Formation and eruption of a flux rope from the sigmoid active region NOAA 11719 and associated M6.5 flare: A multi-wavelength study

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by  Bhuwan Joshi

Draftversion January5,2017
PreprinttypesetusingLATEXstyleAASTeX6v.1.0
FORMATION AND ERUPTION OF A FLUX ROPE FROM THE SIGMOID ACTIVE REGION NOAA 11719
AND ASSOCIATED M6.5 FLARE: A MULTI-WAVELENGTH STUDY
Bhuwan Joshi and Upendra Kushwaha
UdaipurSolarObservatory,PhysicalResearchLaboratory,Udaipur313001,India
7
Astrid M. Veronig
1
0 Kanzelh¨oheObservatory/Institute ofPhysics,UniversityofGraz,Universita¨tsplatz5,A-8010Graz,Austria
2

n
a Sajal Kumar Dhara
J
UdaipurSolarObservatory,PhysicalResearchLaboratory,Udaipur313001,India
4

]
R A. Shanmugaraju
S Department ofPhysics,ArulAnandharCollege,Karumathur,Tamilnadu625514, India
.
h
p
- Yong-Jae Moon
o
r School ofSpaceResearch,KyungHeeUniversity,Yongin,Gyeonggi-Do, 446-701,Korea
t
s
a
[

ABSTRACT
1
v We investigate the formation, activation and eruption of a flux rope from the sigmoid active region
7
NOAA 11719 by analyzing E(UV), X-ray and radio measurements. During the pre-eruption period
6 of ∼7 hours, the AIA 94 ˚A images reveal the emergence of a coronalsigmoid through the interaction
9
0 between two J-shaped bundles of loops which proceeds with multiple episodes of coronalloop bright-
0 enings and significant variations in the magnetic flux through the photosphere. These observations
1. imply that repetitive magnetic reconnections likely play a key role in the formation of the sigmoidal
0 flux rope in the coronaand also contribute towardsustaining the temperature of the flux rope higher
7 thantheambientcoronalstructures. Notably,theformationofthesigmoidisassociatedwiththe fast
1
morphologicalevolution of an S-shaped filament channel in the chromosphere. The sigmoid activates
:
v toward eruption with the ascend of a large flux rope in the corona which is preceded by the decrease
i
X of photospheric magnetic flux through the core flaring region suggesting tether-cutting reconnection
as a possible triggering mechanism. The flux rope eruption results in a two-ribbon M6.5 flare with
r
a a prolonged rise phase of ∼21 min. The flare exhibits significant deviation from the standard flare
modelintheearlyrisephaseduringwhichapairofJ-shapedflareribbonsformandapparentlyexhibit
convergingmotionsparalleltothe polarityinversionline whichis furtherconfirmedbythe motionsof
HXR footpoint sources. In the later stages, the flare follows the standard flare model and the source
region undergoes a complete sigmoid-to-arcade transformation.
Keywords: Sun: activity – Sun:flare – Sun: filaments, prominences – Sun:X-rays – Sun:γ-rays
1. INTRODUCTION nizedthatarefavorableforeruptions. Inthisregard,the
appearance of a “sigmoid” in the active region corona
Coronal mass ejections (CMEs) immensely affect
is considered as an important precursor of CMEs. It is
space weather phenomena. Thus a major objective
widely accepted that sheared and twisted coronal fields
of research in solar physics in recent times has been
associatedwithsigmoidscanstorealargeamountoffree
to explore the source region characteristics of CMEs.
magneticenergywhichis ultimatelyreleasedduringthe
Throughtheseefforts,someconditionshavebeenrecog-
CME.
2
Sigmoids were first identified by the Yohkoh Soft The emergence or formation of magnetic flux
X-ray Telescope (SXT) as large regions produc- ropes in solar active regions and their subse-
ing enhanced soft X-ray emission having S-shaped quent eruption has been recognized as the key
(or inverse S-shaped) morphology (Rust & Kumar component of the sigmoid-to-arcade evolution pro-
1996; Manoharan et al. 1996; Pevtsov et al. 1996; cess (see, e.g., Titov & D´emoulin 1999; Kliem et al.
