A&A422,337–349(2004) Astronomy DOI:10.1051/0004-6361:20035815 & (cid:1)c ESO2004 Astrophysics The basic characteristics of EUV post-eruptive arcades and their role as tracers of coronal (cid:1) mass ejection source regions D.Tripathi,V.Bothmer,andH.Cremades Max-Planck-Institutfu¨rAeronomie37171Katlenburg-Lindau,Germany e-mail:[bothmer;cremades]@linmpi.mpg.de Received5December2003/Accepted5April2004 Abstract.TheExtremeultravioletImagingTelescope(EIT)onboardtheSolarandHeliosphericObservatory(SOHO)space- craftprovidesuniqueobservationsofdynamicprocessesinthelowcorona.TheEIT195Ådatatakenfrom1997totheendof 2002wereinvestigatedtostudythebasicphysicalpropertiesofpost-eruptivearcades(PEAs)andtheirrelationshipwithcoronal massejections(CMEs)asdetectedbySOHO/LASCO(LargeAngleSpectrometricCoronagraph).Overtheinvestigatedtime period,236PEAeventshavebeenidentifiedintotal.ForeachPEA,itsEUVlifetimeasderivedfromtheemissiontimeat195Å, itsheliographicpositionandlength,anditscorrespondingphotosphericsourceregioninferredfromSOHO/MDI(Michelson DopplerImager)datahasbeenstudied,aswellasthevariationoftheseparametersovertheinvestigatedphaseofsolarcycle23. AnalmostonetoonecorrespondenceisfoundbetweenEUVPEAsandwhite-lightCMEs.Basedonthisfinding,PEAscanbe consideredasreliabletracersofCMEeventsevenwithoutsimultaneouscoronagraphobservations.Adetailedcomparisonof thewhite-light,softX-rayandEUVobservationforsomeoftheeventsshows,thatPEAsformintheaftermathofCMEslikely inthecourseofthemagneticrestructuringstakingplaceatthecoronalsourcesites.TheaverageEUVemissionlife-timefor theselectedeventsrangedfrom2to20h,withanaverageof7h.TheheliographiclengthofthePEAswasintherangeof2to 40degrees,withanaverageof15degrees.Thelengthincreasedbyafactorof3to4inthelatituderangeof20to40degrees inthenorthernandsouthernhemispheres,withlongerPEAsbeingobservedpreferentiallyathigherlatitudes.ThePEAswere locatedmainlyintheactivitybeltsinbothhemispheres,withthesouthernhemisphericonesbeingshiftedbyabout15degree inlatitudefurther away fromthe solar equator during 1997−2002. Thedecrease inlatitudeof thePEApositions was10 to 15degreesinthenorthernandsouthernhemispheresoverthisperiod.TheaxesofthePEAswereoverlyingmagneticpolarity inversionlineswhentracedbacktotheMDIsynopticchartsofthephotosphericfield.Themagneticpolaritiesonbothsides oftheinversionlinesagreedwiththedominantmagneticpatternexpectedincycle23,i.e.beingpreferentiallypositivetothe WestofthePEAaxesintheNorthandnegativeintheSouth.Onethird(31%)ofthePEAeventsshowedreversedpolarities. TheoriginofPEAsisfoundnotjustinsinglebipolarregions(BPRs),butalsoinbetweenpairsofneighboringBPRs. Keywords.Sun:corona–Sun:coronalmassejections(CMEs)–Sun:flares–Sun:filaments–Sun:photosphere– Sun:solar-terrestrialrelations 1. Introduction unprecedented observations of CMEs since launch in 1995 (e.g.,St.Cyretal.2000;Howardetal.1997). Coronal mass ejections (CMEs) are dynamic events in which plasma with closed magnetic field structure is ejected out of It is a challenge to better understand the source regions thesolaratmosphere.Theydisrupttheflowofthesolarwindin of CMEs in the low corona and photosphere. Unfortunately, theheliosphereandcancausemajorgeomagneticeffects(e.g., the regions underlying CMEs are best observed on the visi- Gosling et al. 1974; Zhang et al. 2003; Bothmer & Schwenn ble disk, whereasCMEs are best observedat the limb. Those 1995). The origin of CMEs and their evolution in interplane- CMEsoriginatingarounddiskcenterarecalledhalos(Howard taryspacearenotwellunderstood.TheLASCO(LargeAngle etal.1982).Theyappearunstructuredbecausetheypropagate SpectrometricCoronagraph)coronagraphonboardtheSOHO mainlyparalleltotheline-of-sightsothattheirrealphysicalpa- (Solar