Saddle-shaped solar flare arcades
DD raft version F ebruary
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Saddle-shaped solar flare arcades J uraj L¨ orin ˇ c ´ ık , J aroslav D ud ´ ık , and G uillaume A ulanier Astronomical Institute of the Czech Academy of Sciences, Friˇcova 298, 251 65 Ondˇrejov, Czech Republic Institute of Astronomy, Charles University, V Holeˇsoviˇck´ach 2, CZ-18000 Prague 8, Czech Republic LESIA, Observatoire de Paris, Universit´e PSL , CNRS, Sorbonne Universit´e, Universit´e de Paris, 5 place Jules Janssen, 92190 Meudon, France Rosseland Centre for Solar Physics, University of Oslo, P.O. Box 1029 Blindern, NO-0315 Oslo, Norway
ABSTRACTArcades of flare loops form as a consequence of magnetic reconnection powering solar flares and eruptions.We analyse the morphology and evolution of flare arcades that formed during five well-known eruptive flares.We show that the arcades have a common saddle-like shape. The saddles occur despite the fact that the flareswere of di ff erent classes (C to X), occurred in di ff erent magnetic environments, and were observed in variousprojections. The saddles are related to the presence of longer, relatively-higher, and inclined flare loops, con-sistently observed at the ends of the arcades, which we term ‘cantles’. Our observations indicate that cantlestypically join straight portions of flare ribbons with hooked extensions of the conjugate ribbons. The originof the cantles is investigated in stereoscopic observations of the 2011 May 9 eruptive flare carried out by theAtmospheric Imaging Assembly (AIA) and Extreme Ultraviolet Imager (EUVI). The mutual separation of theinstruments led to ideal observational conditions allowing for simultaneous analysis of the evolving cantle andthe underlying ribbon hook. Based on our analysis we suggest that the formation of one of the cantles canbe explained by magnetic reconnection between the erupting structure and its overlying arcades. We proposethat the morphology of flare arcades can provide information about the reconnection geometries in which theindividual flare loops originate. Keywords:
Solar filament eruptions (1981), Solar flares (1496), Solar extreme ultraviolet emission (1493), Solarmagnetic reconnection (1504), Solar coronal mass ejections (310) INTRODUCTIONSolar flares are sudden releases of magnetic energy accu-mulated in the solar atmosphere (see e.g., Schmieder et al.2015). The flare emission originates in flare loops (seee.g., Tsuneta et al. 1992) formed by magnetic reconnection,initially described in the two-dimensional ‘CSHKP’ modelof flares (see e.g., Shibata & Magara 2011, and the refer-ences therein). They are nowadays routinely observed by theAtmospheric Imaging Assembly (AIA; Lemen et al. 2012;Boerner et al. 2012) onboard the
Solar Dynamics Observa-tory ( SDO ; Pesnell et al. 2012).Arcades of flare loops develop along polarity inversionlines (e.g., Moore & Roumeliotis 1992; Priest & Forbes2002). Their footpoints form flare ribbons, bright elongatedstructures observed in lower atmosphere (e.g., Fletcher et al.2004). Since both the flare loop arcades and ribbons developalong the dimension missing in the CSHKP model, 2.5D
Corresponding author: J. L¨orinˇc´ı[email protected] models, based on stacking the 2D models along the thirddimension, have been developed (e.g., Shiota et al. 2005;Tripathi et al. 2006). These models often show translationsymmetry, and are thus insu ffi cient for description of ar-cades with complicated morphology. Among these can be,for example, arcades forming along curved PILs (e.g., Ryu-tova et al. 2011; Baker et al. 2020), or arcades distorted bythe supra-arcade downflows (SADs; e.g., Savage et al. 2010,2012; Cassak et al. 2013; Xue et al. 2020). Moreover, arcadesevolve in time due to ongoing reconnection and formation ofnew flare loops (Masuda et al. 2001), causing the arcades togrow (Schmieder et al. 1995) and rise (Gallagher et al. 2002),see also Section 1.3 in Fletcher et al. (2011). A little atten-tion has however been paid to arcade asymmetries caused byvariations of both lengths and heights of flare loops they arecomposed of.Recent flare analyses (Zemanov´a et al. 2019; L¨orinˇc´ık et al.2019; Chen et al. 2019) reported on flare loops rooted in thevicinity of J -shaped extensions of flare ribbons called rib-bon hooks (see e.g., Janvier (2017)). Arcades reaching to-ward the ribbon hooks can be identified in 3D simulations of a r X i v : . [ a s t r o - ph . S R ] F e b Figure 1.