Sterling & Hudson 1997; Moore et al. 2001). Using 2004; Archontis et al. 2009; Chatterjee & Fan 2013;
SXT data, Hudson et al. (1998) studied the source re- Schmieder et al.2015; Jiang & Feng2016;Kumar et al.
gion of several halo CMEs and found that, for the ma- 2016). Comtemporary observations, taken from mul-
jority of events, the source region exhibited a charac- tiple EUV channels of AIA, indeed provide evidence
teristics pattern in which pre-eruption sigmoids turned toward the existence and activation of the hot flux
into loop arcades following the passage of a CME (see rope in the active region corona (see, e.g., Cheng et al.
also Sterling et al. 2000). Using a large data set of 2011, 2013b; Kumar & Cho 2014; Chen et al. 2014;
SXT images, Canfield et al. (1999) classified solar ac- Cheng & Ding 2016). Further, the comparison between
tive regions in sigmoidal and non-sigmoidal categories the kinematic evolution of the flux rope and associated
and found that the former type of activity centers are CME reveals that the hot flux rope acts as the earliest
more likely to be eruptive than the other ones. Accord- signature of the CME (Cheng et al. 2013a, 2014b).
ing to morphology and evolution time-scale, sigmoids In this study, we present a comprehensive multi-
can be classified into two groups: transient and persis- wavelength analysis of the morphological evolution and
tent (Gibson et al. 2006). Transient sigmoids brighten eruption of the sigmoidal active region NOAA 11719
uponlyforashortperiodoftime,usuallyjustbeforethe on 2013 April 11. We discuss the dramatic evolu-
eruption (see, e.g., Pevtsov et al. 1996). They tend to tion of this active region over a period of nine hours
be more well defined in the form of apparently a sin- (00:00–09:00UT).Inthisperiod,anextremeultraviolet
gle, sigmoid loop. Notably, observations reveal that (EUV) sigmoid structure emerged through the interac-
many sigmoids have the shape of two J’s or elbows, tions between two J-shaped bundles of loops that in-
whichtogetherformtheS-shapeofthesigmoid(see,e.g., volves multiple events of localized energy release and
Moore et al.2001). Persistentsigmoidspresentmuchin- photospheric flux changes. Subsequently, we observe
tricatemorphologyinwhichmanydiscreteshearedloops the ascend and eruption of a flux rope from this sig-
collectively form a sigmoidal structure. They are long- moidalregion. Thisstudyispreliminarybasedonmulti-
lived features that sustain for considerably longer time channel E(UV) imaging taken from Atmospheric Imag-
than the transient sigmoids (from days to weeks)(see, ingAssembly(AIA;Lemen et al.2012)onboardtheSo-
e.g., McKenzie & Canfield 2008). larDynamicsObservatory(SDO)havingunprecedented
Underneath the twisted coronal soft X-ray (SXR) spatial and temporal resolutions. Notably, the evolu-
structures, filament channels are frequently observed tion of the sigmoid was observed at AIA 94 ˚A im-
in Hα observations (Pevtsov et al. 1996; Pevtsov 2002; ages which implies that the structure was comprised
Gibson et al. 2002). Although the sigmoid-to-arcade of very high temperature plasma (∼6 MK). During the
evolutionisquitedramatic,theunderlyingfilamentmay eruption, we observed a large M6.5 flare (SOL2013-04-
or may not show significant changes with the sigmoid 11) which is characterized by a prolonged SXR rise
eruption(Pevtsov2002). Further, sigmoidscanalsode- phase of ∼21 min. It is striking that the flare evolu-
velopoverdecayedactiveregionsshowingweakanddis- tion during the early rise phase significantly deviates
perseddistributionofmagneticflux(Glover et al.2001). from the standard flare model. The Hα observations
We believe that there is some coupling between these from Kanzelho¨he Observatory (KSO; Po¨tzi et al. 2015)
twostructures(sigmoidandfilament)althoughobserva- revealed that a long active region filament existed be-
tions at these two channels (SXR and Hα) correspond low the coronal sigmoid which partially erupted during
to the hottest and the coldest material associated with the M6.5 flare and caused a large two-ribbon flare in
the sigmoidalregions. Therefore,itis essentialto probe the chromosphere. The temporal and spatial evolution
whathappensin-betweenthesetwolayersandtempera- of hard X-ray (HXR) emission during the M6.5 flare
ture regions during the formationand disruption stages was studied using multi-band X-ray time-profiles and
of the sigmoids. This objective can be accomplished by images obtainedfrom the Reuven Ramaty High Energy
analyzing suitable multi-wavelength data sets. Recent Solar Spectroscopic Imager (RHESSI; Lin et al. 2002).
