ApJ, 655, 1142

15 Pages·2007·1.86 MB·English

TheAstrophysicalJournal,655:1142Y1156,2007February1
Copyrightisnotclaimedforthisarticle.PrintedinU.S.A.
IN/OUT PAIRS AND THE DETACHMENT OF CORONAL STREAMERS
N. R. Sheeley, Jr. and Y.-M. Wang
E.O.HulburtCenterforSpaceResearch,NavalResearchLaboratory,WashingtonDC;
[email protected],[email protected]
Received2006June27;accepted2006October19
ABSTRACT
We previously described coronal events that originate in the 2Y6 R field of view of the LASCO white-light
(cid:1)
coronagraphandinvolvethesimultaneousejectionofmaterialinwardtowardtheSunandoutwardawayfromit.
Now,inastudyofmorethan160in/outpairs,wehavefoundthatthesefeaturesaredensityenhancementsatthe
leadingandtrailingedgesofdepletionsthatoccurwhenslowlyrisingcoronalstructuresseparatefromtheSun.The
outwardcomponentisshapedlikealargearchwithbothendsattachedtotheSun,andtheinwardcomponentis
oftenresolvedintoloops.Wealsofoundabout60additionaleventsinwhichtheoutwardcomponentsbegannear
theedgeoftheoccultingdiskandinwardcomponentswerenotvisible,asiftheseeventswerein/outpairsthatoriginated
belowthe2R radiusoftheoccultingdisk.Weconcludethatin/outpairsbelongtoabroadclassofstreamerdetachments,
(cid:1)
which include ‘‘streamer blowout’’ coronal mass ejections, and we suppose that all of these events occur when
risingmagneticloopsreconnecttoproduceanoutgoinghelicalfluxropeandaningoingarcadeofcollapsingloops.
Subject headinggs: Sun: corona — Sun: coronal mass ejections (CMEs) — Sun: magnetic fields
1. INTRODUCTION butmanyofthemdidnot.Now,inastudyof162in/outpairs,we
havefoundthatatleast80%areprecededbya1Y2dayinterval
ThispaperdescribesobservationsobtainedwiththeLargeAngle
ofgradualcoronalexpansion,whichendswiththeonsetofthe
SpectrometricCoronagraph(LASCO)ontheSolarandHelio-
pair. (The remaining 20% are distributed evenly between inde-
sphericObservatory(SOHO).LASCOincludestwowhite-light
terminatecasesandcasesforwhichthepairliesinthewakeofa
coronagraphs(C2andC3)withseparatefieldsof viewspanning
theranges2Y6and4Y30R ,respectively,andhasobserveda sequenceofCMEs.)Inthenextsectionofthispaper,wedescribe
(cid:1) thisnewdiscoveryandshowadditionaleventsthatmaybein/out
varietyofcoronalstructuresandtransientssinceoperationsbegan
pairsthatoriginatebelowtheoccultingdisk,wheretheinward
inearly1996.Inadditiontocoronalmassejections(CMEs)and
componentsarenotvisible.Inthefinalsection,wesummarize
outwardmovingstreamer‘‘blobs,’’wehaveobservedmorethan
theseobservationsandinterpretthemintermsofarisingarcade
10,000smallandusuallyfaintfeaturesmovingtowardtheSun.
of loops that reconnect to form an outgoing helical flux rope
About98%ofthesecoronalinflowsoccurinlong-livedstreams
andaningoingsystemofcollapsingloops.
atnorth-southexcursionsofthestreamerbelt.Theserainingin-
flowsseemtoindicatetheongoingmagneticfieldlinereconnec- 2. OBSERVATIONS
tionthatoccursasdifferentialrotationshearsthephotospheric
2.1. In/Out Pairs and Streamer Disconnection Events
neutralline,weakenstheequatorialcomponentoftheSun’sdipole
field,andgraduallyflattensthestreamerbelt(Sheeleyetal.2001; Figure1showseightsetsofC2andC3imagesofin/outpairs
Sheeley&Wang2001).Liketheinflowsabovepostflareloops observedduringtheyears2000Y2005,arrangedindecreasing
(McKenzie&Hudson1999;McKenzie2000;Gallagheretal. orderoftheirapparentsize.Eachframecontainsarunningdiffer-
2002;Innesetal.2003a,2003b;Asaietal.2004;Sheeleyetal. enceimage,obtainedbysubtractingthepreviousimageonthat
2004),theseraininginflowsdonothavevisibleoutwardcom- dayfromthecurrentimage.Suchdifferenceimagesshowsubtle
ponents,perhapsbecausetheyarefaintandheadedupwardinto propermotionswithoutthedistractingeffectsofstationaryback-
regionsofevenlowerdensity. groundfeatures.InthefirstfewC2frames,thein/outpairsappear
About2%oftheLASCOinflows((cid:2)160)areaccompaniedby asoppositelydirectedarchesresemblingthejawsofanopenmouth.
