Adv.SpaceRes.Vol. 13,No.6, pp. (6)63—(6)74, 1993 0273—1177/93$24.00Printedin GreatBritain. All rights reserved. Copynght© 1993 COSPAR

SOLAR WIND COMPOSITIONANDEXPECTATIONSFORHIGH SOLARLATITUDES

R. von SteigerandJ. Geiss

PhysikalischesInstitut, UniversityofBern, Sidlerstrasse5, CH-3012Bern,Switzerland

ABSTRACT

Instrumentson boardtheUlyssesspacecraftareto investigate,for thefirst time, thesolarwind in thethreeprincipal domainsof solar latitude. (1) During Ulysses’ flight in the ecliptic planethe solarwind nearthe current sheetwascontinuouslymeasured,and its compositionwas determinedwith unprecedentedsensitivity and massresolutionby SWICS,theSolarWind Ion CompositionSpectrometer.(2) Recently,ithasbeenarguedthat EUV and flare particleobservationsindicatestrongabundanceanomaliesin eruptingprominencesand impulsiveflares. WhenUlysseswill crossthe latitudeswheresucheventsmostly occur,we expectto providecomplementarysolarwind compositiondata, aimingto identify the processesthatcausetheseanomalies. (3) Whenreachingthe high solar latitudes,Ulysseswill probe the solarwindemanatingfrom the largepolar coronalholes. There,we expect to usecompositionmeasurementstosludy thedifferencesbetweenhigh and low solar latitudesas regardsthe chromosphericstructureandthecoronalandinterplanetaryexpansion.

INTRODUCTION

Ulysses/1/ was launched in October 1990 and injected into a fast, Hohmann-typetransferorbit toJupiter. In February1992, the spacecraftsuccessfullyflew by Jupiterat a minimum distanceof 6Rj(unharmedby the intenseradiation belts), and was diverted into a new orbit with a high heliographicinclinationof 80.22degrees.Thus,Ulyssesis the first spacecraftto reachheliographiclatitudesin excessof ‘-.~ 30 degrees,and to probe in situ a largenumberof propertiesof the heliosphereat thesehighlatitudes.The Ulyssesdatawill be valuablenotonly to comparewith measurementsfrom otherspacecraftor othermethods(see next section), but also for solarwind theory: Models of the solarwind accountingforlatitudinal effects, such as those by Pizzo /2, 3, 4/ can be tested againstdirect observations. Also, itis hopedthat the availability of Ulysses’ datawill trigger new efforts in three-dimensionalsolarwindmodelling.Up to this time, out-of-eclipticmeasurementshavebeennotoriouslydifficult: Onthe one hand,spacecrafthavebeenconfinedto low latitudes. Only Voyager-i has reachedabout 30 degrees,and then only inthe outerheliospherebeyondthe orbit of Jupiter. On the otherhand,remotesensingtechniques,likeobservationof cometson highly inclined orbits, and IPS (InterplanetaryScintillation) measurements,which can be madeat all latitudes,are limited to somegeneralsolar wind properties. Still, as will beoutlinedbelow, it is remarkablewhat we havelearnedfrom both methods,andespeciallythat the twoyield a consistentpictureaboutthe high-latitudesolarwind.In this paper,we concentratemainlyon the compositionof the solarwind plasma,especiallyof its minorconstituentsheavierthanhelium. It is building on and extendinganearlierpaper/5/, which waspublishedbeforethe launchof Ulysses,then expectedin 1986.The SWICS (Solar Wind Ion CompositionSpectrometer)instrumenton boardUlyssesis a novel type ofmassspectrometer,which is ideally suited to this task. The sensoris a combinationof an electrostaticdeflector, a post-accelerationgap with voltagesUa of up to 30 kV (neededbecausethe solarwind ions

(6)63

(6)64 R. von SteigerandJ. Geiss

90

0

5 ..... ,..---

o ... ...

