge’s shape. light canopy a a that proposed ( nt n rg h antcfil onad n even and downward, field magnetic dom- the become drags could convection and restored inate bridge of light pressure a of gas field the magnetic horizontal nearly the Within ue nasnptlgtbig.A ih rdeusu- bridge light field umbral a the As than inclined bridge. mea- more ( light field even magnetic strength sunspot kG, has field 8 ally a exceeding magnetic in strength strongest sured field the magnetic was a this had interest of ea1997 Leka 2013 2020 al. et Sobotka h antcfil tegho ih rdei com- is bridge light a of strength field magnetic The tde oneeape nwihtelgtbridge light the which in counterexample, a studied ) 1 intoSu Jiangtao , ; askw ta.2007 al. et Katsukawa Rempel SDO .However, ). hw htteacoigrgo of region anchoring the that shows ( 2011 and ) 2 , 4 oln-ag ol not could long-range to iguZhao Mingyu , atlao u´ne al. Dur´an et Castellanos i-ai tal. et Siu-Tapia ), oa yaisOb- Dynamics Solar r esuidmag- studied we er, a op ol not could loops nal eormprovided netogram Jurˇc´ak al. et h uso.We sunspot. the t tce rmthe from etached A n omshort- form and T E ayphysical many econjugate We tl l v3.0 file style X anmag- main e connectivity ( t Beijing ct, ( in– tion 2018 a if hat 2006 5 ). ) , , 2 Y. H. Miao et al. results in reversed polarities at a light bridge at extreme tion of sunspots and light bridges; whereas the AIA 171 ˚A cases (Lagg et al. 2014; Felipe et al. 2016; Zhang et al. 2018; channel captures the emission at the extreme UV Guglielmino et al. 2017). Due to the strong discontinuity be- (EUV) bandpass, this channel is designed for EUV obser- tween the magnetic field of the light bridge and umbra, a vation of coronal loops with temperature at about 0.8 MK. strong layer could be detected at the edges The LOS magnetogram was used to reveal the distribution of a light bridge (Toriumi et al. 2015b). of the magnetic polarities, the Space-weather HMI Active The magnetic configuration of a light bridge and Region Patches (Bobra et al. 2014, SHARPs) product was umbra could excite many dynamic activities, such as applied to display the vector magnetic field, and then trans- jets (Asai et al. 2001; Shen et al. 2011, 2012; Miao et al. formed into standard heliographic spherical coordinates to 2018, 2019b) and MHD waves (Yang et al. 2015; Hou et al. match the AIA 171 ˚A images. The data was provided by 2016; Zhang et al. 2017; Felipe et al. 2017; Li et al. 2018; the Helioseismic and Magnetic Imager (HMI; Schou et al. Miao et al. 2019a; Yang et al. 2019; Miao et al. 2020, 2021; 2012) on board SDO. The AIA has a pixel size of about ′′ ′′ Li et al. 2021). Louis et al. (2014) found that reconnection- 0 .6, whereas the HMI’s pixel corresponds to 0 .5. The AIA excited jets usually originated from one flank of a light and HMI data were calibrated with the standard procedure bridge and propagated towards one side. In order to ex- provided by the Solar Software (SSW)1. The CCD readout plain the jet excitation and its asymmetric spatial distri- noise and dark were removed from the images, and then the bution, Yuan & Walsh (2016) proposed a three dimensional images were corrected with a flat-field and normalized with model for light bridge. The light bridge’s magnetic field be- its exposure time. comes nearly horizontal along the spine. Along the spine, the Figure 1 visualizes three sunspots with light bridges and magnetic field has a helical component. Within this mag- associated coronal loop system, the sunspot samples were netic configuration, the magnetic field lines are aligned with AR 11309, AR 11529 and AR 12738, observed on 6 October the umbral field at one flank, whereas at the other flank 2011, 29 July 2012, and 10 April 2019, respectively. The bot- the magnetic field lines of light bridge and umbra become tom row illustrates the the horizontal magnetic field vectors anti-parallel. At this flank, could (Bx, By) overlaid on the background gray-scale image of ver- be triggered repetitively. This model is supported by the tical field component (Bz). In order to study the influence high resolution ground observation of Robustini et al. (2016) of light bridge over the coronal loops, we followed the evo- and Tian et al. (2018), wherein they detected λ-shaped jets lution of sunspot AR 12738 (see Figure 2), and investigated straddling light bridges at its launching phase. two comparative scenarios with or without a light bridge. Feng et al. (2020) find a potential correlation between In order to verify if this is common feature in sunspots, we the light bridge’s anchoring position and the formation of check the SDO/AIA observations from 2010 to 2020, and coronal loops. The formation of coronal loops is a complex collected 66 such typical cases (see Table 1) that could vali- physical process associated with plasma heating and solar date our proposition. wind generation. Wang et al. (2016, 2019) found that at the footpoint of coronal loops a small and compact bipolar struc- ture is usually formed. They also indicated that some of them have an inverted Y-shape. This configuration could 3 THE CORRELATION BETWEEN LIGHT BRIDGE trigger magnetic reconnections, which may provide forming AND CORONAL LOOPS material, moment and energy flux for large-scale magnetic structures. In the mean while, Chitta et al. (2017) noted Figure 1 shows three unipolar sunspots with light bridge, that coronal loops tend to root at regions with an accompa- the associated active region loop systems, and magnetogram nying mixed minority polarity. revealing the LOS and vector magnetic components. In this letter, we aim to justify the magnetic connec- AR 11309 was an alpha sunspot with negative polarity. Its umbra was segmented by a light bridge, which was ori- tivity between light bridge and coronal loops, and propose a ◦ novel model of light bridges in a sunspot. This letter is struc- ented about 45 from the Solar-X axis in the Helioprojective- tured as follows: Section 2 presents the observation and data Cartesian coordinate, see Figure 1(a2). Figure 1(a1) shows analysis; and Section 3 present the case studies; the model that coronal loops were not formed at the North-West re- of a light bridge and sunspot is proposed with an artistic gion within the field-of-view, the magnetic field line could drawing in Section 4. connect to local minor opposite polarities. The contour of LOS magnetic field shows that minor polarities opposite to that of the sunspot clustered close to the anchoring point of the light bridge, see Figure 1(a3), these opposite polarities 2 OBSERVATIONS AND DATA ANALYSIS could be the other footpoints of small-scale magnetic field lines. Figure 1(a4) reveals that the horizontal magnetic field To justify our initial proposition, we selected sample vector was aligned with the the spine of the light bridge. sunspots with light bridges and coronal loops. We had to The magnetic vectors appear to point to a common source focus on simple sunspot with a dominant polarity, so that at the center of the bridge. the influence of strong opposite polarity was minimized, and AR 11529 was also an almost circular alpha sunspot the interaction between light bridge and coronal loops could with positive polarity. The light bridge in this sunspot was be better traced. We used the 1700 A˚ and 171 A˚ channels ◦ oriented about −20 from the Solar-X coordinate as shown of the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamic Observatory (SDO). The AIA 1700 A˚ channel records plasma emission at ultravio- let (UV) continuum and is optimized for imaging observa- 1 http://www.lmsal.com/solarsoft/ssw_install.html