outwardcomponents(Wangetal.1999;Sheeley&Wang2002; TheC3imagesshowtheoutwardcomponentsafewhourslater
Simnett2004).Unliketheraininginflows,whichbecomeless aslargearcheswhoselegsextendbacktowardtheSun.Aswe
densethantheirsurroundingsastheymoveinward,theseparating havediscussedpreviously,theinwardandoutwardcomponents
componentsofin/outpairsaredensityenhancements.Inaddition, moveatspeedsofabout100and300kms(cid:3)1,respectively,relative
theiroutwardcomponentshavecharacteristicarch-shapedsig- tothecenteroftheSun,andthereforeseparatefromeachotherat
natures.Inarecentpaperofthisseries(Wang&Sheeley2006, relativespeedsofabout400kms(cid:3)1(Sheeley&Wang2002).In
hereafterPaperI),wedescribedobservationsandsimulations theserunningdifferenceimages,theinwardandoutwardcom-
thatsuggestthattheoutwardcomponenthasthehelicalstructure ponentshavetheleading-white/trailing-blacksignaturesofden-
ofamagneticfluxrope. sityenhancements,ratherthantheleading-black/trailing-white
A puzzling aspect of the in/out pairs has been their origin. signaturesofthedarkloopsandtrailing‘‘tadpole-like’’features
Unlike raining inflows, which were visible whenever a north- thataccompanyraininginflows.
southsegmentofthestreamerbeltrotatedpasttheeastorwest AswelookthroughthesequenceofC2imagesinFigure1,we
limb,thein/outpairsoccurredsporadicallywithnoapparentrela- seeachangeintheapparentsizeofthejaw-likeseparations,as
tiontomagneticstructuresortheoccurrenceofCMEs.Somein/out ifwewereobtainingamoreobliqueviewoftheseevents.Even-
pairsseemedtobetriggeredbyshockwavesfrommajorCMEs, tually,thepairsaresonarrowthattheyresemblethestreamer
1142
IN/OUT PAIRS AND DETACHMENT OF CORONAL STREAMERS 1143
Fig.1.—Runningdifferenceimages,showingaselectionofeightin/outpairsintheC2fieldofview(croppedto6R onaside)andtheoutwardcomponentsofthese
(cid:1)
pairsafewhourslaterintheC3field(croppedto15R onaside).Theseeventsarearrangedindecreasingorderoftheirangularspanstoshowtheapparenttrendfrom
(cid:1)
face-ontoedge-onviews.
disconnectioneventsweobservedduringtherisingphaseofthe shape similar to that of a postflare loop system, the outgoing
sunspotcycle(Wangetal.1999).Asweshownext,otherprop- componenthasacorrugated,cusplikeshape,consistentwiththe
ertiesofin/outpairsarealsoconsistentwiththisideathattheyare helicaltopologyofafluxrope,asdescribedinPaperI.Intheright
face-onviewsofstreamerdisconnectionevents. column,similarminimum-subtractedimagesshowthedepletion
asitcontinuesoutwardthroughtheC3fieldofview(croppedto
2.2.Spontaneous In/Out Pairs
15R oneachside).Clearly,thisin/outpairleftthecoronaless
(cid:1)
Figure2referstoanin/outpairon2005October12.Intheleft densethanitwasbefore.
column,therunningdifferenceimagesshowtheformationand Figure 3 shows height/time maps, obtained by observing the
initialseparationoftheinwardandoutwardcomponentsasthey evolutionofthein/outpairthrougharadialslitandthenplac-
movethroughthesouthwestquadrantoftheC2fieldofview(6R ingtheresultingstripsinchronologicalorder.Thisprocedurehas
(cid:1)
oneachside).Asusual,eachcomponenthastheleading-white/ beendescribedpreviouslybySheeleyetal.(1999).Ingeneral,we
trailing-blacksignatureofadensityenhancement.Themiddle choseaslitwidthof17pixels,butforoneparticularlycomplicated
columncontainsimagesobtainedbysubtractingamonthlymini- event (2004 January 20, shown in Figs. 6Y9), we increased the
mumintensityfromeachpixel.Bysubtractingthemonthlymini- widthto25pixelstocapturethedetailsofthemotion.Running
mumintensity,wereducethequasi-stationarycontributionsof differenceimageswereusedinthetoppanel,whichtracksspatial
scatteredlightfromcircumsolardustandfromtheLASCOoptical fluctuationsofcoronalintensityastheymovethroughthefieldof
train,aswedidwiththerunningdifferenceimages,butwithout view.A36hrintervalofgradualaccelerationfromaninitialspeed
removingthecontributionsofcoronalstreamers. (cid:2)20kms(cid:3)1endswiththesuddenformationofthein/outpair.The
These ‘‘minimum subtracted’’ images show a dark depletion tracksoftheinwardandoutwardcomponentsappearasatrailing
betweentheingoingandoutgoingcomponentsofthepair.Be- boundaryoftheflowfield,whichdoesnotextendbeyondthem.