Heliospheric Observatory) spacecraft has provided rametersaredifficulttodetermine.Front-sidehaloCMEsareof Sendoffprintrequeststo:D.Tripathi, strongimportanceintermsofspaceweather(e.g.,Webb2000; Bothmer1999).Detailedstudiesofthesourceregionsoffront- e-mail:[email protected] (cid:1) Table 1 is available in electronic form at the CDS via side halo CMEs will provideimportantinformationto under- anonymousftptocdsarc.u-strasbg.fr (130.79.128.5)orvia standthesolarcausesofCMEsandalsotohelpforecasttheir http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/337 possiblearrivalsatEarth’sorbit. Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20035815 338 D.Tripathietal.:EUVpost-eruptivearcades solardisktogetherwithEITmoviesofhigherspatialandtime resolution as provided in the mvi-format (512× 512 pixels). Thesemoviescommonlyhaveatemporalresolutionof12min. TheinstrumentalcharacteristicsofEIThavebeendescribedin detail by Delaboudinie`re et al. (1995). The photon emission detectedbyEITat195ÅisduetoFeXIIionsformedattem- peratures around 1.4× 106 K (Dere et al. 2000). From 1996 untilMarch1997,thecadenceofEITdatawaslimiteddueto telemetryrestrictions(Subramanian&Dere2001). EUV PEAshavebeenidentifiedin the EITmoviesbythe appearance of transient brightenings of large-scale loop sys- temsoverperiodsofseveralhours.An eventwas selected for furtherstudyif it appearedas a clearlydiscernablefeatureon thesolardiskvisibleoveritsfullspatialextent(seee.g.,Fig.2). Notethatthiscriterionexcludeslimbevents.Fromtheinspec- Fig.1. A post-eruptive arcade imaged by TRACE at 195 Å on tion of the EIT 195 Å data, 236 PEA events were identified 04-Nov.-2003at22:35UT. during1997−2002(seeTable1).ThenumberofPEAsin1998 was strongly decreased in the second half of the year due to In order to identify the source regions of CMEs, erupting the SOHO recovery phase after its preliminary loss in June. prominences (disappearing filaments) represent probably For each PEA, its spatial extent was located in the EIT im- one of the best solar activity phenomena (e.g., Webb & ages at times around its maximum brightness at 195 Å. The Hundhausen 1987), but for CMEs originating from active heliographiccoordinatesofthearcade’sextremeendswerede- regions,oftennoassociateddisappearingfilamentisobserved termined in latitude and longitude listed as b , l , b and l 1 1 2 2 (Subramanian & Dere 2001). Contrary to the cooler promi- in Table 1, with (b , l ) and (b , l ) denoting the PEA’s end 1 1 2 2 nencematerialseeninabsorption,brighteningsofS-orreverse pointstotheWestandEastrespectively.denotingthearcade’s S-shapedhot(T 2.0×106K)loopstructures,calledsigmoids, endpoint to the East (the values provided in Table 1, maybe and developments of X-ray arcade-like loop systems were compared with the top panel of Fig. 2 in Sect. 3). For PEAs also found to be associated with the occurrences of CMEs that can be considered as fairly linear features, to first order (e.g., Rust & Webb 1977; Svestka et al. 1998; Hudson et al. (b ,l ), (b ,l ) representthe endpointsof the mid axis of the 1 1 2 2 1998). The post-eruptive arcades (PEAs), often also called arcade.The calculationof the length L of each arcadesbased post-flare loops, are also visible at EUV wavelengths (e.g., on these values is describedin detail in Sect. 3. The approxi- Sterling et al. 2000). A recent example for such a post- matedlifetimeofeachPEAwasdeterminedfromitspresence eruptive arcade imaged by TRACE (Transition Region and as a discerniblefeaturein the EIT195 Å imageswith a com- Coronal Explorer, http://vestige.lmsal.com/TRACE/ montime-cadenceof12min. POD/TRACEpod.html) on November 4, 2003 at 22:35 UT, The correlation of PEAs with CMEs was studied using which was preceded by a very fast (V > 2000 kms−1) CME data from the SOHO/LASCO coronagraphs. A detailed de- and an extremelyintense X-rayflare (class X28),is shown in scriptionofLASCOwasgivenbyBrueckneretal.