studies indicate that sigmoidal structures are visible in Detailed comparison of chromospheric and coronal ac-
a wider range of temperature (Liu et al. 2007). Finally, tivitiesduringthesigmoidevolutionwiththechangesin
we need to understand how the overlying coronal and photospheric magnetic flux was undertaken to investi-
chromospheric structures are related to the underlying gatethetriggeringmechanisminvolvedinthiseruption.
magnetic field evolution through the photosphere. Formagneticfieldmeasurements,wehaveanalyzedlon-
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HMI 11-Apr-2013 06:49:18 UT HMI 11-Apr-2013 06:44:03 UT
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Figure 1. Multi-wavelength view of active region 11719 on 2013 April 11. Panel (a): White light picture of the active region
showing the distribution of sunspots. Panel (b): HMI line-of-sight magnetogram presenting the magnetic flux distribution of
the active region. Panel (c): AIA 94 ˚A image of the pre-flare phase showing a sigmoidal structure. Panel (d): KSO Hα image
showing a long filament channel (marked with arrows) along thepolarity inversion line.
gitudinalmagnetogramsfromtheHelioseismicMagnetic βγ magnetic configuration of the AR. An inverse S-
Imager (HMI; Schou et al. 2012) on board SDO. These shaped structure (i.e., a sigmoid) is observed in the
multi-wavelengthobservationsarefurthersupplemented AIA94˚Aimageswhichconsistsofasetofhighlysheared
by radio dynamic spectra obtained from the HiRAS ra- coronalloops (Figure 1(c)). FromHα filtergrams of the
diospectrograph. Wepresentanobservationaloverview activeregion,wefindthatalongfilamentchannelexists
of the activities in Section 2. The analysis and observa- under the coronal sigmoid which is indicated by arrows
tional results are presented in Section 3. We interpret in Figure 1(d). We observed significant variations of
ourresultsandemphasizetheuniquenessofthisworkin magnetic flux in AR 11719 from several hours prior to
Section 4. Summary of this study is given in Section 5. the eruption till the post eruption phase (∼00:00–10:00
UT). During the eruption of the flux rope,a large M6.5
2. OVERVIEW OF OBSERVATIONS two-ribbon flare was observed at the location N09E12
togetherwithanassociatedhaloCME.Accordingtothe
InFigure1,wepresentamulti-wavelengthviewofthe
GOES1–8˚Aflux,theflarestartedat06:55UT,reached
active region NOAA 11719 at white light (WL), 94 ˚A,
its maximum at 07:16 UT and ended at 7:29 UT.
and Hα wavelengths to show distribution of sunspots
and associated coronal features. The WL image clearly 3. ANALYSIS AND RESULTS
indicatesthattheactiveregionconsistsofseveralsmall-
3.1. Pre-eruption activities
to-intermediatesizedsunspotswith the largestone pos-
3.1.1. Formation and evolution of EUV sigmoid
sessingnegativepolarity(cf. Figures1(a)and(b)). Itis
interesting to note thatmost ofthe prominentsunspots Pre-eruptionactivitiesrefertotheprocessesleadingto
are of negative polarity. Further, there is scarcity of formation and activation of the EUV sigmoid that sub-
sunspots exhibiting positive polarity while the positive sequently erupts during the M6.5 eruptive flare. The
flux regionis dispersed overa largerarea (Figure 1(b)). evolution of the sigmoid in the pre-eruption phase is
The overall photospheric flux distribution suggests a illustrated in Figure 2 by a sequence of AIA 94 ˚A im-
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Figure 2. Series of AIA 94 ˚A images showing thedevelopment of the sigmoidal structure in active region NOAA11719 during
the pre-eruption phase. Note the ascend of the flux rope (FR) from the active region (marked with a dotted-box region and
arrows in panels (k)and (l), respectively).
ages. The AIA 94 ˚A channel (Fe XVIII; log(T)=6.8) is the whole regionevolves into a large coronalsigmoid at
apt for the understanding of structures associated with ∼ 04:30 UT (see Figure 2(h)). Further, during the sig-
high plasma temperature in the hot flaring corona. In moid formation, the western part of the active region
the beginning (∼00:00 UT), two closely situated bun- remains active in the form of continued episodic bright-
dlesofcoronalloopsareidentified(markedbyarrowsin ening anddiffuse emission(showninside the dottedbox
Figure 2(a)). At this stage, we cannot clearly identify in Figure 2(c)). This region is densely occupied with a
connectivity between these loops. From ∼1:40 UT, the cluster of low-lying loops.
