Adv.SpaceRes.Vol. 13,No.9, ~. (9)363—(9)366,1993 0273—1177/93$24.00Printedin GreatBritain. All rightsreserved. Copyright@ 1993COSPAR

TIME-DEVELOPMENT OF PROTONENERGYSPECTRAIN SOLAR ENERGETICPARTICLEEVENTS

J. Meyer,G.WibberenzandM.-B. Kallenrode

lnstitutfiir Kernphysilc,UniversitärofKiel, Otto-HahnPlatz1, 2300 Kiel,Germany

ABSTRACT

Solarenergeticparticleeventsoften consistof two components:a ‘prompt’ componentof particlesacceleratedin theflare or by a coronalshock,andan ‘energeticstormparticle’ componentaccelera-ted by aninterplanetaryshock.In eventsobservedby twoor morespacecraftthetime-developmentof the proton spectrum,togetherwith the analysisof the intensityand anisotropytime profiles,allows one to distinguishbetweenthesecomponents.

INTRODUCTION

Energeticstorm particlesare a well known phenomenon,especiallyin protons with energiesofa few hundredof keV up to tensof MeV. Theseshockacceleratedparticlesarenot only a “di-sturbance”super-posedon theflare-acceleratedparticlesbut often are the dominantspecies/1,2/andthereforethe intensity time profiles canbe orderedwith respectto theflare normal /3/ withstrongestinfluenceof theshockcloseto theflare normal. In eventswith good magneticconnectiontwo particlecomponentscan be observed,a ‘prompt’ componentaccelerateddirectly in the flarewith abundancesthat resembleclosely photosphericabundances(visible in the earlyphaseof theevent),while the energeticstormparticlesaredominantfor later times, resemblingmore closelythe abundancesobservedin the highercoronaandin interplanetaryspace/1,2/. For eventswithless favourablemagneticconnectiondifferent opinionsarepublishedin the literature: somearguethat in theseeventsthe particlesare entirely acceleratedby the interplanetaryshock /4/, whileotherauthorsarguefor ‘prompt’ particlesalso despitethe poor magneticconnection/3,5/.

OBSERVATIONS

Numerical Calculations

We first numericallyestimatedthe time-developmentof the proton spectrumin adiffusive SEPeventwithout additionallocalaccelerationby a shock.Theenergyspectraare describedby asinglepowerlaw No(E) E~. In this parameterstudywe startedwith an injectedproton spectrumwith aspectralindex ~yof -2.5, a5-injection andan extendedinjection with 25 h durationat the10% level, and radial meanfree paths )t,, of 0.02, 0.08 and 0.3 AU. Fig. 1 showsthe resultingtime-evolutionof the proton spectrum.In theupperpaneldashedlinesare obtainedwith a simplediffusion model, solid lines areobtainedwith the diffusion-convectionmodel, clearly showingahardeningof the proton spectrumfor later timesasaresult of the convection.Thesecurveswereobtainedwith a 5-injection,the curvesfor anextendedinjection are shownin thelowerpanel. Foragivenmeanfreepath the main differencebetweenanextendedandashort injection is theflatterspectrumfor early timeswhile for late times the spectrumis similar for both injections.

EventsWithout InterplanetaryShocks

Let us now apply theseconsiderationsto the impulsive eventon 19 Dec 1979 (cf., /6,8/). Fig. 2showstheintensity (upperpanel)andanisotropy(middlepanel)of 4-13MeV protonstogetherwiththe time-developmentof the protonspectrumin the range4 to 51 MeV (lower panel)for Helios 1(solid line) and Hellos 2 (dashed). In the insert of the lower panel the configurationis shown,Hellos 1 is magneticallyconnectedto theflare sitewhile Helios 2 is connectedto apositionabout

(9)363

(9)364 J. MeyeretaL

~ ~ 10- ~ I I

— with convection r HEUOS 1-“ without convection r ~2 1 -

-1 1-1-i ~. >‘

.4~ ~ c?- C —2 ;~.. .

x .2io-I \. HEUOS2

-~ ~\ \\, 10 ~ I

~ O-• J -

-4 O3AU .

1 0 L. —

~\, ~‘ ~ injection ~ —L I I~ ~ ~

P~~ 1~ I

- —3-~ ii -

0 5 10 ‘ 25 353.8 354 354.2 354.4 354.6t(hours) DOY 1979

Fig. 1. Time developmentof the proton spec- Fig.2: Intensity,anisotropyandtime-develop-trum for differentinjectionsanddifferent scat-ment of the proton spectrumfor the 19 Dectering meanfree paths. 1979event.

70°westof theflare. The muchfasterrise andthehigher intensityon Hellos 1 canbeunderstoodeasily in terms of this configuration; the long-lastinganisotropytogetherwith the slowly risingintensityon Helios 2 gives evidencefor a long-lastinginjection. The proton spectrumis muchharderon Helios 2 thanon Helios 1, which is alsoin agreementwith the differencesbetweenlongandshort injectionsshownin Fig. 1.

EventsWith Inter~lanetarvShocks

Fig. 3 showsintensities(upperpanel)andanisotropies(lower panel)for4-13 MeV protonsas wellas the spectralindex (lower panel)for the 22 Nov 1977 event. In contrastto the previouseventhereoneshock(Si) is observedon Helios 1, threeshocksareobservedon Hellos2. Bothspacecrafthavea relativelygood magneticconnectionto the flare site, leadingto, apartfrom the absoluteintensity,similar intensityandanisotropyprofilesfor early times;the main differenceresultsfromthe shock-acceleratedparticlessuperposedon Hellos 2 (hatchedarea). The time developmentof the proton spectrumon Hellos 1 is in good agreementwith the predictionsfor an extendedinjection (asis observedin this event /7/). For earlytimesthe spectraon bothspacecraftareingood agreement,as the first shock-acceleratedpartides,leakingawayfrom the approachingshock,arriveat Hellos 2, the spectrumbecomesmuchsteeperwith aminimumaroundthe passageof thesecondshock,andthenhardensagain.