~ ~90.~i.~Er~iii1I1970 1960 1990 2000

Year

Fig. 1. Orbits of out-of-ecliptic spacecraftlaunchedto date: Pioneer-il reacheda heli-ographiclatitude of 15 degreesnorth in 1976 and again since the mid-eighties, andVoyager-i is at 30 degreesnorth sincethen. Ulyssesis now at 15 degreessouthandwill reach80 degreessouthaftermid-1994and 80 degreesnorth one yearlater.

haveenergiesmuch below the thresholdof the SSD),a time-of-flight telescope,anda solid stateenergydetector(SSD). Therebyenergy per charge E/q, massperchargem/q, and total energy E + Ea aremeasuredforeachincident ion individually, from which both massand chargecanbecalculated.Duetolimited telemetry,only a small, selectedsampleof all ion eventscanbe transmitteddirectly; the othersarebundledinto variousrates,which containthe full statistics,butat thecost of resolutionin time, E/q,m, m/q, or a combinationof these.The SWICS data are unique in three respects:(1) The mass separationjust mentioned,(2) the lowbackgrounddueto the triple coincidencetechniqueusedfor the two time-of-flight signalsand theenergysignal, and (3) the post-acceleration,which guaranteesthat solarwind of widely differing propertiesis measuredundervery similar, well-known conditionsinside the instrument. (For further details onSWICS, see/6/.)

PRE-ULYSSESOUT-OF-ECLIPTICMEASUREMENTS

Thissectionisnota comprehensivereview,but only a verybrief summaryof afew selectedout-of-eclipticmeasurementsthat havebeenperformedin the past.BeforeUlysses,two spacecrafthavereachedheliographiclatitudesclearly in excessof the±7.25degreescoveredby theEarth’s orbit: Pioneer-il (since1975, today at 17 degreesnorth) andVoyager-I (since1981, todayat 32 degreesnorth)(seeFigure 1). Neitherspacecraftcarriesa compositionexperimentofthe SWICStype,butboth are equippedwith a plasmaexperiment,from which information on thesolarwind dynamicscanbe gained. It hasbeenshown /7/, by comparisonof Pioneer-il datawith in-eclipticdataof Voyager-2,that the solarwind speedwassignificantly fasterat Pioneer-il aftermid-1985,whenthe separationin latitude was> 15 degrees. However, from thesemeasurementsaloneit is not easytodecidewhetherPioneer-li hadcrosseda latitudinal velocity gradient,or rathera solarcycle effect wasseen. The authorsargueconvincinglyfor the latter explanation,as (1) thevelocity gradientwould havehad to be extremelysteep,and(2) the changewasmainlydue to a decreasein velocity nearthe eclipticplane,observedby Voyager-2,not to an increaseat high latitude.This observationfits well with the picture obtainedfrom IPS measurements.Thesedata haveshownthat the solarwind speedhas a V-shapedprofile as a function of heliographiclatitude, i.e. the speedisminimum nearthe solarequatorand increasestowardshigher latitudes/8, 9/. Furthermore,the shapeof this profile is modulatedas a function of phasein the solar cycle: The latitudinalvelocity gradientis strongnearsolarminimum andweak or almost absentnearsolarmaximum. Usually, it is strongestbetween15 and 30 degrees.The observationthata gradientappearedbetweenPioneer-IlandVoyager-2only aftermid-1985,just beforethe last solarminimum, is thusbestexplainedby a solarcycle effect.Coronalholes are the sourcesof high-speedsolarwind /10, 11, 12/, andthe redistributionof them over

SolarWind at High Latitudes (6)65

the solar surfaceduring the solar cycle canprovide an understandingof theseobservations.At solarminimum,the coronalholes are largest,and the magneticdipole is well alignedwith the rotation axis.Thereforethe solarwind is mainly of the slow type nearthe ecliptic planeduring this part of the solarcycle. On the otherhand,aroundsolarmaximum,the polar coronalholeshavedecreasedin sizeor evenvanished.Finally, in the declining activity phase,themagneticaxis is tilted relativeto the rotation axis,and the fast streamsfrom the coronalholes aroundthe magneticpoles canreachthe ecliptic plane, orthe holes candeveloptonguesor isolated regionsreachingtowardsthe equatoror eveninto the otherhemisphere/13, 9/. Due to theseequatorialcoronalholes,the averagenear-eclipticsolarwind speedisincreased.Of course,this picture is complicatedby the existenceof transient-typesolarwind, causedbysolar flares,disappearingfilaments,coronalmassejectionsetc.

