TheAstrophysicalJournal,594:1068–1084,2003September10 E #2003.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. DYNAMICAL AND PHYSICAL PROPERTIES OF A POST–CORONAL MASS EJECTION CURRENT SHEET Yuan-KuenKo,1 JohnC.Raymond,1 JunLin,1 GarethLawrence,2 JingLi,3 and AndrzejFludra4 Received2002October18;accepted2003May10 ABSTRACT IntheeruptiveprocessoftheKopp-Pneumantype,theclosedmagneticfieldisstretchedbytheeruptionso muchthatitisusuallybelievedtobe‘‘open’’toinfinity.Formationofthecurrentsheetinsuchaconfigura- tionmakesitpossiblefortheenergyinthecoronalmagneticfieldtoquicklyconvertintothermalandkinetic energies and cause significant observational consequences, such as growing postflare/CME loop system in thecorona,separatingbrightflareribbonsinthechromosphere,andfastejectionsoftheplasmaandthemag- neticflux.Aneruptionon2002January8providesusagoodopportunitytolookintotheseobservational signatures of and place constraints on the theories of eruptions. The event started with the expansion of a magnetic arcade over an active region, developed into a coronal mass ejection (CME), and left some thin streamer-likestructureswithsuccessivelygrowingloopsystemsbeneaththem.Theplasmaoutflowandthe highlyionizedstatesoftheplasmainsidethesestreamer-likestructures,aswellasthegrowingloopsbeneath them,leadustoconcludethatthesestructuresareassociatedwithamagneticreconnectionsite,namely,the currentsheet,ofthiseruptiveprocess.WecombinethedatafromtheUltravioletCoronagraphSpectrometer, LargeAngleandSpectrometricCoronagraphExperiment,EUVImagingTelescope,andCoronalDiagnostic Spectrometer on board the Solar and Heliospheric Observatory, as well as from the Mauna Loa Solar Observatory Mark IV K-coronameter, to investigate the morphological and dynamical properties of this event, as well as the physical properties of the current sheet. The velocity and acceleration of the CME reachedupto1800kms(cid:1)1and1kms(cid:1)2,respectively.Theaccelerationisfoundtooccurmainlyatthelower corona(<2.76R ).Thepost-CMEloopsystemsshowedbehaviorsofbothpostflareloops(upwardmotion (cid:2) with decreasing speed) and soft X-ray giant arches (upward motion with constant speed, or acceleration) accordingtothedefinitionofSˇvestka.Inthecurrentsheet,thepresenceofhighlyionizedions,suchasFe+17 andCa+13,suggeststemperatureashighasð3 4Þ(cid:3)106K,andtheplasmaoutflowshavespeedsrangingfrom 300to650kms(cid:1)1.Absoluteelementalabundancesinthecurrentsheetshowastrongfirstionizationpotential effectandhavevaluessimilartothosefoundintheactiveregionstreamers.Themagneticfieldstrengthinthe vicinityofthecurrentsheetisfoundtobeoftheorderof1G. Subjectheadings:Sun:corona — Sun:coronalmassejections(CMEs) On-linematerial:colorfigures 1. INTRODUCTION flares,solarenergeticparticles,andradiobursts.Filaments sometimes also constitute part of the CME from active Coronal mass ejections (CMEs) are powerful, transient regions. There have been many multiwavelength studies expulsions of coronal material into interplanetary space. of individual CME events and statistical studies of the Since the launch ofthe Solar and Heliospheric Observatory CME properties (e.g., Howard et al. 1985; Hundhausen, (SOHO), a large number of CMEs have been observed, Burkepile,&St.Cyr1994;Goldstein,Newgebauer,&Clay fromtheirinitiationatthesolarsurfacetothepropagation 1998;Innesetal.1999;St.Cyretal.1999,2000;Ciaravella through the corona into interplanetary space. Because of et al. 2000; Akmal et al. 2001; Gopalswamy et al. 2001). their intimate association with the solar cycle and their ManyeffortshavebeenmadetomodelCMEeruptionproc- potentially strong effect on the space environment at the esses in an attempt to understand their initiation, explain Earth, CME study has been a very vigorous field in both theobservedstructureanddynamicalproperties,andmodel observationalandtheoreticalresearch. their propagation in the heliosphere (e.g., Mikic´ & Linker ThereareingeneraltwotypesoforiginforCMEs.Oneis 1994; Chen 1996; Antiochos, DeVore, & Klimchuk 1999; thefilamentorprominenceeruptionfromquiet-Sunregions Amarietal.2000;Lin&Forbes2000;Low2001;Rileyetal. that usually contains cool chromospheric material and 2002; Manchester et al. 2003; for reviews, see, e.g., Wu, exhibits complex twisting structure. The other is a more Andrews,&Plunkett2001). powerfuleruptionfromactiveregionsoftenassociatedwith Thegenerallyacceptedexplanationfortheenergysource of eruptive phenomena in the solar atmosphere, such as solarflares,eruptiveprominences,andCMEs,istheenergy stored in the coronal magnetic field prior to the eruption. 1Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge,MA02138. These eruptive phenomena commonly involve fast mass 2CenterforSolarPhysicsandSpaceWeather,CatholicUniversityof ejectionsandintenseheating,whichimplyrapiddissipation America,Washington,DC20064. of the magnetic energy. In the coronal environment, the 3InstituteforAstronomy,UniversityofHawaii,2680WoodlawnDrive, electricalconductivityoftheplasmaandthelengthscaleof Honolulu,HI96822. themagneticfieldarefairlylarge,sorapiddissipationofthe 4Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire 0X110QX,UK. magneticfieldisalmostimpossible(e.g.,Priest1982;Priest 1068 POST–CORONAL MASS EJECTION CURRENT SHEET 1069 are heated by shocks due to reconnection (Forbes, Malherbe, & Priest 1989). The cool loops, which lie inside the hot loops, are formed from the hot loops by radiative cooling(Sˇvestkaetal.1987;Forbes&Acton1996).Obser- vations supporting this scenario include rising X-ray loops (e.g., Sˇvestka 1996), cool loops lying inside the hot loops (e.g., McCabe 1973; Sˇvestka et al. 1987; Schmieder et al. 1996), and Dopper shift measurements (Schmieder et al. 1987) providing evidence of chromospheric evaporation. The density of the postflare loops was found to be a few times 1010 cm(cid:1)3 in the X-ray/EUV loops (e.g., Withbroe Fig.1.—Postflare/CMEloopmodelbyKopp&Pneuman(1976).(a) 1978)andashighas1012cm(cid:1)3intheH(cid:1)loops(e.g.,Heinzel Themagneticfieldispushedopenbyaneruption,andacurrentsheetsepa- &Karlicky´ 1987).Botharemuchdenserthanwhatisfound rates two antiparallel magnetic field lines. (b) The opened configuration relaxesintoaclosed,nearlypotentialfieldviathemagneticreconnectionin abovenontransientsolarstructure,whichisgenerallybelow thecurrent sheet.Thisprocessproducestwobrightandseparatingflare 109cm(cid:1)3justabovethesolarsurface(e.g.,Guhathakurta& ribbonsonthesolardiskandacontinuallygrowingflareloopsysteminthe Fisher1995;Parentietal.2000). corona(reproductionofFig.1ofKopp&Pneuman1976). Recent research indicates that the closed magnetic field does not necessarily become fully open as the Kopp- &Forbes 2000) unless the magneticconfiguration includes Pneuman–typeeruptionoccurs.Instead,themagneticstruc- someneutralregions,suchasneutralpoint,nullpoint,cur- tureishighlystretched by the eruption, and a current sheet rent sheet, or quasi-separatrix layers (e.g., De´moulin et al. forms(seeFig.2).Bothnumericalsimulationsandanalytical 1996; Priest & Forbes 2000), in which the fast magnetic studies show that when the footpoints of a closed magnetic reconnectioncanoccureasily. structure are sheared (Mikic´ & Linker 1994), or the whole Kopp & Pneuman (1976) proposed a scenario for two- structure is stretched outward by the catastrophic loss of ribbonflares(Fig.1;theideawasraisedbyCarmichael1964 equilibriuminamagneticfluxrope(Forbes&Isenberg1991; forthefirsttime.)Inthismodel,energyisstoredinaforce- Isenberg,Forbes,&De´moulin1993;Forbes&Priest1995),a free magnetic arcade or flux rope prior to the eruption. current sheet develops in the stretched configuration. With Eventually, the field erupts to form a fully open magnetic dissipation inthe current sheet, the stretched magneticfield configuration including a neutral current sheet that sepa- lines start to reconnect, producing new closed field lines ratestwoantiparallelmagneticfieldlines(Fig.1a).Finally, below the current sheet, so the preexisting closed magnetic the open configuration relaxes into a closed, nearly poten- field never becomes fully open. There are several ways to tialfieldviamagneticreconnectioninthecurrentsheet,and trigger dissipation in the current sheet. The current sheet is the magnetic energy is converted into kinetic and thermal subjecttothetearing mode instability,and magneticrecon- energies (Fig. 1b). The observational consequences of this nection occurs as soon as the length of the current sheet process are two bright and separating flare ribbons on the becomes longer than (cid:4)2(cid:2) times its width (Furth, Killen, & solardiskandacontinuallygrowingflareloopsysteminthe Rosenbluth 1963). Alternatively,as the current sheet builds corona.Themotionsoftheflareribbonsandloopsarenot up,itscurrentdensitymayexceedthethresholdofamicro- duetomassmotionsoftheplasmabuttotheupwardpropa- instability, which creates anomalous resistivity (Galeev & gationoftheenergysourceontonewmagneticfieldlinesat Zelenyi 1975; Heyvaerts & Priest 1976) and triggers rapid higheraltitude(Schmiederetal.1987).AccordingtoFigure reconnection. The scenario sketched by Figure 2 suggests 1,theupwardpropagationoftheenergysourceimpliescon- thattheroleplayedbyreconnectioninaneruptiveprocessis