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Origin and structures of solar eruptions I: Magnetic flux rope

July 2017
Science China Earth Sciences 60(8)

DOI:10.1007/s11430-017-9074-6

Authors:

X. Cheng

Nanjing University

Yang Guo

Nanjing University

Mingde Ding

Nanjing University

Coronal mass ejections (CMEs) and solar flares are the large-scale and most energetic eruptive phenomena in our solar system and able to release a large quantity of plasma and magnetic flux from the solar atmosphere into the solar wind. When these high-speed magnetized plasmas along with the energetic particles arrive at the Earth, they may interact with the magnetosphere and ionosphere, and seriously affect the safety of human high-tech activities in outer space. The travel time of a CME to 1 AU is about 1–3 days, while energetic particles from the eruptions arrive even earlier. An efficient forecast of these phenomena therefore requires a clear detection of CMEs/flares at the stage as early as possible. To estimate the possibility of an eruption leading to a CME/flare, we need to elucidate some fundamental but elusive processes including in particular the origin and structures of CMEs/flares. Understanding these processes can not only improve the prediction of the occurrence of CMEs/flares and their effects on geospace and the heliosphere but also help understand the mass ejections and flares on other solar-type stars. The main purpose of this review is to address the origin and early structures of CMEs/flares, from multi-wavelength observational perspective. First of all, we start with the ongoing debate of whether the pre-eruptive configuration, i.e., a helical magnetic flux rope (MFR), of CMEs/flares exists before the eruption and then emphatically introduce observational manifestations of the MFR. Secondly, we elaborate on the possible formation mechanisms of the MFR through distinct ways. Thirdly, we discuss the initiation of the MFR and associated dynamics during its evolution toward the CME/flare. Finally, we come to some conclusions and put forward some prospects in the future.

a) Hα image overlaid by the extrapolated magnetic field lines. The MFR indicated by mixed colors is cospatial with a segment of the filament (adapted from Guo et al., 2010b. Reproduced by permission of the AAS). (b) Twist releasing by the reconnection during the filament eruption (for details please see Xue et al., 2016. Reproduced by permission of the AAS). (c) Coronal cavities as seen in the AIA 193 Å passband. (d) A sequence of XRT images showing the evolution of a sigmoid prior to the eruption (from McKenzie and Canfield, 2008).

…

a) AIA 131 Å images showing the pre-existence (left) and eruption (right) of a hot channel-like MFR (from Zhang et al., 2012; Cheng et al., 2013a. Reproduced by permission of the AAS). (b) Transformation of a hot channel-like MFR, as seen in the AIA 131 Å passband (left), to the CME imaged by the LASCO C2 white-light coronagraph (right).

…

a) Flux emergence model, in which a twisted MFR bodily emerges from below the photosphere to the corona. The lines in violet show baldpatch-associated separatrix surfaces. The black segments display magnetic dips where the filament materials can be collected (for details please see Gibson et al., 2004. Reproduced by permission of the AAS). (b) Flux cancellation model, in which the MFR is formed by the reconnection of two groups of sheared arcades driven by the shearing and converging motions (for details please see van Ballegooijen and Martens, 1989. Reproduced by permission of the AAS).

…

a) NVST TiO and Hα images overlaid by line-of-sight magnetograms with the positive (negative) in blue (red) showing the formation of a filament driven by the sunspot rotation (from Yan et al., 2015. Reproduced by permission of the AAS). (b) NVST Hα images displaying the formation of a filament by the reconnection (from Yan et al., 2016. Reproduced by permission of the AAS).

…

Emission measure maps at different temperature intervals and instants showing the formation of a sigmoidal hot channel-like MFR in the active region 11520, which can be clearly seen in panels (g)-(i) (adapted from Cheng et al., 2014b. Reproduced by permission of the AAS).

…

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SCIENCECHINA

EarthSciences

• 

REVIEW 

•doi:10.1007/s11430-017-9074-6

OriginandstructuresofsolareruptionsI:Magneticuxrope

CHENGXin1,2*,GUOY ang1,2&DINGMingDe1,2

1SchoolofAstronomyandSpaceScience,NanjingUniversity ,Nanjing210093,China;

2KeyLaboratoryforModernAstronomyandAstrophysics(NanjingUniversity),MinistryofEducation,Nanjing210023,China

ReceivedFebruary17,2017;acceptedJune15,2017;publishedonlineJuly10,2017

AbstractCoronalmassejections(CMEs)andsolararesarethelarge-scaleandmostenergeticeruptivephenomenainoursolar

systemandabletoreleasealargequantityofplasmaandmagneticuxfromthesolaratmosphereintothesolarwind.Whenthese

high-speedmagnetizedplasmasalongwiththeenergeticparticlesarriveattheEarth,theymayinteractwiththemagnetosphere

andionosphere,andseriouslyaffectthesafetyofhumanhigh-techactivitiesinouterspace.ThetraveltimeofaCMEto1AUis

about1–3days,whileenergeticparticlesfromtheeruptionsarriveevenearlier.Anefcientforecastofthesephenomenatherefore

requiresacleardetectionofCMEs/aresatthestageasearlyaspossible.Toestimatethepossibilityofaneruptionleadingto

aCME/are,weneedtoelucidatesomefundamentalbutelusiveprocessesincludinginparticulartheoriginandstructuresof

CMEs/ares.UnderstandingtheseprocessescannotonlyimprovethepredictionoftheoccurrenceofCMEs/aresandtheireffects

ongeospaceandtheheliospherebutalsohelpunderstandthemassejectionsandaresonothersolar-typestars.Themainpurpose

ofthisreviewistoaddresstheoriginandearlystructuresofCMEs/ares,frommulti-wavelengthobservationalperspective.First

ofall,westartwiththeongoingdebateofwhetherthepre-eruptiveconguration,i.e.,ahelicalmagneticuxrope(MFR),of

CMEs/aresexistsbeforetheeruptionandthenemphaticallyintroduceobservationalmanifestationsoftheMFR.Secondly,we

elaborateonthepossibleformationmechanismsoftheMFRthroughdistinctways.Thirdly,wediscusstheinitiationoftheMFR

andassociateddynamicsduringitsevolutiontowardtheCME/are.Finally,wecometosomeconclusionsandputforwardsome

prospectsinthefuture.

KeywordsCoronalmassejections,Flares,Magneticuxropes,Magneticeld,EUV/UVemissions,Photosphere,Corona,

Particleacceleration

Citation:ChengX,GuoY ,DingMD.2017.OriginandstructuresofsolareruptionsI:Magneticuxrope.ScienceChinaEarthSciences,60,doi:10.1007/s11430-

017-9074-6

1. 

Introduction

Solareruptionsrefertovariousphenomenathatinvolvean

outowofplasmaandmagneticuxfromthesolaratmos-

phereintothesolarwindsuchasspicules,jets,surges,coronal

massejections(CMEs),andaresetc..Amongthem,CMEs

andaresarethelarge-scaleeruptiveandenergeticprocesses

thatusuallyaccompanywitheachotherthoughnotalways

(SheeleyJr.etal.,1983;Kahler,1992;Yashiroetal.,2006).

Aftershotout,CMEsoftendisplayatypicalthree-compo-

*Correspondingauthor(email:xincheng@nju.edu.cn)

nentstructure:aleadingfrontfollowedbyanencloseddark

cavityandanembeddedbrightcore(IllingandHundhausen,

1983)asseeninwhite-lightcoronagraphs.Thedarkcavity

andbrightcorearebelievedtobemanifestationsofamag-

neticuxrope(MFR),whichisdenedasacoherentlyhe-

licalmagneticstructurewithalleldlineswrappingaround

thecentralaxisatleastoneturn.Thedarkcavitymaycorre-

spondtothecrosssectionoftheMFRandthebrightcoreto

thecoollament/prominencematerialslocatedatthebottom

oftheMFRwhenviewededge-on(Dereetal.,1999;Gibson

etal.,2007;Rileyetal.,2008).

Besidesbeinglledwithahelicalmagneticstructure,

CMEsalsoexperienceanaccelerationprocessofshortpe-

riod(~tensofminutes;Zhangetal.,2001;ZhangandDere,

2006),nallyreachinghighvelocitiesrangingfromhundreds

tothousandsofkm s−1(Yashiroetal.,2004;Tianetal.,2012;

Fengetal.,2013).After1–3days,thesehigh-speedhelical

plasmoidsmayarriveattheEarth(Liuetal.,2011,2013,

2017;Shenetal.,2012a;HessandZhang,2015;Shietal.,

2015;Huetal.,2016;Temmeretal.,2017).Thetypical

featuresofCMEsintheinterplanetaryspace,suchasrotation

ofmagneticeld,increasedsolarwindspeed,depressed

protontemperature,andlowplasmabeta,canbeobserved

directlyviainsituinstruments(e.g.,Burlaga,1988;Lepping

etal.,1990).WhentheinterplanetaryCMEsinteractwith

themagnetosphereandionosphere,theyprobablygiverise

toseriousinuencesonthesafetyofhumanhigh-techac-

tivitiesinouterspace,suchasdisruptingcommunications,

overloadingpowergrids,presentingahazardtoastronauts,

andsoon(Gosling,1993;Webbetal.,1994;Shenetal.,

2013;Solankietal.,2004;Liuetal.,2014a;Shietal.,2015)

Inordertopredicttheproductsandtheirinuencesinduced

bysolareruptions,elucidatingsomefundamentalbutelu-

siveprocessesincludingtheiroriginandstructuresandsub-

sequentSun-to-Earthpropagationisamatterofgreatimpor-

tance.Inthepastdecades,manysignicantprogresseshave

beenmadeinthisaspect,thereadercanrefertomanyprevi-

ousreviews(e.g.,Forbesetal.,2006;Chen,2011;Schmieder

etal.,2015;Linetal.,2015;Wangetal.,2016a;Byrneet

al.,2010;Lugazetal.,2015;Möstletal.,2017).Inthecur-

rentreview,weelaborateonrecentprogressesonthestudy

oftheoriginandstructuresofCMEs/aresfrommulti-wave-

lengthobservationalperspective,whicharemostlyascribed

tothelaunchofSolarDynamicsObservatory(SDO;Pesnell

etal.,2012).W ealsointroducesomerelevantresultsfrom

SolarT errestrialRelationsObservatory(STEREO;Kaiseret

al.,2008),InterfaceRegionImagingSpectrograph(IRIS;De

Pontieuetal.,2014),andnewlyconstructedground-based

instrumentsliketheNewSolarT elescope(NST;Caoetal.,

2010)atBigBearSolarObservatoryandtheNewVacuum

SolarT elescope(NVST;Liuetal.,2014b)atYunnanObser-

vatory(FuxianLake).Firstofall,westartwiththequestions

ofwhetherahighlyhelicalMFRisnecessaryfortheerup-

tionandwhethertheMFRexistspriortotheeruption.We

thenemphaticallyintroducetheobservationalmanifestations

oftheMFR.Secondly,weelaborateonthepossibleforma-

tionmechanismsofthedifferentmanifestationsoftheMFR

inSection3.Thirdly,wediscusstheinitiationmechanismsof

theMFRandthedynamicsduringtheevolutionoftheMFR

towardtheCME/areinSection4.Intheend,wecometo

conclusionsandpresentsomeprospectsthatshouldbead-

dressedinthefuture.

Thisreviewisfocusedontheobservationalaspect.

Themagneticmodellingaspectoftheoriginandstruc-

turesofCMEs/aresisgiveninanotherreviewbyGuoetal.

(2017).

2. 

Pre-eruptivecongurationsofsolarerup-

tions

Inthe2DstandardCME/aremodel(Carmichael,1964;Stur-

rock,1966;Hirayama,1974;KoppandPneuman,1976;Shi-

bataetal.,1995),thepre-eruptiveconguration,whichis

modelledtobeahelicalMFR(Shibataetal.,1995;Chen,

1996;TitovandDémoulin,1999;Vourlidasetal.,2013)or

shearedarcade(Sturrock,1966;Antiochosetal.,1999),is

constrainedbythebackgroundmagneticelds.Theeruption

ofthepre-eruptivecongurationstretchesthebackground

eldstoformamagneticdissipationregion,aso-calledcur-

rentsheet(CS),inbetweentheirtwolegs.Oncethethick-

nessoftheCSislessthanathreshold,magneticreconnection

willbeswitchedon(LinandForbes,2000).Ontheonehand,

thereconnectionacceleratestheeruptionviacontinuouslyin-

jectingpoloidaluxintotheeruptingstructure.Ontheother

hand,thereconnectionreleasesalargequantityofenergythat

inducesarapidlyenhancedradiationoverthewholeelectro-

magneticspectrumrangingfromdecameterradiowavestoγ

rays.

Atpresent,thenatureofthepre-eruptivecongurationis

stillelusive.Ontheonehand,observationsshowthatthe

pre-eruptivecongurationcouldbeshearedarcades,indicat-

ingthattheMFRcouldbeunnecessaryforinitiatingtheerup-

tion(e.g.,Songetal.,2014a;Ouyangetal.,2015).Onthe

otherhand,afewobservationsimplythatthepre-eruptive

congurationisahelicalMFR(e.g.,LowandHundhausen,

1995;Gibsonetal.,2006;GreenandKliem,2009;Zhanget

al.,2012;Chengetal.,2013a;Patsourakosetal.,2013).Con-

sideringthatthemagneticeldinthesolaratmospherecannot

bemeasuredaccuratelyexceptonthephotosphere,thecom-

munityusuallyresortstoindirectobservationsorextrapola-

tiontechniquessuchasnon-linearforce-freeeld(NLFFF)

modellingtosearchfortheevidenceoftheMFR.Forex-

ample,throughtheNLFFFmodelling,stronglytwistedeld

lineswithasubstantialmagnetichelicityareoftenfoundto

existalongthepolarityinversionline(PIL)ofactiveregions

beforetheeruption(e.g.,Yanetal.,2001;CanouandAmari,

2010;Guoetal.,2010b;Savchevaetal.,2012;Chengetal.,

2013b,2014b;Inoueetal.,2013;Jiangetal.,2014a;Jianget

al.,2016c;Y anetal.,2015).Inthefollowing,weintroduce

variousobservationalevidencesfortheexistenceoftheMFR

indetail.

