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ProceedingsofICRC2001:1 c CopernicusGesellschaft2001 ICRC 2001 (cid:13) Energy Spectra of Very Large Gradual Solar Particle Events A.J.Tylka1,C.M.S.Cohen2,W.F.Dietrich3,C.G.Maclennan4,R.E.McGuire5,C.K.Ng5,6,andD.V.Reames5 1Code7652,E.O.HulburtCenterforSpaceResearch,USNavalResearchLaboratory,Washington,DC20375USA 2CaliforniaInstituteofTechnology,Pasadena,CA91109USA 3LaboratoryforAstrophysics&SpaceResearch,UniversityofChicago,Chicago,IL60637USA 4BellLaboratories,LucentTechnologies,MurrayHill,NJ07974USA 5NASA/GoddardSpaceFlightCenter,Greenbelt,MD20771,USA 6DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742USA Abstract. Energyspectraprovideapowerfultoolinunder- to mass (Q/A) ratio. Thus, SEP ionic charge states can be standingsolarenergeticparticle(SEP)events,inthatspectra inferred by comparing spectra of various elements, as first containinformationonallaspectsofSEPproduction,includ- demonstratedbyTylkaetal. (2000). ingsourceplasma,interplanetarytransporteffects,andchar- Thetwoeventsconsideredhere(1998April20and2000 acteristicsoftheaccelerator. Wecompareenergyspectraof July 14) were both produced by very fast CMEs, with ini- twoverylargegradualevents(1998April20and2000July tialspeedsof 1600km/sand 1800km/s,respectively. In 14), produced by shocks driven by fast coronal mass ejec- botheventsth(cid:24)eassociatedflare(cid:24)swereat heliolongitudesfar tions (CMEs). We showthatdifferencesin theirFe spectra fromthefootpointoftheSun-Earthmagneticfieldline(W90 canbeunderstoodintermsofasmall,variableadmixtureof andW07,respectively). Itisthusunlikelythatineithercase remantflaresuprathermalsintheSEPsourcepopulation. we saw particles directly accelerated in the flare. For the same reason, both events are also probablyfree of compli- cationscaused byconcurrentaccelerationand ion-stripping inthelowcorona,whichaffectedthewell-connected(W63) 1 Introduction eventof1997November6(Reamesetal. 1999;Barghouty &Mewaldt1999,2000;Stovpyuk&Ostryakov2001). The suprathermal tail of the solar wind (SW) is generally A major differencebetween these two events is the level believed to comprise the source populationof gradual SEP of preceding solar activity. The 1998 April 20 SEP event events. Onthebasisofmeasured He/ Heontheorderofa 3 4 markedtheendofaperiodofverylowsolaractivity,withno fewpercentinsomegradualSEPevents,Masonetal.(1999) C-,M-,orX-classx-rayflaresinthepreceding4days. The suggestedthat,atleastatsometimes,theSWsuprathermals 2000July14event,ontheotherhand,followedanextended could be augmented by suprathermals from impulsive SEP periodofveryhighflareactivity, with 40x-rayflares (2X, events. Thesesuprathermalswouldhavebeenoriginallyac- 17M,21C)intheprecedingfourdays. celeratedataflaresite,andhencebeardistinctivecomposi- tional and charge-state characteristics. Since suprathermal particles take several days to move through the inner he- 2 Observations liosphere,theyconstitutearemnantpopulation,replenished nearly continuously by flare activity. A shock would then Fig. 1 shows sample heavy-ionspectra in the 2000 July 14 accelerateparticlesfrombothsolar-windandflaresuprather- (Bastille Day) event from two intervals covering 6–14 and malsinamoreorlessdemocraticfashion.Otherstudieshave 22–26hoursaftertheflare(Tylkaetal. 2001). Thespectra furtherbolsteredtheremnant-flarehypothesis(Richardsonet clearlyevolve,withe-foldingenergiesdecreasingwithtime al.1993;Desaietal. 