tinuous erosion of the current sheet due to reconnection. twofold. First, reconnection breaks the magnetic field lines As pointed out by Forbes (2000), images obtained from thatpassoverthefluxropeandareanchoredintheboundary YohkohSoftX-rayTelescope(SXT)providemanypiecesof surface at both ends. These field lines produce strong mag- evidence that suggest magnetic reconnection in the corona netic tension force in a stretched configuration that would during an eruption. These include a hard X-ray source otherwise prevent the flux rope from escaping to form a located above the soft X-ray loops (Sakao et al. 1992; CME.Second,reconnectiondumpslargeamountsofenergy Masudaetal.1994;Bentleyetal.1994),cuspstructuresug- intheloweratmosphereoftheSun,creatingintenseheating, gesting either an X-type or a Y-type reconnection site which accounts for the traditional flare ribbons and loops. (Acton et al. 1992; Tsuneta 1993; Doschek, Strong, & Figure2incorporatestheflaremodelssummarizedbyForbes Tusneta 1995), bright features at the top of the soft X-ray &Acton(1996)andtheCMEmodelofLin&Forbes(2000, loops (Tsuneta et al. 1992; McTiernan et al. 1993), and hereafterLF00).Itclearlyillustratestherelationshipamong high-temperature plasma along the field lines mapping to solarflare,eruptiveprominence,andCMEandsuggeststhat the tip of the cusp (Tsuneta 1996). Recently, Yokoyama thesethreeeruptivephenomenaareactuallydifferentmanifes- et al. (2001) reported the observation of plasma inflow tationsofasinglephysicalprocessthatinvolvesadisruptionof mergingtoalocalareaabovethecuspstructure. thecoronalmagneticfield. Observationsshowthatbrightribbonsandloopscanlast OneofthemostsignificantpredictionsoftheLF00model formorethan10hr(e.g.,Sˇvestka1976).Ribbonsandloops is that a current sheet develops following the onset of the appeartomovethroughthechromosphereandthecorona, eruption.Becausethetimescaleofreconnectionislongcom- andtheoutermostedges ofthehotloopsmaptotheouter pared with the timescale of the onset stage (Alfveń time- edgeoftheflareribbons(Schmiederetal.1996;Antiochos scale), dissipation of the current sheet by reconnection is etal.2000),whilethecoolH(cid:1)loopsmaptotheinneredge slow,sothecurrentsheetisabletobecomefairlylong.The oftheribbons(Rust&Bar1973).Accordingtothemodels, evolutionofthecurrentsheetissignificantlyconstrainedby thehotloopsarethenewlyreconnectedclosedloops,which the local Alfveń speed. In an isothermal corona, the local 1070 KO ET AL. Vol. 594 tion perpendicular to its vertical extent (refer to Fig. 2). Therefore,itsthicknessissosmallthatweusuallytreatthe currentsheetasaninfinitelythinlayer.Inaddition,thelow plasmadensityinthecoronaimpliesthattheplasmainside thecurrentsheetshouldbetenuous.Therefore,itisdifficult toobserve thecurrent sheetinan eruptiveprocess because both its size and emission are easily dominated by other large-scale and bright structures nearby. Ciaravella et al. (2002) analyzed the spectral data obtained from the Ultra- violetCoronagraphSpectrometer(UVCS;Kohletal.1995) on SOHO after an eruption on 1998 March 23. After the bulkCMEmateriallefttheinnercorona,abrightpostflare/ CMEarcadewasseenbytheEUVImagingTelescope(EIT) and Yohkoh/SXT. UVCS detected a very narrow and hot featurethatwasmostprominentinthe[Fexviii]emissionin the space between the arcade and the CME core. The appearance of the [Fe xviii] emission means a temperature as high as 6(cid:3)106 K in that area (Ciaravella et al. 2002). Theirresultisquiteconsistentwiththetheoreticalexpecta- tionsofthecurrentsheetinthefluxropemodelsketchedin Figure2.Webbetal.(2003)analyzed26‘‘candidatediscon- nectionevents’’(CDEs)associatedwithCMEsobservedby theSolarMaximumMission(SMM).TheseCDEswerefol- lowedbyraylikestructuresthatappearedafewhoursafter the CDE and sometimes were found to connect to newly formedstreamersclosetothelimb.Theyregardthiskindof raylikestructuresasevidenceofthecurrentsheetdeveloped followingaCME(cf.Fig.2). Arecenteruptionprovidesuswithauniqueopportunity forscrutinizingthecurrentsheetaswellastheglobalmor- phologicalfeaturesoftheeruption.Thiseruptionoccurred attheeastlimboftheSunon2002January8.TheCMEleft behind some long and thin streamer-like structures with continuous outflows as seen in white light. Lower in the corona, UV/EUV observations showed formation of the Fig. 2.