Saddle-shaped arcades of flare loops during five eruptive flares as observed in the 131 Å filter channel of AIA. In bottom-right ofeach panel, cartoons showing the arcade in the observed projection is drawn. Loops at ends of the arcades are labeled as ‘cantles’ and shown inred, while loops in central parts of the arcades are plotted in yellow. Where possible, flare ribbons are indicated using orange lines. flares and eruptions (e.g., Inoue et al. 2014, 2015; T¨or¨ok et al.2018), which suggests that their formation is associated with3D magnetic reconnection. According to the The Standardflare model in 3D (Aulanier et al. 2012; Janvier et al. 2013),flare loops rooted in the hooks should form due to the ar–rf reconnection between the erupting structure and its overlyingarcades (Aulanier & Dud´ık 2019; Dud´ık et al. 2019). Thiscould introduce additional asymmetries to flare arcades.In this manuscript we analyse the evolution and morphol-ogy of saddle-shaped flare arcades formed during five eventswell-known from literature. Section 2 briefly introduces thedata used in this study. In Sections 3 and 4 we analyse flarearcades formed during the five events. Finally, Section 5summarizes our results. DATA AND OBSERVATIONS The events discussed in this manuscript are primarily anal-ysed using imaging data from
SDO / AIA. AIA provides full-disk images of the Sun in 10 filters, imaging plasma withtemperatures between ≈ to 10 K. The spatial resolutionof AIA is ≈ (cid:48)(cid:48) (Boerner et al. 2012) and its cadence is 12–24 s depending on bandpass. We processed level-1 AIA datausing the aia prep routine. Flares located on solar disk werecorrected for the di ff erential rotation.Where appropriate, we supplement AIA data with obser-vations performed by the Extreme Ultraviolet Imager (EUVI;Wuelser et al. 2004; Howard et al. 2008) onboard STEREO-B, as well as the X-Ray Telescope (XRT; Golub et al. 2007)onboard the Hinode mission. These datasets were processedusing standard routines available within SolarSoft and co-registered with AIA. These shifted coordinates are denotedas Solar- X ∗ and Solar- Y ∗ . Finally, the phases of individual Figure 2.
The 2011 May 9 eruption observed in the 131 Å and 171 Å channels of AIA (panels (a)–(g)) and the 171 Å channel of
STEREO-B / EUVI (panels (h)–(i)). The hot channel before and during its eruption, cantles of the saddle-shaped arcade of flare loops, and hooked flareribbons are indicated. Red circle plotted in panels (g) and (i) marks the southern footpoint of the southermost flare loop (cantle; indicated usingred dotted line in panel (i).Animated version of the observations from the 131 Å and 171 Å channels of AIA and 171 Å channel of EUVI is available online. The animationcontains the eruption of the hot channel, evolution of the surrounding corona, and the formation of the hooked flare ribbons. It covers the period20:00 – 22:00 UT and its real-time duration is 30 seconds. flares were analysed using the soft X-ray flux measured by
GOES .We chose eruptive flares of di ff erent classes (C to X)known from literature, all showing well-developed arcades and hooked ribbons, with the exception of one event seen o ff -limb where the ribbons could not be observed (Section 4.4).Figure 1 provides an overview of the morphology of flare ar-cades formed during the selected events, all observed in the131 Å channel of AIA. Each individual panel also containsa cartoon showing the simplified arcade morphology as ob-served. All of these arcades are asymmetric along the iden-tifiable ribbons (orange lines) to some degree. Flare loopsat each end of the arcade (red) are inclined and relatively-higher than the central parts of the arcades (yellow). Overall,all five arcades are reminiscent of saddles. We label the in-clined, higher loops at each end of each arcade as ‘cantle’(red). Note that the saddles are also observed in flares whichhave nearly parallel ribbons (and thus straight PILs), see pan-els (a), (b), and (d) of Figure 1. To our knowledge, this over-all saddle-shaped morphology of the flare arcades was notnoticed so far. We now proceed to detail the evolution andmorphology of these arcades. SADDLE DURING THE 2011 MAY 9 ERUPTIONWe first analyze the 2011 May 9 eruption accompanied bya C5.4-class flare. This event is ideal for our investigationas it was observed stereoscopically by AIA and EUVI withangular separation of about 94 ◦ . AIA observed it at the limb,while in EUVI it was near the disk center.3.1. Arcade of flare loops
The erupting structure was a hot channel (e.g., Zhang et al.2012) visible in the 94 Å and 131 Å channels of AIA. Be-fore its eruption, the hot channel was embedded in an ar-cade of coronal loops (Figure 2(a)–(b)) within a bipolar ac-tive region NOAA 11193 (Aulanier et al. 2012). The hotchannel started to slowly rise after ≈ ff erence (RD) 131 Å images, constructed from im-ages averaged over 1 minute, both the rising hot channel andthe first flare loops can be distinguished (panel (b)). Duringthe impulsive phase of the flare at ≈ ≈
37 Mm, while the typical length ofthe flare loops elsewhere in the arcade range between about22 and 27 Mm. The cantle loop can easily be identified inAIA, where it composes the cantle at the southern end of thesaddle, as it is still the southernmost loop (red circle in panel(g)). In the AIA projection, this cantle loop seen in 171 Å isalso relatively-higher than the remainder of the flare arcade.The relation of this cantle loop to the flare ribbons is outlinedin the following section.3.2.
