Adv. SpaceRes.Vol. 13,No. 6,pp. (6)95—(6)98,1993 0273—I177/93$24.00Printedin GreatBritein. All rights reserved. Copyright ~ 1993COSPAR

COSPINLET OBSERVATIONSOFRECURRENTENERGETICPARTICLEEVENTSDURING THE IN-ECLIPTIC PHASEOFTHE ULYSSESMISSION

R. G. Marsden,*T. R. Sanderson,*K.-P. Wenzel*andS. J. Bame**

* SpaceScienceDepartmentofESA,ESTEC,P.O.Box299,2200 AGNoordwijk, TheNetherlands** LosAlamosNationalLaboratory,LosAlamos,NM87545, U.S.A.

ABSTRACT

TheUlyssesspacecraft,launchedin October1990, traversedthe in-ecliptic regionof theheliospherefrom 1 to 5 AU duringthepost-maximumphaseof solarcycle 22 prior to its encounterwith Jupiterin February1992. In theperiodfromlaunchup to mid-1991,particleeventsweredominatedby theeffectsof large-scaletransientstructuresin the solarwind. Thereafter,a more regularpatternofrecurrenteventsrelatedto CorotatingInteractionRegions(CIR) evolved. We presentanoverviewof theseCIR-relatedeventsas recordedby the Low EnergyTelescope(LET), part of the COSPINexperimenton boardUlysses,in the energyrange 1 to 20 MeV/n, with particular emphasisonthe compositionof the particlepopulations.

INTRODUCTION

Thein-ecliptic part of theUlyssesmissioncoincidedwith thepost-maximumphaseof solarcycle22,in contrastto previousexplorationsof this regionof theheliosphereby Pioneer10/11 andVoyager1/2, which took placeat pre-minimumandpre-maximumsolar cycle conditions,respectively.TheUlyssestransit wasmarkedby asignificant changein the level of solar activity. For the first halfof the 16-month journey from 1 AU out to the orbit of Jupiter, the interplanetarymediumwasdisturbed asaresult of frequenttransienteventsrelatedto major flaring activity on the Sun. Ofparticular note are the seriesof flares which occurredin Marchand June 1991. From mid-Juneonwards,however,solar activity tendedto decrease.Pioneer10/11 encountereda well-establishedsystemof Corotating InteractionRegions(CIR) with associatedforward andreverseshockpairsin this region of theheliosphere/1/, whereasthe equivalentVoyagerdataweremainly dominatedby transientevents/2/.

Observationsby the Ulyssesinstrumentsof the March andJune1991 transienteventshavebeenreportedelsewhere(e.g. /3,4,5/). In this paper,we concentrateon therecurrentenergeticparticleincreasesthat wereobservedfrom mid-1991 onwards.Previousstudiesof flux increasesassociatedwith CIRs haverevealedcharacteristicsignaturesin the energy spectraand compositionof theparticles/2/. In particular,valuesof theproton/alpharatio in excessof 100 havebeenobservedattheforwardshock,decreasingto 20 at thereverseshock/6/. Studiesof theheavyion compositionin CIR-relatedeventsat I AU haverevealedlargevaluesof the He/O ratio (“~200) comparedwithnormal solar flare values of -~ 65 /7,8/. We report here measurementsof the compositionofenergeticparticlesin CIR eventsbeyond 3.5 AU using datafrom Ulyssesandcomparethesewithpreviousfindings.

INSTRUMENTATION AND ANALYSIS

Theenergeticparticleobservationsreportedhereweremadewith theLow EnergyTelescope(LET)instrument carriedon boardUlyssesas part of the COSPIN experimentpackage/9/. The LETsensoris a four-element,solid-statedetectortelescopeoperatingin the single- anddouble dE/dXvs. E mode,allowing the measurementof protons,alphaparticles andheavyelementsup to and

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(6)96 R. G. Marsdener a!.

including iron to be madewith good elementalresolution using pulseheight analysis. For thisstudy,we havecalculatedthe abundanceof He andC with respectto 0 in theenergyrange4.25—6.75 MeV/nucleon. Furtherdetailsconcerningtheinstrumentcan befoundin /9/ andadiscussionof the analysistechniqueis given in /10/.