ULYSSES IN-ECLIPTIC MEASUREMENTS

While on its way to Jupiter,Ulysseshas gatheredmore than oneyear’sworth of in-ecliptic data. Thecompositionresultsobtainedby SWICSare unique in many ways, as it is the first time an instrumentofthis kind is successfullyflown in the solarwind.* In Figure 2 wesummarizesomeof the Ulysses/SWICSdataobtainednearthe ecliptic plane.The daily solarwind He~velocity averagesaregiven from switch-on in late 1990 to day 189, 1992. They arecomparedto someof the compositiondata,namelythedailyaveragesof the C/O and Mg/O density ratios. (We analyzeonly daily averageshere becauseone dayis long enoughto accumulatesatisfactorystatistics,and it is comparablein duration with typical solarwind characteristics.)Notethat SWICS is the first instrumentto measureboth C andMg, thanksto theseparationin both massand charge,as C~would normallybemaskedby themuch moreabundantHe~,andMg10~by C5~,in a m/q spectrum. in the velocity plot, we observethat (1) the averagevelocityis increasingwith time (i.e. heliosphericdistance),and (2) its variability is decreasingwith time. Thiscanbe understoodin termsof the corotatinginteractionregions(CIRs) growing togetherat increasingheliosphericdistances,and“eating up” the slow, intcrstream-typesolarwind /18, 19/, thus leadingto aless structured,on averagefastersolarwind.When comparingthe He~velocity va to the compositiondata in Figure 2, we observeno obviouscorrelationbetweenthe two parameters.Sometimes,high-speedsolarwind is accompaniedby enhancedMgJO and,to a lesserdegree,C/O ratios,but on otheroccasionsit is not. Thetwo mostprominentcaseswith the highestv,

5 are comparedin Figure 3.(1) Thelarge flare eventson days 77—84, 1991 /20/, causeda velocity increaseto > 700 km/s,andtheireffects were seenat Ulysses(at 2.5 AU) a few days later. Theyoccurredwithin 25 degreesof thegeometricalfootpoint; hencethe solarwind could havecontainedmaterial from the flare site. SWICSobserveda strongly varying, mostly increasedMg/O ratio, which reachedthree times the solarwindaveragevalueon day 89. Note, however,that thereis also a very low Mg/O value,only barelyhigherthan the photospliericvalue,on day 85.(2) On the other hand,the increasedvelocity after day 325, 1991,wasmost likely dueto an equatorialcoronalhole. Coronalholeassociatedfast streamscanbe reliably identified from kinetic properties;highspeed,low density, and elevatedkinetic ion temperature(cf. /21/). Additionally, we usehere the mostdirect indicatorof the coronal temperatureas a parameter,which can be calculatedfrom chargestateratios of minor ions, suchas C

6~/C5~or 07+/06+ (see below). Unlike in the flare event, we observeherea significantly depressedMgIO ratio for severaldays. Note,however,that this is still almostin themaximumactivity phaseof the solar cycle, when coronalholes are particularly difficult to identify, astheyare small, and frequently canbedisturbedby transientevents.Thus the coronalhole in Figure 3 isnot a particularly clearexample,but it was the bestone found in the analyzedtime period. It will notbe until Ulyssessamplesthe largepolar coronalholes that definitive statementsaboutthe compositionof thesolarwind emanatingfrom therecanbe made.Forthe whole surveyperiodof 500 days,wehavecalculateddaily averagesof the oxygenfreezing-intemperatureT

076. It canbe foundfrom theobservedchargestateratio 07+/06+,usingthe tablesof AmaudandRothenflug/22/. This temperatureis indicative for thepoint wherethe solarwind expansiontimescalebecomesfasterthan theionization/recombinationtime scales,which is nearthecoronaltemperature

BeforeUlysses/SWICS, a similar instrument,AMPTE/CCE/CHEM,was flown in Earth’smagnetosphereandon rareocca-sionsit measuredshockedsolar wind plasmain the magnetosheath,see /14, 15/.