2001). (Tylka et al. 2000), as generally expected as a fast CME- Theconnectionbetweenspectraandsourcepopulationman- drivenshockmovesoutwardfromtheSun(Zanketal.2000). ifests itself at high energies: as energy and gyroradius in- Reames,Ng,&Tylka(2001)alsoexaminedthetimeinter- crease, it becomes less probablethat a particle can be con- valsinFig.1,usingonlyWind/LEMTdataat 4–20MeV/nuc. tained within the shock region. Ellison & Ramaty (1985) Thedashedcurvesaretheirexponentialfits.(cid:24)Thesefitsshow suggestedthatthisescapewouldcausethespectraofshock- thatFehassmallere-foldingenergiesthanotherspecies,cor- acceleratedparticlestorollovermoreorlessexponentially, responding to lower Q/A, at least at LEMT energies. In withe-foldingenergydirectlyproportionaltotheion’scharge fact, Reames et al. (2001) assumed e-folding energies to Correspondenceto:[email protected] be directly proportional to Q/A and that the e-folding en- Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 3. DATES COVERED 2001 2. REPORT TYPE 00-00-2001 to 00-00-2001 4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER Energy Spectra of Very Large Gradual Solar Particle Events 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION Naval Research Laboratory,E. O. Hulburt Center for Space REPORT NUMBER Research,4555 Overlook Avenue, SW,Washington,DC,20375 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S) 11. SPONSOR/MONITOR’S REPORT NUMBER(S) 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES Proceedings of 27th International Cosmic Ray Conference (Hamburg), 2001, in press. 14. ABSTRACT Energy spectra provide a powerful tool in understanding solar energetic particle (SEP) events, in that spectra contain information on all aspects of SEP production, including source plasma, interplanetary transport effects, and characteristics of the accelerator. We compare energy spectra of two very large gradual events (1998 April 20 and 2000 July 14), produced by shocks driven by fast coronal mass ejections (CMEs). We show that differences in their Fe spectra can be understood in terms of a small, variable admixture of remant flare suprathermals in the SEP source population. 15. SUBJECT TERMS 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF ABSTRACT OF PAGES RESPONSIBLE PERSON a. REPORT b. ABSTRACT c. THIS PAGE Same as 4 unclassified unclassified unclassified Report (SAR) Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 2 Fig. 2. Sample heavy-ion spectra from the 1998 April 20 (DOY 110) event, asin Fig.1. (Additional intervals are shown in Tylka 2001.) Leftpanel is log-log; right panel is log-linear. Curves are fitsconsistentwithapureSWsourcepopulation,withoutremnant- flaresuprathermals. moresteeplythantheothersthroughouttheobservedenergy range. Tylkaetal. (2000)usedthefittede-foldingenergies inthiseventtoinfermeanionicchargestates. Thesevalues were similar to slow solar wind (von Steiger & Schwadron 1998) and in excellent agreement with SEPICA measure- mentsat 0.3MeV/nuc(Kleckeretal.1999),implyingvir- tuallyno(cid:24)energydepdendenceinchargestatesinthisevent. Fig.1.(fromTylkaetal.2001):Heavy-ionspectrainthe2000July 14(DOY196)eventintwotimeintervals.Toprowshowslog-linear plots, whilebottomshowsthesamespectrainlog-logplots. Data 3 Modeling comefromWind/LEMT(vonRosenvingeetal.1995;filledcircles), ACE/EPAM (Gold et al. 1998; half-filled circles), and ACE/SIS For the spectra in Figure 1, we consider a two-component (Stoneetal. 