MNRAS 000, 1–?? (2021) Light Bridges 3 in Figure 1(b2). The coronal loops were absent at the South- the minor opposite polarities at the super-penumbra. The West periphery of AR 11529, there was even little coronal link could be developed by the magneto-convection pro- EUV emission above the light bridge itself, see Figure 1(b1). cess, as demonstrated in Rempel (2011) and Toriumi et al. It appears that the formation of a light bridge made the (2015a,b). Therefore, the penumbra and light bridge could coronal loops bifurcated into two bundles. Again, we also have a common submerged source, and this could ex- spotted the clusters of compact opposite polarities about a plain why we could detect five-minute oscillation at these few arc seconds from an anchor of the light bridge, as shown two structures and that they exhibit very identical fea- in Figure 1(b1) and (b3). tures (Yuan et al. 2014; Yuan & Walsh 2016; Feng et al. AR 12738 is an alpha sunspot with dominant nega- 2020). This type of magnetic connectivity is very similar tive polarity. The light bridge started from the umbra and to the moving magnetic features (MMFs, Thomas et al. was oriented towards the North-West direction. Large-scale 2002), which are indicators of submerged low-lying loops. coronal loop structures were rooted at sunspot AR 12738, However, the MMFs are magnetic flux pumped downwad however, no coronal loop extended towards the North-West (Thomas et al. 2002), whereas the minor opposite polarity direction. A compact opposite polarity source was located in this study could be the rising magnetic structure driven close to the anchoring point of the light bridge. by magneto-convection from (Rempel 2011; Siu-Tapia et al. Figure 2 draws the evolution of AR 12738 over five days 2018). from 10th to 14th April 2019. We could see clearly that on In order to explain connectivity of the active region the 10 April, when the light bridge was anchored at the loops and light bridge, a three dimensional model is pre- penumbra, no coronal loop was formed along the North-West sented with an artistic drawing as shown in Figure 3. direction (see Figure 2(a1)). From 12 April and afterwards, The magnetic field is inclined more horizontally and bear the light bridge gradually became detached with the penum- twist along its spine. The twisted magnetic field of a light bra (see Figure 2(a3-c3)), then coronal loop started to form bridge was initially proposed by Yuan & Walsh (2016), it again at the previous anchoring point (see Figure 2(a2-a3)). explains the excitation of λ-shaped jets (Tian et al. 2018; We present three sample sunspots with light bridges, it Robustini et al. 2016) and the spatial asymmetry of jet ori- is clear within these samples that light bridge suppressed gin and direction of jet propagation (Louis et al. 2014). The the formation of coronal loops at its anchor. When the light magnetic field line of a light bridge anchors at another po- bridge become detached with the penumbra, coronal loops larity at the supra-penumbra, this opposite polarity drags start to form again. This phenomenon is in fact very common down the magnetic field lines that used to be corona loops. in sunspots and active region loops. We inspect ten years This model explains why coronal loops are not formed close observation of SDO/AIA and found 66 such cases, as listed to the anchor of a light bridge. This is very important in in Table 1. We shall note that this list is not exclusive, and the comprehension of corona plasma confinement and the we have to focus on simple sunspots, as complex magnetic generation of , and other fundamental question of topology would not reveal this phenomenon. coronal loop heating. We should also note that a light bridge could have more than one anchor. The the horizontal mag- netic field vector in Figure 1 reveals that a light bridge could have two branches, meaning two anchors share one polarity 4 CONCLUSIONS AND DISCUSSIONS at the umbra. If a light bridge have two anchors, it means In this letter, we aim to prove that light bridge could sup- that the magnetic flux bifurcate and connect to the oppo- press the formation of coronal loops, and propose a novel site polarities at two anchors. This could explain in some model for a light bridge in order to explain the dynam- cases at both ends of a light bridge, we could detect minor ics occurring at the small-scale magnetic structure within opposite polarities. The list is limited to simple sunspots, a sunspot, such as magneto-convection, magnetic reconnec- because the phenomenon does not show up in spots with tion, jet generation, and oscillations above light bridge, the complex magnetic topology. We also have to note that we formation and heating of coronal loops. Three typical exam- do not prove that this phenomenon always takes place in ples are presented to display such phenomenon. The LOS simple sunspots. and vector magnetic field maps reveal that close to the an- chor of a light bridge, clusters of minor polarity opposite to that of the spot are usually found. It means the mag- netic field line could start from the spot and connect to 5 ACKNOWLEDGEMENTS this local opposite polarities, rather than extend radially and form coronal loops. As short and compact magnetic Y.H.M. and D.Y. is supported by the National Natural Sci- loops have a different heating rate (Fisher & Hawley 1990; ence Foundation of China (NSFC, 11803005,12111530078, Yokoyama & Shibata 1998), the plasma could be heated to 41731067), the Shenzhen Technology Project another temperature. We have checked the AIA channels (JCYJ20180306172239618, GXWD20201230155427003- that are sensitive to EUV emission of hotter plasmas (131A,˚ 20200804151658001), the Shenzhen Science and Technology 94A,˚ and 335A),˚ but no apparent compact hot loop systems Program (Group No.KQTD20180410161218820), the China were detected. The reason could be that this loop system Postdoctoral Science Foundation (2020M681085) and the could be heated chromospheric temperatures and could not Open Research Program of the Key Laboratory of Solar be detected by AIA channels as studied by Huang (2018); Activity of Chinese Academy of Sciences (KLSA202110). Huang et al. (2020). Or, AIA hot channels are not sensitive C.W.J. is supported by the NSFC 41822404 and Shenzhen enough to reveal this small structures. Technology Project JCYJ20190806142609035. M.Y.Z. is A light bridge could be magnetically connected to supported by the NSFC 11973086.