intensityofthetwoloopsystemsincreasesandweclearly Itisstrikingtonotethe successiveemergenceofcoro-
noticetheestablishmentofaconnectivitybetweenthem nalloopsinaregionthatliebetweentwoJ-shapedbun-
in a sequential manner. This phase (∼1:40–2:00 UT) is dles of loops (indicated by arrows in Figure 2(f)-(g)).
characterizedbyabuild-upofbright,diffuseemissionin Further, the loops in this region brighten up several
the region that lies between the two loop systems. The times(∼3:00–3:15UT,∼3:55–4:10UT,∼4:30–4:40UT)
coupled loop system undergoes further expansion and untilthefulldevelopmentofthe sigmoidstructure. Itis
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Figure 3. Series of AIA 304 ˚A images showing the occurrence of sequential brightenings (indicated by arrows) from different
locationsofalongfilamentchannel. TheemergenceofanS-shapedfilamentcanbeclearlyseeninpanel(l)wheretherelatively
straight middle part along with hook-shaped eastern and western portions are indicated by arrows.
noteworthy that the two J-shaped bundle of loops suc- steadyexpansionofahotfluxrope(FR)wellbeforethe
cessive transform into a coherent sigmoid structure via onset of the impulsive flare emission (See Figure 2(l)).
transient loop brightenings that occur between them.
Finally, we highlight the rise of a large bundle of flux 3.1.2. Episodic energy release in the vicinity of the
filament channel
rope that evolved into a kinked structure toward the
south-east side of the sigmoid (see the region inside the In Figure 3, we present a sequence of AIA 304 ˚A im-
dotted box in Figure 2(k)). We clearly notice that the agesoftheARshowingtheincidencesofepisodicbright-
top portion of this flux rope exhibits writhing motions enings in the vicinity of a long filament channel during
withthesimultaneousexpansionofitslegsduringwhich thepre-eruptionphase(i.e.,between00:30UTand06:00
the core of the sigmoid brightens up at multiple loca- UT).TheAIA304˚Achannel(HeII;log(T)=4.7)images
tions(∼6:30UT).Wefurthermentiontheriseofanother thesolarstructuresformedatthechromosphereandthe
thread of this flux rope from the eastern leg of the sig- transitionregion. In Figure 4, we providecomposites of
moid. Theserisingstructuresclearlyrevealtheslowyet the AIA 94 ˚A and 304 ˚A images. These images reveal
6
Figure 4. Composite of the AIA 94 ˚A (red) and
304 ˚A (green) images. The images reveal an S-shaped fil-
ament channel underneaththe hot coronal sigmoid.
good spatial correlation between the filament channel
and overlying hot coronal sigmoid. The AIA 304 ˚A im-
ages reveal interesting evolutionary stages of the fila-
ment channels. At the very beginning (i.e., at ∼00:28
UT),weobservedaU-shapedfilamentatthesouth-west
partoftheARwithlocalizedbrighteningatitssouthern
side (marked by an arrow in Figure 3(b)). Thereafter,
we note episodic brightenings from various portions of
thefilamentchannel(markedbyarrowsinFigures3(d)-
(h))andsimultaneousextensioninthelengthoffilament
toward the north-east part of the AR. A small portion
of the filament, situated at its northern side, undergoes
confinederuptionandisassociatedwithanintenseEUV
Figure 5. Panel(a): HMILOSmagnetogramoverplottedon
brightenings at ∼03:05 UT (marked by arrow in Figure anAIA94˚Aimage. Thepositiveandnegativepolaritiesare
3(i)). At around 03:40 UT, the filament channel at- shown by red and blue contours, respectively, with contour
levels as ±50, ±100, ±150, ±200, ±250 G. This figure also
tainsitsmaximumlengthandaclearS-shapedstructure
shows theregions ofinterest for magnetic fluxcomputation:
emerges (see Figure 3(l)) which is the chromospheric full sigmoidal region (within the rectangular box) and core
counterpartofthecoronalsigmoidseeninAIA94˚Aim- region (enclosed by the curve at the central part). Panels
ages(Section 3.1.1). After complete developmentofthe (b)and(c): Thetemporalevolutionofmagneticfluxofpos-
itiveandnegativepolaritiesfromthefullsigmoidalandcore
filament,itseasternpartstartedlifting upandpartially
regions, respectively. Panel(d): GOESSXRfluxinthe0.5–
disrupted in next few minutes. At this stage, we ob- 4 and 1–8 ˚A channels. The vertical dashed line marks the
served ribbon like brightenings below the rising portion onset of the M6.5 flare. Note a sudden decrease in negative
of the filament (see Figure 3(m)). However, the south- flux in the core flaring region during the rise phase of the
flare (panel (c)). The dotted line in panel (c) indicates the
west portion of the filament has remained quiet.