(1995).The Fig. 1. This PEA had formedat the solar limb an example of LASCO/C2 coronagraph images and movies provide white- adiskPEAisshowninFig.2.Kopp&Pneuman(1976)have light observations over a field of view (FOV) from 1.5 to interpretedtheformationoftheloopsystemsasaconsequence 6.0 solar radii with a correspondingpixel size of 11.4 arcsec of magnetic reconnection processes in the course of solar and a time resolution of 20 min. The spatial correlation eruptions. Sometimes two ribbon flares demark the footpoint of PEAs with CMEs was investigated by comparing the locationsanddevelopmentsoftheseloopsystems. CME’s position angle (PA) as provided by the CME cata- Whereas TRACE provides detailed views of PEAs in in- logue (http://cdaw.gsfc.nasa.gov/CMElist/, Yashiro dividualevents,SOHO providesa uniquedataset offulldisk etal.2002)withthesolarpositionsoftherelatedPEAs(com- observations for solar cycle 23. Complementary to studies of pare with Fig. 2 in Sect. 3). The PA is measured positive in smaller sets of individual events (e.g., Hudson et al. 1998; the counter-clockwise direction, starting with zero degrees in Sterlingetal.2000),themainscopeofthispaperistoexplore the solar North. In order to prove the association of a given the unique set of SOHO/EIT (Extreme ultraviolet Imaging PEA with a CME detected by LASCO/C2, the temporal cor- Telescope)observationsinorderprovidethefirstdetailedsta- relation of both phenomena was further investigated by com- tistical analysis of the generalobservationalcharacteristicsof paring the observation time of the PEA with the height-time EUVPEAsandtheirrelationshiptowhite-lightCMEsdetected (hereafterh-t)diagramoftheCMEastakenfromtheCMEcat- byLASCO. alogue including both LASCO C2 and C3 (FOV 1.5−6.0 R(cid:2) & 4.0−30 R(cid:2)) measurements. The time of the first detection of each related CME in C2 is specified in the last column 2. Datasetandeventselection of Table 1. From the h-t diagrams the approximate CME on- SOHO/EIT 195 Å daily mpeg-movies taken from 1997 until settimeswereestimatedtogetherwiththeiroutwardevolution theendof2002wereinspectedtoidentifyPEAsonthevisible andcomparedwiththeapproximateonsetsofthedisturbances D.Tripathietal.:EUVpost-eruptivearcades 339 Fig.2.Toppanel:runningdifferenceimagestakenbyEIT195Åon12-Sep.-2000.Thefirsttwoimagesshowtheeruptingprominence(EP) eventandconsequent post-eruptivearcade(PEA)formationinthesouthernhemispherenearCM.Points1and2representthestartandend pointsofthePEA.Thelastfigurerevealsthedimmingofthearcade.Bottompanel:LASCO/C2imagesshowingtheevolutionoftheassociated CMEinwhite-light.LEdenotestheleadingedgeoftheCMEandEPtheeruptingprominence.Inallfigures,northisupandwestistowards theright.ThespeedoftheCMEintheplaneoftheskywas1550kms−1atPA220. In order to study the underlying photospheric field sig- nature of the PEAs, magnetograms from the SOHO/MDI (Michelson Doppler Imager, see Scherrer et al. 1995) in- strument were used. MDI measures the line-of-sight, i.e. the longitudinal component of the magnetic field with a resolution of 2 arcsec via the Ni I (6768 Å) line. In order to compare the EIT PEA events with other solar activ- ity features, such as disappearing filament/prominence eruptions, X-ray flares or soft X-ray brightenings, data from the Paris/Meudon (http://bass2000.obspm.fr/ home.php), and Big Bear (ftp://ftp.bbso.njit.edu/ pub/archive/) observatories, the GOES satellite (ftp:// ftp.ngdc.noaa.gov/STP/SOLARDATA/SOLAR FLARES/ XRAY FLARES/) and the Soft X-ray telescope (SXT) on board Yohkoh (http:// cdaw.gsfc.nasa.gov/CMElist/ daily mpg/;seeTsunetaetal.1991)wereconsulted. 