2.1 

Filamentsandlamentchannels

Filamentsareaphenomenonofrelativelycoolanddense

plasmaembeddedinthehotandtenuouscorona,commonly

observedinabsorptioninHαandtheExtremeUltraviolet

(EUV)passbandsonthesolardisk,whileappearinginemis-

sionasbrightfeatures,i.e.,prominences,againstthedark

backgroundwhenseenabovethesolarlimb(Hirayama,1985;

2ChengX,etal.SciChinaEarthSci

Mackayetal.,2010).

Themagneticstructureoflamentsisusuallythoughttobe

shearedarcades(Antiochosetal.,1994;Aulanieretal.,2006)

orhighlytwistedMFR(KuperusandRaadu,1974;vanBal-

legooijenandMartens,1989;Aulanieretal.,1998;Aulanier

etal.,1999),whichpossessmagneticdipsthatareableto

provideanupwardmagnetictensionagainstthegravityof

lamentmaterials(Martin,1998;Mackayetal.,2010).In

ordertovalidatesuchapicture,manyauthorsextrapolated

three-dimensional(3D)structuresofthelamentsourcere-

gionsbasedontheassumptionoflinear(e.g.,Aulanieretal.,

1998;Aulanieretal.,1999)ornon-linearFFF(e.g.,Guoet

al.,2010b;Savchevaetal.,2012;Chengetal.,2014b;Jiang

etal.,2016c;Y anetal.,2015).Inmanyevents,inparticular

activeregionlaments,thedipsoftheshearedarcades(Anti-

ochosetal.,1994;Aulanieretal.,2006)ortwistedelds(e.g.,

Chengetal.,2014b;Jiangetal.,2016c)aremostlyco-spa-

tialwiththelamentlocations.Sometimes,apartofthela-

mentlocationsareconsistentwiththedipsofshearedarcades,

whiletheotherpartwiththedipsoftwistedelds(Figure1a;

Guoetal.,2010b).Duringthelamenteruption,thetwistof

magneticeldlinesisalsoobservedtobereleasedbymag-

neticreconnection(Figure1b;Xueetal.,2016).

Insomeevents,itisverydifculttoreconstructthestrongly

twistedeldlinescomparablewiththelaments,inparticu-

larforthequiescentlaments,whichareprobablyduetothe

reasonthatthepreprocessingover-smoothesthevectoreld

beforedoingtheextrapolation.However,usingthenewlyde-

velopedCESE-MHD-NLFFFcodebyJiangandFeng(2012),

Jiangetal.(2014b)reproducedalarge-scalecoronalMFR

thatexistsinbetweenanactiveregionandaweakpolarityre-

gionandsupportsaquiescentlament.Itisevenfoundthat

thelargepolarcrownprominencelocatedattheweakmag-

neticeldregioncanalsobemodelledwithanMFRcong-

urationalthoughwithacertaindegreesoffreedom(e.g.,Su

andvanBallegooijen,2012;Suetal.,2015).

Itispossiblethattherearenocoolmaterialsdeposited

indipsofMFRs.Inthiscase,theMFRsmaymanifestas

lamentchannelsandusuallylieoverthePILofthelong

decayedactiveregions(e.g.,vanBallegooijenetal.,1998;

AulanierandSchmieder,2002;Chenetal.,2014c).Itisalso

observedthattheeruptingMFRdoesnotaccompanywith

apre-existinglament,suchasforadouble-deckercong-

urationthatconsistsofahigh-lyingMFRandavertically

separatedlament-associatedlow-lyinguxsystem(e.g.,

Liuetal.,2012;Chengetal.,2014b;Dudíketal.,2014;

Kliemetal.,2014b).Withtheeruptionbeginning,onlythe

high-lyingMFRuxeruptstogiverisetoaCMEandaare,

whilethelow-lyinglamentremainsinoriginalplace.

2.2 

Coronalcavities

Whenquiescentlamentsandlamentchannelsrotatetothe

solarlimb,theyareprobablyseenasdark,semi-circularor

circularcavitiessurroundingprominencesandembeddedin

bipolarhelmetstreamer(Figure1c).Cavitiesinactivere-

Figure1 

(a)Hαimageoverlaidbytheextrapolatedmagneticeldlines.TheMFRindicatedbymixedcolorsiscospatialwithasegmentofthelament

(adaptedfromGuoetal.,2010b.ReproducedbypermissionoftheAAS).(b)Twistreleasingbythereconnectionduringthelamenteruption(fordetailsplease

seeXueetal.,2016.ReproducedbypermissionoftheAAS).(c)CoronalcavitiesasseenintheAIA193Åpassband.(d)AsequenceofXRTimagesshowing

theevolutionofasigmoidpriortotheeruption(fromMcKenzieandCaneld,2008).

ChengX,etal.SciChinaEarthSci3

gionsareverydifculttoobserve,becausetheylierelatively

lowtothesolarsurfaceandaresignicantlyinuencedby

strongemissionfromtheforegroundandbackground.At

present,theyhavebeenobservedonlyinfeweventsasan

eruptinghotblob(e.g.,Songetal.,2014b).Itisalsoar-

guedthatthemagneticstructureofcavitiesisanMFR,i.e.,

thecrosssectionoftheMFRcorrespondstothewholeor

lowerpartofcavities(LowandHundhausen,1995).Prior

totheeruption,thecavitiestypicallyexistinthelowcorona

andareabletosurvivefordays,evenformonths(Gibson

etal.,2006,2007).Itcanbeobservedatarangeofwave-

lengths,mostlyinthewhite-lightpassbandofsuchasthe

ground-basedwhite-lightcoronagraphMarkIVcoronameter

installedattheMaunaLoaSolarObservatory,aswellinthe

EUVpassbandsofsuchastheAtmosphericImagingAssem-

bly(AIA;Lemenetal.,2012)onboardSDO.

Manyfeaturesofcoronalcavitiesindicatethattheirfunda-

mentalmagneticstructureisanMFR.Therstevidenceis

continuousspinningmotions,whicharefrequentlyseenin-

sidecavitiesandhaveaowspeedof5−10 km s−1(Wang

andStenborg,2010).Moreover,thepolarizationringincav-

itiesobservedbyCoronalMulti-ChannelPolarimeteralso

supportstheMFRmodel,whichillustratesthatabrightring

oflinearpolarizationmayappearinadensitydepletedre-

gion(Doveetal.,2011).Inlinearpolarizationobservations,

Bak-Stęślickaetal.(2013)furtherfoundthatthecavitypos-

sessesacharacteristic“lagomorphic”signature,whichagain

indicatestheexistenceoftheMFRasapatternofconcentric

rings.

Solar“tornadoes”,anewphenomenondiscoveredrecently

andoftenappearingincavityassociatedprominences,are

alsoconsideredasapieceofevidenceoftheMFR.Thedi-

rectevidenceof‘tornadoes”havingahelicalstructureisthe

swirlingmotions(ZhangandLiu,2011;Lietal.,2012;Su

etal.,2012;Wedemeyer-Böhmetal.,2012).Spectroscopic

observationsalsodisclosedapatternwithblueshiftedand

redshiftedemissions,i.e.,oppositevelocities,existingatthe

twosidesofprominences,implyingthemagneticstructureof

“tornadoes”beinghelical(Suetal.,2014).However,spectro-

scopicobservationsincoollines(e.g.,Hαand10830 Å)re-

vealedthattheDopplershiftpatterndoesnotfollowthepat-

ternobservedincoronallines.Itismostlikelyoscillationsof

theplasmaalongtheeldlineslikecounterstreamingoroscil-

lationsofthewholemagneticstructure(MartínezGonzálezet

al.,2016;Schmiederetal.,2017).Tornadoescouldbejustthe

foopointsofprominences(Wedemeyeretal.,2013;Levenset

al.,2016)oramanifestationofspirallyejectedjetsdrivenby

torsionalAlfvénwaves(Pariatetal.,2009a).

2.3 

Sigmoids

Sigmoids,forwardorreversedsigmoidalemissionpatterns

appearinginEUVandsoftX-ray(SXR)passbands(Figure

1d),havebeenfoundtobeanimportantpre-eruptivecong-

urationofCMEs/ares(Hudsonetal.,1998;RustandKu-

mar,1996;SterlingandHudson,1997;Gibsonetal.,2002),

whicharestatisticallymorelikelytobeeruptive(Caneld

etal.,1999).Basedonthedurationtime,sigmoidscanbe

classiedastransientorpersistentones.Theformertendto

besharperandbrighter,apparentlyassigmoidalloops,and

evolveintocuspsorarcadesofloopsmanytimes;thelatter

appearmorediffuseandcouldbeacollectionofsomesheared

loops(Pevtsov,2002;Gibsonetal.,2002;Greenetal.,2007).

Thesigmoidalemissionpatternisexpectedtobeduetothe

heatinginacurvedCSattheinterfacebetweenthehelical

coreeld(e.g.,MFR)andtheambienteld(Kliemetal.,

2004;Gibsonetal.,2006).Greenetal.(2007)evenfound

that,duringtheeruptionphaseofsigmoids,thehelicitysign

ofsigmoidsisconsistentwiththerotationdirectionofassoci-

atederuptinglaments(alsoseeYangetal.,2015b),showing

theconversionoftwistintowritheundertheassumptionof

helicityconservation,supportingtheexistenceofthetwisted

eldlinesinsigmoids.

Theappearanceofthesigmoidalemissionpatterndoes

notmeantheexistenceofcontinuoussigmoidaleldlines.

McKenzieandCaneld(2008)analyzedalong-lasting

coronalsigmoidandfoundthattheoverallSshapeofthe

sigmoiddenitelyconsistsoftwoseparateJ-shapedloops

withastraightsectionpossiblylyinginthemiddle.Green

andKliem(2009)andLiuetal.(2010)pointedoutthattwo

oppositeJ-shapedloopscanformthecontinuousS-shaped

loopsthroughthetether-cuttingreconnection.Usingan

MHDsimulation,Aulanieretal.(2010)reproducedsynthetic

SXRimagesfromthedistributionoftheelectriccurrentsand

revealedtheformationofasigmoidalactiveregion.They

foundthatabrightsigmoidalenvelopeisbuiltupgradually

bythebald-patch(BP,wheremagneticeldlinesarecurved

upwardandaretangenttothephotosphere)andtether-cutting

reconnectionbetweentwopairofJ-shapedeldlines.Using

theuxemergencemodel,Archontisetal.(2009)evendis-

closedthatoppositeJ-shapedloopsandS-shapedloopsexist

simultaneously,whichresultintheoverallmagneticstructure

ofsigmoids.Moreover,someauthorsalsoreconstructed3D

NLFFFcongurationofsourceregionsofsigmoidsanddid

ndthatthecoreeldconsistsofatwistedMFRembedded

inhighlyshearedelds(e.g.,Suetal.,2009;Savchevaand

vanBallegooijen,2009;Savchevaetal.,2012;Jiangetal.,

2013,2014a;Chengetal.,2014b).

2.4 

Hotchannels

Hotchannelsorhotblobsareatypeofnewandpromisingev-

idenceoftheexistenceofMFRs.Throughanalysingalimb

event,Chengetal.(2011b)forthersttimeobservedthe

formationofanMFRduringtheimpulsivephase.Itinitially

appearsasaneruptinghotblobasseenintheAIA131and

4ChengX,etal.SciChinaEarthSci

94 Åpassbands(T≥8MK).Whileintheotherlowtemper-

aturepassbands(1MK≤T≤5MK),itappearsasadarkcav-

ity.Combingwithsometypicalfeaturessuchastheinows,

stretchedoverlyingeld,andcusp-shapedareloopsthatare

expectedbytheMFReruptionmodelsofCME/ares,theau-

thorsstronglyarguedthatthehotblobisanunambiguousevi-

denceoftheMFRexistinginthecorona(alsosee;Songetal.,

2014b).FollowingtheworkofChengetal.(2011b),Zhang

etal.(2012)andChengetal.(2013a)startedtosearchfor

moreevidenceoftheMFRinthe131and94 Åpassbands.

TheydiscoveredthattheMFRevenexistspriortotheerup-

tionasawrithedchannel-likestructurewithtwoelbowsin-

cliningtotheoppositedirectionsandthemiddlebeingcon-

cavedtowardthesurfacewhenseenoffthesolarlimb(Fig-

ure2a).Thevisibilityofthechannel-likestructureonlyat

theAIAhightemperaturepassbands(e.g.,131and94 Å)but

notatothercoolerpassbandsshowsthatithasatempera-

tureof>6MK.Subsequently,moreandmorehotchannel

eventsthatexistpriortotheCME/arebeginningareiden-

tied(e.g.,Patsourakosetal.,2013;LiandZhang,2013a,

2013c;Tripathietal.,2013;VemareddyandZhang,2014;

Dudíketal.,2014;Chintzoglouetal.,2015;Joshietal.,2015;

Zhouetal.,2016).Interestingly,apre-existingMFRiseven

conrmedtoexistinthechromospherewithobservationsby

NSTatBBSO(Wangetal.,2015).Moreover,Chengetal.

(2012)quantiedthedifferentialemissionmeasure(DEM)of

thehotblobsandchannels,whichshowsthattheemissionof

thesehotMFRsareactuallyfromabroadtemperaturerange

of6.5≤logT≤7.3withaDEM-weightedaveragetemperature

largerthan~8MK.Thecorrespondingelectronnumberden-

sityvariesfrom5.0×108to3.0×109 cm−3.

Zhangetal.(2012)andChengetal.(2013a)furtherfound

thatthehotchannelshowsaremarkablemorphologicalevo-

lutionduringtheearlyphaseoftheeruption.Initially,the

dippedcentralpartofthewrithedhotchannelrisesupslowly

andgraduallybecomesmorelinear.Thecontinuingriseofthe

centralparteventuallyturnsthesigmoidalshapeofthechan-

nelintoalooplikeshapedpartialtorus(Figure2a).Duringthe

transformationprocess,thetwofootpointsoftheevolvinghot

channelarenearlyxed.Afterwards,theloop-likestructure

quicklystretchestheoverlyingeldandbuildsupaCME,si-

multaneouslygivingrisetoaareunderneath.Furthermore,

Chengetal.(2014c)identiedthatthehotchanneliscapa-

bleofevolvingsmoothlyfromtheinnerintotheoutercorona

withalmostretainingitscoherence,morphologicallyconsis-

tentwiththeCMEcavityasseeninthewhite-lightimages

(Figure2b).ChengandDing(2016)studiedthefootpointsof

thehotchannelandfoundasubstantialdeviationofthehot

Figure2 

(a)AIA131Åimagesshowingthepre-existence(left)anderuption(right)ofahotchannel-likeMFR(fromZhangetal.,2012;Chengetal.,2013a.