causethedepletionexpandsrapidly,itsedgesovertakeandcom- Theslopeoftheupwardboundaryisapproximately360kms(cid:3)1.
pressmaterialthatliesahead,causingthedensityenhancements Thebottompanelof Figure3containsaheight/timemap,ob-
thatgiverisetotheleading-white/trailing-blacksignaturesinrun- tainedusingdifferenceimagesrelativetoafixed‘‘baseframe’’
ningdifferenceimages.Whiletheinwardcomponenthasaspiny near the start of the expansion, and indicates the accumulated
1144 SHEELEY & WANG Vol. 655
Fig.2.—C2runningdifferenceimages(left)andthecorrespondingminimum-subtractedimages(center)ofanin/outpairinthesouthwestquadrantofthecoronaon2005
October12.Theseimagesshowthatthein/outpairconsistsofapairofdensityenhancementsseparatedbyanexpandingdensitydepletion.C3minimum-subtractedimages
(right)showthecontinuedexpansionofthisdepletionfartheroutinthecorona.AlloftheC3imagesinthispaperhavebeencroppedto15R onaside
(cid:1)
changes since that time. This map shows the slowly rising re-
gionsofcoronalintensity,whosespatialfluctuationsproduced
thetracksinthetoppanel.Thepairleavesadarkercoronainits
wake.
Figures4and5refertoaverynarrowin/outpairwhoseangular
spanwasonly4(cid:4).Thisnarrow2005September17pairisthelast
pairillustratedinFigure1.IntheC3imagesof Figure4(right),
theoutwardcomponentsresemblethe‘‘streamerblobs’’thatwe
studiedduringtherisingphaseofthesunspotcycle(Sheeleyetal.
1997).Thus,itisnaturaltoaskwhetherstreamerblobsmaybe
in/out pairs whose inward components are unobserved below
theoccultingdisk.Wethinkthisisunlikely.Streamerblobsemerge
fromthetipsof helmetstreamerswithoutleavinginwardcom-
ponentsbehind.Theyalsoaccelerategraduallyandreachtheir
terminal speeds of 300Y400 km s(cid:3)1 beyond 15 R from Sun
(cid:1)
center.Bycomparison,theheight/timemapsinFigure5show
thattheoutwardcomponentonSeptember17startedabruptly
near5R witharelativelyhighspeedof300kms(cid:3)1andwas
(cid:1)
accompanied by a prominent inward component. Presumably,
thesedifferenceshaveaphysicalorigin;theblobsarethoughtto
beformedbyreconnectionbetweenopenandclosedfieldlines
(Wangetal.1998),whereasthein/outpairsprobablyindicate
reconnectionbetweentheclosedloopsofarisingarcade(PaperI).
Fig. 3.—Height/timemapsconstructedfromrunningdifferenceimages(top)
Inthe top panel ofFigure5,the tracksofthe gradual expan-
anddifferenceimagesrelativetoafixedbasenearthestartofthegradualexpansion
(bottom).The36hrinterval(2005October10Y12)ofgradualexpansionendswith sionareparticularlywellresolvedandlastforabout30hrbefore
theformationofthein/outpair.Thepositionangleis221(cid:4). thepairforms.Intheupperbranch,thetrackslosetheiridentity
No. 2, 2007 IN/OUT PAIRS AND DETACHMENT OF CORONAL STREAMERS 1145
Fig. 4.—C2(left)andC3(right)runningdifferenceimagesofaverynarrow
in/outpairon2005September17.Thearrowsindicatetheinwardandoutward
componentsnearthestartoftheeventat09:36UT.
whentheyencounterandmergewiththesteepertrackoftheout-
wardcomponent.Inthelowerbranch,thetrackslosetheiriden-
titywhentheyencounterandmergewiththetrackoftheinward
Fig.6.—C3runningdifferenceimagesofthenortheastquadrantofthecorona,
component.ThisbehaviorisunaffectedbyaCMEwhoseshort,
showingtheformationandmergingofseveral‘‘outcomponents’’inthewakeofa
steeptrackcutsacrossthepatternat15:00UTonSeptember16. haloCMEon2004January20.Thearrowindicatesthefirstofthesearch-shaped
featuresasitappearsbehindthedisturbancefromtheCME.
Inthebottompanel,thefixed-baseheight/timemapshowsthe
slowlyrisingintensitypattern,endingatthetimeofthepair.(We
spatiallysmoothedthismaptoreduceperiodicdefectsassociated
withthefinitewidthofthestackedstrips.)