JAIR 13:6-F

(6)66 R. von 5teigerand3. Geiss

Heliocentric Distance (AUJ8001 1.5 2 2.5 3 3.5 4 4.5 5 5.4 5.3

CS/Ulysses

200 I I I I I I I I I’ .

1.0

• ~ . • • •~~::~. •~

1991.0 1991.5 1992.0 1992.5Year

Fig. 2. Upperpanel: Daily averagesof the solarwind He~velocity measuredat Ulyssesas a function of time and heliocentricdistance(indicatedon the top axis). Lower panel:C/0, andMg/O solarwind abundanceratios. Solid line: Long-termaveragesof the periodshown. Dashedline: Solarratios /16/. The MgIO ratio is clearly enrichedrelative to thesolarvalue, while there is only a slight enrichmentin C/O. (The datagap in early 1992 isdue to the Jupiterencounter,see/17/.)

maximum. Thechargestateratios are known to normally remainunalteredduring solarwind expansion/23, 24/; To76 thusservesas a coronal thermometer.In Figure 4, the Mg/O abundanceratio is correlatedto Tom. Obviously, the correlation is quite good(r = 0.74),while thereis no correlationof Mg/O to va (r = 0.05). A correlationof Mg/O and,to a lesserdegree,C/O to the freezing-intemperaturehas beenfound earlier in the shockedmagnetosheathplasmastemmingfrom high-pressuresolarwind /15/, andin the free solarwind /25/. A similar resulthasbeenfoundby Galvin et a!. /26/ for Si/O asa function of T076.The reasonfor this behaviouris thoughtto lie in differencesof the FIP (First Ionization Potential)fractionationeffect in different solar wind types. Both Si and Mg are low-HP elements,which areobservedto be enrichedby a factor of 3 to 5 in the averagein-ecliptic solarwind /27/, as well as inthesolarenergeticparticle (SEP)population/28/. This enrichmentfactor appearsto be muchweakerinthefast solar wind from coronalholes /29/. So far, the evidenceis confinedto the equatorialcoronalholes,which are the only onesit hasyet beenpossibleto observe. (Seebelow for a discussionof theHP effect).Apparently,the fractionationeffect is correlatedmuchmore to thecoronaltemperaturethanto the solarwind speed(andthusto theacceleration).This is remarkable,as theFIP fractionationnecessarilyoperatesin a regionwherea substantialfraction of thegasis neutral,i.e. in thechromosphere.On theotherhand,the chargestateratios are frozen-in nearthe coronal temperaturemaximum, two ordersof magnitudehotter than the chromospherebelow. Consequently,the structureof the chromosphere(andprobablythetransition zone)mustbe significantly different beneathcoronalholesthan in otherparts of the solaratmosphere,but it is not clearhow the two regionsare causallyrelated. Differencesin transitionzonethicknesshavebeenobserveddirectly on Skylab/10/, cf. /5/.

SolarWindatHigh Latitudes (6)67

1000 1000

SWCSIUIySses]

eo~ ~ . ~ 600:11 600 ~4d”~’~~J ~‘ 800 ~ •—....--__. -—

~ I~IlIIfi1E’a I I I

IE*7 1E*7

IE+8 ~If\I ~ , ~I~ ikl~ 106

IE~$ ~ ~1’~_ I is.s 1I, ~ I I~~ U

H _____

~81 88 81 323 328 333

Dayof 1991 Day of 1991

Fig. 3. Blow-upsfrom Figure 2 with full time resolution(13 minutes).Left: FlareeventofMarch 1991. As a consequenceof several largeflares,thesolarwind velocity increasedtoextremelyhigh valuesin severalsteps.The Mg/O ratio was stronglyvarying, reachingup toan anomalouslyhigh valueof threetimesthe averagesolarwind value, i.e. almostanorderof magnitudeabovethe photosphericvalue. Right: High-speedstreamfrom an equatorialcoronalhole; the velocity increaseis accompaniedby an enhancedkinetic temperatureanda low freeze-intemperature. Unlike in the flare event, the Mg/O ratio is depressedbelowthe solarwind averagefor severaldays.