—Upper part: Sketch of the flux rope/CME model of Lin & post-CME loops, and an electron temperature of Forbes (2000) showing the eruption of the flux rope, the current sheet ð3 4Þ(cid:3)106Kwasfoundat1.53R .Temperaturesthishigh (cid:2) formed behind it, and the postflare/CME loops below, as well as the at this height are not commonly seen in the quiescent inflows and outflows associated with the reconnection. Lower part: corona, even above most active regions (Ciaravella et al. Enlargedviewofthepostflare/CMEloops(adoptedfromForbes&Acton 2002).Webelievethatthesestreamer-likestructuresconsist 1996). The upper tip of the cusp rises as reconnection happens continuously. of a current sheet in which the magnetic reconnection occurs. The observed high temperature results from the heating in the magnetic reconnection processes that also Alfveńspeedincreaseswithheightatlargealtitudes,sothe produce continuous outflows along the current sheet and current sheet is quickly eroded by reconnection, and its the formation of the post-CME loops beneath it. In many length consequently shrinks several hours after the onset respects,thiseruptionislikemanyobservedpreviously,but (LF00; Forbes & Lin 2000). In a more realistic corona, on the location of the source region (AR 9782/9785; see the theotherhand,thelocalAlfveńspeeddecreaseswithheight nextsection)andtheorientationofthedisruptedmagnetic at large altitudes; erosion of the current sheet is thus not configuration lead to a current sheet just along the line of veryfast,andthecurrentsheetmayremainalongtime(Lin sight(i.e.,edge-on),asillustratedinFigure2.Thismakesit 2002, hereafter Lin02). The model predicts that the mag- possibleforustoanalyzetheplasmapropertiesandinvesti- netic energy around the current sheet is converted into gate the evolution of the current sheet, as well as the post- kineticandthermalenergyoftheplasmaduringtherecon- CME loops below it. In the next section, we describe this nection process. The plasma then flows along the current event as observed in white light and UV/EUV and find sheet both upward and downward at approximately the manyaspectsconsistentwiththepredictions ofthemodels Alfveń speed and is heated to high temperature inside the described in this section. In x 3, we analyze the dynamical currentsheet(comparedtotheambientcorona).Themodel and physical properties of this current sheet and interpret alsopredictscertaindynamicalpropertiesoftheCMEand thedatabasedontheworkbyLF00andLin02.Finally,we the current sheet that we will discuss in detail in a later summarizeouranalysisinx4. section. Thereis,however,littledirectobservationalevidencethat 2. THE OBSERVATIONS can confirm the occurrence of the current sheet. The high electrical conductivity and nearly force free environment Wesuggestedintheprevioussectionthatsolarflares,erup- confinethecurrentsheettoaverylocalregioninthedirec- tiveprominences,andCMEsaredifferentmanifestationsof No. 2, 2003 POST–CORONAL MASS EJECTION CURRENT SHEET 1071 a single physical process that involves a disruption of the coronalmagneticfieldfollowedbymagneticreconnection.In thissection,wedescribeaCMEfollowedbytheformationof thepost-CMEloopsandthecurrentsheetthatprovidesgood illustrationofthisidea.ThiseventwasobservedbytheLarge Angle and Spectrometric Coronagraph Experiment (LASCO;Brueckneretal.1995),EIT(Delaboudinie`reetal. 1995),UVCS,andCoronalDiagnosticSpectrometer(CDS; Harrisonetal.1995)onboardSOHOaspartofSOHOJoint ObservingProgram151,aswellasbytheMaunaLoaSolar Observatory (MLSO) Mark IV K-coronameter (MK4). Because of the dynamical nature of all eruptions, it may be helpful to refer to the movies5 of this event recorded by SOHO/LASCOandSOHO/EIT. 2.1. TheCoronalMassEjectionandtheCurrentSheet On2002January8,amajoreruptionoccurredattheeast limboftheSun.Thecorrespondingsourceactiveregion(see x 2.4) was located just behind the limb. Therefore, it is unclear whether a filament preexisted in the relevant magnetic configuration and erupted or whether this eruption started with a flare. The movie obtained by EIT 195 A˚ Fig.4.—Height-timeplot(filledcircles)oftheleadingedgeoftheCME showsmovement/expansionofthecoronalloopsthattook measuredinLASCOC2andC3withthefittingcurveofasecond-order placeforafewhoursbeforetheeruptionwasclearlyseenat polynomial(solidline).Aconstantaccelerationof0.19kms(cid:1)2providesa 17:36UT.Intheimageoneframebefore(17:24UT),there goodfittothedata.AlsoplottedisthetimeframethatbracketstheCME is some clearer signature of the expansion of the magnetic onsettime,indicatingalargeraccelerationof(cid:4)1kms(cid:1)2inthelowercorona below2.76R . arcade.Figure3showstheCMEonthecompositeimageof (cid:2) EIT195A˚ runningdifferenceat18:24UTandLASCOC2 at 18:30 UT. The overall coronal disruption due to the CME eventually spanned 180(cid:5) between the two poles by 17:54UT.Itthensubsequentlytraveledto3.34R(cid:2)at18:06 18:54 UT. Figure 4 is the height-time plot of the leading UT,5.25R(cid:2)at18:30UT,and14.12R(cid:2)(inC3)at19:42UT. These data points fit well to a curve that corresponds to a edge of the CME. The leading edge of the CME first appearedintheLASCOC2fieldofview(FOV)at2.76R , constant acceleration rate of about 0.19 km s(cid:1)2. Further- (cid:2) more, from this curve, we can evaluate the tangents at pointsandinferspeedsof,forexample,around700kms(cid:1)1 5Availableathttp://lasco-www.nrl.navy.mil/daily_mpg. at3R toover1800kms(cid:1)1by14R .ThisisafastCMEby (cid:2) (cid:2) comparisonwiththeoverallCMEpopulation(St.Cyretal. 