OBSERVATIONS

In Figure 1 we show an overviewof the proton intensity in two energychannelsmeasuredby theLET for 1991 andthefirst halfof 1992. Theintensity increasesassociatedwith theflaring activityin March are evident. Beginningwith the peak centredaroundday 160, which is superimposedon an elevatedbackgroundpopulationpresumablyhaving its origin in the major flares of earlyJune,a seriesof recurrentenhancementsseparatedby approximately27 days canbe seen. Theseenhancementsshowno velocity dispersionin the energyrangecoveredby the LET (-‘.~1—20 MeVfor protons), and are therefore not consideredto be associatedwith injections from solar flareevents.

, J1sse8 —I I I I ICOSPINILET 27 d 10

~ ~

2 , 3 4 , 5AU 9-19MeV

31 01 91 121 151 181 211 241 271 301 331 I 31 01 91 121 151 181 211

Day of Year 1991 Day of Year 1992

Fig. 1 Protonintensity measuredby Ulyssesat 0.9—1.2MeV (uppertrace)and9.0—19MeV(lower trace) between1 Jan1991 and26 Aug 1992. 27-day tick-marksareincludedto guidethe eye. The peaksaroundday 39, 1992 areassociatedwith Jupiter flyby.

Examplesof two suchrecurrenteventsarepresentedin Figures 2a and2b. Figure 2a shows the27-day period starting on day 231 (19 August), 1991. Characteristicfeaturesof this event arethe lack of velocity dispersionat onsetandthe near-exponentialdecayover a periodof 10 days.Vertical lines mark the times of occurrenceof forward (F) andreverse(R) shocksas identified inthedatafrom theVHM/FGM magnetometer/11/ andSWOOPSsolarwind experimentsonboardUlysses. Thesedataareplotted in thesecondandthird panelsof Figure 2a, respectively.As canheseen,thereare apparentlytwo CIR structurespresent,thefirst of whichhasaclearF-R shockpairassociatedwith it, the secondbeinglesswell-developed.Thepassageof this latter streamstructurepreventsparticlesthat wereacceleratedat the R-shockof the first CIR andhavepropagatedintotherarefactionregionfrom reachingthespacecraft,andterminatesthesmoothdecayphase.In thebottompanelof Figure 2a, we plot the proton-to-alpha(p/a) ratio at equal energyper nucleon.The characteristicsignature,first reportedby Barnesand Simpson/6/, showinga valuefor p/ain excessof 150 at the forward shock droppingto -.~15—20 in the intershockregion, is very clearin this example.

The secondexampleof a CIR-related event shownin Figure 2b occurred5 solar rotations afterthatof Figure 2a. Plottedis the 27-dayperiodstarting on day 1 of 1992. This eventshowsmanysimilarities with the first example,including the smoothdecayproffle. Thep/a ratio dropsby afactor 3 at the discontinuity separatingtheforward andreverseshocks. Particularly evident inthe 9.0—19MeV proffle of Figure 2b is the differencein accelerationefficiency betweenthe F- and

UlyssesObservationsof EnergeticParticles (6)97

0’ - COSPIN LET P,otone~ 0.9- 1.2Mev - ~‘ COSPIN LET P,0t0fl,~0.9- 1,2 Mod -R 1 8 -3.8 Med - 1.8-3.9MeV

100 - F ~ .._ 9-19 MoV . ~‘ A 9-19 M0V -S ~ - F A? . is’

— Megoelie held MOgoetic hold

800 ‘ / ~ 0 0~Wet 600 Sole, w,od

~305 ~°°

231 234 237 240 243 246 249 252 255 258 1 4 7 10 13 16 19 22 25 28DeS oh Yea,, 1991 D~y01 Year. 1992

Fig. 2 Two examplesof recurrent energeticparticle events at Ulysses,showingprotonintensityat 0.9—1.2, 1.8—3.8, 9.0—19 MeV (top panels),magneticfield magnitude/11/ andsolarwind velocity (2nd and3rd panels),andp/a ratio at 1.8—3.8MeV/n (lower panels)forthe periods (a) 19 Aug—14Sep 1991 and (b) 1—27 Jan 1992.