3. AssociationofPEAswithCMEs Fig.3. LASCO C2/C3 h-t diagram for the CME detected on The association with a white-light CME was investigated for 12-Sep.-2000, around 12:30 UT. The solid triangle marks the es- everyidentifiedPEAlistedinTable1basedonLASCO/C2ob- timated CME onset time at 1 solar radii and the solid circle rep- servationsasdescribedinSect.2.Figure2showsanexample resents the identified onset time of the PEA based on EIT 195 Å of a PEA located near central meridian (CM) in the southern images. The h-t diagram was taken from the CME catalogue at solar hemisphere and its associated white-lightCME first de- http://cdaw.gsfc.nasa.gov/CME list/. tectedin the FOV of C2 on12 September2000,at 12:30UT. The detection of the CME in C2 was preceded by a filament in the low corona and the PEA developments (see Fig. 3 in eruptionobservedby EIT at around11:48UT, becomingvis- Sect.3). ibleinC2 at12:54UT.TheestimatedonsettimeoftheCME 340 D.Tripathietal.:EUVpost-eruptivearcades Table2.Thecolumnsfromlefttorightare:Dateofobservation;start-,endofrising-andpeak-timeofGOESX-rayflares;flarelocationson thesolardiskinheliographiccoordinates;timesofcoronalbrighteningsobservedbyYohkoh/SXT,onsettimesofPEAobservedbySOHO/EIT at195Å;estimatedonsettimesoftheassociatedCMEsbasedontheinvestigationofh-tdiagrams. Date GOES Flare Flare CB PEA CME X-ray location class onset onset (UT) (deg) (UT) (UT) (UT) 9-Feb.-00 19:10–19:18;19:40;20:00 S17W40 C74 (19:43) 20:12 19:26 17-Feb.-00 20:10–20:15;20:30;20:32 S29E07 M13 (20:23) 20:36 20:19 2-Jun.-00 08:50–09:00;09:30;09:36 N10E23 C24 (09:22) 09:36 09:12 6-Jul.-00 12:15–12:20;12:35;12:36 N18E25 C43 (13:15) 13:25 12:30 wereno LASCO data taken,so thatonly19(8%) PEAswere foundtobelackinganassociatedCME. Figure 4 shows the distribution of all identified PEAs in heliographic longitude presented in bins of ten degrees. As expected, the distribution peaks near CM, where PEA events can best be observed over its full extent. The few number of PEAswithoutassociatedLASCO/C2CMEswereallobserved arounddiskcenterintherange40◦Eastto30◦West. Taking into account, that front-side halo CMEs originat- ing from near disk center are often hard to detect because of the dependence of the efficiency of Thomson scattered light on the viewing angle of the CME with respect to the line of sight(e.g.,Plunkettetal.1998;Brueckneretal.1998),itseems reasonable to assume that the small numberof PEAs without Fig.4.DistributionofPEAsinheliographiclongitudeinbinsof10de- CME association may have beencaused by sensitivity limita- greesasidentifiedinEIT195Åimagesfrom1997to2002.Thepor- tionsofLASCO.ItthusseemsplausibletoassumethatPEAs tionsofthebarsrepresentedasspottedareasrepresentthosePEAsfor aredefiniteindicatorsofCMEsthatoriginatedfromthecorre- whichnowhite-lightCMEhadbeendetectedbyLASCO/C2. spondingregionsofthevisibledisk. Disappearingfilaments(prominenceeruptions)areconsid- ered as one of the most unique signatures of CMEs (e.g., was11:45UTaccordingtotheh-tdiagram(seeFig.3),which Bothmer & Schwenn 1994). As an example for this connec- in this case is in good agreement with the onset time of the tion,Fig.5showsthatthefront-sidehaloCMEonFebruary17, prominenceeruptionimagedbyEIT.However,thevalidityof 2000, that originated from the SE part of the Sun near CM, theestimatedCMEonsettimebasedontheextrapolationofthe was accompanied by a filament disappearance in Hα (Fig. 5, h-tdiagramcanbemisleading,e.g.,forCMEsthatwereslowly middle and right images in top panel). Figure 6 shows a acceleratedinitsinitiationphaseorincaseofhaloCMEs.The