2000). Wenotethatwiththisconstantaccelerationrate,thetime when the CME started would be much earlier than as observedinEIT.Therefore,theremustbeadifferentaccel- erationratebeneaththeC2occultingdisk.Alsoplottedon Figure4aretwopointsbracketingthetimeframewhenthis CMEprobablystarted(between17:24and17:36UTofEIT images). Taking 17:30 UT as the CME initiation time, we obtainanaverageaccelerationofabout1kms(cid:1)2between1 and2.76R .ThisimpliesthattheCMEunderwentamuch (cid:2) larger acceleration in the lower corona than that seen in LASCO C2. Previous studies found low-coronal accelera- tions of as much as 2–7 km s(cid:1)2 (e.g., Zhang et al. 2001; Alexander,Metcalf,&Nitta2002).Usinglow-coronadata of MLSO MK3 and MK4, along with the SMM and LASCO data to the outer corona, Burkepile et al. (2002) found that the largest CME acceleration occurs in the low corona(<2.9R ). (cid:2) AfterthefrontofthisfastCMEhadpassed,thinthreads of streamer formed behind it at a position angle (P.A.) of (cid:4)101(cid:5),as indicated by the arrow in the upper left panel of Figure 5. Blobs of material can be seen flowing along the streamer continuously as it gradually moved northward Fig.3.—CompositeimagefromEIT195A˚ runningdifferenceat18:24 (Figs.5,6,and7).Weinterpretthisthinstreamerasthecur- UT,andLASCOC2at18:30UT,showingthe2002January8CMEerup- rent sheet left behind the CME. The blobs are the plasma tion.ThenonradialexpansionofthefieldlinesinthisCMEextendedfrom outflow that is produced by magnetic reconnection in the thesolarsurfacewellintotheoutercorona.[Seetheelectroniceditionofthe Journalforacolorversionofthisfigure.] current sheet (see x 1). They result from the magnetic 1072 KO ET AL. Vol. 594 Fig.5.—EITandLASCOC2compositeimagesshowingtheevolution Fig.6.—EITandLASCOC2compositeimagesshowingtheevolution ofthecurrentsheetfeatureandthepost-CMEloops.Upperleft:EIT195A˚ of the current sheet feature(indicatedby the arrow) andthe post-CME at19:48UT,C2at19:54UT,January8.Thearrowpointstothecurrent loops.Upperleft:EIT195A˚ 11:24UT,LASCOC2at11:30UT,January9. sheet feature (CS) at its early appearance. Upper right: EIT 195 A˚ at Upperright:EIT195A˚ at16:48UT,C2at16:54UT,January9.Lowerleft: 21:17UT,C2at21:32UT,January8.Thearrowspointtothefirstappear- EIT195A˚ at06:48UT,C2at06:54UT,January10.Lowerright:EIT195 anceofthepost-CMEloopsintheEITimageandthecurrentsheetfeature. A˚ at23:18UT,C2at23:13UT,January10.NotethattheEITimageisnot Lowerleft:EIT195A˚ at00:48UT,C2at00:54UT,January9.Lowerright: toscalewithC2inordertoshowthefeaturenearthelimbmoreclearly(see EIT195A˚ at06:24UT,C2at06:30UT,January9.Thearrowspointtothe also Fig. 9 for enlarged EIT images). [See the electronic edition of the northandsouthpost-CMEloopsystems.NotethattheEITimageisnotto Journalforacolorversionofthisfigure.] scalewithC2inordertoshowthefeaturenearthelimbmoreclearly(see also Fig. 9 for enlarged EIT images). [See the electronic edition of the Journalforacolorversionofthisfigure.] obvious that the thin streamer of interest (i.e., the current sheetfeatureatP:A:(cid:4)83(cid:5))seeninC2isassociatedwiththe reconnection in the nonuniform plasma and the magnetic southernpartoftheloopsystem.Wenotethattheappear- field. anceofthetwoloopsystemsintheMK4image(seetheinset Several other CMEs occurred after this January 8 CME in the left panel of Fig. 10) is similar to the giant X-ray event.Particularly,aCMEfromtheeastlimbat12:06UT loops/cuspformedafteraflare/CME(e.g.,Tsuneta1996), ofJanuary10seemstopushthiscurrentsheetfarthernorth- andthethinfeatureabovethesouthernloopresemblesand ward.Inaddition,abrightstreamerstartedtoappearatthe is along the same direction as the streamer/current sheet northeastlimbonJanuary10andoverlappedwiththiscur- featureseeninLASCOC2.Sincethedensityinsidethecusp rent sheet feature on the plane of thesky (see Fig. 6, lower isexpectedtobehigherthanthatintheambientcorona(see panels).However,asweexaminetheC2runningdifference