ReproducedbypermissionoftheAAS).(b)Transformationofahotchannel-likeMFR,asseenintheAIA131Åpassband(left),totheCMEimagedbythe

LASCOC2white-lightcoronagraph(right).

ChengX,etal.SciChinaEarthSci5

channelaxisfromtheassociatedlament.Itshowsthatthe

hotchannelhasascendedtoahighaltitudeandlikelysepa-

ratedfromthatofthelamentwhenapproachingtheeruption.

InordertouncovertheappearancefrequencyofthehotMFR,

Nindosetal.(2015)madeastatisticalstudyanddocumented

thatalmosthalfofmajoreruptivearescontainahotblobor

channel-likeconguration.TheobservedMFRmorphology

mainlydependsontheorientationoftheMFRaxiswithre-

specttothelineofsight.Thatistosay,theMFRappearsas

ahotblobandahotchannelparallelandperpendiculartoits

axis,respectively.

Inaddition,somespectroscopicobservationsalsosupport

thepre-existenceoftheMFR.ByanalysingCoronalDiag-

nosticsSpectrometer(CDS)orEUVImagingSpectrometer

(EIS)data,Gibsonetal.(2002),Harraetal.(2009),and

Harraetal.(2013)foundasignicantpre-areenhancement

innon-thermalvelocity,thelocationsofwhichmaycorre-

spondtothefootpointsoftheMFR.CombiningEISandAIA

observations,Syntelisetal.(2016)evenfoundthattheen-

hancednon-thermalvelocities,aswellastheblueshifts,can

lastfor5hoursbeforetheeruptionofthehotchannel-like

MFR.

2.5 

ReconcilingdistinctaspectsoftheMFR

Asdiscussedabove,laments,lamentchannels,cavities,

sigmoids,andhotchannelscanbewelluniedintheframe-

workoftheMFR;theymaybejustthedistinctmanifesta-

tionsoftheMFR,dependingondifferentobservationalwave-

lengthsandperspectives,aswellasmagneticenvironment.

Therehavebeenmanystudiesrevealingtherelationshipbe-

tweenanytwoofthem.

Throughobservingtheevolutionofaneruptinglament,Li

andZhang(2013b)foundthattheeruptingmaterialshavea

helicaltrajectorywhenmovingalongthethreadsofhotchan-

nels(alsoseeYangetal.,2014;Zhangetal.,2015a).During

theeruptionphaseofhotchannels,coollamentarymaterials

arealsoseentodescendspirallydowntothechromosphere

alongtheirlegs(Chengetal.,2014c).Furthermore,Chenget

al.(2014a)foundthatthehotchannelisinitiallyco-spatial

withtheprominenceintheearlyrisephase,whilewiththe

eruptionbeginningthehotchannelquicklyexpands,result-

inginaseparationofitstopfromtheprominence.Through

adetailedanalysisofthetemperaturestructureofanerupting

lament,Chenetal.(2014a)conrmedthattherelatively

coolplasmaalwaysstaysatthebottomofthehotchannel.

Theseresultsstronglysuggestthatthehotchannelisadirect

manifestationoftheheatedMFRwithlamentmaterialscol-

lectedatitsbottom.

AlthoughpreviousstudiesrevealedthatCME-productive

activeregionsoftentakeonasigmoidalshapeintheEUV

and/orSXRimagespriortotheeruption,itdoesnotmeanthat

thecorrespondingmagneticeldlinesmustbehighlytwisted.

Alternatively,theycouldconsistoftwogroupsofshearedar-

cades,makingupasigmoidalshapeapparently(Titovand

Démoulin,1999;Kliemetal.,2004;Schmiederetal.,2015;

ChengandDing,2016).However,westillcannotexcludethe

possibilitythatanexistingbutinvisibleMFR,probablyhav-

ingaveryweakemission,isembeddedinthemiddleofthe

sigmoidandoverlaidbyambientshearedarcades.Recently,

peoplehavestartedtorecognisethatthecontinuoussigmoidal

orhighlytwistedeldlinescanoriginateinthesigmoidalac-

tiveregions.UsingXRTdata,McKenzieandCaneld(2008)

observedadiffuselinearstructurethatappearsinthemiddle

ofthesigmoidpriortotheeruptionandliftsoffastheare

begins(alsoseeLiuetal.,2010;Greenetal.,2011;Zharkov

etal.,2011).Takingadvantageoftheunprecedentedhigh

cadence,highresolution,andmulti-wavelengthobservations

oftheAIA,Chengetal.(2014b)foundthatthelinearfeature

ismostlikelytobecontinuoussigmoidalhotthreads.They

evenfoundthatadouble-deckerMFRsystemthatconsists

ofahigh-lyingcontinuoussigmoidalthreads(hotchannel)

andaverticallyseparatedlament-associatedlow-lyingux

couldbeformedinthesigmoidalactiveregion.Closetothe

eruption,themorphologyofthehigh-lyinghotchannelvaries

fromanS-shapetoaloop-shape,similarlytothelinearfeature

intheeruptingsigmoids.Inaddition,itshouldbenotedthat

sigmoidsaremostlyamanifestationduetotheheatingina

sigmoidalCSbetweentheMFRanditsambienteld(Kliem

etal.,2004;Gibsonetal.,2006),whiletheydonotdelin-

eatethespecicmagneticeldcongurations.However,hot

channelsrefertocoherentmagneticstructures,whichcanbe

tracedcontinuouslyfromthesigmoidalactiveregionstothe

outercorona.Duringthewholeeruptionprocess,theevolu-

tionofhotchannelsismainlycontrolledbytheirowndy-

namics.Therefore,wecansaythathotchannelsandsig-

moidsaredistinctphenomena,morespecically,theformer

arewell-denedandspecicstructuresthatoriginateinthe

latter.

Thevisibilityofhotchannel-likeMFRsonlyinthe131and

94 Åpassbandsshowsthattheyaresubstantiallyheatedbe-

foretheeruption.Interestingly,quiescentcavitiesarealso

foundtobeheatedwithahighertemperaturethantheback-

ground.Reevesetal.(2012)examinedthethermalproperties

ofaquiescentcavitythatcontainsstrongX-rayemissioninits

coreandfoundthatthereisanobvioustemperatureincrease

inthecavitycore,andthatthecoretemperaturevariesfrom

1.75to2.0MKwiththeevolutionofthemorphologyfrom

aring-shapedatthebeginningtoanelongatedstructuretwo

dayslater.Thereasonisconjecturedtobethatdifferentparts

ofthecavitycoreareheatedatdifferenttimes.Byconstruct-

inglimbsynopticmapsoftheAIA211,193,and171 Åpass-

bands,Karnaetal.(2015)analyzedanumberofquiescent

cavityeventsandalsofoundthatquiescentcavitiesarehotter

thantheirsurroundingsalthoughonlyslightly .Theseresults

implythatactiveregionhotchannelsandquiescentcavities

6ChengX,etal.SciChinaEarthSci

mayhavethesame,atleastsimilar,heatingmechanismin

thepre-eruptivephase,thoughtheexactmechanismremains

mysteriousatpresent.Webelievethatmagneticstructures

ofboththeactiveregionhotchannelsandquiescentcavities

areanMFR,theonlydifferenceofthemisthedistinctsize;

theformerusuallyhasalengthof20–100Mm(thescaleof

theactiveregionPIL)andaheightof10–20Mm,whilethe

latterextendsalongthewholePILoflong-termdecayedac-

tiveregions,havingalengthof200–500Mmandaheightof

30–100Mm(e.g.,Liuetal.,2010;Suetal.,2015;Chengand

Ding,2016).

3. 

Formationofpre-eruptivecongurations

3.1 

BodilyemergenceoftheMFR

IfanMFRreallyexistsinthecorona,thequestionisthen

when,where,andhowtheMFRisbuiltup.Theoretically,two

possibilitieshavebeenproposed.Onepossibilityisthatthe

MFRintheconvectionzoneemergesintothecoronabybuoy-

ancy(Figure3a;Fan,2001;Magara,2004;Martínez-Sykora

etal.,2008;ArchontisandTörök,2008;Leakeetal.,2013).

However,Manchesteretal.(2004)foundthatwhenthepri-

maryaxisoftheMFRapproachesthephotosphere,theMFR

issplitintotwopartsbymagneticreconnectionwithsur-

roundingelds,whichonlyallowstheupperuxofthemid-

dlesectionwithveryweaktwist(lessthanoneturnabout

theaxis)toseparatefromthelowermass-ladenanddipped

ux(alsoseeMagara,2006).Evenso,thetotalrelativemag-

netichelicityofthewholesystemiswellconserved(Zhang

andLow,2003).Afterascendingtothecorona,thecenter

oftheMFRriseswithanincreasingvelocityaslongasthe

MFRfootpointsrotatecontinuously.Asaresult,signicant

twististransportedfromtheMFRinteriorparttowardthe

coronalpartthroughnonlineartorsionalAlfvénwaves(Fan,

2009;Leakeetal.,2013).Afteremergingintothecorona,the

reconnectionwiththepre-existingcoronaleldalsoplaysan

importantroleinformingtheMFRandevendrivingitserup-

tion(e.g.,ArchontisandTörök,2008;Leakeetal.,2014).

Fan(2012)foundthatinthequasi-staticrisephaseofthe

MFR,magneticreconnection,mostlikelytether-cuttingin

thesigmoidalhyperbolicuxtube(HFT,intersectionoftwo

quasi-separatrixlayers(QSLs),wherethelinkagesofmag-

neticeldlinesarecontinuousbutchangedrastically(Titov

etal.,2002)),effectivelyinjectstwisteduxtotheMFRso

astodriveitseruption.

Someobservationalstudiesalsostandfortheemergenceof

theMFRfrombelowthephotospheretothecorona.Through

analyzingthevectormagnetogramsobtainedbytheDunnSo-

larTelescopeoftheNationalSolarObservatory,Lites(2005)

foundaconcave-upgeometryinthephotospherebelowtwo

activeregionlaments.Okamotoetal.(2008)andOkamoto

etal.(2009)examinedasequenceofvectormagnetograms

ofAR10953observedwiththeSolarOpticalTelescopeon

boardHinodeandfoundthefollowingfeatures:theadjacent

opposite-polarityregionswithhorizontallystrongbutverti-

callyweakmagneticeldsgrowinglaterallyandthennar-

rowing,thereversalofthedirectionofthehorizontalmag-

neticeldsalongthePILfromanormalpolaritytoanin-

verseone,andtheappearanceoftheblueshiftanddiverging

owsinthehorizontalmagneticeldregion.Theseobser-

vationalfeatures,aswellastheconcave-upgeometry,imply

thattheMFRprobablymayemergefromthesolarinterior

tothecorona.However,V argasDomínguezetal.(2012)re-

centlyprovidedacontradictoryinterpretationforthoseobser-

vationalcharacteristicsinthephotosphere.Comparingwith

thenumericalresultsofMacTaggartandHood(2010),they

pointedoutthatmagneticcancellationisalsocapableofpro-

ducingthelateralgrowingandthennarrowingofthepositive

andnegativepolarities,aswellasthereversalofthedirection

ofthehorizontalmagneticelds.

Magneto-convectionhasasignicantinuenceonthe

emergenceoftheMFRfrombelowthephotospheretothe

corona.Becauseofconvectiveows,undulationsappearin

theemerginghorizontaleldtoformΩ-loopsandU-loops

(CheungandIsobe,2014).Thelaternaturallyhaveacon-

cave-upgeometry.Bernasconietal.(2002)andPariatetal.

(2004)studiedtheFlareGenesisTelescopedataandfound

thatserpentinestructuresand“U”-shapedloopsfrequently

appearedinemergingactiveregions.Ellermanbombsare

alsodetectedatthelocationswhereserpentinestructures

and“U”-shapedloopstouchthephotosphere(alsoseeLi

etal.,2015).Thisismainlyduetothebuildupofcurrents

alongtheserpentineand“U”-shapedmagneticeld,which

thenleadtothereconnectionintheloweratmosphere(e.g.,

Isobeetal.,2007;Pariatetal.,2009b;ArchontisandHood,

2009;W ang,2006).Formoredetailsconcerninghowthe

sub-photospheremagneticeldemergesintothecoronaand

producesvariousactives,thereadercanconsultthereviews

bySchmiederetal.(2014)andCheungandIsobe(2014).