StatisticalAnalysis

For 18 eventsobservedby both Hellos spacecraftwe definedan ‘increasefactor’ of the energeticstorm particles abovethe prompt particlesas the ratio betweenthe maximum of the energetic

Time-Developmentof ProtonSpectra (9)365

1 ~3, S I i~ ~

2 F S2 S2 S1S2 -10- ~

10- :.. :i—~ --.-‘ ~ HELlOS 2

* - .1 .~ - -0_i ‘.

~‘ —1 ~~--~~-.-_-. :.~1O-—2 HELlOS 1

dO-—3

10-—4

10 .:. r1 ~ I I i I i

*0- 1-

-S

o

1 -I ~l I _____________0 - 4 ~--~“~ -

‘, H2.~~- /“S / -

~ —2- ‘\~ .

—.4. — ~ I I l’’’’I’ —

326 327 328 329 330

DOY 1977

Fig. 3. Protonintensity,anisotropy,andspectrumfor the22 Nov 1977 event.

storm particlesand the assumedcontinuationof the prompt intensity profile at that time (cf.,dashedline on the Helios 2 intensity in Fig. 3). Fig. 4 showsthis increasefactor plottedversusthe angle4 betweenthe flare normal andthe sun-spacecraftline. Eachline representsone eventobservedsimultaneouslyby Helios 1 (rhombs),Helios2 (square),andsometimesalsoIMP (circles).The ‘increasefactor’ showsapronouncedeast-westasymmetrywith highestvaluesfor observersfarwestof theflare normal (cf. /9/). Here the magneticconnectionto the flare siteis poor, thereforeonly few promptparticlesareexpected,while theshockis a veryefficient particleaccelerator.Theincreasefactor is lowest eastof the flare normal wherethe magneticconnectionto the flare siteis good and thereforethe prompt particlecomponentis large. The two componentsaloneshowdifferent variations: the prompt componentvariessymmetricalwith respectto the anglebetweenflare andobserver’smagneticfootpoint /6,8/ andthe energeticstormcomponentvaries,at leastwithin aconeof ±60°,symmetricallywith respectto theflare normal /8/. The strongorderingofthe different events(only two eventsdo not fit in themain stream)indicateson underlyingrelationthat is muchstrongerthantheevent-to-eventvariations.Thusthegeometricalconfigurationseemsto havea strongerinfluence on the accelerationefficiencyof the shock,as proposedin /3,9/, thanthe individual propertiesof theshockhave.

CONCLUSIONS

The time developmentof the proton spectrumin eventsaccompaniedby an interplanetaryshockoftenshows atypically hardspectrumearly in the eventwith spectralindicescomparableto theonesobservedin impulsiveflares, while it weakensas the interplanetaryshockapproaches,with

(9)366 J. MeyeretaL

I I I I I I I I I I I_____

P43- -

I-

o 2- -

(5,o I

9-

1i_

0- -

I I I I I

—120 —80 —40 0 40 80 120

angle flare — sun—spacecraftline ~

Fig. 4. Relativeincreaseof the ESPcomponentabovethe SEP componentversusangle

betweenflare normal and sun-spacecraftline.

the steepestspectrumcloseto the time of shockpassage.This changein proton spectrumclearlyindicatesthat for early timestheaccelerationleadsto relativelymorehighenergyparticles,eitherbecausethe particlesare accelerateddirectly in the flare site or while the shock/CMEis stifi inthe densercorona, and that the accelerationbecomes,at least for higher energies,less efficientas the shockpropagatesoutward. The sometimesextremelysteepspectrumcloseto the passageof the shockmight also be due to adecreasedaccelerationefficiencyor dueto the storageof lowenergyprotonsin the turbulencearoundthe shockfront. In eventswith less favourablemagneticconnectionthe hard spectrumearly in the eventsoften is difficult to identify, especiallybecausethe flare spectrumis unknown.However, the useof two or morespaceprobes(cf., Fig. 3) helpstoidentify that part of the intensityprofile that is dominatedby promptparticles. This separationmethodallows the identificationof promptcomponentsevenfor observersabout100°west of theflare normal.

Acknowledgement:We are grateful to all membersof the Universityof Kiel Hellos team(principalinvestigatorH. Kunow). Partof this work wassupportedby the DFG undercontractWi-259/8-1.

REFERENCES

/1/ D.V. Reames,H.V. Cane,andT.T. von Rosenvinge,Astrophys.J. 373, 259 (1990)/2/ D.V. Reames,Astrophys.J. Leit. 358, L63 (1990)/3/ H.V. Cane,D.V. Reames,andT.T. von Rosenvinge,J. Geophys.Res.93, 9555 (1988)/4/ D.V. Reames,preprint, Cosmic Plasma Workshop,Barthol (1991)/5/ M.-B. Kallenrode,this issue/6/ M.-B. Kallenrode,submittedto J. Geophys.Res.(1992)/7/ J. Beeck, G.M. Mason, D.C. Hamilton, G. Wibberenz,H. Kunow, D. Hovestadt,and B.Klecker, Astrophys.J. 322, 1052 (1987)/8/ J. Meyer,G. Wibberenz,andM.-B. Kallenrode,in preparation(1992)/9/ E.T. Sarris. R.B. Decker,and S.M. Krimigis, J. Geophys.Res.90, 3961 (1985)