x1),the‘‘loop’’seenintheMK4imagemayactuallybethe images,wefindthatthiscurrentsheetfeaturepreservedits white-lightcounterpartoftheX-rayloops/cusp.However, general morphology despite this January 10 CME, and it it is not easy to definitely associate the north loop system appeared to be a separate feature, distinct from the bright seeninMK4andEITwithstructuresseeninC2,althoughit streamertoitsnorth(Fig.8). ispossiblethatitisassociatedwiththetwothin‘‘spikelike’’ streamers just north of the long current sheet of interest. This‘‘fan’’ofspikesseeninC2(cf.alsoFigs.5and6)seems 2.2. Post-CMELoops to be analogous to the ‘‘fan of spikelike rays’’ seen above Thepost-CMEloopsstartedtoappearinEITataround theflareloopsbyYohkohSXT(McKenzie&Hudson1999). P:A:(cid:4)90(cid:5) at 21:17 UT, January 8, as indicated by the Thelatterwasfoundtohaveaone-to-oneassociationwith arrowinthetopleftpanelofFigure9.Afterthat,aseriesof the X-ray loops in the flare arcade and was interpreted as loops appeared at the limb and successively appeared at thecurrentsheet,consistentwiththestandardreconnection higher altitude (see Fig. 9). They gradually faded on Janu- picture. ary 10. This indicates continuous reconnection along the Figure11plotstheheightversustimeoftheouteredgeof current sheet as described in x 1 to power the flare loops. the two post-CME loop systems measured from the EIT Theseloopsappeartobeintwogroupsascanbeseenfrom images.Themeasurementsendwhenthetopoftheloopsis Figure9(e.g.,indicatedbythetwoarrowsinthemiddleleft out ofthe EIT FOV (in the case of thenorthloops)or the and bottom left panels). Figure 10 shows the MLSO MK4 loops are too faint to be visible (in the case of the south white-lightimageon17:52UTofJanuary9(leftpanel)and loops).Theseloopsystemsseemtoshowbehaviorsofboth a composite image of EIT (at 17:36 UT), MK4 (at 17:52 the postflare loops (the south loops, upward motion with UT), and LASCO C2 (at 17:54 UT) (right panel). It is decreasingspeed)andthesoftX-raygiantarches(thenorth No. 2, 2003 POST–CORONAL MASS EJECTION CURRENT SHEET 1073 Fig.8.—LASCOC2runningdifferenceimagesshowingthecurrentsheet featureatP:A:(cid:4)78(cid:5).Left:at16:54UT,January10;middle:at23:13UT, January10;right:at02:30UT,January11.Thecurrentsheetpersistedeven afteraCMEeruptionat12:06UT,January10,althoughitwaspushedabit tothenorth.ItgraduallydissipatedonJanuary11. (lessthan5%when(cid:6)1daytothelimbasinthiscase)com- paredwiththegrowthoftherisingloops.TheMK4images maybeusedtoroughlyestimatetheheightofthe‘‘loops.’’ Wefindthatat17:52UTofJanuary9,thenorthloop(see Fig.10)isat(cid:4)1.7R andthesouthloopisat(cid:4)1.4R .Note (cid:2) (cid:2) thatifthewhite-lightloopsindeedcorrespondtotheX-ray Fig. 7.—Six consecutive LASCO C2 running difference images from 03:06UTto04:54UTofJanuary9showingcontinuouslyappearingblobs flowingalongthecurrentsheetfeatureatP:A:(cid:4)89(cid:5). loops,upwardmotionwithconstantspeed,oracceleration) as defined by Sˇvestka (1996). The former group of loops Fig.9.—EIT195A˚ imagesshowingamoredetailedviewoftheevolu- approachedtheheightof0.3R abovethesolarlimbovera tionofthepost-CMEloops.TheseloopsfirstappearedintheEITimageat (cid:2) 21:17UT,January8(indicatedbythearrowinthetopleftpanel),andseem periodof40hr,andthelattergroupofloopsroseabove0.4 tobeintwogroups(indicatedbythearrowsinthemiddleleftandbottom R abovethelimboveraperiodof(cid:4)20hrandappearedto (cid:2) left panels). The subsequent appearance of loops toward higher height continue growing. Note that the projection effect is small impliescontinuousmagneticreconnection. 1074 KO ET AL. Vol. 594 942.2–1097.7A˚ fortheOviprimarychannel(471–549A˚ in second order). Several other lines of interest can also be obtained from the O vi redundant channel. They are Ly(cid:1), Mgx(cid:3)609,[Fexii](cid:3)1242,Nv(cid:3)1238,and[Sx](cid:3)1196.For a detailed description of the UVCS instruments, see Kohl et al. (1995). The raw data have been wavelength and flux calibrated. The uncertainty in the flux calibration is about 20%forthefirst-order lines (Gardneretal. 2000) and 50% forthesecond-orderlines(Gardneretal.2002).Straylight is negligible in the data, so no stray light correction is per- formed. Figure 12 plots the spatial distribution of several majorlinesandtheintensityratiooftheOvi(cid:3)1032/(cid:3)1037 doublet (Fig. 12j). We separate the data into four spatial zones,whichareindicatedontheplots.Figure13plotsthe spatiallyaveragedspectraforzones2,3,and4takenatone ofthe grating positions where we measure most ofthe line fluxes.Wecanseethatzone3containsstrongemissionfrom highly ionized ions (such as [Fe xviii] (cid:3)974 and [Ca xiv] Fig.10.