A&A 615, L9 (2018) Astronomy https://doi.org/10.1051/0004-6361/201833404 & © ESO 2018 Astrophysics

LETTER TO THE EDITOR Nature of the source powering solar coronal loops driven by nanoflares? L. P. Chitta1, H. Peter1, and S. K. Solanki1,2

1 Max Planck Institute for Research, 37077 Göttingen, Germany e-mail: [email protected] 2 School of Space Research, Kyung Hee University, Yongin, Gyeonggi 446-701, Republic of Korea

Received 10 May 2018 / Accepted 28 June 2018

ABSTRACT

Context. Magnetic energy is required to the corona, the outer of the , to millions of degrees. Aims. We study the nature of the magnetic energy source that is probably responsible for the brightening of coronal loops driven by nanoflares in the cores of solar active regions. Methods. We consider observations of two active regions (ARs), 11890 and 12234, in which nanoflares have been detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV) images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) for diagnostics. These images are combined with the co-temporal line-of-sight magnetic field maps from the Helioseismic and Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal loops and their magnetic roots in the . Results. The core of these ARs exhibit loop brightening in multiple EUV channels of AIA, particularly in its 9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex mixed polarity magnetic field distribution at the base of these loops. We detect the cancellation of photospheric magnetic flux at these locations at a rate of about 1015 Mx s−1. The associated compact coronal brightenings directly above the cancelling magnetic features are indicative of heating due to chromospheric . Conclusions. We suggest that the complex magnetic topology and the evolution of magnetic field, such as flux cancellation in the photosphere and the resulting chromospheric reconnection, can play an important role in energizing active region coronal loops driven by nanoflares. Our estimate of magnetic energy release during flux cancellation in the quiet Sun suggests that chromospheric reconnection can also power the quiet corona. Key words. Sun: atmosphere – Sun: – Sun: corona – Sun: magnetic fields – Sun: photosphere

1. Introduction netic flux, can heat the plasma to several million Kelvin (e.g. Engell et al. 2011). However, the active corona persists for sev- The extreme ultraviolet (EUV) and the thermal X-ray emission eral days or even weeks, well after the emergence of large- from the corona of the Sun is radiated by plasma at temperatures scale magnetic field has ceased (e.g. Ugarte-Urra & Warren in excess of 1 MK, which is much hotter than the Sun’s surface 2014). Therefore, current scenarios of coronal heating rely on temperature of about 6000 K. Spatially resolved solar observa- either the dissipation of magnetohydrodynamic (MHD) waves tions reveal that, at EUV and X-ray wavelengths, the corona is (e.g. Hollweg 1981; Asgari-Targhi et al. 2015; Karampelas et al. particularly bright in active regions (ARs) hosting strong mag- 2017), or Ohmic dissipation of current sheets in nanoflares, netic field concentrations including . The cores of these induced by slowly moving the footpoints of coronal loops and ARs have an emission component from hot plasma at tempera- braiding the magnetic field (e.g. Parker 1988; Priest et al. 2002; tures of about 5 MK (Testa & Reale 2012) confined by the mag- Klimchuk 2006). MHD waves are observed to be ubiquitous in netic field structured in the form of compact loops. The origin of the solar atmosphere (Tomczyk et al. 2007) and they can heat such high-temperature plasma and, more generally, the nature of the quiet corona, i.e. the corona outside ARs (McIntosh et al. the energy source responsible for coronal heating remain poorly 2011), but in general the observed wave power is two-orders-of- understood. Any heating process has to explain not only the magnitude too weak to power the AR corona (Withbroe & Noyes high temperature, but also the observed spatial structuring and 1977). Although coronal loops may develop ubiquitous mag- temporal intermittency in the corona (e.g. Webb & Zirin 1981; netic braids due to continual motion of their footpoints at the Shimizu et al. 1992; Falconer et al. 1997; Ugarte-Urra & Warren surface, direct observations of magnetic braiding, which would 2014). indicate the release of energy through unwinding of those braids, During the emergence of an AR, when new sunspots form, are sparse (Cirtain et al. 2013). Moreover, questions were raised the energy released from the rapid reconfiguration of surface on the kind of footpoint motions that would generate such highly magnetic fields, including emergence and cancellation of mag- braided structures (van Ballegooijen et al. 2014). Furthermore, ? The movie associated to Fig.1 is available at Tiwari et al.(2014) observed that the coronal brightenings at https://www.aanda.org these braids were not triggered internally, but were initiated