Flare ribbons and coronal loops
During the eruption, the coronal loops overlying the hotchannel moved aside from the erupting hot channel towardthe south and north (Figure 2 (e)–(f)). While the northerncoronal loops remained visible during the eruption, obscur-ing the northern cantle, most of the southern coronal loopsdisappeared. EUVI 171 Å observations reveal that thesecoronal loops were rooted at either sides of the J -shaped(hooked) flare ribbons. The straight parts of the ribbons werenearly parallel, separated by a straight PIL (see also Figure 1in Aulanier et al. 2012). The ribbon hooks, one to the north-east (N-hook) and the other to the south-west (S-hook), areindicated in Figure 2(h).A magnified view of the coronal loops overlying the S-hook is shown in Figure 3. Panels (a)–(d) contain 2-minuterunning-di ff erence images produced using AIA 171 Å. Ini-tially, the coronal loops with various inclinations (a) wereoverlying the hot channel (seen only in 131 Å, Figure 2(b)).At ≈ ≈ Figure 3.
View of the disappearing coronal loops in 2-minute running-di ff erence images produced using the 171 Å filter channel of AIA (panels(a)–(d)) and in STEREO-B 171 Å channel (panels (e)–(h)). In panels (f)–(h), the hooked flare ribbon is indicated using orange dashed line. Redcircle in panels (d), (f), and (h) marks the southern footpoint of the southermost flare loop. described in Section 3.1, the 171 Å flare loop rooted in thelocation of the red circle is longer and relatively-higher thanthe remainder of the flare arcade. This behavior is also con-sistent with the ar–rf reconenction. Comparison of Figures4d and 5d of Aulanier & Dud´ık (2019) reveals that the flareloops originating in the ar–rf geometry can be longer and alsohigher than the flare loops formed due to the reconnection be-tween overlying arcades, which occurs beneath the eruptingstructure. A similar situation is also seen in Figure 1c therein.The formation of this cantle may therefore be explained bymagnetic reconnection between the hot channel and coronalloops rooted nearby. SADDLES IN FOUR ADDITIONAL EVENTSWe now show that saddle arcades also occurred in di ff erentevents studied previously by numerous authors.4.1. Filament eruption of 2011 June 7
The 2011 June 7 filament eruption is a well-known event(e.g, van Driel-Gesztelyi et al. 2014; Yardley et al. 2016;Dud´ık et al. 2019, and references therein). It occurred inan active region NOAA 11226 and was accompanied by aM2.5–class flare.This eruption started at ≈ We next analyse a flare arcade which formed during the2011 December 26 C5.7–class flare (Qiu et al. 2017). Thisflare occurred in the active region complex NOAA 11383and 11384 and was accompanied by a Coronal Mass Ejec-tion (CME) (Cheng & Qiu 2016).The flare morphology is shown in Figure 4, panels (d)–(f). It showed a pair of very straight ribbons studied by Qiuet al. (2017). Nevertheless, we identified a pair of hooked
Figure 4.
Saddle arcades of the 2011 June 7 (top row) and 2011 December 26 (bottom row) flares observed by XRT and AIA. Orange dashedlines plotted in panel (a) indicate the ribbon hooks of 2011 June 7 flare. The red circle marks the footpoints of cantle flare loops. Hookedribbons and cantle loops of the 2011 December 26 flare are highlighted using arrows. extensions formed at their far ends, shown by arrows in panel(d). The western hook was curved and partially obscuredby coronal loops rooted in the south-west at ≈ [180 (cid:48)(cid:48) ,360 (cid:48)(cid:48) ],toward which the ribbon elongated.First flare loops started to appear at around 11:15 UT inAIA 131 Å. In the composite image (panel (d)), they are seenas red-colored emission, located between the ribbons, andextending toward the western hook. As the arcade furtherdeveloped (panel (e)), a saddle with two cantles started to bevisible. The south-western cantle is more pronounced thanthe north-eastern one. This is because it was composed oflong and faint flare loops, extending for more than 100 (cid:48)(cid:48) to-ward the western hook in which they are rooted (arrow inpanel (e)). On the other hand, the conjugate cantle started tobe visible after the saddle cooled, at which it was visible inAIA 131 and 171 Å (arrow in panel (f)).4.3. On 2017 September 6, the large active region NOAA12673 produced two flares (e.g., Yan et al. 2018a). Thefirst one, confined X2.2–class one, started at ≈ Figure 5.