multi-wavelength view of the CME’s source region based on identified onset time of the corresponding PEA at 195 Å in SOHO/EIT and MDI, Yohkoh/SXT, and ground-based ob- the EIT images was 12:24 UT, showing its maximum devel- servations from Paris/Meudon. The filament closely followed opmentaround13:48UTanddisappearingatabout17:48UT. the orientation of the neutral line separating regions of op- The EUV PEA formedabout39 min after the eruptionof the posite magnetic polarity as inferred from the SOHO/MDI prominence and CME lift-off, and was still lasting when data. Bright loops in form of an S-shaped sigmoid, promi- theCMEhadalreadyreachedaheightofmorethan40R(cid:2).The nent in the Sun’s southern hemisphere (e.g., Canfield et al. timingoftheindividualphenomenaissummarizedinFig.3. 1999),areoftenvisibleshortlybeforetheeruptionofaCME, A remarkable feature of Fig. 2 is the apparent similarity visible here at around 20:00 UT on February 17 (Fig. 6, between the shape of the CME core and structure of erupt- second panel from top, first image). A strong brighten- ing prominence and PEA system. As can be inferred from ing of the sigmoidal system occurred near the CME’s lift- the bottom panel of Fig. 2, the CME resembles a large-scale off time at around 20:23 UT (see Yohkoh/SXT images in curved cylindricalflux tube that has originated from a source Fig. 6), followed by the onset of the EUV arcade around siteboundedbythePEA’sstartandendpointswhicharelikely 21:36 UT. The X-ray flare observedby GOES 8 (available at tobeconsideredasthetwolegsoftheCME. http://www.lmsal.com/SXT/plotgoes.html)startedbe- Thesystematicinvestigationoftheassociationoftheentire tween 20:10and 20:15 UT (see Table 2), increasing in inten- setofPEAeventslistedinTable1,withwhite-lightCMEsde- sity and reaching its peak value near 20:32 UT. The onset of tectedbyLASCOinclosespaceandtimerelationship,showed the PEA seen by EIT follows in time the peak of the X-ray that210(92%)outof236eventsidentifiedfrom1997untilthe flare. The CME onset time corresponds to the rising phase endof2002hadclearCMEassociations.Inseveneventsthere of the X-ray flare, coinciding with its acceleration phase, in D.Tripathietal.:EUVpost-eruptivearcades 341 Fig.5.Toppanel:thefirstimageshowsarunningdifferenceimagetakenbyEITat195Åon17-Feb.-2000displayingthePEAthathadformed inthesouthernhemisphere.Thesecondandthirdimagesshowthatafilament(F)disappearedfromthissolarregionbasedonHαobservations taken on17 and18February 2000 (courtesyParis/Meudon Observatory). Bottom panel: theassociated haloCME onFebruary17, 2000 as detectedbyLASCO/C2around21:30UT.ThespeedoftheCMEwas600kms−1atPA196. Fig.6.Multi-wavelengthsobservationsforthesourceregionofthe17-Feb.-2000PEAeventaround21:30UT.Toptobottom:Firstpanel:EIT 195 Å images taken at 20:12 UT, 20:24 UT and 20:36 UT on February 17, 2000 showing the pre-eruption configuration of the halo CME detectedlaterbyLASCOandtheinitialphaseofthearcadeformation.Secondpanel:Yohkoh/SXTimagestakenat20:03UT,20:23UTand 21:23UTonthesamedayshowingthebrighteningofanS-shapedsigmoidintheS-hemisphereintheCMEsourceregion.Thirdpanel:Hα observations taken on 16, 17and 18 February2000 showing that afilamenthad disappeared fromthecorresponding source region. Fourth panel:MDImagnetogramstakenat12:53UTonFebruary16,16:43UTand21:20UTonFebruary17,showingthephotosphericmagnetic fieldstructureoftheCME’ssourceregion. 342 D.Tripathietal.:EUVpost-eruptivearcades Fig.7.Lefttoright:zoomedviewofthepost-eruptivearcade(PEA)observedbyEITat195Åon15-Apr.-2001at05:12UTfollowedbytwo H-alphaimagestakenon14and15Aprilshowingthepositionofthecorrespondingdisappearingfilament(F)onthesolardisk. Fig.9. YOHKOH/SXT image taken on 25-Jan.