2.3. Pairs Triggered by CMEs
Theeventswehavejustdescribedoccurredspontaneouslywith-
out the perturbing effects of CMEs. However, sometimes the
disturbancefromamajorCMEwillterminatestreamergrowth
prematurelyandtriggeranin/outpair.Fromasubsetof60well-
observedin/outpairs,wefound30eventsthatseemedtobetrig-
geredbyCMEs,25eventsthatseemedtooccurontheirown,
andfiveindeterminateevents.
Next,weshowanextremecaseinwhichthedisturbancefroma
haloCMEon2004January20triggeredaseriesofin/outpairs.
Although the bulk of the ejected mass went into the south-
ernhemisphere,amajordisturbancepropagatedinalldirections
Fig.5.—Height/timemapsforthepairinFig.4,againshowingthatthepair aroundtheoccultingdisk.TheC3runningdifferenceimagesin
markstheendofalongintervalofcoronalexpansion.Inparticular,theindividual
Figure6provideanoverviewofthisdramaticeventasitappeared
tracksoftheflowfieldcontinuebothaboveandbelowthepairformationheight
inthenortheastquadrant(croppedto15R oneachside).Thefirst
untiltheyreachtheboundingtracksofthein/outpair.Thefixed-basemaphasbeen (cid:1)
smoothedtoreduceperiodicdefects.Thepositionangleis75(cid:4). twopanelsshowthedisturbancemovingacrossthe4Y15R(cid:1)field
1146 SHEELEY & WANG Vol. 655
andthesubsequentformationofthein/outpairs.Thebottompanel
showsanexpandedviewofthisaction,beginningwithapairof
widelyspacedwhitetracksoftheCMEat01:00UT.
The scenario is essentially the same as that observed in Fig-
ure7.Thetracksofoneortwoslowlyrisingoutwardcomponents
emergefromtheblurrypairofCMEtracksat01:00UT.Thetrack
ofanother,fasteroutwardcomponentformsatabout04:00UT
and overtakes the slower of the first two tracks at 06:00 UT.
Around10:00UT,themergedtrackisovertakenbyathird,even
steepertrack,whichseparatedfromaninwardtrackofpointed
loops at 08:00 UT. By 10:00 UT, the height/time map shows
littleflowbelow15R ,exceptfortheremnantsofthesecollaps-
(cid:1)
ingloops.
InFigure9,theC2runningdifferenceimagesshowthenorth-
eastquadrant(6R oneachside)during07:31Y12:30UT,when
(cid:1)
thefinaloutwardcomponentswereseparatingfromtheirinward
counterparts.At07:31UT,theoutwardcomponentsarevisible
astheymergeintoasinglecusp-shapedfeatureandseparatefrom
anapparentarcadeofpointedloops(arrow).Theremainingim-
agesshowthecollapseandprogressiveroundingoftheseloops.
Inthispaper,wehaveshownin/outpairsthatformedsponta-
neouslyandin/outpairsthatseemtohavebeentriggeredbythe
disturbancefromaCME.However,wehavealsofoundcasesin
whichaCMEtriggeredasinglein/outpairandothercasesin
whichasequenceofpairsoccurredspontaneouslywithoutthe
occurrenceofaCME.Butthecommoncharacteristicofallthese
in/outpairsistheirlocationattheendofagraduallyrisingcoronal
structure.
2.4. Events without Inward Components
As mentioned above, some events have all of the proper-
tiesofanin/outpair,exceptforanobservedinwardcomponent.
Figure10showssuchaneventon2004October16Y17.Inthe
left panels, the running difference images show the weak sig-
natureofaslowlyrisingloopsystemat06:30UTandthestrong
signatureofamuchfasterrisingarchduring22:30Y00:30UT.
Thearchhastheleading-white/trailing-blacksignatureofaden-
sityenhancement.Intherightpanels,theminimum-subtracted
Fig.7.—C3minimum-subtractedimages,correspondingtothedifferenceim- imagesshowthisdensityenhancementandthedepletionleftbe-
agesinFig.6andshowingthedensitydepletionsbehindtheindividualoutward hindit.Liketheoutwardcomponentsofmanyin/outpairs,this
components. ‘‘loop’’hasthecorrugatedshapeofahelicalfluxropewhoselegs
remainattached tothe Sun. The outward component looksso
of view during 01:42Y02:42 UT. By 02:42 UT, a small arch- muchlikeaneruptiveprominencethatwewonderwhetheritis
shapedfeature(arrow)hasbecomevisiblebehindthetrailingedge prominencematerialcarriedupfromthechromosphereandion-
ofthiswave.Itisthefirstofseveralarchesthatformnear5R ized,asweoftenobserveforCMEswithathree-partstructure
(cid:1)
andthenmoveoutwardtocatchthepreviousones.By04:42UT (leadingedge,trailingcavity,andcentralcore),orwhetheritwas
a second arch forms behind the first one, which has moved to producedinsituliketheoutwardcomponentsofthe(cid:2)160ob-
about6R .Likealloutwardcomponentsseeninrunningdiffer- servedin/outpairs.Ofcourse,thisleadstotherelatedquestionof
(cid:1)
ence images, these arches have the leading-white/trailing-black whetherthetrailingdarkregionisarisingstreamercavityoranew
signaturesofdensityenhancements.Thesecondarchmovesfaster depletioncreatedduringtheformationofanin/outpairbelow
thanthefirstandovertakesitby06:42UTwhentwonewarches theoccultingdisk.