AIA observations of the saddle arcades during the 2017 September 6 (top row) and 2017 September 10 (bottom row) flares. Orangedashed lines in panel (a) indicate the flare ribbons. The filament as well as some of the loops composing the erupting flux rope of 2017September 10 are indicated using arrows in panel (d). composing it are rooted near the southern hook. A fully-formed saddle is shown in panel (b). Similar to the eventsdescribed before, the cooling saddle arcade is later visible formany hours in cooler filter channels, such as AIA 171 Å and211 Å, see panel (c).4.4.
Four days later, when the same active region was behindthe limb, it produced another large X8.2 flare (Yan et al.2018b; Warren et al. 2018; Polito et al. 2018). The eruptingflux rope consisted of a filament and a hot channel, studied indetail e.g., by Yan et al. (2018b). The hot channel was visiblein the AIA 94 Å and 131 Å and consisted of numerous loopslocated both to the north and to the south of the filament, seee.g., Figure 3 of Yan et al. (2018b), and Chen et al. (2020). The first flare loops can be identified at ≈ SUMMARYWe presented observations of five saddle-shaped arcadesof flare loops formed during five well-known eruptive flares.The saddles are visible as the arcades develop a pair of can-tles, longer, relatively-higher, and inclined flare loops locatedat both ends of the arcades. An example of a cantle loop ob-served in 171 Å by both
SDO / AIA and
Stereo / EUVI showedthat the cantle loops connect straight portion of one flare rib-bon with the hooked extension of the conjugate ribbon. Thissuggests that the shapes of flare loops composing saddle-shaped arcades result from variations in magnetic connec-tivity.The observational characteristics of the saddles are similarin all of the investigated eruptions. One peculiar di ff erenceconcerns the development of the saddles during the 2011June 7 and December 26 events (Sections 4.1, 4.2) wherearcades initially formed a ’half-saddle’ before the full saddlewas seen. We suggest the formation of one cantle prior toits counterpart is caused by an asymmetric shape and / or evo-lution of the erupting flux rope, initially reconnecting withoverlying arcades in a preferred direction.Formation of flare loops joining the hooks and the straightparts of the conjugate ribbons is addressed in the latest exten-sions to the Standard flare model in 3D (Aulanier & Dud´ık2019, and references therein). There, such loops form dueto the ar–rf magnetic reconnection between the erupting flux rope and its overlying arcades rooted near the ribbon hooks.We found indications of this process using stereoscopic ob-servations of the 2011 May 9 eruption, where it likely actedin the formation of one of the cantles. Furthermore, in the2011 June 7 event, we identified footpoints of one cantle inthe locations corresponding to footpoints of flare loops orig-inating in the ar–rf reconnection reported earlier by Dud´ıket al. (2019). Even though a complete analysis of the recon-nection geometries in 3D was out of scope of this letter, theseresults still show that the shape of flare arcades could reflectthe origin of individual flare loops.Finally, we shown that the flare arcades are saddle-shapeddespite the fact that selected events were observed in di ff er-ent projections, magnetic environments, and accompanied byflares of di ff erent classes. This could indicate that the pres-ence of saddles is common and a generic property of eruptiveflares in 3D.The authors thank the anonymous referee whose commentshelped us improve the manuscript. J.L. and J.D. acknowledgethe project 20-07908S of the Grant Agency of Czech Repub-lic as well as insitutional support RVO: 67985815 from theCzech Academy of Sciences. G.A. thanks the CNES andthe Programme National Soleil Terre of the CNRS / INSU forfinancial support. AIA and data are provided courtesy ofNASA / SDO and the AIA science team.
Hinode is a Japanesemission developed and launched by ISAS / JAXA, with NAOJas domestic partner and NASA and STFC (UK) as inter-national partners. It is operated by these agencies in co-operation with ESA and the NSC (Norway). Full-disk EUVIimages are supplied courtesy of the STEREO Sun Earth Con-nection Coronal and Heliospheric Investigation (SECCHI)team.
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