-1998 at 15:54 UT showingthecoronalbrightening(CB)thatwasdetectedontheNEpart ofthesolardiskandthecorrespondingpost-eruptivearcade(PEA)ob- servedbyEITat195Åat17:53UT. Fig.8. GOES 8 X-ray plots for the CME and PEA events on February 9, 2000 (top) and 6 July, 2000 (bottom). The approximate onsettimesoftheCMEsandPEAsarelabeled. agreementwiththefindingsofZhangetal.(2001).Assuming thatthepeakintensityoftheflareindicatestheCME’slift-off fromtheSun,thePEAmaybeinterpretedasaconsequenceof magneticreconnectionprocessesthatwereinitiatedunderneath therisingCMEassuggestedbyKopp&Pneuman(1976). ForthreemorePEAevents,theflareposition,asprovided by the GOES catalogue (ftp://ftp.ngdc.noaa.gov/ STP/SOLAR DATA/SOLAR FLARES/XRAY FLARES), was lo- Fig.10.FrequencydistributionofthelifetimesoftheidentifiedPEAs cated not further away from the PEA location than ±5◦ in during 1997−2002 based on SOHO/EIT 195 Å observations in bins heliographic latitude and longitude and which were observed of2h.Theaveragevalueforthelifetimewas7h. within a time interval of three hours prior to the onset of the PEA, the chronological evolution of the coronal features seen by EIT 195 Å and SXT were compared with the GOES February9andJuly6,2000.WhereasonJuly6,2000,thePEA 8 X-ray flare timings (Table 2). The time evolution in the causes a second increase of the X-ray intensities, it can not eventsissimilar:ArisingphaseinsoftX-rayintensityduring be distinguished as a separate feature in the GOES 8 X-rays the accelerationphaseof theCME, its consequentlift-offand on February 17, 2000. The coronal brightenings observed propagation phase, followed some minutes later by the peak by SXT preceded the onset of the EUV PEAs by about 5 to in X-ray intensity, finally followed by the formation of the 30 min (see Table 2), but due to the irregular cadence of the PEA underneath it. Figure 8 shows the time evolution of the SXT observations, no detailed statistics could be performed. GOES 8 X-ray flare for the PEAs and CMEs observed on Figure9 showsan exampleof a transientcoronalbrightening D.Tripathietal.:EUVpost-eruptivearcades 343 Fig.11.CarringtonsynopticmaprepresentingthepositionsofallidentifiedPEAsduring1997−2002basedontheanalysisofSOHO/EIT195Å observations. Fig.12. Frequency distribution showing the heliographic lengths of Fig.13. Variation of PEA lengths with heliographic latitude during PEAsidentifiedfromSOHO/EIT195Åimagesduring1997−2002in 1997−2002. The data points represent the latitude for the midpoint binsof5◦.TheaveragelengthoftheidentifiedPEAswas15◦. of the PEA axes. Note that no events were identified above 60◦. Thestraight lines represent the fittedlatitudinal trends in both solar hemispheres in the range 20 to 40 degree latitude, with: L (deg) = N detected by SXT on 25-Jan.-1998 and post-eruptive arcade 2.9(b−17.1) and L (deg) = −1.6(b+13.4),withbbeingthehelio- S seenbyEIT. graphiclatitudeofthemidpointofaPEAaxis.Positivevaluesofthe latitudecorrespondtothenorthernhemisphere. A PEA eventassociated with a disappearingfilament, but without correspondingLASCO white-light CME is shown in Fig. 7. For better visibility, only a portion of the full disk associatedwithCMEincreasesto98%(224outof229events) EITimage,comprisingthefullspatialextentofthePEAisdis- i.e. almost a one to one correspondence is found between played.TheHαandEITfeaturesarebasicallythesameasob- EUVPEAsandCMEs. servedfortheeventon17February2000,visualizedinFigs.5 and6,exceptthataCMEwasnotidentifiedintheLASCOdata. IfoneconsidersHαfilamenteruptionsasvalidCMEprox- 4. BasicpropertiesofPEAs ies, 14 more events out of the 19 cases where no white-light 4.1.EIT195Åemissionlifetime CMEwasdetected,canbeclearlyconsideredasprobablyCME