becomevisiblenear5R .Thesenewarchesmoveevenmore Figure11showsthecorrespondingheight/timemaps.Likethe
(cid:1)
rapidly outward and begin to merge with the earlier ones at height/timemapsofin/outpairs (cf.Figs. 3 and 5), these maps
08:18UTtoformasingle,large,cusp-shapedloop,whichisstill show about 32 hr of gradual expansion ending with the steep
visibleat10:20UTwhenthesequenceends. (330kms(cid:3)1)upwardtrackofanoutwardcomponent.However,
Ifthesearchesarereallytheoutwardcomponentsofin/out inthisevent,theoutwardcomponentbeginsattheoccultingdisk
pairs,thenminimum-subtractedimagesoughttoshowcoronal andaninwardcomponentisnotvisible,againraisingtheques-
depletions behind these outward components. The minimum- tionofwhetherthiseventmaybeanin/outpairthatformedbe-
subtractedimagesinFigure7showtheexpecteddepletions,even- lowtheoccultingdisk.
tuallymergingintoasingledarkregion. Figures12and13provideanotherexample,thistimeon2003
Figure 8 shows running difference height/time maps for this April12Y13.Intheleftpanelsof Figure12,theC2runningdif-
event.Thetoppanelreferstothe3dayinterval2004January18Y ferenceimagesshowthefamiliarcharacteristicsofanoutward
20andshowsthegraduallyrisingflowfieldforabout34hrbefore component:anarch-shapeddensityenhancementinthewakeof
itendedwiththepassageoftheCMEat01:00UTonJanuary20 agradualexpansion.Intherightpanels,theminimum-subtracted
No. 2, 2007 IN/OUT PAIRS AND DETACHMENT OF CORONAL STREAMERS 1147
Fig.8.—Runningdifferenceheight/timemaps,showing1.5daysofgradualexpansionpriortotheoccurrenceofthehaloCMEandthetriggeredin/outpairsonJanuary20
(top)andanexpandedviewofthetracksthatendedtheexpansion(bottom).Thewidthoftheslitusedtomaketheheight/timemapsis25pixelshere,butitis17pixelsforthe
otherfiguresinthispaper.Inthebottompanel,thefinaloutwardcomponenttearsawayfrompointedloops,whichmoveinwardandbecomerounder.
imagesshowthatthiscomponentwastornfromacoronalstreamer. aninwardcomponent(ifitoccurred)musthavenarrowlymissed
Liketheoutwardcomponenton2004October16Y17inFigure10, detectionjustbelowtheoccultingdisk.
thisfeaturehasacomplicatedfinestructure,suggestiveofaflux The preceding examples raise the question of whether some
rope. in/outpairsmayformbelowtheoccultingdisk,wheretheirinward
UnliketheOctober16Y17imagesinFigure10,theseApril12Y componentsescapedetection.Next,weexaminehistogramsof
13 minimum-subtracted images show only a slight depletion themeasuredstartingheightsof153in/outpairstoseewhether
behindtheoutwardcomponent.However,whenthefullcomple- theyareconsistentwiththishypothesis.Weexcludedabout60
mentoftheseimagesisviewedinatime-lapsemovie,itshowsa additionalevents,suchasthoseinFigures10Y13,forwhichin-
largedepletionintheregionbetweenthelegsofthearchinthe ward components were not observed. Figure 14 contains his-
runningdifferenceimages.Inthestaticimages,thisdepletionis togramsoftheheightsoftheinwardandoutwardcomponentsat
weakenedbyemissionfromtheremainingstreamermaterialalong thetimeswhentheinwardcomponentwasfirstseen.Italsocon-
thelineofsight.Thisoverlappingintensityisgreatlyreducedin tainshistogramsoftheaverageofthesetwoheights(providing
therunningdifferenceimagesanddoesnotmaskthevisibility anestimateofthecommonpointoforigin)andoftheirdifference.