We have checkedfor the possibility of a delayedcausalrelationshipby correlating the two data sets(Mg/O andT

076) with a time shift of ±1day. In both cases,the correlationcoefficient (seeFigure 4)droppeddrastically, and with a time lag of ±2days, no correlation was discernibleany more. Wethereforeconcludethat thedepressedHP fractionationof Mg/O is not a result of the coolertemperaturein coronalholes (this would bedifficult to understandindeed),butof thechromospherebeingstructureddifferently beneathcoronalholes than elsewhereon the solar surface.

ULYSSES OUT-OF-ECLIPTICEXPECTATIONS

Ulysseswasdivertedby Jupiteron February8, 1992, into anorbit out of the ecliptic plane. Meanwhile(Aug 1992),the spacecrafthasreacheda heliographiclatitudeof 15 degreessouth,and this is presentlyincreasingat a rateof 0.05 degreesperday. Onecrucial pointwhendiscussingour expectationsforhighlatitudesis thephasein thesolarcycle whenUlysseswill be there. Cycle 22, afteranexceptionallysharpincrease,reachedits maximumin mid-1989,anda secondarymaximumaftermid-1990. Solaractivity isnow beginningto decrease,as the(unsmoothed)sunspotnumberdroppedto Rz < 100 for the first timein April 1992. Ulyssesis thusjust beginningto crossthe latitudezonewheremostof the activeregionsare located. With decreasingactivity, the active regionsoccur closerto the equator, while Ulyssesismoving towardshigher latitudes. In this phaseof the cycle, the lower borderof the maximum activitybelt hasalreadyreachedthe low latitudes,and the poleward border is is expectedat 25 degrees,azoneUlysseswill crossbetweennow and the first few monthsof 1993. Thetwo polarpassesin summer

(6)68 R. von Steigerand3. Geiss

0.4

SWCS/Ulysses~

03

0 ,•~,• ~.‘~ 0.2 9 8

,,.L&~’.a~

? ~ -,

01

8%. •~,

— — •,, .~ .

Go I1.0 20 3,0

1076(MK]

Fig. 4. Correlogram of the Mg/O solarwind abundanceratio to the coronatemperatureT076 in its sourceregion. While Mg/O and the He~velocity V(~appearto be uncorrelated(r = 0.05, not shown),thereis a clearcorrelationof Mg/O to To76 (r = 0.74). Eachdatapoint correspondsto a daily average,and a typical errorellipse is indicated in the upperleftcorner.

1994 (south) andsummer1995 (north) will then occurtowardsthe endof cycle 22, whensolaractivityapproachesminimum and whenthe polarcoronalholes approachmaximumsize andare centeredat thepoles.As expected,the post-Jupiterdatain Figure 2 (after day 47, 1992), show no clear differencefrom thein-eclipticsolarwind dataseenjust beforeJupiter.* Also, in the nearfuture we do not expecta drasticchangein the solar wind parametersassociatedwith latitude. At the intermediate,active latitudesweare now crossing,we will encounterflare-associatedtransientsmuch like in the ecliptic plane, as thesetransientsare thoughtto spreadout overa full 2ir hemispherecenteredat the flare site /30/. However,whenobservingdirectly from abovethe flare site, Ulyssesis much more likely to encountermaterialfrom there,while at larger anglesonly a shock in plasmapossiblyunrelatedin compositionto the flaremaybe observed.This will enableus to follow up on the hint at stronganomaliesin flare associatedtransientsfound by von Steigerer a!. /15/.Later in the mission,whenUlysseswill be at high latitudes,thereexiststhe possibilityof making opticalobservationsof the sourceregion in the coronadirectly at the solar limb (particularly whenthe angleUlysses-Sun-Earthis near90degrees),andto associatethe solarwind observedat Ulysseswith its sourceregionobservedfrom Earth. This techniquewasalreadysuccessfullyusedin the ecliptic planewith theHELlOS andICE (formerly ISEE-3)spacecraft.Unfortunately,this constellationwill happenonly whenUlyssesis at very high latitudes, and whensolar activity has droppedto almostminimum, making itexceedinglyunlikely to observea transientboth at the solarlimb and at Ulysses. On the otherhand,asUlysseswill crossthelargepolar coronalholes then,it will be extremelyinterestingto combineUlysses’measurementswithEarth-basedcoronagraphimages.Specifically, theexactboundariesandthe structuresof the coronalholesobservedfrom Earthmay be readily relatedto the solarwind measurementstakenat Ulysses.After crossingthe belt of activity, Ulysseswill enter the region of the polar coronal hole. This willhappenin the declining phaseof the activity cycle, whenthe holes are not centeredat the pole,butaround±50degreesheliolatitude. Thus,we expectUlyssesto crossa polar hole oncepersolarrotation,at first only for a short period, but the durationof the passeswill increaseas both Ulyssesand theholewandertowardsthe pole. Finally, judging from the observationsof previoussolarcycles,Ulysseswill