associated. Unfortunately, in 3 cases no Hα data were avail- Foreachof the PEA events,thetime intervalwas determined able and in 2 cases the events were very complex so that the over which the EUV arcade could be clearly distinguished in interpretation of the data was difficult (see Table 1). If one theEITimagesat195Åwithausualtimeresolutionof12min. takesintoaccounttheseinformations,thetotalnumberofPEAs The starttime of the eventswas definedas the momentwhen 344 D.Tripathietal.:EUVpost-eruptivearcades Fig.14.SOHO/MDICarringtonsynopticchartsforrotations1928a)in1997and1963b)in2000andidentifiedsourceregionsofninepost- eruptivearcades(PEAs),markedbysolidanddashedlines.Inthesynopticcharts,whitecolorsrepresentareasofpositivemagneticpolarity, blackcolorsthoseofnegativepolarity.NotethatintheNorththeleadingareasrevealpositivemagneticpolaritiesinthiscycleandviceversa intheSouth.ThesolidanddashedlinesrepresenttheindividualPEAaxes.Casei)andj)indicatesalinearshapedPEA.APEAlocatedin betweentwobipolarregionsislabeledask)andacaseuntypicalforthedominantpolarityinthenorthernhemisphereincycle23asl)polarity configuration. the large scale loop system could be identified the first time 4.2.Heliographicposition,lengthandvariation in the EIT images and the end time was defined as the withlatitude time when the loops could not be distinguished anymore.All estimated lifetimes are listed in Table 1. Figure 10 shows the For each PEA, the start and end points of the loop system’s EIT195ÅlifetimedistributionofthePEAsinbinsof2h.The longaxiswerecalculatedasdescribedinSect.2(seeTable1). typicallifetimeofaEUVPEArangedfrom2to10h,withan Figure11displaystheheliographicpositionsofallPEAsiden- averagelifetimeofabout7h. tifiedduringtheyears1997to2002inasingleCarringtonmap. D.Tripathietal.:EUVpost-eruptivearcades 345 Figure 11 shows that most of the PEAs formed in the helio- graphic latitude range ±40◦ North and South and that transe- quatorial cases were extremely rare. No PEA was observed at latitudes above 60◦ North or South. The orientation of the PEA axes followed Joy’s law for the tilt of sunspots (Hale et al. 1919), i.e. the tilt angle of sunspots is half of the lati- tude, i.e. a sunspot at 60◦ North would be expected to have a tilt of about 30◦, as measured from East towards North. The tilt anglesin sunspotsare measuredfromthe equatortowards Northinthenorthernhemisphereandviceversainthesouthern hemisphere. Based on the estimated heliographic coordinates, the length(L)ofeachPEAswascalculatedas: L=arccos(sin(b ) sin(b )+cos(b ) cos(b ) cos(l −l )). (1) 1 2 1 2 1 2 Theparameters(b ,l )and(b ,l )aretheheliographiccoordi- 1 1 2 2 natesofthePEA’sfirstandsecondpointsasdefinedinSect.2. Figure12showsthefrequencydistributionofthecalculated lengths of the PEAs. The length of the PEAs varied from 2 to 40 degrees, with an average value of about 15◦. As indi- cated by Fig. 11,PEAs exceedingthe averagelength seem to beobservedpreferentiallyathigherheliographiclatitudes.The lengthvariationwithheliographiclatitudein bothsolarhemi- spheres is shown in Fig. 13. From a linear polynomial fit it is found, that the length (L) of PEAs increases with latitude in the North as L (deg) = 2.9(b−17.1) and in the South as N L (deg) = −1.6(b+13.4),with b being the heliographiclati- S tude. For latitudes of 20, 30 and 40 degrees North and South Fig.15.Sketchshowingthepossiblepre-eruptionfieldconfiguration this corresponds to PEA lengths of 8, 37 and 66 degrees in of PEAs forming along neutral lines/filament sites in single bipolar the North, and 11, 27 and 43 degrees in the South. Since regions(A)andinbetweenpairsofthem(B).Thepre-eruptioncon- PEAs can be considered as tracers of the