3.2 

MFRformationbymagneticreconnection

3.2.1



MFRformationpriortotheeruption

TheMFRcanalsobebuiltupdirectlyinthecoronaviamag-

neticreconnectionpriortotheeruption.Inthemodelofvan

BallegooijenandMartens(1989),itisproposedthatuxcan-

cellationtransfersshearedloopstohelicaleldlines,creat-

ingacoherentMFRconguration(Figure3b).Theuxcan-

cellationisusuallyinterpretedintermsoftransportofpos-

itiveandnegativeuxestowardthePIL,reminiscentofthe

well-knownmoatowaroundtwopolaritiesofanactivere-

gion(Amarietal.,2010,2011,2014).Throughimposing

convergingmotionstowardthePIL,Amarietal.(2003a)

successfullysimulatedthattwogroupsofsheareduxare

broughttogetherandreconnecttowardatwistedMFR(also

ChengX,etal.SciChinaEarthSci7

Figure3 

(a)Fluxemergencemodel,inwhichatwistedMFRbodilyemergesfrombelowthephotospheretothecorona.Thelinesinvioletshowbald-

patch–associatedseparatrixsurfaces.Theblacksegmentsdisplaymagneticdipswherethelamentmaterialscanbecollected(fordetailspleaseseeGibson

etal.,2004.ReproducedbypermissionoftheAAS).(b)Fluxcancellationmodel,inwhichtheMFRisformedbythereconnectionoftwogroupsofsheared

arcadesdrivenbytheshearingandconvergingmotions(fordetailspleaseseevanBallegooijenandMartens,1989.ReproducedbypermissionoftheAAS).

seeMackayandvanBallegooijen,2006).Amarietal.(1999,

2003b)showedtheimportanceofphotosphericturbulentdif-

fusiononpre-shearedmagneticeld(possiblyremnant)that

leadstotheformationofmagneticuxrope.Subsequently,

Aulanieretal.(2010)didamoredetailedMHDsimulation,

inwhichaninitiallypotentialbipolareldevolvesasdriven

bymagneticelddiffusionandshearingmotions.Similarto

theresultsofAmarietal.(2003a),ux-cancellation-driven

reconnectionappearsinaBPseparatrixandgraduallytrans-

formstheshearedarcadesintotheMFR.Inthewholeforma-

tionprocess,theMFRgraduallyrisesupbutinaquasi-static

manner.Then,theBPstructurechangestotheHFTtopology,

wherethereconnection,ofatether-cuttingtype,takesplace

tocontinuouslyinjectthepoloidaluxtotheMFR.Using

isothermalMHDsimulations,Xiaetal.(2014)evolvedalin-

earforce-freebipolarmagneticeldbymeansofintroducing

vortexowsaroundtheoppositepolaritiesandconverging

owstowardthePIL.Theyalsofoundthecreationofthehe-

licaleldlinesthroughthereconnectionanduxcancellation

atthePILdrivenbytheconvergingows.

Observationally,adirectviewoftheformationofanMFR

isimpossibleasmagneticeldmeasurementabovethepho-

tosphereistechnicallydifcultatpresent.Thus,forthesake

ofexploringtheformationoftheMFR,peopleusuallyinves-

tigatehowthevariousmanifestationsoftheMFR,including

laments,sigmoids,andhotchannels,areformed.

ThroughHαobservationsbytheMulti-channelSubtractive

DoublePassspectrograph,Schmiederetal.(2004)observed

thatdifferentsegmentsoflamentsmerge(reconnect)toform

alonglament.Atthemergencelocations,bothbrightenings

attheEUVpassbandsanduxcancellationsofsmallbipo-

lararefound.Thisisconsistentwiththelamentformation

modelintermsofmagneticreconnectionproposedbyvan

BallegooijenandMartens(1989)andAulanieretal.(1998).

Afterthereconnection,thecoolmaterialsareexpelledalong

thereconnectedeldlines,whichareconrmedbymeasured

horizontalvelocities(e.g.,Dengetal.,2002).

Recently,usingHαdatawithhigherresolutionandcadence

providedbytheNVST ,Yanetal.(2015)observedtheobvi-

ousshearingmotionoftheoppositepolaritiesandthesunspot

rotationduringtheformationprocessoftwoactive-regionl-

aments(Figure4a).Theysuggestedthattheshearingmotion

stretcheslament-associatedmagneticeldmorehorizontal

andthenthesunspotrotationinjectssometwisttoformala-

8ChengX,etal.SciChinaEarthSci

Figure4 

(a)NVSTTiOandHαimagesoverlaidbyline-of-sightmagnetogramswiththepositive(negative)inblue(red)showingtheformationofalament

drivenbythesunspotrotation(fromY anetal.,2015.ReproducedbypermissionoftheAAS).(b)NVSTHαimagesdisplayingtheformationofalamentby

thereconnection(fromY anetal.,2016.ReproducedbypermissionoftheAAS).

ment-hostinghelicalmagneticstructure.Besidesthesunspot

rotation,Y anetal.(2016)andV emareddyetal.(2016)also

addressedtheroleofthereconnectioninbuildingupthehe-

licalconguration,whichisevidencedbytheappearanceof

theEUV/UVbrighteningatthetouchpointofthedifferent

branches(Figure4b).Bymeansofstudyingtheinteraction

oftwosetsofdarkthreadsorlamentchannelsdrivenbyux

convergenceandcancellation,bothJoshietal.(2014a)and

Yangetal.(2016a)arguedthatthereconnectionisaneces-

saryconditionfortheformationofthelament.Moreover,

theyalsoobservedthatthereconnection-drivenhotplasma

undergoarollingmotionalongthelamentthreads.

Tripathietal.(2009)analyzedthetemperaturestructure

ofasigmoidanddiscoveredthattheplasmaintheJ-shaped

arcadeshasahighertemperaturethanthatintheS-shaped

uxifbotharesimultaneouslyvisible.Theyarguedthatthe

J-shapedarcadesaremostlikelyreconnectingtotheS-shaped

ux,thushavingahighertemperaturebutstartingtocool

downafterleavingthereconnectiondiffusionregion.Green

andKliem(2009)andGreenetal.(2011)supportedthepoint

thatthesigmoidisfromthereconnectionofshearedarcades

thatisdrivenbytheuxconvergenceandcancellationunder

thesigmoidalthoughonlypartofthecancelleduxbeing

injectedintothesigmoidaleldlines.

Chengetal.(2014b)studiedtheformationofthehotchan-

nel-likeMFRafterthatwasdiscovered.Throughanalyzing

thelong-termevolutionofanevolvingsigmoidalactivere-

gion,theyfoundthatthetwistedeld,indicatedbycontin-

uoussigmoidalhotthreads,isformedviathereconnection

oftwogroupsofshearedarcadesnearthePILhalfdaybe-

foretheeruption.Thetemperatureofthetwistedeldand

shearedarcadesderivedbytheDEMtechniqueishigherthan

thatoftheambientvolume(Figure5a),indicatingthatthe

reconnectiontakesplaceandheattheplasmatherein.They

alsoconrmedthatthereconnectionisdrivenbytheshearing

andconvergingmotionsnearthePIL.Throughconstructing

atimesequenceofNLFFFstructures,itisfurtherrevealed

thatthereconnectionhappenssimultaneouslyattheBPsep-

aratrixinthephotosphereandintheHFTinthecorona(the

tether-cutting).TheMFRcanevenbeformedinthelowerat-

mosphere(e.g.,Wangetal.,2015),via,forexample,aseries

ofmagneticreconnectioninthechromosphere,andsome-

timesbeheateduptothecoronaltemperatureasvisiblein

theAIA131and94 Åpassbands(Kumaretal.,2015,2017;

LiandZhang,2015).Moreover,theconversionofmutualhe-

licitytoself-helicitythroughtheinterchangereconnectionof

twogroupsofloopsisalsoarguedtobestrongevidencefor

theformationofthehelicaleldpriortotheeruption(e.g.,

Tziotziouetal.,2013;Lietal.,2014).

Chengetal.(2015a)furtherperformedspectroscopicdi-

agnosticsontheformationofhotchannelsbasedontheAIA

andIRISjointobservations.Atthefootpointsofthehotchan-

nel,itisfoundthattheSiIV ,CII,andMgIIlinesexhibit

weaktomoderateredshiftsandnon-thermalvelocitiesinthe

pre-arephase.However,relativelylargeblueshiftsandex-

tremelystrongnon-thermalvelocitiesappearatthereconnec-

tionsiteoftwoshearedarcades,i.e.,theformationsiteofthe

hotchannel(Figure6aand6b).Thesespectralfeaturesim-

plythatthereconnectionplaysanimportantroleinthefor-

mationandheatingofhotchannels,andthatthelocationof

thereconnectionismostlikelyintheloweratmosphere(Fig-

ure6c),basedonthefactthattheSiIV ,CII,andMgIIlines,

forminginthechromosphereandtransitionregion,allexhibit

blueshiftsandnon-thermalvelocities.Theoutowsfromthe

ChengX,etal.SciChinaEarthSci9

Figure5 

Emissionmeasuremapsatdifferenttemperatureintervalsandinstantsshowingtheformationofasigmoidalhotchannel-likeMFRintheactive

region11520,whichcanbeclearlyseeninpanels(g)–(i)(adaptedfromChengetal.,2014b.ReproducedbypermissionoftheAAS).

reconnectionsitemaypropagatetowardthefootpointsofthe

hotchannelalongthenewlyreconnectedeldlines,produc-

ingweakredshiftsandnon-thermalvelocities.Notethat,red-

shiftsarealsoexpectedatthereconnectionsite(Innesetal.,

1997;Peteretal.,2014),which,however,couldbeabsent

intheobservedlinesbecauseofkineticallybeinglessobvi-

ousthanblueshifts.Weshouldalsomentionthat,therecon-

nectionisnotauniqueinterpretationfortheappearanceof

blueshifts,redshifts,andnon-thermalvelocities.Therota-

tionmotioncouldbeanalternativereason.

ItisworthnotingthattheMFRcanevenbeformedduring

aseriesofconnedarespriortotheeruption.Patsourakoset

al.(2013)studiedaconnedareandaneruptivearefrom

thesamesourceregionandbelievedthattherstconned

areformstheMFRbythereconnection.TheMFRthen

lossesitsequilibriumandproducestheseconderuptionabout

7hourslater.Thisdeductionisconsistentwiththeanalysisof

Guoetal.(2013),whofoundthattheQSLreconnectioninthe

interfacebetweenthecentraluxandthesurroundingelds,

manifestingasaseriesofconnedaresbeforetheeruptive

one,hasanimportantroleininjectingmagnetichelicityand

twisttotheMFR.WiththeeruptionoftheMFR,thetwist

numberandmagnetichelicityintheresidualuxthenquickly

decrease(Yangetal.,2016b;Liuetal.,2016a,2016d).

3.2.2



MFRformationduringtheeruption

IthasalsobeenproposedthattheMFRcanbequicklybuiltup

duringtheeruptionviathearereconnection.Inthetether-

cuttingmodelproposedbyMooreetal.(2001),twooppo-

sitelyshearedarcadesreconnecttoformatwistedloopdur-

ingtheonsetandearlyphaseoftheeruption.Inthebreakout

modeldevelopedbyAntiochosetal.(1999),theinitialcon-

gurationiscomprisedofcentralshearedarcadesandtwo

neighboringuxsystems.Withthecentraluxtakingoff,

aCSisformedbelowandthereconnectionthereinquickly

transformsthesheareduxestothecentraluxtoforman

eruptingMFR.Evidenceofbreakoutreconnectioninitiating

amajoreruptionisidentiedbyAulanieretal.(2000).Fol-

lowingsuchanidea,MacNeiceetal.(2004)performedan

MHDsimulationandreproducedthecompleteprocessofthe

MFReruptionincludingtheinitiation,formation,andaccel-

eration,aswellastheeventualrelaxationoftheshearedcen-

traleldtoamorepotentialstate(alsoseeLynchetal.,2008;

Karpenetal.,2012).

Correspondingly,someobservationalstudiessupportthat

theMFRevolvingsubsequentlytoaCMEisdirectlyformed

duringtheeruption.Theunambiguousevidenceisgivenby

Liuetal.(2010),whoobservedthatinthepre-arephase

twooppositeJ-shapedloopsreconnecttoformcontinuous

sigmoidalloopswiththecentralpartdippeddownand

alignedalongthePIL.Simultaneously,thecompactbright

loopscrossingthePILarealsoseen.Afterlastingformin-

utes,thesigmoidalloopsquicklyriseupandthenproduce

aCMEandaare.Throughobservingtheinteractionof

twopre-existingloopsorlamentsintheinitiationphaseof

twoeruptiveares,Chenetal.(2014b,2016a)alsonoticed

thatsomesmallbrightloopsappearedbelowtheinteraction

regionandsomenewhelicallinesconnectingthetwofar

endsofthepre-existingloopsareformedatthesametime.

Theypointedoutthattheformationprocessofthehelical

10ChengX,etal.SciChinaEarthSci

Figure6 

(a)SDO/AIA131Å,304Åimages,andSDO/HMIline-of-sightmagnetogramshowingtheformationofanMFRpriortotheeruption(top).Spec-

trogramsoftheSiIV ,CII,andMgIIlinesattheMFRformationsite(bottom).(b)Dopplervelocityandnon-thermalvelocitymapsoftheSiIVlineatthe

MFRformationsite.(c)AcartoonillustratingtheMFR(brown)formationthroughthereconnectionoftwoarcades(green)intheloweratmosphere(fordetails

pleaseseeChengetal.(2015a).ReproducedbypermissionoftheAAS).

ChengX,etal.SciChinaEarthSci11

structurebasicallyagreeswiththetether-cuttingscenario.

ObservationsrevealingtheformationoftheMFRduring

themainphaseofaresareveryrare.Songetal.(2014a)

reportedaninterestinglimberuptiveevent,whichshowsthat

theblob-likeMFRcouldbebuiltupduringtheCMEerup-

tionphase.Itisseenthattheexpansionofalow-lyingcoro-

nalarcadestretchestheoverlyingmagneticeld,whoselegs

arethencurvedin,forminganX-pointinbetween.Then,

thereconnectionneartheX-pointleadstotheformationand

eruptionofthehotbooblikeMFR.However,itisdifcultto

ensurethattheeruptingMFRisfullyfromthereconnection;it

ispossiblethatanascentMFR(e.g.,MFRseedwithastrong

twistbutasmallux)hasexistedbeforetheeruption.More-

over,agoodagreementisfoundbetweenthereconnection

uxcalculatedfromareribbonsandtheuxofmagnetic

cloudscomputedusinginsituobservationsat1AU,whichis

alsoregardedasastrongindicationofMFRformationduring

thearephase(e.g.,Linetal.,2004;Qiuetal.,2007;Huet

al.,2014;Gopalswamyetal.,2017).

4. 

Initiationandearlydynamicsofsolarerup-

tions

OncetheMFReruptsoutward,itquicklyformsaCMEand

producesareemissionssimultaneously.Forare-associ-

atedCMEs,theyusuallyexperienceathree-phaseevolution:

theslowrisephase,impulsiveaccelerationphase,andprop-

agationphaseofanearlyconstantvelocity(Zhangetal.,

2001,2004).Thethreephasesroughlycorrespondtothe

threephasesofassociatedares:thepre-arephase,rise

phase,anddecayphase,respectively(Zhangetal.,2001,

2004;Qiuetal.,2004;Temmeretal.,2008,2010;Chenget

al.,2010),implyingthecouplingbetweentheCMEeruptions

andtheenergyreleaseofthearesthroughasamephysical

mechanism,mostlikelythemagneticreconnection(Linetal.,

2000;PriestandForbes,2002;ZhangandDere,2006;Linet

al.,2015).