MNRAS 000, 1–?? (2021) 4 Y. H. Miao et al.

(a1) +30 (b1) +150 (c1) +30 Table 1. Selected sunspots with light bridge that suppress the -150 -30 -150 formation of active region loop

Universal Time Active region Coordinates

2010-09-21 20:00 11108 −103′′, −570′′ ′′ ′′ 2011-05-31 02:00 11226 −535 , −320 6-Oct-2011 20" 29-Jul-2012 10-Apr-2019 ′′ ′′ 2011-07-17 16:00 11251 +55 , +195 21:29:00 UT 11:59:59 UT 19:30:09 UT ′′ ′′ 2011-10-06 21:30 11309 −250 , +285 (a2) (b2) (c2) 2012-06-12 17:30 11504 −415′′, −300′′ 2012-06-12 23:33 11507 −135′′, −455′′ 2012-07-12 12:06 11521 +280′′, −413′′ 2012-07-29 12:00 11529 +36′′, −270′′ 2012-07-31 23:10 11532 −10′′, −410′′ − ′′ ′′ 2012-08-13 07:00 11543 20 , +250 20" 2012-08-25 12:00 11554 −60′′, +140′′ 2012-09-22 02:00 11575 −520′′, +25′′ ′′ ′′ (a3) µ=0.76 (b3) µ=0.99 (c3) µ=0.97 2012-10-24 03:00 11596 −25 , +50 2013-01-29 22:30 11663 −45′′, −70′′ 2013-03-29 12:00 11704 +12′′, +360′′ 2013-09-02 16:00 11836 +95′′, +70′′ 2013-09-20 00:00 11843 +370′′, −90′′ 2013-09-20 18:59 11849 +353′′, +250′′ BLOS(G) 2013-09-21 16:59 11846 −410′′, −400′′ 20" 2013-10-08 16:00 11857 +100′′, −225′′ -200 0 200 ′′ ′′ 0