Article published by EDP Sciences L9, page 1 of6 A&A 615, L9 (2018) externally by photospheric flux cancellation along a polarity presented observations covering western footpoints of this loop inversion line over which the braided structure had formed. (including the region roughly covered by the magenta box in Recent studies that connect the evolution of coronal bright- Fig.1) during the same period. This loop brightening is clearly enings to the evolution of photospheric magnetic field in ARs distinguishable in the 9.4 nm filter of AIA for about 40 min. This find that some of the bright coronal loops are rooted in regions filter can detect plasma at temperatures of about 7 MK, but is of mixed magnetic polarities (i.e. regions where a dominant also sensitive to cooler plasma at temperatures of about 1 MK. magnetic polarity and minor opposite magnetic polarity patches During the phase when the main section of the loop is bright are present; Chitta et al. 2017a; Tiwari et al. 2017; Huang et al. in the 9.4 nm filter (Fig.1a), it was invisible (or indistinguish- 2018). Interactions of such mixed-polarity magnetic fields lead- able) to the other filters of AIA at 17.1 nm, 19.3 nm, or 21.1 nm, ing to the cancellation of surface magnetic flux followed by that are more sensitive to plasma at ∼1 MK (Boerner et al. 2012), disturbances in the solar atmosphere that are widely associ- which suggests that most of the brightening in the 9.4 nm image ated with magnetic reconnection (e.g. plasma jets and compact is due to hot plasma. This is demonstrated using the 17.1 nm fil- brightenings) are also observed at the footpoints of coronal loops ter image displayed in Fig.1c. (Chitta et al. 2017a,b; Huang et al. 2018). The loop brightening presented some interesting characteris- Based on the above observations, we find it necessary to tics. Once the hot core loop is fully developed (Fig.1a), it con- investigate the nature of the magnetic energy source that is likely nects the main positive (white/west) and negative (black/east) responsible for powering the coronal loops, in particular loops in magnetic polarities at both ends. The AIA 9.4 nm emission light the cores of ARs hosting hot plasma at several million Kelvin. In curves of hot plasma near these regions show a similar evolu- the present work, we consider brightenings in such loops that are tion and both feet brighten almost simultaneously. This applies associated with nanoflares. We find that these loops are appar- in particular to the initial, smaller brightenings visible in the light ently rooted in photospheric regions with a complex magnetic curves (see short vertical lines in panel b), which are clearly seen landscape containing mixed magnetic polarities. We notice that as transient bright dots in the accompanying online movie (dis- intermittent brightenings in the solar atmosphere follow surface tinguishable at the western footpoint from 11:50 UT onward). magnetic flux cancellation at these locations. Our findings hint The bulk of the loop shows similar evolution to the footpoints. at the possibility of reconnection driven by the cancellation of The light curve from the AIA 17.1 nm filter showing the evolu- surface magnetic flux as the energy source, at least in some of tion of cooler plasma (panel d) also displays high variability, but the coronal loop brightenings. only near the eastern footpoint. Similar, high variability in the emission, although only at the eastern footpoint, is also apparent 2. Observations in the AIA 19.3 nm and 21.1 nm filter images. These intensity fluctuations near the eastern footpoint originate from a bright To investigate the origin of coronal loop brightenings in compact feature in the corona (see panel c and the movie). AR cores, we consider observations of AR 11890 and AR When these coronal emission maps are examined in con- 12234 obtained with the Solar Dynamics Observatory (SDO; junction with the photospheric magnetic field maps, we find Pesnell et al. 2012). We use time series of images recorded in the that the bright compact coronal feature near the eastern foot- ultraviolet (UV) at a cadence of 24 s and the EUV at a cadence of point lies directly above the region where the surface magnetic 12 s by the Atmospheric Imaging Assembly (AIA; Lemen et al. field is mixed with the presence of opposite magnetic polari- 2012) onboard SDO to detect brightenings through the solar ties (see arrow in panel e; magnetic field contours in panels a atmosphere. To the magnetic roots of these coronal loops and c). Moreover, the surface magnetic field in the region of at the solar surface, we analyse co-temporal, photospheric line- interest (black box in panel e) is evolving, in that we observe of-sight magnetic field maps of these ARs, recorded every 45 s a monotonic decrease of the magnetic flux in the area at a by the Helioseismic and Magnetic Imager (HMI; Scherrer et al. rate of about 1015 Mx s−1 (panel f). We note that this decrease 2012) onboard SDO. is due to the local disappearance of the magnetic flux and is As described at the end of Sect.1, these ARs are chosen sim- not due to the advection of the flux out of the rectangular box ply because the loop brightenings in their cores are attributed to considered. nanoflares. For instance, Testa et al.