Atpresent,weareonlyabletoforecastthelikelihoodofthe

productionofCMEs/aresempiricallyinthelightofdifferent

propertiesofactiveregionsincludingmagneticmorphology,

horizontalgradientofthemagneticeld,current,magnetic

helicity,magneticshear,non-potentiality,aswellasLorentz

forceetc.(e.g.,LekaandBarnes,2003a,2003b;Falconeret

al.,2008;Bobraetal.,2014;BobraandIlonidis,2016,Eu-

ropeanFLARECASTproject).Anaccuratedeterminationof

theonsetoferuptionsisstilldifcult,whichisprimarilydue

tothefollowingreasons:(1)theoretically,theinitiationof

CMEs/areshasnotbeenunderstoodthoroughly,(2)validat-

ingordistinguishingtheexactinitiationmechanismfromthe

possibleonesobservationallyisamatterofgreatdifculty.

Recently,SDOobservationsprovideunprecedentedhighca-

dence,highresolution,andmulti-wavelengthdata,whichal-

lowustostudytheinitiationofCMEs/aresindetail.More-

over,thenewobservationsalsoopenawindowtounderstand

thedetailedformationprocessofCMEs,inparticularthose

MFR-drivenCMEs.Inthefollowing,werstintroducevar-

ioustheoreticalmodelsthatarefrequentlyusedforinterpret-

ingtheonsetofCMEs/ares.Then,wepresentsomeobser-

vationaleffortstowardansweringtheabovequestions,inpar-

ticularsomenewknowledgeachievedontheearlydynamics

ofMFR-drivenCMEs.

4.1 

Initiationofthepre-eruptiveconguration

4.1.1



Initiationbymagneticreconnection

Intermsofwhetherthereconnectionisinvolvedornot,the

existinginitiationmodelscanbedividedintotwocategories.

Therstcategoryisreconnection-basedmodelsincludingthe

tether-cuttingmodel(Mooreetal.,2001),breakoutmodel

(Antiochosetal.,1999;Karpenetal.,2012),anduxemer-

gencemodel(ChenandShibata,2000).Inthetether-cut-

tingmodel,thekeymechanismisthereconnectioninthe

sigmoidalcoreeldregion,whichtransformstwogroups

ofshearedarcadesintotwistedloops,thusprovidinganup-

wardLorentzforcetoinitiatetheeruption.Asmentionedin

Section3.2.2,thetether-cuttingreconnectioninthepre-are

phasehasbeenobservedinsomeevents(e.g.,Liuetal.,2010;

Chenetal.,2014b,2016a).

Thebreakoutmodelresortstothereconnectiontaking

placeatthenullpointthatexistsbetweenthecentralsheared

uxandoverlyingeld.Themostimportantfeatureisthat

thereconnectionsiteislocatedabovethecoreeld,rather

thaninthecoreeldasstatedinthetether-cuttingmodel.

Thereconnectionatthenullpointisabletoremovethe

constraintoftheoverlyingux,therebyreducingthedown-

wardtensionforceandallowingthecentraluxtoescape

away.Theoretically,aquadrupolarstructure,whichincludes

acentralshearedarcadeandtwoneighboringloopsystems

withanX-pointlocatedinbetweenisapromisingstructure

forbreakout-typeeruption.Observationally,abrighteningat

theX-shapedstructure,andsomeremotebrighteningsatthe

footpointsofthetwoneighboringuxes,aswellastheside-

waysmotionofthelateralloops,havebeenseentosupport

theoccurrenceofthebreakoutreconnection(Aulanieretal.,

2000;GaryandMoore,2004;Ugarte-Urraetal.,2007;Shen

etal.,2012b;Chenetal.,2016b;Revaetal.,2016).

IntheuxemergencemodelofChenandShibata(2000),

whentheemerginguxemergeswithinthelamentchannel,

itcanreconnectwiththemagneticeldbelowtheMFR.Ow-

ingtotheincreaseofmagneticpressure,theMFRmaylose

itsequilibriumandthenrisetoformaCSbelowit.Thisis

similartothetether-cuttingreconnection.Onetheotherhand,

whenreconnection-favoredemerginguxappearsandrecon-

nectswiththeouteredgeoftheMFR,thedownwardtension

forceisreduced,makingtheMFRriseup.Thiscaseissimi-

lartothelateralbreakoutreconnection.

12ChengX,etal.SciChinaEarthSci

4.1.2



MFRinitiationbyMHDinstabilities

Differentfromthereconnectionmodels,theothercategory

referstoMFR-basedidealMHDmodelsincludingcata-

strophicloss-of-equilibrium(ForbesandIsenberg,1991;

Isenbergetal.,1993;Lin,2001;LinandvanBallegooi-

jen,2002),kinkinstability(Töröketal.,2004),andtorus

instability(KliemandTörök,2006;OlmedoandZhang,

2010).ForbesandIsenberg(1991)andIsenbergetal.(1993)

documentedthatastraightMFRcanloseitsequilibrium

intheidealMHDprocesswhenthephotosphericsources

oftheconstrainingeldapproacheachother.Thetorus

instabilitymeansthattheexpansionoftheMFRtendsto

developnonlinearlyiftheconstrainingelddeclineswith

heightrapidlyenough.ForatoroidalMFRthatstartsto

becometorusunstable,thecriticaldecayindexoftheover-

lyingeldisfoundtobe1.5(KliemandTörök,2006).

OlmedoandZhang(2010)showedthatthecriticalvalueisa

functionofthefractionalnumberofthepartialMFRwiththe

footpointsanchoredinthephotosphere,i.e.,aratiobetween

thelengthofthepartialMFRabovethephotosphereand

thecircumferenceoftheMFR.Interestingly,Démoulinand

Aulanier(2010)andKliemetal.(2014a)provedthatthe

torusinstabilityisactuallyanequivalentdescriptionofthe

catastrophiclossofequilibriumoftheMFRintheMHD

framework.IfignoringtheminorradiusoftheMFR,the

criticaldecayindexis1.5and1forthecircularandstraight

MFR,respectively.However,whentheMFRisdeformable

andasthickastherealcase,theircriticalindicesvarybut

slowly,typicallyintherangeof1.1–1.3.

TheMFRwithenoughtwistcanalsobecomeunstable,an

MHDprocessknownasthekinkinstability.Itrequiresthat

thetwistnumberoftheMFRexceedsathresholdsuchas

3.5π(Töröketal.,2004;Wangetal.,2016b).Whenthe

kinkinstabilityhappens,thetopoftheMFRshouldslowly

ascendatrstiftheperturbationisupward.Then,theMFR

isquicklywrithedbytheconversionoftwistintowrithe,the

deformationoftheMFRaxis,forminganinverseγ-shaped

orΩ-shapedstructure(e.g.,Jietal.,2003;Williamsetal.,

2005;RustandLaBonte,2005;Gilbertetal.,2007;Guoet

al.,2010a;Y anetal.,2014;HassaninandKliem,2016).At

thesametime,theheightoftheMFRincreasesexponentially

(Schrijveretal.,2008a),whichthencausesthereconnection

atthecrosspointoftwoMFRlegs(e.g.,LiuandAlexander,

2009;Kliemetal.,2010;Tripathietal.,2013).Itisworth

noticingthattherapidrotationoftheMFRaxisisusuallyre-

gardedasaconditionbutnotasufcientoneforjudgingthe

occurrenceofkinkinstability(Lynchetal.,2009).

Recently,Aulanieretal.(2010)comparedthedistinct

mechanismsthroughazero-βMHDsimulation.They

disclosedthat,priortotheeruption,uxcancellationand

tether-cuttingreconnectioncontinuouslyworktobuildup

theMFRandmakeitascending.Whenrisingtothecritical

heightatwhichtheidealtorusinstabilityoccurs,theMFR

thenstartstoerupt.Aulanieretal.(2010)thusarguedthatthe

reconnection-involvedprocessesdonottriggertheeruption

butactasthekeymechanismsoftheMFRformationandits

slowrise.Themechanismthatinitiatestheeruptionofthe

MFRistheidealtorusinstability.

4.1.3



Validationoftorusinstability

Inthepastyears,manystudiesattemptedtovalidateanddis-

tinguishtherightmodelfromtheavailableinitiationmodels,

inparticularthetorusinstability,mainlybecausewhichcan

betestedfromobservationsquantitatively.Ageneralway

istocomparethedecayindexofthebackgroundeldatthe

criticalheightwiththetheoreticalvalue.Thecriticalheight

isestimatedroughlyastheheightoftheMFRjustbeforethe

eruption(e.g.,Chengetal.,2011a;Nindosetal.,2012;Jiang

etal.,2013;Chenetal.,2014b;Inoueetal.,2014;Zuccarello

etal.,2014;Chintzoglouetal.,2015;Wangetal.,2016c).

Thebackgroundmagneticeldisthencalculatedusingpo-

tentialeldmodel.Ifthedecayindexofthebackgroundeld

attheonsetheightislargerthanthethresholdof~1.5,itis

usuallysuggestedthatthetorusinstabilityplaysaroleintrig-

geringtheeruptions.

However,estimationofthecriticalheightoftheMFRerup-

tionusuallysuffersfromasignicantuncertainty.Inorderto

resolvethisissue,Chengetal.(2013b)devisedamathematic

modelthatassumestheheightevolutionoftheMFRinthe

lowercoronafollowingafunctionh(t)=c0e(t−t0)/τ+c1(t−t0)+c2,

whereh(t)isheight,tistime,andτ,t0,c0,c1,c2arevefree

coefcients.Themodelconsistsofalineartermandanexpo-

nentialterm,whichcorrespondtotheslowrisephasewitha

constantvelocityandtheimpulsiveaccelerationphasechar-

acterizedbyanexponentialincreaseofvelocity,respectively.

Physically,thisexponentialtermisreasonablebecauseitde-

scribestheimpulsiveaccelerationoftheMFR(Schrijveret

al.,2008b)whenitistriggeredeitherbythearereconnection

(e.g.,Antiochosetal.,1999;Mooreetal.,2001;Karpenetal.,

2012)orbyotherMHDinstabilities(e.g.,TörökandKliem,

2005;OlmedoandZhang,2010).Applyingthemathematic

modeltotwoMFReruptionevents,Chengetal.(2013b)

quantitativelydeterminedtheonsettimeoftheMFRimpul-

siveacceleration,andfoundthattheonsettimeis~2minutes

earlierthanthatoftheassociatedares(Figure7a).Simi-

larly,throughanalyzingthetemporalcorrelationbetweenthe

velocityofalamenteruptionandtheassociatedSXRemis-

sion,Songetal.(2015)alsofoundthatthebeginningofthe

lamentaccelerationoccursearlierthanthatoftheareSXR

emissionbyminutes.CombingthefactthattheMFRhasas-

cendedtoaheightattheonsettimewherethedecayindexof

theoverlyingeldislargerthanthethresholdof1.5(Figure

7b),itissuggestedthattheidealtorusinstabilityplaysakey

roleininitiatingtheimpulsiveaccelerationoftheMFR.

Studyingthemagneticenvironmentofconnedarescan

ChengX,etal.SciChinaEarthSci13

Figure7 

(a)Temporalevolutionoftheheight,velocity,andaccelerationoftheMFRduringtheearlyeruptionwiththeblacksolidlinesshowingthemodel

tting.TheredsolidlinesshowtheGOESSXR1–8Åuxandresultingtimederivation.Theverticalblue(horizontal)lineestimatestheonsettime(height)

oftheeruption.Theverticalredlinepointsouttheonsettimeoftheare.(b)Distributionsofthebackgroundmagneticelddecayindexwithheightoverthe

differentsegmentsofthePILofaCME-productiveactiveregion.TheverticallinesdisplaytheonsetheightsoftheMFReruptionwithblueandgreenbars

showingtheuncertainties.Thehorizontallineindicatesthethreshold1.5oftorusinstability(fromChengetal.,2013b.ReproducedbypermissionoftheAAS).

(c)Distributionsofthebackgroundmagneticeld(black)andtheresultingdecayindex(green)withheightoveraCME-pooractiveregion.Thehorizontal

dashedlinealsoshowsthethreshold1.5(adaptedfromSunetal.,2015b.ReproducedbypermissionoftheAAS).

alsohelptodistinguishthedistinctinitiationmechanisms.A

goodexamplethathasbeenwellanalysedistheare-produc-

tivebutCME-pooractiveregion12192,whichproduced32

M-classand6X-classareswithonlyoneassociatedwith

aCME.Throughcomparingthisactiveregionwithother

are-and-CME-productiveactiveregions,Sunetal.(2015b)

foundthatthebackgroundmagneticeldintheactiveregion

12192ismuchstrongerthanthatofothers(Figure7c).The

decayindexinthelowercorona(e.g.,30–100Mm)isalso

smallerthanthethresholdoftorusinstability(alsoseeWang

andZhang,2007;Chenetal.,2015;Thalmannetal.,2015;

Jiangetal.,2016b).Ofcourse,thedecayofthebackground

magneticeldbeingrapidenoughisnotauniquecondition

fortorusinstabilitytotakeplace.Anotherconditionisthe

pre-existenceofanMFRinthesourceactiveregion(Liuet

al.,2016b).Moreover,Zuccarelloetal.(2017b)noticedthat

thechangeofthearesfromeruptivetoconnedisalsoin-

uencedbythevariationintheorientationofthepre-eruptive

magneticcongurationwithrespecttotheoverlyingeld,

ratherthanmerelytheoverallchangeoftheMHDstability.

Inthepastyears,muchattentionhasalsobeenpaidtothe

onsetconditionoffailederuptions.Guoetal.(2010a)stud-

iedalamenteruptionthatrstlydisplaysafastrisingand

writhingmotionbutisnallyconnedinthelowercorona.