1998;filledtriangles). Modestadjustments(lessthan source, with 95% of the Fe ions coming from solar wind afactoroftwo)havebeenusedtoremovenormalizationdiscrepan- suprathermalsand5%fromflaresuprathermals. Tospecify cies,withoutalteringthespectralshapereportedbyanyinstrument. theSWcomponent,weusedthein-ecliptic,slowsolar-wind Dashed curves are exponential fits to LEMT only (Reames et al. Q distribution from Ulysses (von Steiger & Schwadron 2001). Solidcurvesaremodelfits,asdescribedinthetext. 19F9e8).ThisdistributionextendsfromQ 6toQ 16, with mean Q 10 and rms widtFhe=2. ForFthee=flare supratherma<ls, wFee >us(cid:24)ed the Q distributi(cid:24)on observed in a ergyofcarboncorrespondedto Q =5.5,atypicalvalue typical impulsive SEP event bFyeSEPICA (Mewaldt 1999). in gradual events and the solar<windC.>From ratios of fitted Thisdistributionhas Q 17andrmswidth 4. With e-foldingenergies,theythenderivedreasonablemeanionic this5%flarecompon<ent,F3e>%(cid:24)oftheresultingFe-io(cid:24)nsource chargestatesforotherspecies,including Q 10. This populationhaveQ 16andlessthan0.5%haveQ 22. value agrees with measurements from SE<PICFAe>(M(cid:24)o¨bius et NotethataflarecoFmep>onentmuchlargerthan5%iseFxcel>uded al.1998)onACE(Smithetal.2001). by Q 10fromSEPICAintheBastilleDayevent. ExceptforFe, theseLEMTexponentialsaccountreason- T<o mFoed>el(cid:24)the Fe spectra, we first fit the spectra below ablywellforhigher-energydatafromSIS.However,Fespec- 20 MeV/nuc, using the full Ellison-Ramaty (1985)formof ttrraaladbioffveere(cid:24)n2c0esMbeetVw/eneuncFareeannedalrilgyhpteorwseprelcaiwess.hSavimebileaernspperce-- Ffo(ldEin)g(cid:24)enEer(cid:0)gy(cid:13)e(xEp()(cid:0)dEer=iEve0d).frWome tthheen as2su0mMedeVth/nautcthdeaeta- viouslynotedathighenergiesinlargeSEPeventsinearlier correspondsto Q0 10.Thee-fold<ingenergiesofother solar cycles (Tylka and Dietrich 1999). Also, although the Fe ions were th<enFscea>le=d from this value, proportionallyto spectra soften as time increases, the uniquecharacterof Fe Q . Thecontributionofeachchargestatetothespectrum persistslongintotheevent.Thisisanotherindicationthatthe waFseweightedbyitsfractioninthetwo-componentmixture. high-energyFecannotbeattributedtoadirectflareorigin SolidcurvesinFig.1showtheresultsofthissuperposition. Fig. 2 shows comparable spectra for the 1998 April 20 Thissmalladmixtureofremnant-flaresuprathermalsdoes event.UnlikeintheBastilleDayevent,theFespectrumhere remarkablywellinaccountingfortheadditionalhigh-energy doesnothardenwithincreasingenergy.Instead,itfallsmuch Fe. Agreementcouldbefurtherimprovedbymassagingthe 3 Fig.4. (fromTylkaetal. 2001): Energydependence ofthemean Fig.3. (from Tylkaetal. 2001): Contributionsofvarious charge ionicchargestateofFe.Solidcurveisevaluatedfromthespectrain states to the Fe spectrum. Blue curves are Q = 6–16 that arise Fig.3.Dashedcurveiscalculatedwiththesamesourcemixturebut primarilyfromtheSWcomponent;redcurvesFaereQ ,from theremnantflare-suprathermals.ThegreencurveisFthee>sum16. subjectedtoashockwithe-foldingenergiesanorderofmagnitude larger.Measurementsarefromvarioussources,asnotedinthetext. assumedflare-Q distribution.Forexample,theslightdeficit strongerenergydependence,particularlybelow2MeV/nuc, in the highest-enFeergyFe can be removed by increasing the which cannot be re-producedby the two-componentmodel proportionofionswithQ 22. Alargerfractionofvery discussed here. The 1997 November 6 event has been ex- highly-ionizedFeionsmayFneo>tbeunreasonable:theassumed plainedasconcurrentshockaccelerationandstrippinginthe flare-Q distributioncamefromaC1.0x-rayflare,whereas low corona (Reames et al. 1999; Barghouty & Mewaldt theBasFtielleDayeventwasprecededbymuchlargerflares. 