(2014) presented imaging To examine the evolution of the lower atmosphere during the and spectroscopic observations of AR 11890, where they detect loop brightening, we adopt the ratio of AIA UV filters at 160 nm footpoint brightenings at the base of a coronal loop that is visible and 170 nm. The 160 nm filter samples photospheric continuum in the AIA 9.4 nm filter. These footpoint brightenings are asso- and also has a contribution from the C iv doublet forming at ciated with modest upflows with velocities of 15 km s−1 in the a characteristic transition region temperature of ∼0.1 MK. The transition-region Si iv observations obtained from the Interface 170 nm filter records photospheric continuum only (Lemen et al. Region Imaging Spectrograph. Based on numerical modelling of 2012). The intensity ratio of 160 nm and 170 nm filters can pro- impulsively heated loops, the authors argued that the footpoint vide information on the C iv emission, if present, by normaliz- brightening and the upflows in the transition region are due to ing out continuum from both the filters. Therefore, we expect the interaction of the lower atmosphere with non-thermal elec- that the ratio of these light curves will be (i) close to constant if trons accelerated in coronal nanoflares. Similarly, Ishikawa et al. the C iv doublet contribution to the 160 nm filter is steady, and (2017) presented observations from the Focusing Optics X-ray (ii) smaller compared to regions with higher densities of C iv Solar Imager, with a detection of faint emission in hard X-rays (meaning less plasma at 0.1 MK assuming equilibrium condi- (above 3 keV) in AR 12234 that they ascribed to plasma heated tions). In Fig.2 we plot such ratios from the eastern and west- above 10 MK in nanoflares. ern loop footpoints (black and magenta), and from a quiet-Sun region for reference (green). Unlike the almost time-independent 3. Brightening in AR 11890 (steady) and smaller (less plasma at 0.1 MK) filter ratio from the quiet-Sun region, the ratios from the footpoint regions are akin to Here we describe the case of a coronal loop brightening in AR their coronal counterparts (particularly the 17.1 nm light curves). 11890 observed on 9 November 2013 (Fig.1); Testa et al.(2014) The ratio from the eastern footpoint is similar to the intensity

L9, page 2 of6 L. P. Chitta et al.: Energy source powering solar coronal loops driven by nanoflares L. P. Chitta et al.: Energy source powering solar coronal loops driven by nanoflares

Fig.Fig. 1. 1.HotHot core core loop loop in in AR AR 11890 11890 and and its its association association with with the the cancellation cancellation of of surface surface magnetic magnetic flux. flux. (a)Panel AIA a 9.4: AIA nm 9.4 filter nm image filter image showing showing a fully a formedfully formed coronal coronal loop seen loop from seen almost from almost directly directly above. above. The black The (east)black and(east) magenta and magenta (west) (west) boxes boxes mark the mark regions the regions near the near footpoints. the footpoints. The field The offield view of is view 18000 is× 1809000.× (b)90 AIA00. Panel 9.4 nm b: AIA light 9.4 curves nm light from curves the footpoint from the regions. footpoint The regions. short vertical The short bars pointvertical to barsthe near-simultaneous point to the near-simultaneous peaks in the twopeaks light in curves. the two (c) light and curves. (d) as inPanels (a) and c and (b) butd: as with in panels the AIA a 17.1and b nmbut filter. with (e) the HMI AIA line-of-sight 17.1 nm filter. magneticPanel fielde: HMI map, line-of-sight in which darker magnetic shading field representsmap, in which negative darker polarity shading and represents lighter shading negative positive polarity polarity and field.lighter The shading blackpositive box encloses polarity magnetic field. The flux black near the box eastern encloses footpoint magnetic of the flux loop; near itthe lies eastern at the same footpoint position of the as loop;the black it lies boxes at the in samepanels position (a) and as(c). the The black blue boxes (positive in panels polarity) a and andc red. The (negative blue (positive polarity) polarity) contours and within red (negative this box coverpolarity) regions contours with awithin magnetic this boxflux coverdensity regions of ±75 with G or a more magnetic (see flux also density panels a of and±75 c). G (f) or Magnetic more see alsoflux ofpanels the negative a and c. polarityPanel f: near magnetic the eastern flux of footpointthe negative as a polarity function near of time the eastern (see arrow). footpoint The as vertical a function dashed of time line (see in the arrow). right panels The vertical marks dashed the time-stamp line in the of right the panelscorresponding marks the spatial time-stamp maps displayedof the corresponding on the left. See spatial online maps movie displayed to follow on the the left. evolution See online of this movie loopbrightening. to follow the North evolution is up. of See this Sect. loop 3 brightening. for details. North is up. See Sect.3 for details.