Throughexaminingtheheightdistributionofthedecayindex

ofthebackgroundmagneticeld,theyfoundthatthedecay

indexinthehighercoronadoesnotcontinuouslyincrease,

insteaditstartstodecreaseandstaysbelowthethresholdfor

thetorusinstability,thusleadingtotheconnementofthel-

amenteruption(alsoseeWangandZhang,2007;Liu,2008;

Chengetal.,2011a;Joshietal.,2014b;Liuetal.,2015).

Inaddition,throughalaboratoryexperiment,Myersetal.

(2015)foundthattheconnementoftheMFReruptionis

alsocontrolledbytheguidemagneticeld,thecomponentof

thebackgroundeldthatrunstoroidallyalongtheMFRaxis,

whichinteractswithelectriccurrentsintheMFRtogenerate

atoroidaleldtensionforcetorestricttheeruption.

Oneshouldbeverycarefulwhendeterminingthedecayin-

dexatthecriticalheight.WiththehelpofMHDsimulations,

Zuccarelloetal.(2016)foundthatthedecayindexatthe

14ChengX,etal.SciChinaEarthSci

heightoftheMFRaxisisdifferentfromthatattheheightof

theMFRtop.ItissuggestedthatthesizeoftheMFRshould

notbeignoredobservationallywhenestimatingtheheightof

theMFR.

Amarietal.(2000,2010,and2011)showedthatuxcan-

cellationisabletobringtheinitialequilibriumcontaininga

twisteduxropetonalnon-equilibriumstateassociatedto

theonsetoftheeruptiontoacriticalvalueoffreeenergy.

Theynallyuniedthisenergycriteriaandthetorusinstabil-

ityoneinthecaseoflargescaleeruption(Amarietal.,2014).

4.2 

EarlydynamicsofMFR-drivenCME

4.2.1



FormationoftheCME

Althoughitisknownthattheeruptionofvariouspre-eruptive

structurescanproduceCMEs,howdotheybuildupCMEsis

stillaquestion.Themainobstaclesarethat(1)lackofthe

lowercoronaobservationsthathaveanenoughlargeeldof

view(e.g.,extendingto~1.5R⊙)toguaranteethecomplete

CMEformationprocessobservableand(2)lackofhighca-

denceandhighresolutiondataasthedynamicaltimescaleof

theCMEformationisveryshort,usuallyoftheorderofmin-

utes.

AfterthelaunchofSTEREOandSDOsatellites,the

abovetwoobstaclesareovercometosomeextent.Usingthe

STEREO-EUVIdata,Patsourakosetal.(2010a)studieda

limbCMEandfoundthatitoriginatesfromtheexpansionof

aplasmabubble.Shortlyaftertheonsetoftheacceleration,

aneruptingbubbleshowsafastoverexpansion,whichis

roughlycoincidentwithitsimpulsiveacceleration,andit

isthenfollowedbyaself-similarexpansionprocess.The

authorsattributedtheoverexpansiontotheuxconservation

aroundarisingMFRofdecreasingaxialcurrentandtheux

injectiontoagrowingMFRbythereconnection.Withthe

highcadenceSDO-AIAdata,Patsourakosetal.(2010b)

foundthattheplasmabubbleevenexperiencesanevolution

ofthreephases:aslowself-similarexpansion,afastbut

short-livedperiodofstronglateraloverexpansion,anda

self-similarexpansion.Theyarguedthatitisthelateral

overexpansionoftheplasmabubblethatcreatestheCME.

However,theyalsofoundthattheoverexpansionhappens

duringthedecliningphaseoftheare,thusweakeningthe

roleofthearereconnectionininducingtheoverexpansion.

Sometimes,theoverexpansionisalsobelievedtobethe

originofcompressionregionswheretypeIIandIIIbursts

areproduced(e.g.,Démoulinetal.,2012).

Thediscoveryofthehotchannelfurtherimprovesourun-

derstandingoftheCMEformation.Chengetal.(2013a)in-

vestigatedindetailtwoCMEeventsandfoundthatthefor-

mationoftheCMEsiscompletelycontrolledbythedynamics

ofthehotchannels.IntheAIAhightemperaturepassbands,a

hotchannelappearsastheS-shapedstructurewithitsaxisal-

mostparallelwiththePIL.Afterexperiencingashortperiod

ofrisingmotion,thehotchanneldevelopsintothesemicircu-

larstructureandthenquicklyexpandsoutwardandspeedsup.

Atthesametime,intheAIAlowtemperaturepassbands,itis

clearlyseenthataplasmabubbleappearsandalsohasafast

expansionandascendingmotion,verysimilartotheevents

analyzedbyPatsourakosetal.(2010a)andPatsourakosetal.

(2010b).Grechnevetal.(2016)alsodisclosedthesimilar

formationprocessofalimbCMEthatisdrivenbytheerupt-

inghotMFR.Throughacarefulanalysis,itisfoundthatthe

speedofthehotchannelisalwaysfasterthanthatofthebub-

ble(Chengetal.,2013a).Moreover,thehotchannelnotonly

hasanoverexpansionbutalsocoincideswiththeoverexpan-

sionoftheplasmabubble.Therefore,itisarguedthatearly

dynamicsofaCMEessentiallydependsonthatofembedded

hotchannel,whichactsasacentralenginetodrivetheCME

formationandacceleration.

4.2.2



Emissioncausedbyenergeticparticles

Intheaccelerationphase,asexpectedbythestandard

CME/aremodel(Carmichael,1964;Sturrock,1966;Hi-

rayama,1974;KoppandPneuman,1976),theeruption

stretchestheoverlyingmagneticelds,formingaCSin

thewakeoftheeruptingMFR(e.g.,Linetal.,2005,2007;

CiaravellaandRaymond,2008;Chengetal.,2011b;Lietal.,

2016a;Zhuetal.,2016;Seatonetal.,2017).Thereconnec-

tionintheCSefcientlyinjectspoloidaluxestotheMFR

andthusacceleratesitseruption.Thereconnectiontypically

lastsforminutestohours,whichismainlymaintainedby

sinkowscausedbyinward-directedmagneticpressure

gradientatbothsidesoftheCS(Zuccarelloetal.,2017a).Si-

multaneously,thereconnectionaccelerateselectrons,which

thenquicklystreamdownalongthenewlyformedareloops

toheattheirfootpoints,mappingtwoparallelbrightribbons

inthechromosphere(ForbesandPriest,1995;Tianetal.,

2014;Chengetal.,2015b).

TodisclosetherelationshipbetweentheeruptingMFRand

particleaccelerationandfurtherdeterminethelocationofpar-

ticlesacceleration,oneusuallyneedstocomparetheHXR

emissionsourceswiththedynamicsofCMEs/ares.Liuet

al.(2013)analyzedtheCScausedbytheeruptingMFRand

foundthatbothbi-directionaloutowsinformsofplasmoids

andcontractingcusp-shapedloopsoriginateinbetweenthe

hotMFRandareloops(leftpanelofFigure8a).Moreover,

theseoutowsareco-spatialwithseparateddoublecoronal

X-raysources(alsoseeSunetal.,2014).Thecentroidsep-

arationofdoublecoronalsourcesdecreaseswithenergybut

increaseswithtime(rightpanelofFigure8a).Afterwards,in

thelaterphaseofthereconnection,manydarkvoidsarealso

seentomovetowardthearearcadeswithintheCS(McKen-

zie,2000;Innesetal.,2003;Liu,2013).Theseobservations

showacloserelationshipbetweentheeruptingMFRandthe

productionofenergeticparticles,suggestingthatthelatter

maymainlyoccurinthereconnectionoutowregionsrather

ChengX,etal.SciChinaEarthSci15

Figure8 

(a)ThecentroidlocationsandevolutionsofHXRemissionsinducedbytheeruptingMFR(fordetailspleaseseeLiuetal.,2013).(b)Thesource

ofmetrictypeIIradioburstproducedbytheCME-drivenshock(fordetailspleaseseeChenetal.,2014d).ReproducedbypermissionoftheAAS.

thanintheCS.

Observationsattheradiowavelengthalsoprovideanim-

portantperspectivetoexplorewhereparticlesareaccelerated

duringtheeruption.AstheMFRisacceleratedcontinuously,

acoronalshockmayappearatthefrontofCMEs,whichis

provedbytheappearanceofmetrictypeIIradiobursts(e.g.,

Shenetal.,2007;Liuetal.,2009,2017;Maetal.,2011;

Bainetal.,2012;Fengetal.,2012;Carleyetal.,2013).

TheformationoftheshockisdrivenbytheCMEexpansion

(Kouloumvakosetal.,2014;Cunha-Silvaetal.,2015;W an

etal.,2016).Theaverageheightofshocksattheonsettime

oftypeIIburstswasestimatedtobe0.5solarradius(Gopal-

swamyetal.,2009)withthesmallestvalueof0.2(W anet

al.,2016).ThetypeIIburstsourcesareusuallybelieved

tobelocatedatthetopoftheshockfront(Zimovetsetal.,

2012;Grechnevetal.,2015,2016).However,throughcom-

paringthephysicalparametersoftheshockfrontderivedby

theDEMmethodwiththatderivedfromtheband-splitting

ofthetypeIIburst,Suetal.(2016)foundthatthesources

ofthetypeIIradioburst,atleastfortheeventtheystudied,

arelocatedattheankoftheshock.Thisresultisconsistent

withthedirectcomparisonoftheNancayradioimageswith

reconstructed3Dmorphologyoftheshockwaveasdoneby

Chenetal.(2014d),whoforthersttimeidentiedthatthe

typeIIradiosourcesoriginateinaninteractionregionofthe

shockankandnearbycoronalray(Figure8b).

Radioimagingisalsoapowerfultooltotracethedynami-

calevolutionoftheMFRandassociatedfeatures(Picketal.,

2005;PickandVilmer,2008;Démoulinetal.,2012).V ery

recently,theeruptinghotMFRhasbeenobservedintheradio

wavelength.WiththeNobeyamaRadioheliographobserva-

tionat17GHz,Wuetal.(2016)presentedtherstmicrowave

observationscorrespondingtoahotMFRthatappearsasan

overallarcade-likecongurationconsistingofseveralinten-

sityenhancementsconnectedbyweakemissions.V asanthet

al.(2016)evenobservedanobviousMFRstructureinthe

metricwavelengthandfoundthattheassociatedradioemis-

sionmanifestsasamovingtype-IVburstwiththeirsources

co-movingwiththemotionofthehotMFR.Theseobserva-

tionsindicatethatelectronsarealsoprobablyacceleratedand

trappedwithintheMFRduringtheeruption.

4.2.3



3Dstructureandproperties

ThestandardCME/aremodelthatwellinterpretsmanyas-

pectsofthecharacteristicsofCMEs/aresisessentially2D.

Inreality,theCME/areprocessis3D(Aulanieretal.,2012;

Janvieretal.,2013).ChengandDing(2016)foundthatthe

axisofthepre-eruptiveMFRofCMEs/areshasasignicant

16ChengX,etal.SciChinaEarthSci

writhingevidencedbythebigratioofitsprojectedlengthto

footpointseparationdistance.Theorientationoftheaxisof

theMFRalsosignicantlydeviatesfromthatofthemainPIL

priortotheeruption.Moreover,theareloopsarestrongly

shearedinitially,butthesheargraduallybecomeweakwith

thedevelopmentofCMEs/ares(Suetal.,2007).Thesechar-

acteristicsshowthatboththepre-eruptiveMFRandtheback-

groundmagneticeldare3DinnatureandtheMFR-induced

CME/areprocessisalso3D.Inordertounderstandthe3D

processofCMEs/ares,Aulanieretal.(2012)andJanvier

etal.(2013)extendedthe2DstandardCME/aremodelto

3D,withwhichmanynewfeaturescanbeinterpreted.The

strong-to-weaktransitionoftheshearoftheareloopsis

foundtobearesultoftheshearofreconnectedoverlyingeld

graduallydecreasingwithincreasingheight(Schmiederetal.,

1996).

DuringtheMFReruption,theinducedareribbonsalso

displayparticularfeaturesintheirmorphologiesandevo-

lution.TheyinitiallyappearasEUV/UVbrighteningsat

thetwoendsoftheMFRandthenextendtotwosheared

J-shapedribbonswithtwohookssurroundingthefootpoints

oftheMFRs(ChengandDing,2016),probablycorrespond-

ingtothefootprintsofthecurvedQSLsinthechromosphere

(Savchevaetal.,2015).AtthefootpointsoftheMFR,both

theaverageinclinationangleandthedirectcurrentdecrease

withtimesuggestiveofastraighteninganduntwistingofthe

magneticeldoftheMFRlegs(ChengandDing,2016).

Theseobservationsarebasicallyconsistentwiththe3D

standardCME/aremodelofAulanieretal.(2012)and

Janvieretal.(2013).Notethat,however,thecurrentattwo

parallelareribbonscanbedoubledascomparedwiththe

prearevalue,contrarytothatatthefootpointsoftheMFR,

probablyasaconsequenceofthecollapseofthecoronal

currentlayerduringtheareassuggestedbyJanvieretal.

(2014)andJanvieretal.(2016).

Themorphologiesofareribbonsalsodependonthethree-

dimensionalityofthebackgroundeldabovethepre-eruptive

MFR.Sunetal.(2012)studiedanon-radialMFReruptionin

aquadrupolarcongurationwithanullpointlocatedabove.

Throughananalysisofmagnetictopology,theypointedout

thatthesimultaneousbrighteningofmultiplepairsofare

ribbonsisaresultofthereconnectionbetweenthedifferent

uxesinthequadrupolarsystem(alsoseeJiangetal.,2014a;

Joshietal.,2015;Zhangetal.,2015b).Inadifferentevent,

however,Y angetal.(2015a)foundthatthenullpointcanalso

beembeddedwithinthequadrupolarstructure.Inthiscase,

theeruptionofanMFRlocatedblowthenullpointleadstoa

three-ribbonarewithtwohighlyelongatedonesinsideand

outsideaquasi-circularone,respectively.Veryrecently,a

closeattentionisalsopaidtoX-shapedareribbons.Lietal.