time after which both positive and negative flux in the core
region undergo a general decrease for ∼1.5 hours. Notably,
3.2. Evolution of magnetic flux thisregion isassociated with multipletransient loop bright-
eningsduringtheextendedpre-eruptionphaseaswellasthe
It is well established that the coronal transients are
flux rope activation.
driven by the solar magnetic fields (e.g., reviews by
Priest & Forbes2002;Schrijver2009;Wiegelmann et al.
2014) . Therefore, it is crucial to explore how the pho- central part of the sigmoid (enclosed by a curve in Fig-
tospheric magnetic flux evolves prior and during the ure 5(a)). This centralregionis of particularinterestas
eruptive phenomena. In Figure 5(a), we overplot an transientloopbrighteningscontinuouslyoccurredinthis
HMI magnetogram (as contours) displaying the distri- regionduring whichthe two J-likebundles of loopssuc-
bution of photospheric line-of-sight magnetic fields on cessively transform into the sigmoid (discussed in Sec-
the AIA 94 ˚A image showing the coronal sigmoid. We tion 3.1.1; also see Figure 2(f)-(g)). Notably, during
haveselectedthefollowingtworegionsontheHMImag- the M6.5 flare, the HXR footpoint emission also occurs
netogram to investigate the evolution of magnetic flux withinthisregion,implyingthistobethecoreregionas-
during the sigmoid-to-arcade transformation: (1) the sociated with magnetic field lines involved in the large-
large region that encompasses the whole sigmoid (see scale magnetic reconnection process (see Section 3.3).
the region defined within the rectangular box in Fig- Thepositiveandnegativefluxthroughtheextendedac-
ure 5(a)), and (2) the smaller region which forms the tivitysiteandthesmallercoreregionareplottedinFig-
7
ures 5(b) and (c), respectively. HMI provides the line- 10-3
(a)
of-sightmagnetogramatacadenceof45swithaspatial 10-4
resolutionof0′′.5pixel−1. Weprocessedtheline-of-sight -2m) 10-5
magnetogramsfor10hrstartingfrom00:00UTon2013 W
April 11. The magnetograms are differentially rotated Flux ( 10-6
toacommonheliographiclocationandcorrectedforthe 10-7
line-of-sight effect by multiplying 1/cos(θ), where θ is
10-8
106
the heliocentric angle. We averaged4 magnetogramsto -1or) (b)
rmpeloadtgunctheeteitcGheflOunExoSies1ve.o0ll–euv8te.i0ol.nanFadonr0d.a5c–oc4roo.0mn˚AaplaflerunisxeornignybFeritegwlueeraeesne5,(tdwh)ee. -1unts sdetect 110045 6125-2501---2251 500k 0eKK VKeeVVeV
We find that the photospheric magnetic fluxes ofpos- e (co 103
at
itive and negative polarities undergo a continuous in- nt r 102
u
o
creasewithinthesigmoidalactiveregionduringthepre- c 101
eruptionphase,i.e.,00:00UTto06:55UT(Figure5(b)). 100 AIA 1600 (c)
Notably, this whole duration is characterizedby signifi- nsity AAAIIIAAA 11973411
cantcoronalactivities in the forms of localizedepisodes d intes 10
of energy release and evolution of bright loops from the alize
EUV sigmoidal region. Further, we note that the pos- orm
N 1
itive flux continues to rise following the flare onset at
06:55 UT while the negative flux maintains an almost
06:50 07:00 07:10 07:20 07:30 07:40
steadylevelfrom∼05:20UT tillthedecayofflareemis- Start Time (11-Apr-13 06:45:01)
sion at ∼07:30 UT. More interesting variations of mag- Figure 6. X-ray and EUV lightcurves of the M6.5 flare on
netic flux are observed in the core region (Figure 5(c)). 2013 April 11. Top panel: GOES SXR profiles at both 0.5–
Thenegativefluxundergoesaslowriseuntil∼05:20UT 4 and 1–8 ˚A energy channels. Middle panel: RHESSI HXR
lightcurvesin4differentenergybandsalongwithGONGHα
while the positive flux exhibits both increasing as well
intensity profile of the flaring region. Bottom panel: EUV
asdecreasingepisodes. Inthelaterphases(>05:20UT), lightcurves at 1600, 171, 131, and 94 ˚A channels. A vertical
thenegativeflux,ingeneral,decreases;Interestingly,the dashed line (at 07:06 UT) is drawn to differentiate between
early rise phase and late rise phase of theflare.