fluctuations observed in prolonged UV bursts, which are thought The similarities in the AIA 9.4 nm light curves from both the Based on the presence of mixed polarities and the decrease in the compact bright region near the eastern footpoint is directly to result from magnetic reconnection in the chromosphere (e.g. footpoints suggest that they are causally connected. Moreover, the magnetic flux at the surface, which coincides with the over- overlying a source, where the magnetic energy is likely released Chitta et al. 2017b). the compact bright region near the eastern footpoint is directly lying brightenings near the eastern footpoint, we infer that this during the reconnection process. This indicates that the magnetic Based on the presence of mixed polarities and the decrease in overlying a source, where the magnetic energy is likely released cancellation of magnetic flux is associated with magnetic recon- reconnection at the eastern footpoint is probably also responsible the magnetic flux at the surface, which coincides with the over- during the reconnection process. This indicates that the magnetic nection. Here the magnetic energy released during the reconnec- for the disturbances seen at the western footpoint. lying brightenings near the eastern footpoint, we infer that this reconnection at the eastern footpoint is probably also responsible tion is likely responsible for the compact brightening, and the cancellation of magnetic flux is associated with magnetic recon- for the disturbances seen at the western footpoint. decrease in the surface magnetic flux is a consequence of the nection. Here the magnetic energy released during the reconnec- 4. Brightening in AR 12234 submergence of the reconnected field. We note that these mixed tion is likely responsible for the compact brightening, and the polarity magnetic concentrations are present only near the east- 4. Brightening in AR 12234 decrease in the surface magnetic flux is a consequence of the On 11 December 2014, during its second suborbital rocket flight, ern footpoint but not at the western one (Fig. 1(e)). We cannot, submergence of the reconnected field. We note that these mixed theOn Focusing 11 December Optics 2014, X-ray during Solar its second Imager suborbital (FOXSI-2) rocket detected flight, however, rule out the presence of smaller and weaker patches of polarity magnetic concentrations are present only near the east- athe nanoflare Focusing in OpticsAR 12234, X-ray with Solar faint Imager emission (FOXSI-2) in hard detected X-rays opposite polarity magnetic elements at the western footpoint due ern footpoint but not at the western one (Fig.1e). We cannot, (abovea nanoflare 3 keV), in from AR plasma 12234, heated with faint to temperatures emission in above hard 10 X-rays MK to the moderate spatial resolution of the HMI (e.g. Chitta et al. however, rule out the presence of smaller and weaker patches of (Ishikawa(above 3 keV), et al. from 2017). plasma The ARheated exhibited to temperatures coronal brighteningsabove 10 MK 2017a, cf. Barthol et al. 2011; Solanki et al. 2017). opposite polarity magnetic elements at the western footpoint due recorded(Ishikawa by et AIA al. 2017 that). are The co-temporal AR exhibited to the coronal inferred brightenings nanoflare to theThe moderate similarities spatial in the resolution AIA 9.4 nm oflight the HMI curves (e.g. from Chitta both et the al. inrecorded the same by region. AIA that Unlike are the co-temporal AR-scale brightening to the inferred displayed nanoflare in footpoints2017a, cf. suggestBarthol that et al. they 2011 are; Solanki causally et al.connected. 2017). Moreover, Fig.in the 1, samethe AR region. 12234 Unlike brightening the AR-scale is a small-scale brightening event displayed extend- in Article number, page 3 of 6 L9, page 3 of6 A&A proofs:A&Amanuscript 615, L9 (2018) no. 33404corr

20182018).). The The location location of of the the reconnection reconnection site site can can also also be be under- under- stoodstood in in terms terms of of the the horizontal horizontal separation separation at at the the solar solar surface surface betweenbetween opposite-polarity opposite-polarity magnetic magnetic concentrations concentrations while while can- can- celling.celling. Therefore, if if the the chromospheric chromospheric reconnection reconnection is also is also re- responsiblesponsible for for the the observed observed loop loop brightenings, brightenings, then, then, clearly, the the exactexact energy energy input input into into the the loops loops depends depends on on the the location location of of thethe reconnection, reconnection, meaning meaning that that we we can can only only provide provide an an order- order- of-magnitudeof-magnitude estimation. estimation. WeWe use use the the loop loop brightening brightening presented presented in in Fig. Fig.1 as 1 an as exam- an ex- pleample to toestimate estimate the the energy energy content content in in the the coronal coronal section section of of thethe loop. loop. The The thermal thermal energy energy density density for for a a parcel parcel of of a a mono- mono- atomicatomic gas at at temperature, temperature,TT,, and and particle particle density, density,n,n is, given is given by 3 3 byetherether= =2 nknkBTBT,, where wherekkBBisis the the Boltzmann Boltzmann constant. constant. Here, we we 2 − considerconsider a a typical typical