1999,2000;Stovpyuk&Ostryakov2001). Wesimilarlymodeledotherspecies’spectra,usingSWQ The solid curve in Fig. 4 might suggest that high-energy distributions(vonSteiger&Schwadron1998)andassuming solarFeionsmustalwaysbenearlyfully-stripped. Thisim- theflare-componenttobefullyionized.However,inthatim- pression would be incorrect. The dashed curve in Fig. 4 pulsiveSEPeventsareFe-rich(Reames1995),theassumed showsanothercalculation,againusingthesamesourcemix- flarecomponentforotherspecieswassmaller( 1.8%forSi turebut subjectedto a shockwith e-foldingenergiesan or- andNe, 0.6%forOandC.)Asshownbythe(cid:24)solidcurves der of magnitude larger. The 1989 September 29 event is inFigure(cid:24)1,theserefinementshadlittleimpact. anexampleofsuchashock: protone-foldingenergieswere greaterthan500MeV(Lovelletal. 1998),andFeionswere 4 OtherImplications observedup to nearly 1 GeV/nuc (Tylka & Dietrich 1999). Inthis andthe otherverylargeeventsof 1989,Tylka et al. Fig. 3 illustrates how variousFe chargestates contributeto (1995)measured Q 14at200–600MeV/nuc. the overall spectrum in the Bastille Day event. Solar-wind Figs.1and2cl<earlFyein>d(cid:24)icateenergy-dependentabundance charge states dominate at low energies. The remnant-flare ratios. Fig.5showstheFe/Cratiovs. energy.Theenhanced componentbecomesmoreimportantasenergyincreases.This Fe/Cbelow 10MeV/nucinbotheventsisprobablycaused modelingimpliesenergydependenceinthemeanFecharge by Q/A-dep(cid:24)endent transport (Ng et al. 1999, 2001). The state,whichisshownbythesolidcurveinFig.4.Thiscurve increase in Fe/C above 30 MeV/nuc in the Bastille Day is consistent with measured Q values from SEPICA event appears to be due(cid:24)to the remnant flare-suprathermal (Smithetal. 2001)andSAMP<EXF(Le>eskeetal. 2001)forthe component. Fine-tuningtheassumedflare-Q distribution BastilleDayevent. canimprovethecurve’sagreementwithdata.FTehestatistical Forcomparison,Fig.4 also showsobserved Q for significanceofthis increase is modesthere. However,very the1992November(Leskeetal. 1995;Mason<etalF.e1>995; similar,complicatedenergy-dependentFe/Oratioshavebeen Oetlikeretal.1997)and1997November(Mazuretal.1999) previouslynotedinotherlargeSEPevents(Tylka&Dietrich events. The 1992 event shows energy dependence remark- 1999). Remnant flare suprathermals may also account for ably similar to that derivedhere for the Bastille Day event. reported association between Fe enhancements above 40 The 1997 November 6 event, on the other hand, exhibits MeV/nucandground-levelneutron-monitorevents(Diet(cid:24)rich 4 Fig.6.(fromTylkaetal.2001):HandHeintheBastilleDayevent Fig.5.Fe/Cvs.energyinthe2000July14and1998April20events, fromGOES(half-filledtriangles,Honly), Wind/LEMT(filledcir- normalized to coronal value of 0.288 (Reames 1995). Dashed cles),IMP8/GME(filledsquares),IMP8/CRNC(half-filledsquares, curvesarecalculationsusingSW-componentFeonly;solidcurves Honly),andACE/EPAM(half-filledcircles,Heonly). Curvesare alsoincludeflaresuprathermals.Notethedifferenty-axisscales. exponentials,withe-foldingenergiesscaledfromthatofcarbonin Fig.1,asdecribedinthetext. &Lopate1999),whichalmostalwaysoccurduringperiods ofhighflareactivity. DPRS92662F),andtheOfficeofNavalResearch. WFDwassup- portedbytheSECGuestInvestigatorProgram(NASADPR19,493). 