rateofdecreaseofnegativefluxishigherduringtherise
phase of the M6.5 flare. On the other hand, the evo-
lution of positive flux is rather striking. We note that in different energy bands (6-12, 12-25, 25-50, and 50–
the positive flux decreases during the pre-flare period 100 keV). This figure also presents normalized the Hα
(∼05:20–06:40UT). However, it changes its trend com- time profile of the flare showing the average intensity
pletely with the onset of the M6.5 flare and starts to enhancement of the flaring region with respect to the
increase. non-flaring background. For this purpose, we have an-
alyzed uninterrupted series of Hα filtergrams observed
3.3. The eruptive M6.5 flare
from the Global Oscillation Network Group (GONG).
3.3.1. Flare light curves
We findthatduringtheprolongedSXRrisephase,mul-
After a significantpre-eruptionactivities in the forms tiple high energy HXR peaks (>12 keV) are observed
of photospheric magnetic changes, formation, evolution (see Figure 6(b)). In Figure 6(c), E(UV) light curves
and activation of a long filament channel, and local- of the flare at 94, 131, 171, and 1600 ˚A passbands are
ized brightenings from various locations close to fila- shown.
ment channel, a large M6.5 flare occurred in AR 11719
3.3.2. Sigmoid-to-arcade evolution
which is associated with a fast halo CME. In Figure 6,
we present the flare light curves at multiple X-ray en- In Figure 7, we present series of AIA 94 ˚A images to
ergy channels from06:45to 07:45UT on 2013April 11. show the sequential evolution of the sigmoid into the
The GOES SXR profiles presented in Figure 6 clearly post-flarearcadeduringvariousphasesoftheenergyre-
indicate the onset of flare emission at ∼6:55 UT while lease in the M6.5 flare. The RHESSI HXR contours in
gradually declining SXR flux lasted until >7:40 UT. It different energy bands (12–25 keV: yellow, 25–50 keV:
is important to note a rather prolonged rise phase of red, and 50–100 keV: blue) are also over plotted on se-
∼21 minute (from 06:55 UT to 07:16 UT). RHESSI ob- lectedEUVimages. Werecallthatthefluxropeascends
served this prolonged rise phase but missed subsequent from ∼06:00UT, i.e., about anhour before the onsetof
phases(from∼07:06to07:45UT)duetosatellitenight- the M6.5 flare (Section 3.1.1). We note that the rising
time. In Figure 6(b), we provide RHESSI light curves fluxroperemainedinaquasi-stationarystateforseveral
8
Figure 7. SequenceofAIA94˚Aimages showing expansionand eruptionofthefluxrope(FR)duringM6.5 flare. Co-temporal
RHESSIHXRsourcesin12–25keV(yellow),25–50keV(red),and50–100 keV(blue)arealsooverplottedontherepresentative
AIA94˚Aimages (panels(c)–(e)). Abrightpost-flareloop arcadeisformed after eruptionofthefluxrope(seepanels(f)–(h)).