R-shocks, the latter contributing the majority of the particles. Discontinuitiesin the intershockregion of the streamstructure clearly control the ability of particles acceleratedat the R-shockto reach Ulysses. (This samefeatureis presentin the CIR eventshownin Figure 2a, althoughthe more compressednature of the interaction region makesit less clear). There is perhapsasuggestionin the low-energyintensity profile andp/a ratio of a secondstructure(CIR?) ‘—. half asolarrotation after the first, but the plasmaandmagneticfield signaturesare weak.

Throughoutthe periodfrom mid-1991 up to February1992, the streampatternremainedratherstable, with one dominant CIR and a second,more variablestructure occuring10—12 dayslater.The particle increasesassociatedwith these CIRs showedconsiderablevariability, however,pre-sumably asa resultof changesin shockstrengthfrom onerotation to the next, andthe presenceof discontinuities.

RESULTS AND DISCUSSION

In orderto extendthe comparisonbetweenrecurrenteventsobservedat Ulyssesandthoseidentifiedpreviouslyon Pioneer10/11 andVoyager1/2, we haveexaminedtheheavy ion compositionusingthe LET pulse-heightdata. Specifically, for thoseeventsfor which sufficient countswereavailable,we havecomputed the He/O and C/O ratios. The results are given in Table 1, togetherwiththe p/a ratio measuredin the peak associatedwith the R-shock in eachcase. In general,theUlyssesvaluesare consistentwith the resultsof Hamilton et al. /2/ obtainedfrom Voyager 1/2dataat slightly lower energy(-.~1 MeV/n), andlikewiseshowsignificant differencescomparedwithabundancesmeasuredin corotatingeventsat 1 AU /7,8/. The UlyssesC/0 ratiosdo not show thefactor 2 enhancementcomparedwith solarflare particlesthat is seenat 1 AU. The He/0 ratiosappearto fall into two classes.Four of the eventsfor which the ratio could be determinedhavevalues 70, consistentwith a solar wind and/or flare particle origin /2,8/. The secondgroupoffour eventsshows enhancedHe/O ratios closerto the 1 AU valuesof McGuire et al./8/, thoughnot ashigh as thoseof Scholer et al./7/. As notedearlier thep/a ratios in the R-shockpeaks, -althoughshowingsomevariability, are typically -~20—30, being closer to solar wind abundances

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(6)98 R. G.MarsdenetaL

TABLE 1 AbundancesMeasuredin UlyssesCIR Events (4.25 — 6.75MeV/n)

Period He/O C/O p/a

91.157—163 69±2 0.45±0.02 20.5+0.191.169—170 75±8 0.60±0.10 37.0±1.0

91.185—189 69+2 0.56±0.04 17.4±0.191.199—201 — — 26.0±1.0

91.213—216 123±40 0.40±0.20 74.0+1.091.238—241 67±4 0.42±0.05 21.2±0.191.275—277 147±21 0.40±0.10 45.5±1.091.320—321 122±28 0.60±0.20 21.8±0.192.008—011 197±26 0.40±0.10 38.4±0.2

than solarflare particles(p/a -‘~ 40). In general,then, the Ulyssesdataare consistentwith asolarwind sourcefor the majority of the particles observed. The enhancementin the He/O ratio atequalenergy per nucleon seenin severaleventsis possibly due to a difference in spectralslope,the 0 spectrumbeingsteeperin thesecases.Unfortunately, theverysmallPHA samplescollectedin many of the recurrenteventsprevents a test of this using LET data. Further study, involvingUlyssesmeasurementsat lower energies,is planned.

ACKNOWLEDGEMENTS

We wish to thankA. Baloghfor the useof UlyssesVHM/FGM magneticfield data,andtherefereefor usefulcomments.

REFERENCES

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3. K.-P. WenzelandE.J. Smith, Geophys.Res. Lett. 19, 1235 (1992)

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