5 DiscussionandSummary References The foregoinganalysis assumed that e-foldingenergies are directlyproportionaltoQ/A.Thisassumptioncanbechecked Barghouty,A.F.&Mewaldt,R.A.,Ap.J.520,L127,1999. withHandHe(Tylkaetal.2000).Foravarietyofreasons,H Barghouty,A.F.&Mewaldt,R.A.,AIPConfProc.528,71,2000. andHedataintheBastilleDayeventarelimitedandofpoor Desai,M.I.,etal.,Ap.J.553,L89,2001. quality. Nevertheless, as a consistency check, Fig. 6 com- Dietrich,W.F.&Lopate,C.,Proc26thICRC6,71,1999. Ellison,D.&Ramaty,R.,Ap.J.298,400,1985. pares H andHe data to exponentials,with e-foldingenergy Gold,R.E.,etal.,SpaceSci.Rev.86,541,1998. scaledbyQ/AfromthatofcarboninFig.1,againassuming Klecker,B.E.,etal.,Proc26thICRC6,83,1999. Q =5.5.Thecomparisonappearsreasonable. Leske,R.A.,etal.,Ap.J.452,L149,1995. <ThCe>observationsandmodelingdiscussedheretouchupon Leske,R.A.,etal.,Proc.SOHO-ACEWorkshop,submitted,2001. severalpreviouslyunexplainedfeaturesofFespectrainlarge Lovell,J.L,Dulding,M.,&Humble,J.,JGR,103,23733,1998. SEPevents:(1)spectralhardnessofFerelativetootherspecies Mason,G.M.,etal.,Ap.J.452,901,1995. at high energies; (2) energy-dependent Fe charge states at Mason,G.M.,Mazur,J.E.,&Dwyer,J.R.Ap.J.525,L133,1999. highenergies;and(3)enhancedFe/C(andFe/O)athighen- Mazur,J.E.,etal.,GRL26,173,1999. ergies.Theseeffectshavesometimesbeeninterpretedasevi- Mewaldt,R.A.,Proc.26thICRC,AIPConfProc.516,265,1999. denceforflare-acceleratedparticlesbecomingdominantover Mo¨bius,E.,etal.,SpaceSci.Rev.86,449,1998. shock-acceleratedparticlesathighenergies. Ng,C.K.,Reames,D.V.,&Tylka,A.J.,GRL26,2145,1999. Ng,C.K.,Reames,D.V.,&Tylka,A.J.,theseproceedings,2001. However,wehaveshownthattheseeffectsarenaturalcon- Oetliker,M.,etal.Ap.J.477,495,1997. sequencesofshockaccelerationactinguponasourcepopula- Reames,D.V.,Adv.SpaceRes.15(7),41,1995. tioncontainingasmalladmixtureofremnantflaresuprather- Reames,D.V.,Ng,C.K.&Tylka,A.J.,GRL26,3585,1999. mals. Precedingflareactivitythusplaysahighlysignificant Reames,D.V.,Ng,C.K.&Tylka,A.J.,Ap.J.548,L233,2001. roleingeneratingSEPevent-to-eventvariability. Richardson,I.G.,etal.,Ap.J.363,L9,1990. Theconditionsattheacceleratorareapparentlyquitedif- Smith,C.W.,etal.,Sol.Phys.,submitted2001. ferent in some events, such as 1997 November 6. Never- Stone,E.C.,etal.,SpaceSci.Rev.86,357,1998. theless, this analysis strongly suggests that the hypothesis Stovpyuk,M.F.&Ostryakov,V.M.,Sol.Phys.198,163,2001. of remnantflare suprathermalsmaymakeit possible to un- Tylka,A.J.,JGR,inpress,2001. derstand a large body of SEP heavy-ion data, extending to Tylka,A.J.&Dietrich,W.F.,Radiat.Meas.30(3),345,1999. at least several hundredMeV/nuc, in terms of CME-driven Tylka,A.J.,etal.,Ap.J.444,L109,1995. Tylka,A.J.,etal.,AIPConf.Proc528,147,2000. shocks,withoutappealtootheraccelerationmechanisms. Tylka,A.J.,etal.,Ap.J.Letters,submitted,2001. Acknowledgements. NOAAprovidedflareandGOESprotondata. vonRosenvinge,T.T.,etal.,SpaceSci.Rev.71,155,1995. AJTwassupportedbytheACEGuestInvestigatorProgram(NASA vonSteiger,R.&Schwadron,N.A.,ASPConf.Proc.206,54,2000. DPRW19,501),ISTPSolarMaxExtendedScienceProgram(NASA Zank,G.P.,Rice,W.K.M.,andWu,C.C.,JGR105,25079,2000.