ofthefast,brightoutwardcomponent.However,theremaining Thedistributionofinwardstartingheights,r ,isasymmetric
in
streamermaterialdoescontributetothefainterheight/timetracks withasharppeaklocatedbelowthe3.3R averagevalueanda
(cid:1)
ofthegraduallyrisingfeature,whichextendslightlybeyondthe broadtailextendingabovethisaverage.Thesteepfalloffinthe
steeper ((cid:2)400 km s(cid:3)1) and more intense track of the outward range2.0Y2.5R probablyreflectsthedecreasinginstrumental
(cid:1)
component,asshowninFigure13.Thestreamercontinuedtobe resolutiontowardtheoccultingdiskandsuggeststhatthedis-
activeatthislocationonApril14whenanin/outpairat07:30UT tributionofallin/outpairs(unobscuredbytheoccultingdisk)
wasfollowedbyasmallCME2hrlater. wouldpeakevenclosertotheSun.By comparison,thedistri-
Thetoppanelof Figure13showsthatthetrackoftheoutward butionofoutwardstartingheights,r ,ismoresymmetricand
out
componentdidnotbeginattheoccultingdiskbutinsteadstarted broadlypeakedarounditsaveragevalueof5.3R .Inparticular,
(cid:1)
0.5R aboveitatadistanceofabout2.5R fromSuncenter. itisnotthesimpleshiftoftheinwarddistributionthatonewould
(cid:1) (cid:1)
Wesupposethatthisisarealeffectandnotentirelytheresultof obtainifallofthestartingseparations,r (cid:3)r ,werethesame.
out in
thedecreasedresolutiontowardtheoccultingdiskbecausethe Thisimpliesthatthedistributionofstartingseparationsmustalso
evenfainteroutwardcomponentshowninFigure11wasvisible bebroad,asonecanreadilyverifyinthebottomrightpanel.Here
rightdowntotheoccultingdisk.Thiswouldmeanthattheout- thedistributionofstartingseparationsrangesfrom1to3R and
(cid:1)
wardcomponentformedintheC2fieldofviewanddidnotsimply hasapeakat2R .Thebreadthofthisdistributioniscausedby
(cid:1)
emergefromalocationbelowtheoccultingdisk.Consequently, temporal sampling of the LASCO observations, but the lower
1148 SHEELEY & WANG Vol. 655
Fig.10.—C2runningdifference(left)andminimum-subtracted(right)images
on2004October16Y17,showinganarch-shapederuptionattheendofaninterval
ofcoronalexpansion.Intheminimum-subtractedimages,thearchdevelopsa
corrugatedstructure,similartothatofaneruptiveprominenceorhelicalmag-
neticfluxrope.
Itisplausible thatthegradually expanding coronal structures
indicatetherisingloopsofagrowingmagneticfieldandthatthe
in/out pairs are signatures of magnetic field line reconnection
when the field stops growing and the detached field is carried
away.Inthiscase,theheightofthereconnectionmightdepend
onapropertyoftheexpansion,suchasitsstrengthorsize,so
that‘‘strongerevents’’reconnectclosertotheSunand‘‘weaker
events’’reconnectfartheroutinthecorona.Infact,our60events
withoutinwardcomponentsdidtendtobelargerthantheevents
withobservedinwardcomponents.(Forexample,50%ofthe
Fig.9.—C2runningdifferenceimagesonJanuary20,showingpointedloops
events without visible inward components had angular spans
detachingfromtheiroutgoingcounterpartsandbecomingrounderastheycollapse.
greaterthan50(cid:4),whichwastheupperlimitoftheangularspans
oftheobservedin/outpairs.)
limitat1R isarealeffect,probablyindicatingthatthesefea- Figure15showsoneoftheselargeeventson2004February13.
(cid:1)
turesareexceedinglyfaintwhentheyfirstappear.Inthebottom Intheleftpanels,therunningdifferenceimagesshowabroad
left panel, the distribution of ‘‘extrapolated’’ starting heights, feature risingslowlyacrossthenorthernhemisphere. Itsdisrup-
(r þr )/2,iscenteredatanaveragevalueof4.3R .Itslower tionbeginswhenasmalldarkarchemergesinthenorthwestand
in out (cid:1)
limitnear3R reflectstheinstrumentalcutoffof2.0Y2.5R for eventually spreads eastward to span the entire northern hemi-
(cid:1) (cid:1)
theinwardcomponentsplustheobservedseparationlimitofabout sphere.Noinwardcomponentisvisible.Intherightpanels,the
1.0R . minimum-subtractedimagesshowthedisruptionofacoronal
(cid:1)
No. 2, 2007 IN/OUT PAIRS AND DETACHMENT OF CORONAL STREAMERS 1149
Fig.11.—Height/timemapsoftheeventinFig.10.A1Y2dayintervalof
gradualexpansionendswiththeoccurrenceofasteepoutwardcomponent,which
extendsallthewaytotheoccultingdisk,butnoinwardcomponentisvisible.This
mighthavebeenanin/outpairthatoriginatedbelowtheoccultingdisk.The
positionangleis35(cid:4).
streamer in the northwest and the formation of a depleted re-
gionbehindtheexpandingloop.Figure16containsheight/time
mapsobtainedatthreedifferentpositionanglesacrosstheevent.
Theyshowabout16hrofslowexpansion,terminatedbyafaster
(250kms(cid:3)1)depletion,butnoevidenceofaninwardcomponent.