coronal coronal electron number number density density of of 10 1099 cmcm−33,, andand a a plasma plasma temperature temperature of of 7 7 MK, MK, such such that that the the loop loop is is clearly clearly visiblevisible in in the the AIA AIA 9.4 9.4 nm nm filter. filter. The The thermal thermal energy energy of of the the loop loop Fig. 2. As in Fig. 1(b) but for the ratios of 160 nm and 170 nm AIA Fig. 2. As in Fig.1b but for the ratios of 160 nm and 170 nm AIA filters. isis then etherV, where where VV isis the the volume volume of of the the loop. loop. From From Fig. Fig. 1(a),1a, Thefilters. green The curve green representing curve representing a quiet a Sunquiet region Sun region adjacent adjacent to the to AR the thethe loop loop is is roughly roughly 10 10 Mm Mm wide wide and and 100 100 Mm Mm long long.22. WithWith these these showsAR shows the comparison the comparison with the with ratios the ratiosfrom the from footpoint the footpoint regions regions (black values,values, and and the the additional additional assumption assumption that that it it has has a a circular circular cross- cross- and(black magenta, and magenta, cf. Fig.1 cf.). TheFig. 1).short The vertical short verticalbars and bars the vertical and the dashed vertical section, we estimate the thermal energy content of the loop to be dashed line denote the same time-stamps as in Fig. 1(b). See Sect. 3 for section, we estimate the thermal energy content of the loop to be line denote the same time-stamps as in Fig.1b. See Sect.3 for details. 8 × 102727 erg. Next, we estimate the upper limit for the kinetic en- details. 8 × 10 erg. Next, we estimate the upper limit for the kinetic energyergy to beto ofbe the of same the same order order of magnitude of magnitude as the thermalas the thermal energy, energy,because because one can safely one can assume safely that assume the flows that within the flows the loop within are Fig.1, the AR 12234 brightening is a small-scale event extend- −1 −1 00 thesubsonic loop are (i.e. subsonic below about (i.e. below 400 km about s ). 400 The km energy s ). required The energy for ing about 15 00 (Fig.3). Nonetheless, it shares similarities with ing about 15 (Fig. 3). Nonetheless, it shares similarities with requiredthe ionization for the of ionization gas is negligible of gas in is comparison. negligible in Consequently, comparison. thethe first first example. example. the total energy content of the loop is approximately 1028 erg. The AR 12234 nanoflare is observed as a brightening in all Consequently, the total energy content of the loop is approxi- The AR 12234 nanoflare is observed as a brightening in all Motivated28 by the close association of the flux cancellation the AIA EUV filters including the UV 160 nm filter. In Fig.3a mately 10 erg. the AIA EUV filters including the UV 160 nm filter. In Fig. 3(a) withMotivated the loop brightening, by the close we association propose that of the the energy flux cancella- required wewe show show the the 9.4 9.4 nm nm filter filter emission emission map map during during the the reported reported for this loop brightening can be extracted from chromospheric nanoflare. The light curves from two different (east and west) tion with the loop brightening, we propose that the energy nanoflare. The light curves from two different (east and west) requiredreconnection for this at heights loop brightening of around 500 can km. be extracted Staying with from the chro- ex- regionsregions covering covering the the nanoflare nanoflare exhibit exhibit large large fluctuations fluctuations (panel (panel ample from Fig. 1, we find that the cancelled flux amounts to b). Clear, almost simultaneous peaks from both the regions at mospheric reconnection at heights of around 500 km. Staying (b)). Clear, almost simultaneous peaks from both the regions at withabout theΦ example = 2.25 × from1019 Fig.Mx.1, To we estimate find that the the converted cancelled energy flux 19:14 UT in the AIA 9.4 nm filter are co-temporal with the hard 19 19:14 UT in the AIA 9.4 nm filter are1 co-temporal with the hard amountsin the flux to cancellation,about Φ = 2 we.25 use× 10 theMx. (cancelled) To estimate magnetic the con- en- X-rayX-ray emission detected by FOXSI1 ((IshikawaIshikawa et et al. al. 2017 2017).). For For ergy E = 1 B2V in the volume V = AH where the en- comparison, we show the AIA 17.1 nm filter emission map and verted energymag in8π the flux cancellation, we use the (cancelled) comparison, we show the AIA 17.1 nm filter emission map and ergy is released. We estimated1 2 the height range of energy re- magnetic energy Emag = π B V in the volume V = AH where lightlight curves curves that that reveal reveal similar similar characteristics characteristics to to the the 9.4 9.4 nm nm filter filter lease to be H = 500 km above,8 3 and we estimate the area to be (panels c and d). the energy is released. We estimated the height range of energy (panels (c) and (d)). A = 3 Mm × 3 Mm through the extent3 of the brightening near The surface magnetic field underlying this nanoflare (panel release to be H = 500 km above , and we estimate the area The surface magnetic field underlying this nanoflare (panel tothe be easternA = 3 footpoint Mm × 3 Mm in Fig. through 1(c). Thethe extent cancelled of fluxthe brighten-Φ = BA e)(e)) isqualitatively is qualitatively similar similar to that tonear that thenear east the footpoint east footpoint in Fig.1 in. corresponds to the (cancelled) magnetic field B integrated over Here we also observe cancellation of the surface magnetic flux ing near the eastern footpoint in Fig.1c. The cancelled flux Fig. 1. Here we also15 observe−1 cancellation of the surface mag- Φthe= areaBA correspondsA, meaning to that the we (cancelled) find the estimation magnetic forfield theB mag-inte- at a rate of about 10 Mx s (panel15 f).