—Left:MLSOMK4white-lightimageat17:52UT,January9, (cid:3)943)thatarerarelyseeninthecoronaatthisheight.This showing two loop/cusp systems after the CME. The inset shows our high-temperature emission is concentrated around perception of what the two loop/cusp systems might look like. Right: CompositeofEIT195A˚ (at17:36UT),MK4(at17:52UT),andLASCO P:A:¼81=3(cid:6)3=7(halfoftheFWHM),whichisalongthe C2(at17:54UT)imagesofJanuary9showingthatthecurrentsheetfeature same direction asthe thincurrent sheet feature seen inC2. seeninC2correspondstothesouthloopsysteminMK4andEIT.[Seethe Zone 2 contains enhanced emission from low-ionization electroniceditionoftheJournalforacolorversionofthisfigure.] ionssuchas[Fex](cid:3)1028and[Siviii](cid:3)944.Wesuspectthat itisrelatedtotheeruptedprominencematerialinthelegof loops, we would expect them to be above the EUV loops the CME (that occurred starting 12:06 UT, January 10, in (seex1),whichisconsistentwithFigure11. C2asmentionedbefore)justsouthofthecurrentsheet(see Fig.6,lowerrightpanel,andFig.8).Zone4wouldcontain materialfromthebrightstreamernorthofthecurrentsheet. 2.3. High-TemperatureEmission Table 1 lists the line intensities for all four zones. In the SOHO/UVCS observed this current sheet feature from followinganalysisoftheUVCSdata,wewillconcentrateon 17:48 UT, January 10, to 03:47 UT, January 11. The data zones 2, 3, and 4 only. The fluxes measured for zone 1 are reported here were taken from the O vi channel. The slit listedbutarenotrelevantforthepresentstudy. widthwassetto100lm,whichcorrespondstoaninstanta- Figure 12j plots the O vi (cid:3)1032/(cid:3)1037 intensity ratio, neous FOV of 400(cid:3)2800. The slit angle was at P:A:¼78(cid:5) which can be used as an indicator of the outflow speed of and a projected heliocentric height of 1.53 R . The data the ions because of the Doppler dimming effect (Hyder & (cid:2) wereobtainedwithaspatialbinningof10pixels(7000)anda Lites 1970; Noci, Kohl, & Withbroe 1987; Strachan et al. spectral binning of 2 pixels (0.198 A˚). Using four grating 2000).Whentheoutflowislessthan100kms(cid:1)1,the(cid:3)1032/ positions, UVCS data have extended spectral coverage of (cid:3)1037ratioisbetween2and4dependingmainlyontheout- flowspeedandtheelectrondensity(andweaklyontheelec- tron temperature)—the higher the speed or the larger the density,thesmallertheratio.Whentheoutflowexceeds100 km s(cid:1)1, the ratio goes below 2 because of the pumping of Ovi (cid:3)1037.6 by Cii (cid:3)1037.0 (Noci et al. 1987). Figure 12j shows that the ratios are larger than 2 in all four zones, whichimpliesthattheoutflowspeedislessthan100kms(cid:1)1. Zone3hasthelowestvaluesoftheratio,whichimpliesthat eithertheelectrondensityortheoutflowspeed(orthecom- binedeffectofthetwo)isthehighestinzone3.Sincethere areprobablybothcurrentsheetandbackgroundcorona(as inzone4;seex3.2)overlappinginthelineofsight,itispos- sible that the outflow speed in the current sheet is higher than 100 km s(cid:1)1 (which should give the O vi doublet ratio below2)butthe‘‘average’’Ovidoubletratioisstillabove 2asintegratedalongthelineofsightwithmostcontribution from a much lower outflow speed in the background corona. SOHO/CDSmadeobservationsfrom07:00to15:30UT on January 10. The observation sequence consists of six rasters (ARDIAG_2), each with FOV of 40(cid:3)40. Twenty spectral windows include a wide selection of transition regionandcoronallines,providingtemperaturediagnostics Fig.11.—Height-timemeasurementofthetwopost-CMEloopsystems as well as relative abundances of Fe, Si, Ca, Ne, Mg, and showingthemrisingabovethelimb.Sincetheposition(longitude)ofthese loopsshouldbewithin1dayofthelimb,theprojectioneffectissmall. O. Figure 14 shows the images in Mg ix 368.07 A˚ (106 K No. 2, 2003 POST–CORONAL MASS EJECTION CURRENT SHEET 1075 Fig.12.—SpatialdistributionofseveralmajorlinesfromUVCSdataandtheintensityratiooftheOvi(cid:3)1032/(cid:3)1037doublet(Fig.12j) formation temperature) and Fe xvi 360.8 A˚ (2:5(cid:3)106 K) observed by UVCS (see Fig. 12). Even though these loops composed from the six rasters. Both images show multiple maybemixedwithotherpartsoftheactiveregion,theouter loop structure extending up to the edge of the FOV. For boundaryoftheloopsisprobablyassociatedwiththepost- example,theoutertipoftheMgixloopsappearedtobeat CME reconnection loops like those seen by EIT (Fig. 9; around X ¼(cid:1)140000, Y ¼þ20000, which corresponds to also cf. Fig. 2). However, the two loop systems seen by (cid:4)1.46 R at P:A:¼82(cid:5). The Fe xvi loops appeared to be EIT on January 9 were not clearly distinguishable in the (cid:2) alongthesamedirectionbutevenhigherup.Thisisconsis- CDS images that were taken on January 10. In fact, the tent with the P.A. of the high-temperature emission two loop systems have also become too faint to be clearly 1076 KO ET AL. Fig.13.