RHESSIimages arereconstructedwithCLEANalgorithm with1minintegration time. Thecontourlevelsareset as50%,70%
and 90% of thepeak fluxin each image. (An animation of AIA 94 ˚A observations is available online at ApJwebsite).
minutes (∼6:30–6:55 UT) before undergoing rapid ex- occupiedwiththesigmoidinthepre-eruptionphase. In
pansion from ∼6:55 UT which marks the impulsive rise Figure8,wepresentafewrepresentativeAIA131˚Aim-
of the flare emission (Figures 6 and 7). With the rapid agesoftheactivitysite. TheAIA131˚Achannel(FeVIII,
eruption of the flux rope, we observe intense brighten- XXI; log(T)=5.6, 7.0) observes plasma structures in the
ings in the source region which subsequently evolved transitionregionandtheflaringcorona. Fromtheseim-
into two distinct flare ribbons. The co-temporal AIA ages,weclearlynoticethatfollowingtheeruptionofthe
94˚A andRHESSI HXR imagesrevealthat the highen- flux rope, a cusp forms at the apex of hot EUV loops
ergyemissionisentirelyassociatedwiththemiddlepor- (marked by an arrow in Figure 8(c)).
tion of the EUV sigmoid. Further HXR emission origi-
3.3.3. Early rise phase and deviation from the standard
natesintheformofkernelsthatlieoverconjugateEUV
flare model
flareribbons. FollowingthepeakphaseofSXRemission
(∼7:16 UT; Figure 6), a beautiful system of post-flare From the X-ray and EUV flux profiles, it is evident
arcadesenveloptheflaringregion(Figure7(h))thatwas that the event under study belongs to the category of
long durationevent(LDE)which aremarkedbytypical
9
Figure 8. Representative AIA 131 ˚A images showing the important stages of theeruption. After the eruption of the fluxrope
(FR),theflaringregion isenvelopedbyabrightloop arcade. Following theeruption,wenoteacuspatthenorthernendofthe
loop arcade (panel(c)).
evolution of long, parallel flare ribbons in the chromo- of flare ribbons with respect to photospheric magnetic
sphere. However,duringtheprolongedrisephaseofthis polarities. It is noteworthy that the Hα filament nicely
flarewithadurationof∼21minutes(Figure6),theflare delineatestheseparationofoppositemagneticpolarities
ribbons and associated HXR sources exhibit a compli- in the photosphere and, therefore, can be considered to
cateddynamicalevolution. Forthedetailedstudyofthe approximately outline the polarity inversion line (PIL).
evolutionofflareribbons,we havepresenteda sequence Both Hα and UV images clearly reveal that the east-
ofKSOHαandAIA1600˚AUVimagesinFigure9. The ern flare ribbon extends toward south-west direction,
sequence of Hα filtergrams provide information about parallel to the PIL until ∼7:06 UT while the western
the flare morphology in the chromosphere and response flare ribbon evolves in the opposite direction. The spa-
of coronal energy release phenomena at this layer. The tial evolution of flare ribbons observed during the early
AIA 1600˚Achannel(C IV+cont.; log(T)=5.0)observes rise phase is not consistent with the canonical picture
combined emission from the transition region and the of eruptive two-ribbon flares. In particular, the con-
upper photosphere. The co-temporal HXR contours in jugate J-shaped flare ribbons apparently move toward
25–50 keV (red) are also overplotted on a few represen- each other parallel to the PIL.
tative Hα images (Figures 9(c),(e)–(g)). TocomparetheevolutionoflowandhighenergyX-ray
TheearlyHαflareemissionsareoriginatedintheform emitting regions, we have shown a sequence of RHESSI
ofbrightkernels(see Figures9(a) and(b)) that evolved 6–12 keV images (gray background) overlaid by co-
into the flare ribbons subsequently(markedasthe east- temporal 25–50 keV (yellow contours) in Figure 10. As
ern and western flare ribbons in Figure 9(b)). It is im- describedearlier,weobservedtwodistinctHXRsources
portant to note that the flare ribbons exhibit J-shaped (marked as FP-east and FP-west in Figure 10(b)). Due
structure during the early stages (Figures 9(b)–(f)). In to the limited observations from RHESSI, we could in-
particular,thisJ-shapedmorphologyisquiteprominent vestigatethemotionofHXRsourcesonlyduringtheini-
for the eastern flare ribbon which is also larger and un- tial∼6minutesoftherisephase. Wefindthatthesepa-
dergoes more dynamic evolution during the early rise ration between conjugate HXR sources decreases in the
phase of the flare (i.e., between 06:55 and 07:06 UT). successive images, further confirming the observed con-
A comparison of Hα filtergrams with the evolution of verging motions of flare ribbons (see also Figures 9(c),
the sigmoid seen in AIA 94 ˚A images (cf. Figures 7 (e)-(g)). We also note that initially the FP-westis rela-
and 9) suggests that the hooked part of the J-shape tively weak and does not show much spatial variations.