(2016b)foundthatthearebrighteningspropagatealongthe

ribbonstowardthecenterofanX-structure,andthenspread

outwardinadirectionmoreperpendiculartotheribbons.Itis

interpretedastheevidenceof3Dreconnectionthathappens

betweentwosetsofnon-coplanarloopsthatapproachlater-

allyandproceedsdownwardalongasectionoftheCS.How-

ever,Liuetal.(2016c)attributedtheX-shapedribbonstothe

intersectionoftwoQSLlayers,i.e.,theHFT ,withinwhicha

separatorconnectingdoublenullsisembedded.Infact,even

formostofobservedareswithtwoparallelribbons,there-

connectionisalso3Dinnature.Usingthetwoperspectives

ofSTEREOandSDO,Sunetal.(2015a)reportedawellob-

servedlimbareandclearlyshowedthattwogroupsofeld

linesoverlyingtheeruptingMFRareoppositelydirectedand

non-coplanarwhentheyreconnect,indicatingthepresenceof

aquasiseparator.Afterthereconnection,thepoloidaluxes

newlyaddedtotheMFRarehighlyhelicalandtheirtwoends

arestillanchoredinthephotosphere(Figure9).

5. 

Summaryandprospects

CMEs/aresarelarge-scaleandmostenergeticeruptivephe-

nomenainthesolarsystem.Theejectedhigh-speedmagne-

tizedplasmaandacceleratedparticlesmayhittheEarthand

thusseriouslyaffectthesafetyofhumanhigh-techactivities.

Inthepastdecades,anewinterdisciplinaryeldcalledspace

weatherthatreferstothesolaractivities,thesolarwindand

theirinuencesonthemagnetosphere,ionosphere,andther-

mosphereofourEarthhaveemerged.Inordertounderstand

theoriginofspaceweather,asignicantbutstillunsolvedis-

Figure9 

Reconstructed3Dtopologyoftwomagneticeldlines(cyanandgreencurves)before,duringandafterthereconnection.Thebottomboundaries

areprojectedEUVI304Åimagesdisplayingtheareribbons(fromSunetal.,2015a).

ChengX,etal.SciChinaEarthSci17

sueisunderstandingtheoriginandstructuresofCMEs/ares.

Inthepastyears,signicantprogressesinthisaspecthave

beenachieved.Fromobservationalperspective,wesumma-

rizethemajorndingsandnewunderstandingsasfollows:

(1)Thepre-eruptivecongurationofCMEs/aresismore

likelytobeanMFR,whichcanmanifestitselfasalament,

lamentchannel,cavity,sigmoid,andhotchanneletc.,inde-

pendenceonthesize,twist,andwritheoftheMFRcongu-

ration,theviewingangle,andthewavelengthatwhichobser-

vationisperformed.Inthefuture,weneedamoreadvanced

MHDsimulationthatcanreproducealltheseobservables.In

reality,thepre-eruptivecongurationofCMEs/aresmaynot

beassimpleasanisolatedhelicalMFR.Somespecicchar-

acteristicssuchasdouble-deckerstructuresandpartialerup-

tionsshouldalsobeconsidered.

(2)TheformationoftheMFRisproposedeitherduetoa

directemergenceoraslowreconnectioninthecoronaprior

totheeruption,orsometimesevenduetothefastreconnec-

tionduringtheare.Inmostobservations,thereconnection

scenarioseemsamorefavorableexplanationfortheMFRfor-

mation.MHDsimulationsshowthatthebodilyemergingof

awholeMFRistheoreticallydifcult.Thereconnectionis

thusneededtotransfersomeemergeduxesintoanewMFR

systeminthecoronapriortotheeruption.However,thefol-

lowingquestionsarestillunclearandneedtobeexploredfur-

ther:howdoesthereconnectionexactlybuildupanMFR?

WhatisthetimescaleoftheMFRformation?Whatarethe

indispensablefeatures?Howmuchuxesareneededtobuild

upanunstableMFR?CanwedistinguishtheunstableMFR

fromthestableoneobservationally?

(3)TheeruptionoftheMFRgenerallyexperiencesaslow

risephasefollowedbyanimpulsiveaccelerationphasechar-

acterisedbyanexponentialincreaseinheight.Theinitia-

tionmechanismsforthetwophasesaredifferentandneed

tobeclariedrespectively .Itisarguedthattheinitiationof

theslowrisephasecouldbeduetodiversereasonsincluding

magneticreconnection,MHDinstabilities,andwavepertur-

bationsaslongastheequilibriumoftheMFRisbroken.The

transformationoftheslowrisephasetothefastacceleration

oneismostlikelyaresultofMHDinstabilities.Shortlyaf-

terwards,themagneticreconnectionisthenignitedtocontin-

uouslyacceleratetheMFReruption.However,moreobser-

vationsareneededtoconrmthisargument.

(4)TheMFRis3Dinnature,e.g.,awrithedhotchan-

nel-likecongurationwithitstwoelbowsincliningtoop-

positedirectionsandthemiddlepartdippedtowardthesur-

face.TheMFReruptionandtheinducedareoftenshow

somenewpropertiessuchasJ-shapedandX-shapedarerib-

bons,strong-to-weakchangeofareloopshear,asymmetric

orpartialeruptions,sequentialreconnectionalongthePIL

etc.Therefore,arealMFReruptionprocesscouldbemuch

morecomplexthanthata2Dmodelpredicts.Inordertothor-

oughlyunderstandallobservables,3DMHDsimulationsthat

considersomespecicphysicalprocesses(e.g.,Törökand

Kliem,2005;Töröketal.,2011;Aulanieretal.,2010)and

3Ddata-drivenMHDsimulations(e.g.,CheungandDeRosa,

2012;Kliemetal.,2013;Fisheretal.,2015;Jiangetal.,

2016a),evenincludingtheradiativetransfer(e.g.,Rempel,

2017),areimminentlyneeded.Moreover,theeruptionofan

MFRin3Dmaycomplicatetheprocessofparticleaccelera-

tion,whichshouldbeconsideredinthefutureaswell.

Acknowledgements



Wear egratefultotheassociateeditorand

threeanonymousreferees,whosecommentsandsuggestsimprovedthe

manuscript.W ealsothankProf.FengXSandProf.WanWXforcordial

invitationtowritethereviewpaperandtherstISSIworkshopon“Decod-

ingthePre-EruptiveMagneticCongurationofCoronalMassEjections”

ledbyS.PatsourakosandA.V ourlidasforusefuldiscussions.ChenX,Guo

Y,andDingMDaresupportedbytheFundamentalResearchFundsfor

theCentralUniversities,theNationalNaturalScienceFoundationofChina

(GrantNos.1 1303016,11373023,11533005,1 1203014)andNationalKey

BasicResearchSpecialFoundation(GrantNo.2014CB744203).

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Magnetic structure of solar flare regions producing hard X-ray pulsations

Preprint

Aug 2017

We present analysis of the magnetic field in seven solar flare regions accompanied by the pulsations of hard X-ray (HXR) emission. These flares were studied by Kuznetsov et al. (2016) (Paper~I), and chosen here because of the availability of the vector magnetograms for their parent active regions (ARs) obtained with the SDO/HMI data. In Paper~I, based on the observations only, it was suggested that a magnetic flux rope (MFR) might play an important role in the process of generation of the HXR pulsations. The goal of the present paper is to test this hypothesis by using the extrapolation of magnetic field with the non-linear force-free field (NLFFF) method. Having done this, we found that before each flare indeed there was an MFR elongated along and above a magnetic polarity inversion line (MPIL) on the photosphere. In two flare regions the sources of the HXR pulsations were located at the footpoints of different magnetic field lines wrapping around the central axis, and constituting an MFR by themselves. In five other flares the parent field lines of the HXR pulsations were not a part of an MFR, but surrounded it in the form of an arcade of magnetic loops. These results show that, at least in the analyzed cases, the "single flare loop" models do not satisfy the observations and magnetic field modeling, while are consistent with the concept that the HXR pulsations are a consequence of successive episodes of energy release and electron acceleration in different magnetic flux tubes (loops) of a complex AR. An MFR could generate HXR pulsations by triggering episodes of magnetic reconnection in different loops in the course of its non-uniform evolution along an MPIL. However, since three events studied here were confined flares, actual eruptions may not be required to trigger sequential particle acceleration episodes in the magnetic systems containing an MFR.

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Ravy Alessandro Simanjuntak

A New Implementation of a Fourth-Order CESE Scheme for 3D MHD Simulations

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Full-text available

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This paper is devoted to the description and validation of a new implementation of a fourth-order space–time conservation-element and solution-element (CESE) scheme to numerically solve the time-dependent, three-dimensional (3D) magnetohydrodynamic (MHD) equations. The core of the scheme is that, with the aid of a grid staggered in space and time, the conservative variables are advanced by integration of the controlling equation in the space–time four-dimensional domain by utilizing Taylor expansion, and their spatial derivatives are computed by finite difference with p order derivatives from

p-1

order ones. The new scheme achieves fourth-order accuracy in both space and time simultaneously, using a compact stencil identical to that in the second-order CESE scheme. We provide a general framework for convenience of programming such that the scheme can be easily extended to arbitrarily higher order by including higher-order terms in the Taylor series. A suite of 3D MHD tests demonstrate that the fourth-order CESE scheme at relatively low grid resolutions can obtain reliable solution comparable to the second-order CESE scheme at four-times higher resolution, and showing a very high efficiency in computing by using only around

5\%

of the computing resources.

Helical flows along coronal loops following the launch of a coronal mass ejection

Article

Full-text available

Mar 2025
ASTRON ASTROPHYS

Context. Solar flares and coronal mass ejections (CMEs) are manifestations of energy release in the solar atmosphere. They can be accompanied by dynamic mass motions and waves in the surrounding atmosphere. Aims. We present observations of plasma moving in a helical trajectory along a set of coronal loops formed following the eruption of a CME on 2024 May 14. This helical motion was observed in extreme ultraviolet images from the Solar Dynamics Observatory (SDO) and provides new insights into plasma properties in a set of post-eruption coronal loops. Methods. We utilized images from the SDO Atmospheric Imaging Assembly (AIA) instrument to track the helical motion of plasma and to characterize its speed, acceleration, and physical properties. Additionally, we explored the evolution of the plasma density and temperature along the helical structure using the differential emission measure technique. Results. The helical structure was visible to AIA for approximately 22 minutes; it had a diameter of ∼22 Mm and a total trajectory of nearly 184 Mm. According to our analysis of the AIA observations, the speed of the plasma flow along this helical coronal loop ranged from 77 to 384 km s ⁻¹ , temperatures from 3.46 to 10.2 MK, densities from 4.3 × 10 ⁶ to 1.55 × 10 ⁷ cm ⁻³ , and the magnetic field strength from 0.05 to 0.3 G. Conclusions. Following the launch of the CME, we find clear evidence for impulsive heating and expansion of the plasma, which travelled along a helical trajectory along a set of post-eruption loops. These observations provide an insight into impulsive plasma flows along coronal loops and the topology of coronal loops.

The Formation of a Multifilament System Driven by Photospheric Converging Motions in a Bipolar Sunspot

Article

Full-text available

Jan 2025

Solar filaments are believed to be a clump of cold plasma accumulated in the magnetic dips. However, the magnetic configuration of filaments and the key factors for their formation remains elusive. In this Letter, we present a detailed study of the formation and eruption of a multifilament system with observations and simulations. Before the filament appeared visible, the chromospheric fibrils gradually gathered together, evolving from a diffuse distribution into threadlike structures that were nearly parallel to the polarity inversion lines. On 2022 March 20, an arch filament first appeared showing high dynamics, and subsequently two reserved S-shaped filaments were visibly observed. These two filament segments further reconnected, forming a long coherent filament and resulting in a double-decker configuration. In addition, continuous converging motion and magnetic flux cancellation were found in the photosphere during the evolution. Simultaneously, more bald patch structures appeared at the polarities' collision position. Through a data-driven numerical simulation, we further reconstructed the coronal magnetic field, which is composed of two twisted magnetic flux ropes (MFRs) with their bottom touching the photosphere, along with a group of sheared arcades forming an X-shaped configuration. These findings suggest that the magnetic configuration of the filament is in a highly dynamic state, evolving from a hybrid to a coherent MFR. Moreover, we propose that the formation and eruption of the multifilament system are closely related to magnetic reconnection taking place on the photosphere and in the lower corona, respectively, both mainly driven by the photospheric converging motion.

The Hidden Magnetic Structures of a Solar Intermediate Filament Revealed by the Injected Flare Material

Article

Full-text available

Jan 2025
ASTROPHYS J

Solar filaments are spectacular objects in the solar atmosphere, consisting of accumulations of cool, dense, and partially ionized plasma suspended in the hot solar corona against gravity. The magnetic structures that support the filament material remain elusive, partly due to the lack of high-resolution magnetic field measurements in the chromosphere and corona. In this study, we reconstruct the magnetic structures of a solar intermediate filament using EUV observations and two different methods, to follow the injection of hot material from a B-class solar flare. Our analysis reveals the fine-scale magnetic structures of the filament, including a compact set of mutually wrapped magnetic fields encasing the cool filament material, two groups of helical magnetic structures intertwining with the main filament, and a series of arched magnetic loops positioned along the filament. Additionally, we find that the northern footpoints of the helical structures are rooted in the same location, while their southern footpoints are rooted in different areas. The results obtained in this study offer new insights into the formation and eruption mechanisms of solar filaments.

Data-constrained Magnetohydrodynamic Simulation of a Filament Eruption in the Decaying Active Region 13079 on a Global Scale

Article

Full-text available

May 2025
ASTROPHYS J

Filaments are special plasma phenomena embedded in the solar atmosphere, characterized by unique thermodynamic properties and magnetic structures. Magnetohydrodynamic (MHD) simulations are useful to investigate the eruption mechanisms of filaments. We conduct a data-constrained zero- β MHD simulation in spherical coordinates to investigate a C3.5 class flare triggered by an eruptive filament on 2022 August 15 in the decaying weak active region 13079. We reconstruct the three-dimensional coronal magnetic field using vector magnetograms and synoptic maps from the Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We transform the vector magnetic field into Stonyhurst heliographic spherical coordinates combined with a synoptic map and construct a potential field source surface model with a magnetic flux rope embedded using the regularized Biot–Savart laws. Subsequently, we conduct a spherical zero- β MHD simulation using the message-passing interface adaptive mesh refinement versatile advection code (MPI-AMRVAC) and replicate the entire dynamic process of the filament eruption consistent with observations. With the calculation of the time–distance profile, quasi-separatrix layers, and synthetic radiation from simulated current density, we find a good agreement between our simulation and observations in terms of dynamics and magnetic topology. Technically, we provide a useful method of advanced data-constrained simulation of weak active regions in spherical coordinates. Scientifically, the model allows us to quantitatively describe and diagnose the entire process of filament eruption.