−1 Therefore, also in the netic flux at a rate of about 10 Mx s (panel (f)). Therefore, gratednetic energy over the converted area A, meaningduring the that flux we cancellation find the estimation to be given for casealso ofin thethis case FOXSI of this nanoflare FOXSI the nanoflare coronal the brightening coronal brightening is causally through E = 1 Φ2H/A. With the above values, we find associated with the dynamic evolution of the magnetic field in the magneticmag energy8π converted during the flux cancellation to is causally associated with the dynamic evolution of the mag- beE given= 10 through28 erg,E which= is1 Φ comparable2H/A. With to the the above estimate values, for wethe the form of flux cancellation at the surface. mag mag 8π netic field in the form of flux cancellation at the surface. 28 findenergyEmag content= 10 oferg, the which heated is comparable loop. We note to the that estimate this estimate for the energyfor Emag contentmight of be the a heatedlower limit, loop. Webecause note thatthe reconnectionthis estimate for re- gion is most probably smaller, meaning that A becomes smaller, 5.5. Energetics Energetics of of coronal coronal brightenings brightenings and and flux flux Emag might be a lower limit, because the reconnection region isand most the probably ratio H/A smaller,grows, resulting meaning in that a largerA becomesEmag. smaller, In principle, and cancellationcancellation events events thethe ratio magneticH/A energygrows, releasedresulting from in a larger chromosphericEmag. In principle, reconnection the EnergyEnergy released released in in the the process process of of magnetic magnetic reconnection reconnection dur- dur- magneticduring flux energy cancellation released is from suffi chromosphericcient to explain reconnection the heating of dur- the inging surface surface magnetic magnetic flux flux cancellation cancellation is is generally generally considered consid- ingsystem flux of cancellation loops displayed is suffi incient Fig. 1. to explain the heating of the eredto drive to drive transient transient coronal coronal brightenings, brightenings, explosive explosive events, events, and system of loops displayed in Fig.1. andplasma plasma jets in jets the in upper the upper solar solaratmosphere atmosphere (e.g. Dere (e.g. etDere al. 1991; et al. 1991Innes; etInnes al. 1997; et al. 1997 Chifor; Chifor et al. 2008; et al. 2008 Morita; Morita et al.2010; et al. 2010 Kano; 6. Discussion 6. Discussion Kanoet al. et 2010; al. 2010 Peter; Peter et al. et 2014; al. 2014 Tiwari; Tiwari et al. et 2014). al. 2014 In). both In both the Most of the magnetic energy from the flux cancellation events the cases discussed in Sects.3 and4, atmospheric brightenings cases discussed in Sections 3 and 4, atmospheric brightenings Mostmay already of the magnetic be dissipated energy low from in the the atmosphere, flux cancellation heating events the associated with the loops are directly overlying regions of sur- associated with the loops are directly overlying regions of sur- maychromosphere, already be where dissipated the energy low in requirements the atmosphere, are evenheating higher the faceface magnetic magnetic flux flux cancellation. cancellation. Recent Recent studies studies that that employ employ mag- mag- 2 neticnetic field field extrapolations extrapolations locate locate the the site site of of magnetic magnetic reconnection reconnection 2 WeWe estimate estimate the the loop loop length length based based on on the the lateral lateral separation separation of of foot- foot- inin the the chromosphere chromosphere (at (at heights heights of of about about 500 500 km–1000 km–1000 km) km) in in pointspoints and and assuming assuming it it to to be be a a semi-circle. semi-circle. 3 suchsuch flux flux cancellation cancellation events events (e.g. (e.g. Chitta Chitta et et al. al. 2017b 2017b;; Tian Tian et et al. al. 3 ThisThis estimate estimate of of HH isis also also consistent consistent with with three-dimensional three-dimensional MHD MHD simulationssimulations of of AR AR models, models, which which yield yield a a heating heating scale scale height height below below 1 1 WeWe note note that that being being on on a a suborbital suborbital rocket rocket flight, flight, FOXSI-2 FOXSI-2 recorded recorded thethe corona corona of of about about 500 500 km, km, i.e. i.e. in in the the chromosphere chromosphere where where the the recon- recon- datadata for about about 6 6 minutes min only only and and therefore therefore the the other other peaks peaks we we display display in in nectionnection could could be be triggered triggered (i.e. (i.e. the the heating heating rate rate drops drops every every 500 500 km km by by Fig.Fig.3 3 occurred occurred outside outside the the window window of of time time that that FOXSI FOXSI could could observe. observe. aboutabout a a factor factor of of 2.71; 2.71; e.g. e.g. Bingert Bingert & & Peter Peter 2011 2011).).