for the eastern flare ribbon is associated and probably On the other hand, FP-east is stronger and moves to-
physically linked with the eruption of the eastern por- wards the FP-west. Further, the intensity of FP-west
tionofthe overlyingflux rope(FR).Onthe otherhand, increases with the rise phase and becomes comparable
the westernflareribbonis shorterandexhibits asimple totheFP-eastby∼07:02UT(seeFigure10(e)and(f)).
morphology. We note that the HXR conjugate sources Also by this time, both the FP sources come closest to
in 25–50 keV energy bands nicely correlates with the eachotherandtheircorrespondingflareribbonsbecame
brightestpartoftherespectiveHαflareribbons(seeFig- almost parallel (see Figure 9(n)). The 6–12 keV X-ray
ures9(c),(e)-(g)). InFigures9(d)and(i), wehaveover- source exhibits a single structure and moves slowly to-
plotted HMI mangetograms over the co-temporal Hα ward south-west during the rise phase.
and UV images, respectively, to compare the evolution
10
KSO 06:55:05 UT KSO 06:56:55 UT KSO 06:57:56 UT KSO 06:58:32 UT
350 (a) (b) (c) (d)
300
Y (arcsecs) 250 eastern flawree sritbebrnon
200 flare ribbon
150
KSO 0 6:59:5 2 UT KSO 0 7:00:5 3 UT KSO 0 7:02:4 4 UT KSO 0 7:05:4 2 UT
350 (e) (f) (g) (h)
300
Y (arcsecs) 250
200
150
AI A 160 0 06:5 6:16 U T AI A 160 0 06:5 7:52 U T AI A 160 0 06:5 9:28 U T AI A 160 0 07:0 1:04 U T
350 (i) (j) (k) (l)
300
Y (arcsecs) 250
200
150
AI A 160 0 07:0 1:52 U T AI A 160 0 07:0 3:52 U T AI A 160 0 07:0 5:04 U T AI A 160 0 07:0 5:52 U T
350 (m) (n) (o) (p)
300
Y (arcsecs) 250
200
150
-350 -300 -250 -200 -150 -350 -300 -250 -200 -150 -350 -300 -250 -200 -150 -350 -300 -250 -200 -150
X (arcsecs) X (arcsecs) X (arcsecs) X (arcsecs)
Figure 9. KSO Hα (panels (a)-(h)) and AIA 1600 ˚A (panels (i)-(p)) images showing the temporal evolution of flare brighten-
ings/ribbons during early rise phase (i.e., 06:55–07:05 UT; see Figure 6). During this early phase, the flare ribbons present a
J-shaped morphology and show lateral extension toward each other. Note that by the end of this early phase (∼07:05 UT),
flare ribbons become parallel to each other and exhibits “standard” morphology and spatial evolution thereafter. The eastern
and western ribbons are indicated by arrows in panel (b). The co-temporal HXRcontours in 25–50 keV (red) energy band are
overplotted on a few representative Hαimages (panels (c), (e)–(g)). In panels (d) and (i), thecontours represent themagnetic
polarity distribution (blue: negative, red: positive) over Hα and AIA 1600 ˚A images respectively. RHESSI images are recon-
structed with PIXON algorithm with 1 min integration time. The contour levels are set as 10%, 20%, 40%, and 80% of the
peak flux in each image.
3.3.4. Standard phase of the eruptive flare ribbon undergoes more dynamic evolution and lateral
expansion compared to the western flare ribbon (Fig-
The early rise phase (06:55-07:06 UT; see previous ure 11). Notably, the eastern flare ribbon remains pro-
section) is characterized by the converging motions of longed and bright until the late gradual phase of the
flare ribbons. In the subsequent stages, we note ‘stan- event while the western flare ribbon appreciably decays
dard’evolutionofapairofwelldeveloped,classicalflare in length as well as intensity of the emission. In this
ribbons during which they apparently move away from context, we emphasize that the western ribbon is asso-
eachotherintheperpendiculardirectiontothePIL.To ciated with stronger magnetic field (negative polarity)
show the standard phase of the M6.5 flare, we present while the easternribbonforms overthe weakeranddis-
KSO Hα, AIA 1600 ˚A, and AIA 304 ˚A images in Fig- persedfluxregions(positivepolarity)inthephotosphere
ure11. Wenotethat,outofthetworibbons,theeastern (Figure 11(g)). These observations indicate crucial ev-