Reconnection of magnetic flux ropes driven by two-color Laguerre–Gaussian laser pulses in plasma

Article

May 2025

The generation and reconnection of magnetic flux ropes in a plasma irradiated by two Laguerre–Gaussian laser pulses with different frequencies and opposite topological charges are investigated numerically by particle-in-cell simulations. It is shown that twisted plasma currents and hence magnetic flux ropes can be effectively generated as long as the laser frequency difference matches the electron plasma frequency. More importantly, subsequent reconnection of magnetic flux ropes can occur. Typical signatures of magnetic reconnection, such as magnetic island formation and plasma heating, are identified in the reconnection of magnetic flux ropes. Notably, it is found that a strong axial magnetic field can be generated on the axis, owing to the azimuthal current induced during the reconnection of the ropes. This indicates that in the reconnection of magnetic flux ropes, the energy can be transferred not only from the magnetic field to the plasma but also from the plasma current back to the magnetic field. This work opens a new avenue to the study of magnetic flux ropes, which helps in understanding magnetic topology changes, and resultant magnetic energy dissipation, plasma heating, and particle acceleration found in solar flares, and magnetic confinement fusion devices.

MHD Simulations of the Slow-rise Phase of Solar Eruptions Initiated from a Sheared Magnetic Arcade

Article

Apr 2025
RES ASTRON ASTROPHYS

Prior to solar eruptions, a short-term slow-rise phase is often observed, during which the pre-eruption structure ascends at a speed much greater than the photospheric motions but much less than that of the eruption phase. Numerical magnetohydrodynamic (MHD) simulations of the coronal evolution driven by photospheric motions until eruptions have been used to explain the slow-rise phase, but their bottom driving speeds are much larger than the realistic photospheric values. Therefore, it remains an open question how the excessively fast bottom driving impacts the slow-rise phase. Here we modelled the slow-rise phase before eruption initiated from a continuously sheared magnetic arcade. In particular, we performed a series of experiments with the bottom driving speed unprecedentedly approaching the photospheric value of around 1~km~s

^{-1}

. The simulations confirmed that the slow-rise phase is an ideal MHD process, i.e., a manifestation of the growing expansion of the sheared arcade in the process of approaching a fully open field state. The overlying field line above the core flux has a slow-rise speed modulated by magnitude of the driving speed, but always larger than the driving speed by over an order of magnitude. The core field also expands with speed much higher than the driving speed but much lower than that of the overlying field. By incrementally reducing the bottom-driving speed until the realistic values, we observed a correspondingly better match of the slow-rise speed with typical observed ones.

SIP-IFVM: Efficient time-accurate magnetohydrodynamic model of the corona and coronal mass ejections

Article

Full-text available

Dec 2024
ASTRON ASTROPHYS

Context. Coronal mass ejections (CMEs) are one of the main drivers of space weather. However, robust and efficient numerical modelling applications of the initial stages of CME propagation and evolution process in the sub-Alfvénic corona are still lacking. Aims. Magnetohydrodynamic (MHD) solar coronal models are critical in the Sun-to-Earth model chain, but they do sometimes encounter low- β (<10 ⁻⁴ ) problems near the solar surface. This paper aims to deal with these low- β problems and make MHD modelling suitable for practical space weather forecasting by developing an efficient and time-accurate MHD model of the solar corona and CMEs. In this paper, we present an efficient and time-accurate three-dimensional (3D) single-fluid MHD solar coronal model and employ it to simulate CME evolution and propagation. Methods. Based on a quasi-steady-state implicit MHD coronal model, we developed an efficient time-accurate coronal model that can be used to speed up the CME simulation by selecting a large time-step size. We have called it the Solar Interplanetary Phenomena-Implicit Finite Volume Method (SIP-IFVM) coronal model. A pseudo-time marching method was implemented to improve temporal accuracy. A regularised Biot-Savart Laws (RBSL) flux rope, whose axis can be designed into an arbitrary shape, was inserted into the background corona to trigger the CME event. We performed a CME simulation on the background corona of Carrington rotation (CR) 2219 and evaluated the impact of time-step sizes on simulation results. Our study demonstrates that this model is able to simulate the CME evolution and propagation process from the solar surface to 20 R s in less than 0.5 hours (192 CPU cores, ~1 M cells). Compared to the explicit counterpart, this implicit coronal model is not only faster, but it also has improved numerical stability. We also conducted an ad hoc simulation with initial magnetic fields artificially increased. It shows that this model can effectively deal with time-dependent low- β problems ( β < 10 ⁻⁴ ). Additionally, an Orszag-Tang MHD vortex flow simulation demonstrates that the pseudo-time-marching method used in this coronal model can simulate small-scale unsteady-state flows. Results. The simulation results show that this MHD coronal model is very efficient and numerically stable. It is a promising approach to simulating time-varying events in the solar corona with low plasma β in a timely and accurate manner.

On Flare-CME Characteristics from Sun to Earth Combining Remote-Sensing Image Data with In Situ Measurements Supported by Modeling

Chapter

Full-text available

Jan 2017

Origin and structures of solar eruptions II: Magnetic modeling

Article

Full-text available

Jul 2017

The topology and dynamics of the three-dimensional magnetic field in the solar atmosphere govern various solar eruptive phenomena and activities, such as flares, coronal mass ejections, and filaments/prominences. We have to observe and model the vector magnetic field to understand the structures and physical mechanisms of these solar activities. Vector magnetic fields on the photosphere are routinely observed via the polarized light, and inferred with the inversion of Stokes profiles. To analyze these vector magnetic fields, we need first to remove the 180° ambiguity of the transverse components and correct the projection effect. Then, the vector magnetic field can be served as the boundary conditions for a force-free field modeling after a proper preprocessing. The photospheric velocity field can also be derived from a time sequence of vector magnetic fields. Three-dimensional magnetic field could be derived and studied with theoretical force-free field models, numerical nonlinear force-free field models, magnetohydrostatic models, and magnetohydrodynamic models. Magnetic energy can be computed with three-dimensional magnetic field models or a time series of vector magnetic field. The magnetic topology is analyzed by pinpointing the positions of magnetic null points, bald patches, and quasi-separatrix layers. As a well conserved physical quantity, magnetic helicity can be computed with various methods, such as the finite volume method, discrete flux tube method, and helicity flux integration method. This quantity serves as a promising parameter characterizing the activity level of solar active regions.

Multiwavelength Observations of a Flux Rope Formation by Series of Magnetic Reconnection in the Chromosphere

Article

Full-text available

Mar 2017

Using high-resolution observations from the 1.6 m New Solar Telescope (NST) operating at the Big Bear Solar Observatory (BBSO), we report direct evidence of merging/reconnection of cool H

\alpha

loops in the chromosphere during two homologous flares (B- and C-class) caused by a shear motion at the footpoint of two loops. The reconnection between these loops caused the formation of an unstable flux rope which showed counterclockwise rotation. The flux rope could not reach the height of torus instability and failed to form a coronal mass ejection. The HMI magnetograms revealed rotation of the negative/positive (N1/P2) polarity sunspots in the opposite directions, which increased the right and left-handed twist in the magnetic structures rooted at N1/P2. Rapid photospheric flux cancellation (duration

\sim

20-30 min, rate

\approx

3.44

\times

^{20}

Mx h

^{-1}

) was observed during and even after the first B6.0 flare and continued until the end of the second C2.3 flare. The RHESSI X-ray sources were located at the site of the loop's coalescence. To the best of our knowledge, such a clear interaction of chromospheric loops along with rapid flux cancellation has not been reported before. These high-resolution observations suggest the formation of a small flux rope by a series of magnetic reconnection within chromospheric loops associated with very rapid flux cancellation.

On Flare-CME Characteristics from Sun to Earth Combining Remote-Sensing Image Data with In Situ Measurements Supported by Modeling

Article

Full-text available

Jul 2017
SOL PHYS

We analyze the well observed flare-CME event from October 1, 2011 (SOL2011-10-01T09:18) covering the complete chain of action - from Sun to Earth - for a better understanding of the dynamic evolution of the CME and its embedded magnetic field. We study the solar surface and atmosphere associated with the flare-CME from SDO and ground-based instruments, and also track the CME signature off-limb from combined EUV and white-light data with STEREO. By applying 3D reconstruction techniques (GCS, total mass) to stereoscopic STEREO-SoHO coronagraph data, we track the temporal and spatial evolution of the CME in interplanetary space and derive its geometry and 3D-mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the MC from in-situ measurements (Wind). This is compared to nonlinear force-free (NLFF) model results as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are reduced in interplanetary space by ~50% and 75%, respectively, possibly indicating to an erosion process. A mass increase of 10% for the CME is observed over the distance range from ~4-20 Rs. The temporal evolution of the CME associated core dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME.

Vortex and Sink Flows in Eruptive Flares as a Model for Coronal Implosions

Article

Full-text available

Feb 2017
ASTROPHYS J

Eruptive flares are sudden releases of magnetic energy that involve many phenomena, several of which can be explained by the standard 2D flare model and its realizations in three-dimensions. We analyze a three-dimensional magnetohydrodynamics simulation in the framework of this model that naturally explains the contraction of coronal loops in the proximity of the flare sites, as well as the inflow towards the region above the cusp-shaped loops. We find that two vorticity arcs located along the flanks of the erupting magnetic flux rope are generated as soon as the eruption begins. The magnetic arcades above the flux-rope legs are then subjected to expansion, rotation or contraction depending on which part of the vortex-flow advects them. In addition to the vortices, an inward-directed magnetic pressure gradient exists in the current sheet below the magnetic flux rope. It results in the formation of a sink that is maintained by reconnection. We conclude that coronal loop apparent implosions observed during eruptive flares are the result of hydro-magnetic effects related to the generation of vortex- and sink-flows when a flux rope moves in a magnetized environment.

Estimation of Reconnection Flux using Post-eruption Arcades and Its Relevance to 1-au Magnetic Clouds

Article

Full-text available

Apr 2017
SOL PHYS

We report on a new method to compute the flare reconnection (RC) flux from post-eruption arcades (PEAs) and the underlying photospheric magnetic fields. In previous work, the RC flux has been computed using cumulative flare ribbon area. Here we obtain the RC flux as half of that underlying the PEA associated with the eruption using an image in EUV taken after the flare maximum. We apply this method to a set of 21 eruptions that originated near the solar disk center in solar cycle 23. We find that the RC flux from the arcade method ({\Phi}rA) has excellent agreement with that from the ribbon method ({\Phi}rR) according to: {\Phi}rA = 1.24({\Phi}rR)^0.99. We also find {\Phi}rA to be correlated with the poloidal flux ({\Phi}P) of the associated magnetic cloud at 1 au: {\Phi}P = 1.20({\Phi}rA)^0.85. This relation is nearly identical to that obtained by Qiu et al. (2007) using a set of only 9 eruptions. Our result supports the idea that flare reconnection results in the formation of flux rope and PEA as a common process.

The standard flare model in three dimensions

Article

Jul 2013

Solar Flares and Coronal Mass Ejections

Article

Jan 1992
ANNU REV ASTRON ASTR

S. W. Kahler

The transition from eruptive to confined flares in the same active region

Article

Feb 2017

Context. Solar flares are sudden and violent releases of magnetic energy in the solar atmosphere that can be divided into two classes: eruptive flares, where plasma is ejected from the solar atmosphere resulting in a coronal mass ejection (CME), and confined flares, where no CME is associated with the flare. Aims. We present a case study showing the evolution of key topological structures, such as spines and fans, which may determine the eruptive versus non-eruptive behavior of the series of eruptive flares followed by confined flares, which all originate from the same site. Methods. To study the connectivity of the different flux domains and their evolution, we compute a potential magnetic field model of the active region. Quasi-separatrix layers are retrieved from the magnetic field extrapolation. Results. The change in behavior of the flares from one day to the next – from eruptive to confined – can be attributed to the change in orientation of the magnetic field below the fan with respect to the orientation of the overlaying spine rather than an overall change in the stability of the large-scale field. Conclusions. Flares tend to be more confined when the field that supports the filament and the overlying field gradually becomes less anti-parallel as a direct result of changes in the photospheric flux distribution, being themselves driven by continuous shearing motions of the different magnetic flux concentrations.

Halpha Doppler shifts in a tornado in the solar corona

Article

Dec 2016

Context. High resolution movies in 193 Å from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory (SDO) show apparent rotation in the leg of a prominence observed during a coordinated campaign. Such structures are commonly referred to as tornadoes. Time-distance intensity diagrams of the AIA data show the existence of oscillations suggesting that the structure is rotating. Aims. The aim of this paper is to understand if the cool plasma at chromospheric temperatures inside the tornado is rotating around its central axis. Methods. The tornado was also observed in H α with a cadence of 30 s by the MSDP spectrograph, operating at the Solar Tower in Meudon. The MSDP provides sequences of simultaneous spectra in a 2D field of view from which a cube of Doppler velocity maps is retrieved. Results. The H α Doppler maps show a pattern with alternatively blueshifted and redshifted areas of 5 to 10′′ wide. Over time the blueshifted areas become redshifted and vice versa, with a quasi-periodicity of 40 to 60 min. Weaker amplitude oscillations with periods of 4 to 6 min are superimposed onto these large period oscillations. Conclusions. The Doppler pattern observed in H α cannot be interpreted as rotation of the cool plasma inside the tornado. The H α velocity observations give strong constraints on the possible interpretations of the AIA tornado.

Origin and structures of solar eruptions I: Magnetic flux rope

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