Wesupposethatthiseventhasaslowerprojectedspeedthanthe
previous,smallereventsbecauseitismoreinclinedoutofthesky
plane,likeahaloCME.
Inselectingtheeventsofthissection,weconsideredproperties
thataresimilartothoseofin/outpairssuchasthepriorinterval
ofgradualexpansionendingwithasuddenoutwardejection.We
triedtoavoideventsthathadthetraditionalthree-partstructure
(leadingedge,trailingcavity,andcentralcore)ofCMEs.Because
in/outpairssignifytheendofgraduallyacceleratingoutflows,we
havethoughtofthemasCME-endingevents,butnotasCMEs
themselves.InPaperI,wereferredtothemas‘‘failedCMEs.’’
Now,itisclearthatalloftheseeventsinvolvethedisruptionof
Fig.12.—C2runningdifferenceimages(left)andminimum-subtractedimages
coronalstreamersandthatsomeofthem(suchasthe2004Feb-
(right)ofthesoutheastquadrantofthecoronaon2003April13,showingastreamer
ruary13eventinFig.15)arestreamerblowoutCMEsobserved
disruptionwithoutavisibleinwardcomponent.
faceon.
StreamerblowoutCMEsinvolvethegradualbrighteningand
suddeneruptionofcoronalstreamersandoftenhavecharacteristic
‘‘bugle-like’’signaturesinCarringtonmapsofcoronalintensity
(Howardetal.1985;Hundhausen1993;Vourlidasetal.2002
and references therein). Consequently, in Figure 17 we show
Carringtonmapsofcoronalintensityat3R forthethreeevents
(cid:1)
ofthissection.Inthetoppanel,aneast-limbmapduringCarrington
rotation2022showsaverticalfeaturenear200(cid:4) longitude,cor-
responding to the event of 2004 October 16. The broad latitu-
dinalextentofthisfeatureissimilartothosewehavefoundfor
othersucheventsbutdiffersfromthebugle-likeappearanceof
somestreamereruptions.Inthemiddlepanel,aneast-limbmap
duringrotation2002showstheendofastreamer-relatedfeature
near310(cid:4)longitude,correspondingtotheeventof2003April12.
Thisfeaturehasthebugle-likesignatureofastreamereruption.
Inthebottompanel,awest-limbmapofcoronalintensityduring
rotation2012showsthedisruptionofastreamernear50(cid:4)longitude,
correspondingtotheeventon2004February13.Thisfeature
extendstothenorthpoleandcontinuesbackdowntolowerlati-
Fig. 13.—Height/timemapsofthestreamerdisruptioneventinFig.12.The
tudesnear210(cid:4) longitudeontheeast-limbmapofrotation2013
outwardcomponentformsabout0.5R abovetheoccultingdisk,butnoinward
(cid:1)
(notshownhere).BycomparingthesemapswithCarringtonmaps componentisvisible.Thepositionangleis115(cid:4).
1150 SHEELEY & WANG Vol. 655
Fig.14.—Histogramsofthestartingheightsof153observedin/outpairs,showingtheheightsoftheinwardcomponents(topleft)andoutwardcomponents(top
right),theiraverages(bottomleft),andtheirdifferences(bottomright).Thebinsizeis0.45foreachdistribution.Relativetotheotherdistributions,thedistributionof
startingheightsfortheinwardcomponentsisstronglypeakedandskewedtotheright.Thesuddendroptowardtheoccultingdiskat2.0R isinstrumentalandsuggests
(cid:1)
thatmanyunobservedpairformationsmayhaveoccurredclosertotheSun.
ofthephotosphericfieldanditscoronalextension,wefoundthat geststhatthedisturbancemaypushthereconnectingfieldlines
theSun’sfieldhadaprominenttwo-sectorstructure,correspond- together sooner than would otherwise happen and raises the
ingtoadipolefieldwhoseaxiswastippedslightlytowardEarth question of what determines the time and place of the recon-
onFebruary13.Thus,wesupposethatthislargeeruptionstarted nectionwhenthereisnoCME.