source regions of figurationisadaptedfromTandberg-Hanssen(1974). CMEs, this findingmay imply that the longitudinalextension ofCMEsincreaseswithheliographiclatitude.Theincreaseof away from the photosphere) and negative (field lines point- thePEAlengthwithlatitudeappearstobeassociatedwiththe ingtowardsthephotosphere)magneticpolarities.Thesolidand largersizesofdisappearingfilamentsathigherlatitudes. dashedlinesinFig.14markthecalculatedPEAlongaxespo- sitions of the three selected PEAs of CR 1928 and the seven ofCR 1963.The axespositionsalwaysmatchedthepositions 5. Photosphericsourceregionsandsolarcycle andorientationsofpolarityinversionlines(PILs,neutrallines) dependence separatingregionsofoppositemagneticpolaritiesinbothsolar TheestimatedheliographicpositionforeachPEAwasusedto hemispheressimilartotheconclusionofSubramanian&Dere locate itsphotosphericsourceregioninthe SOHO/MDImag- (2001). netogramsynopticcharts taken duringthe correspondingCR. Figure 14a shows data from October 1997, i.e. in the ris- Forsimplicity,noconsiderationofthetimedifferencesbetween ingphase of solar cycle23,and Fig. 14bshowsdata forJune the arcade occurrences and the observation dates of the sin- 2000,i.e. around solar activity maximum.The numberof ac- gle magnetograms that contributed to the individual synoptic tiveregionsandthephotosphericmagneticfluxisconsiderably maps were taken into account here. The synoptic charts are highernearsolarmaximum.Thisdifferenceisalsoreflectedby generated from the definitive MDI magnetograms (available theminornumberofPEAsidentifiedinCR1928.Thecompari- at http://soi.stanford.edu/magnetic/index5.html). sonoftheEITandMDIobservationsyielded,thatPEAscanei- Forthisstudy,thechartsproducedfromnearcentralmeridian thercompriseanentirePILofabipolarregion(Fig.14a,casei) observationswereused.Thisimpliesthatarcadeeventsidenti- ortheyjustafractionofit(Fig.14a,casej).Besidesthecasesin fiedintheeasternsolarhemispherehadappearedintimeafter whichPEAshadformedassociatedwithspecificsingleBPRs, the magnetograms used to construct the synoptic maps were some PEAs were found to be located in between two neigh- taken, whereas western hemispheric events had preceded the boringbipolarregion(seeFig.14a(k)).Themagneticconfigu- magnetogramobservations. rationsshown in Fig. 15,in analogyto the onessuggestedby Figures 14a,b show the positions of the ten EUV arcades Tandberg-Hanssen(1974,p.118),naturallyrepresentsthepre- identified in CRs 1928 and 1963. White and black colors eruptionmagneticstructureforbothtypesofevents.InFig.15, in the magnetograms represent positive (field lines pointing the presence of a filament is assumed in both configurations, 346 D.Tripathietal.:EUVpost-eruptivearcades Fig.16.Carringtonsynopticchartsoftheheliographicpositionsofpost-eruptivearcadesidentifiedduring1997−2002forwhichthemagnetic polaritiesofthesourceregionswereidentifiedfromMDIsynopticcharts.ThecolorsattheequatorwardendsofthePEAsdenotethemagnetic polaritytotheWestoftheindividualpolarityinversionline,withtheredcolorbeingassignedtothepositivemagneticpolarityandtheblue colorassignedtothenegativeone. althoughitisnotnecessarilyrequired.Althoughthemagnetic polarity configurationas comparedto the one dominatingthe polarities on both sides of the PILs were most often the ex- givencycle(Fig.14b(l)). pectedonesforcycle23,withleadingpositivepolaritiesinthe Out of the 236 PEA eventslisted in Table 1, in 216 cases senseofsolarrotationintheNorthandleadingnegativepolari- themagneticpolaritiesonbothsidesofthearcade’saxiscould tiesintheSouth,someofthePEAsformedinregionsofreverse be uniquely identified. The remainingevents occurred at lati- tudesabove40◦orinregionswithverydiffuseaswellashighly