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Fig.Fig. 3. 3.AsAs in in Fig. Fig. 11 butbut for for AR AR 12234. 12234. The The field field of of view view is is 60 600000××30300000.. North North is is up. up. See See Sect. Sect. 4 for for details. details.

chromosphere, where the energy requirements are even higher over flux cancellation sites, do not show any coronal counter- thanthan in in the the corona corona (Withbroe (Withbroe & & Noyes Noyes 1977).1977). However, However, if if the the overparts flux (e.g. cancellation Peter et al. 2014 sites,). do not show any coronal counter- observedobserved coronal coronal brightenings brightenings are are indeed indeed influenced influenced by chromo- by chro- partsAnother (e.g. Peter interesting et al. 2014). aspect to be considered is the near- sphericmospheric (footpoint) (footpoint) reconnection reconnection during during the flux the cancellation flux cancellation (cf. simultaneousAnother interesting brightening aspect of two tobe footpoints considered (which is the are near- later- Sect.(cf. Sect.5), it5 is), itexpected is expected that somethat some of the of released the released magnetic magnetic en- simultaneousally separated brightening by about of 100 two00) observed footpoints in (which the AIA are 9.4 later- nm ergyenergy reaches reaches coronal coronal heights. heights. In In this this case, case, the the issue issue of of energy energy allyfilter separated (Fig.1). by Using about the 100 same00) observed observations, in the Testa AIA9.4 et al. nm(2014 filter) partitionpartition between between the the chromosphere chromosphere and and the the corona corona needs needs to to be be (Fig.suggested 1). Using that the the same brightening observations, in the Testa western et al. footpoints (2014) sug- and addressed.addressed. Alternatively, Alternatively, the the observed observed flux flux cancellation cancellation and and foot- foot- gestedgentle that upflows the brightening in the transition in the regionwestern are footpoints due to the and acceler- gentle pointpoint reconnection reconnection can can facilitate facilitate the the release release of of the the stored stored mag- mag- upflowsation of in non-thermal the transition particles region in are coronal due to nanoflares. the acceleration Here ofwe neticnetic energy energy in in coronal coronal braids, braids, leading leading to to loop loop brightening. brightening. non-thermalreveal ongoing particles magnetic in coronal reconnection nanoflares. at the Here eastern we reveal footpoint, on- FluxFlux cancellation cancellation and and associated associated reconnection reconnection can can shed shed light light goingpresumably magnetic at lowreconnection heights, that at the could eastern accelerate footpoint, non-thermal presum- onon the the spatial spatial structuring structuring (meaning (meaning only only at at specific specific locations locations ablyparticles at low from heights, the site that of could chromospheric accelerate non-thermal reconnection. particles Further inin ARs) ARs) and and temporal temporal intermittency intermittency (meaning (meaning only only at at specific specific fromobservational the site of analysis chromospheric is required reconnection. to build statistics Further on observa- the pres- times)times) of of hot hot loops loops observed observed in in ARs ARs (e.g. (e.g. Webb Webb & & Zirin Zirin1981; 1981; tionalence analysis(or lack) is of required these near-simultaneous to build statistics on footpoint the presence brighten- (or ShimizuShimizu et et al. al. 1992;1992; Falconer et et al. al. 1997 1997;; Ugarte-Urra Ugarte-Urra & & Warren War- lack)ings of in these association near-simultaneous with brightenings footpoint of hot brightenings coronal loops in asso- (e.g. ren2014 2014).). This This is because is because such such flux flux cancellation cancellation events, events, due dueto their to ciationGupta withet al. brightenings2018). In addition, of hot coronal it would loops be interesting (e.g. Gupta to et con- al. theirsparsity, sparsity, are discrete are discrete in space in space and and time, time, at least at least on a on scale a scale vis- 2018).duct numerical In addition, experiments it would tobe investigate interesting the to conductrole of footpoint numer- visibleible to to HMI. HMI. We We note note that that the the cancellation may in in some some cases cases icalreconnection experiments and to the investigate local energy the role deposition of footpoint in the reconnec- genera- bebe associated associated with with previous previous small-scale small-scale flux flux emergence. emergence. Nev- Nev- tiontion and of thea near-simultaneous local energy deposition signal in at the both generation footpoints. of a This near- is ertheless,ertheless, we we emphasise emphasise that that many many flux flux cancellation cancellation events events (and (and simultaneousworthwhile because signal at any both energy footpoints. transport This mechanism is worthwhile (e.g. heatbe- relatedrelated reconnection) reconnection) need need not not be be associated associated with with coronal coronal bright- bright- causeconduction any energy or non-thermal transport mechanism particles) that(e.g. is heat fast conduction enough may or eningsenings due due to to the the inherent inherent lack lack of of magnetic magnetic coupling coupling to to coronal coronal non-thermalefficiently transport particles) the that energy is fast enoughto the other may e footpointfficiently trans-within heightsheights at at the the locations locations of of such such events. events. For For instance, instance, some some of of portthe the AIA energy cadence to the of other 12 s, footpoint givingwithin rise to the an AIA apparent cadence near- of thethe low-lying low-lying brightenings, brightenings, such such as as UV UV bursts, bursts, which which also also occur occur 12simultaneous s, giving rise signal. to an apparent near-simultaneous signal.