—UVCSspectraforzones2,3,and4takenatgratingposition236900,whichcoversthewavelengthrangeof942.2–1042.5A˚ (471–521A˚ insecond order).Low-ionizationlines([Siviii](cid:3)944and[Fex](cid:3)1028)areparticularlystronginzone2,andhigh-ionizationlines([Fexviii](cid:3)974and[Caxiv](cid:3)943)are particularlystronginzone3. distinguishable in EIT on January 10 (see bottom right loops were projected in the plane of the sky. These loops panel of Fig. 9). In Figure 14, the lower edge of the Mg ix seenbyMK4areprobablyunrelatedtothebrightstreamer loops (probably associated with the south loop system) is seenbyC2atthenorthofthecurrentsheetbecausethesize plottedonbothpanels.Itshowsthatthelowertemperature of the loops in MK4 is much less than that of the bright emission loops (i.e., Mg ix) were located inside the higher streamerinC2.Wecanseethatthehigh-temperatureemis- temperature loops (Fe xvi). This is consistent with the sionseenbyUVCSandCDSisalongthesamedirectionof expectation of the flare loop models (e.g., Forbes & Acton thecurrentsheetfeatureinMK4andC2,asexpectedfrom 1996;seex1). the models, although the center of the high-temperature Figure15showstheMK4imageat19:28UT,January10 emission (seen by UVCS and CDS) is at P:A:(cid:4)81(cid:5) and (leftpanel),andthecompositeimagefromEIT195A˚ (19:36 that of the current sheet seen by C2 is at P:A:(cid:4)78(cid:5). This UT), CDS Fe xvi, MK4 (19:28 UT), and C2 (20:26 UT) maybeduetothefactthatthefieldlinesassociatedwiththe images (right panel; all on January 10). Superposed on the current sheet are nonradial, especially when it was dis- imageattherightpanelistheprojectionoftheUVCSsliton turbed by the CME earlier that day (see Fig. 8). Note that the plane of the sky (note that the width is not to scale) showingthespatialdistributionofthe[Fexviii](cid:3)974emis- sion. The two ‘‘loops’’ (see the inset in the left panel) and thecurrentsheetfeature(thinstreamer)abovetheloopsin theMK4imageseemtobestillpreservedatthistime.How- ever, it is less certain that the current sheet corresponds to thesouthloopsystem,probablybecauseofthewaythetwo Fig.15.—Left:MLSOMK4white-lightimageat19:28UT,January10, showingthatthetwoloop/cuspsystemsstillexist2daysaftertheCME. Theinsetshowsourperceptionofwhatthetwoloop/cuspsystemsmight looklike.Right:CompositeimagefromEIT195A˚ (19:36UT),CDSFexvi Fig.14.—ImagesofMgix(cid:3)368.07(formationtemperatureof(cid:4)106K, (smallrectangleatthelimb),MK4(19:28UT),andC2(20:26UT)images withasmallcontributionofMgvii(cid:3)367.68)andFexvi(cid:3)360.8(formation ofJanuary10.SuperposedontheimageistheprojectionoftheUVCSslit temperatureof(cid:4)2:5(cid:3)106K)composedfromsixrastersoftheCDSobser- ontheplaneofthesky(thewidthisnottoscale)showingthespatialdistri- vationsonJanuary10.ThelinemarksthelowerboundaryoftheMgix bution of the [Fe xviii] (cid:3)974 emission (cf. Fig. 12k). It is clear that the loops,whichindicatesthattheMgixloopsareinsidetheFexviloops. high-temperatureemissionisalongthedirectionofthecurrentsheet. n o alibratiaError 22222113311111111111111 C nt e m sureError(%) <20<10<10<20<10<20<20<30<20<20b<10<10<10<10<10<10<10<10<10<10<10<20<10 a e M 1(cid:1)) r s 1(cid:1) s 2(cid:1) Zone4otonscm 19.92634.01580.532.9491.6...3.6...16.413.85.44.875.837.78.61176.1417.1398.34.554.828171.549.0122.9 h p 70 1 (cid:3) ( 1(cid:1)) r s 1(cid:1) s 2(cid:1) Zone3otonscm 102.09668.56419.6148.92705.745.319.212.027.554.2201.518.8287.7205.627.13086.91598.71171.421.1176.761149.590.1540.8 h p 70 1 (cid:3) ( s e E1nsiti 1(cid:1)sr) TABLLineInte Zone221(cid:1)(cid:1)otonscms ...7414.31488.246.4649.6...68.539.976.5...7.19.7121.762.276.83597.91438.11259.0...156.643703.5111.5308.9 h p 70 1 (cid:3) ( 1(cid:1)) r s 1(cid:1) Zone172(cid:1)Identification(10photonscms(cid:3) xv121spPpD...Fe33312x22sSpPMg221988.4==1232xii22sSpPSi22378.7==1232xiii22335pPspSs33322.1Fe]322xii22sSpPSi22116.1==1212xiv3432pSpD...[Ca]22==3232viii3432SpPp212.7[Si]2==3232viii3432pSpP[Si]227.6==3212ix2321pPpS(cid:4)[Si]22,Ly18.210(cid:5)...Lyxviii5252pPpP...[Fe]22==3212xii3432pSpD[Ar]222.4==3252(cid:6)Ly32.0(cid:6)Lycollisionalcomponent10.3x44dDdF[Fe]3314.7==7272vi22sSpPO221041.2==1232viOcollisionalcomponent331.9vi22sSpPO22343.3==1212xii3432SpDp...2[Ar]2==3232x3432pSpD[S]2237.2==3252(cid:1)Ly15785.5v22sSpPN2240.1==1232xiic3432pSpP[Fe]3364.9==3232

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