atthewestlimbandspreadforabout160(cid:4) alongthenorthern- The events without visible inward components tend to be
hemispherestreamerbelt,whichwasthenlocatedontheback biggerthanthosewithinwardcomponents,butotherwisethey
sideoftheSun. have similar properties. The gradual expansion involves the
samemarble-likefinestructure,thesame20kms(cid:3)1speed,and
3. SUMMARYAND DISCUSSION thesame1Y2dayduration.Theexpansionendsasacollection
Intheprevioussection,wedescribetheresultsofexamining of loopstearsawayfromthestreamerandacquiresacorrugated,
acollectionofmorethan160in/outpairsand60similarevents prominence-likestructuresimilartothatofahelicalmagneticflux
whose inward components, if they occurred, were masked by rope.Thelargesteventshavecharacteristicbugle-likesignatures
theoccultingdisk.Wefoundthattheinwardandoutwardcom- inCarringtonmapsofcoronalintensityandareprobablystreamer
ponentsoccuralongtheleadingandtrailingedgesofdepletions blowoutCMEs(Howardetal.1985;Hundhausen1993;Sheeley
thatformwhenslowlyexpandingcoronalstructuresfinallysep- etal.1997;Vourlidasetal.2002).Thus,in/outpairsandstreamer
aratefromtheSun. blowoutCMEsseemtobepartofthesamebroadclassofstreamer
However,wealsofoundthattheseparationisnotcomplete. eruptionevents,differingintheirstrengths,sizes,andcoronal
Thelegsofthearch-shapedoutwardcomponentextendbackto- heightsofformation.Manyofthelargeeventswithoutinward
wardtheoccultingdiskandapparentlyremainconnectedtothe componentsextendoverthepoles(suchasthe2004February13
Sun.Thisiswhatwewouldexpectiftheoutwardcomponentwere eventin Fig. 15) and therefore are probably streamer blowout
ahelicalfluxrope,createdwhentheloopsofanexpandingarcade CMEsseenfaceon.
reconnectwitheachotherinthemanneroriginallysuggestedby Anadvantageofthein/outpairsisthattheyshowtheseparation
Gosling(1990).AsdiscussedinPaperI,theoutwardcomponent thatmarkstheendofthecoronalexpansion,ratherthanjustthe
oftenhasacorrugatedstructurethatisconsistentwiththehelical ejectionoftheoutwardcomponent.Consequently,fortheseevents,
configurationofamagneticfluxrope,andtheinwardcomponent weknowthatthecorrugatedoutwardcomponentsarecreatedinthe
occasionallyresolvesintoasystemofcollapsingloops. coronaduringtheseparationandarenotsimplycarriedupfromthe
Thefinalseparationofthestreamermaterialoftenoccursspon- chromosphere.Thissuggeststhattheeruptiveprominencesthat
taneouslywithoutanobviouscause.However,almostequally accompanysomestreamerblowoutCMEsmayacquiretheirhelical
often,theseparationseemstobetriggeredbythepassageofa shapesduringasimilarreconnectionprocessthatoccursmuch
wavelikedisturbancefromalarge,high-speedCME.Thissug- closertotheSun.
No. 2, 2007 IN/OUT PAIRS AND DETACHMENT OF CORONAL STREAMERS 1151
Fig.16.—Runningdifferenceheight/timemapsobtainedatthreeradialpo-
sitionsinthe2004February13eventofFig.15,comparingtheevolutioninthe
northeast(top),north(middle),andnorthwest(bottom)directions.
Fig.15.—C2runningdifferenceimages(left)andminimum-subtractedimages
(right),showingthegradualexpansionandeventualdisruptionofacoronalstructure
on2004February13.Theevolutionbeginswiththecoronalstreamerinthenorthwest
(upperright),buteventuallyspreadsalongthestreamerbelttospantheentirenorthern
hemisphere.
As mentioned above, we suppose that the slowly rising co-
ronal structures indicate closed loops of a growing magnetic
field,producedeitherbytheemergenceofnewfluxatthepho-
tosphere or by the separation of the footpoints of a magnetic
arcade byphotospheric motionsorreconnection between adja-
centloopsystems.Infact,runningdifferencesbetweenclosely
timed images sometimes show a crisscross pattern resembling
the overlapping loops of a magnetic arcade seen in projection
alongthelineofsight.Evidently,theLASCOobservationsare
revealing the transient processthat occurs when thefield stops
growing—namely, the reconnection of the fields in the rising
arcade and the separation of the newly reconnected parts. This
transient behavior is not included in a sequence of potential
fields,whoserisingloopsbecomestationarywhenthefieldstops
growing.
However, a similar transient behavior is obtained when one
retainsthetime-dependenttermsinMaxwell’sequationsandcal-
culatestheevolutionofavacuumdipolefieldwhenitssource
currentisgraduallyrampedupfromonestationaryvaluetoanother.
This change produces an outgoing wave whose leading edge
enhancesthebackgroundfieldandwhosetrailingedgereduces
it(unpublishedcalculationsbyN.R.Sheeley1997,privatecom-
munication).Theresultisabulgeatthefrontofthetransientand Fig.17.—Carringtonmapsofwhite-lightcoronalintensityat3R(cid:1).Thearrows
indicateeventswithoutinwardcomponentson2004October16(top),2003April12
apinch-offattherearwheretheoutgoingwaveseparatesfrom
(middle),and2004February13(bottom),whichwehaveshowninFigs.10,12,and
thestrengtheneddipolefieldleftinitswake.Inthiscase,thesep-
14,respectively.Thelattertwoeventshavethecharacteristicsignaturesofstreamer
arationisanunmistakableconsequenceofendingthegrowthand blowoutCMEs.