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In this study we focus on a scenario of coronal bright- a complex and dynamic magnetic coupling through the solar enings in AR cores and their likely association with chromo- atmosphere that probably governs the energetics of these coronal spheric reconnection during surface magnetic flux cancellation. structures and, as we speculate, those of many others. We sug- In principle, such a scenario might also be applied to the dif- gest that the energy released in chromospheric reconnection is a fuse solar corona in quiet Sun regions. The rate of energy loss viable source to power AR coronal loops driven by nanoflares. A from the entire quiet solar corona has been estimated to be similar process of chromospheric reconnection during flux can- about 1028 erg s−1 (Withbroe & Noyes 1977). Parker(1988) pro- cellation would be sufficient to power the quiet solar corona. posed nanoflares as a way to energize the quiet corona. With each nanoflare providing on average an energy of 1024 erg, there Acknowledgements. We thank the anonymous reviewer for constructive com- would have to be about 104 nanoflares at any given time dis- ments, which helped to improve the manuscript. L.P.C. benefited from many use- ful discussions with Eric Priest, and received funding from the European Union’s tributed on the Sun to sustain the quiet corona. Horizon 2020 research and innovation programme under the Marie Skłodowska- In contrast to the nanoflares produced by reconnection dis- Curie grant agreement No. 707837. This project has received funding from the tributed in the corona following energy build-up by braiding, one European Research Council (ERC) under the European Union’s Horizon 2020 could also speculate that the quiet Sun is heated by small recon- research and innovation programme (grant agreement No. 695075). This work nection events at the coronal base associated with flux cancella- was partly supported by the BK21 plus program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea. SDO data are tion in the photosphere, similar to our proposal for the AR cores. courtesy of NASA/SDO and the AIA, and HMI science teams. This research has In a recent study, Smitha et al.(2017) used high-resolution mag- made use of NASA’s Astrophysics Data System. netic field maps and analysed the rate of flux emergence and can- cellationinthequietSunfromtheballoon-borneSunrisetelescope References (Solanki et al.2010;Barthol et al.2011).Forthemagneticfeatures with a flux content in the range of 1015 Mx to 1018 Mx, they obtain Asgari-Targhi, M., Schmelz, J. T., Imada, S., Pathak, S., & Christian, G. M. 2015, a flux-loss rate (due to flux cancellation) per unit area of about ApJ, 807, 146 −2 −1 Barthol, P., Gandorfer, A., Solanki, S. K., et al. 2011, Sol. Phys., 268, 1 1150 Mx cm day at the photosphere. This amounts to a flux- Bingert, S., & Peter, H. 2011, A&A, 530, A112 20 −1 loss rate of Φ˙ = 8 × 10 Mx s , over the entire solar surface. This Boerner, P., Edwards, C., Lemen, J., et al. 2012, Sol. Phys., 275, 41 is a lower limit, as, using another technique, Zhou et al.(2013) Chifor, C., Isobe, H., Mason, H. E., et al. 2008, A&A, 491, 279 obtain a flux-loss rate more than five times higher. We now con- Chitta, L. P., Peter, H., Solanki, S. K., et al. 2017a, ApJS, 229, 4 Chitta, L. P., Peter, H., Young, P. R., 2017b, A&A, 605, A49 sider that this flux loss leads to magnetic energy release through Cirtain, J. W., Golub, L., Winebarger, A. R., et al. 2013, Nature, 493, 501 reconnection over a height range of H = 500 km above the photo- Danilovic, S., van Noort, M., & Rempel, M. 2016, A&A, 593, A93 sphere, which corresponds to the temperature minimum level in Dere, K. P., Bartoe, J.-D. F., Brueckner, G. 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