This alsodemonstratesthat SWICS wasunaffectedby Jupiter’sintenseradiationbelts.

SolarWind atHigh Latitudes (6)69

becontinuouslyexposedto fast solarwind for at leasta few monthsin summer1994 andagainin 1995.Ironically, this mayprovidean opportunityto learnabout the origin of theslow solarwind, which still isverymuch a matterof contention.Possiblesourceregionsare (1) the borderregionsof thecoronalholes,or (2) thehelmetstreamersby somecontinuousdisconnection,or (3) plasmoids.Thethird possibilityhasalreadybeenrenderedquestionableas a substantialsourceby Bochsler/31/, as theseplasmoidsshouldcontainmany more low-chargedions(suchasHe~,O~,etc.) thanwe observewith SWICS /32/. Still,suchplasmoidsmay occuroccasionally.WhenUlysseswill scanslowly acrossthe polarcoronalhole in1994and againin 1995,analysisof solarwind bulk propertiesandof minorion abundancesas a functionof latitude will revealhow thesolarwind type changesbetweentheslow, quiet type andthefast,coronalhole type. From this turnoverwe expectto learnmore aboutthe sourceregionsof thequiet solarwind.So far, we could studythe compositionof coronalhole solarwind only in fast streamsoriginatingfromequatorialextensionsof coronalholes,which were typically crossedin a few daysonly. The turnoverbetweenthe solarwind types wastoo abruptthere,andpossiblyevendifferent from the turnoverwhenenteringthe polarcoronalhole.

DISCUSSION

From the Ulysses/SWICSdata, we expect to learnmore about importantopenquestionsin solarwindresearch.Among theseare: (1) What is the origin of theHPfractionationeffect? (2) Why do theminorions have the samebulk speedandkinetic temperaturesproportionalto their masses?(3) How can weunderstandthe coronalexpansion,particularly in thesimpler geometryof the coronalholes?The minor ions are testparticlesfor expansionmodels/24/, as their abundancesand chargestatesaregood tracersof chromosphericandcoronalprocesses.Theyput very stringentlimits on the constructionof sucha model, so the SWICS datawill be valuableto discriminatebetweendifferent models.

HP Effect

TheFtP fractionationeffect is responsiblefor the observedenrichmentof low-HP (~10 eV) elementswith respectto high-F1P(~10 eV) elements/33, 34, 35, 28/ in the (predominantlyslow) solarwind andin the SEP.Helium is furtherdepletedby a factorof 0.5 relativeto the high-HPgroup (C to Ne)/36/.This is probablynot only the resultof theHP effectalone, but also relatedto the accelerationof He inthe corona. It may evenbe partly due to gravitationalsettlingof He in theouterconvectivezone/37/.The HP effect for the relevant solar wind elementsis illustrated in Figure 5. This effect appearstobe considerablyweakerin coronalhole associatedfast solarwind streams129, 15/ (see Figure 3), butof courseonly near-equatorialcoronalholescould be analyzedso far. Scanningoverthe polarcoronalholeswill revealpossiblecenter-to-bordervariations,and give us a largebaselinewith which to comparethe complicatedin-ecliptic solarwind. Particularinterestwill go to the elementsnearthe stepbetweenlow- andhigh-HPelementsat ~ 10 eV, C and S. whichwe think are crucial to theunderstandingof theVIP effect(seeFigure 5). Ulysses/SWICSis the first solarwind instrumentbeingableto unambiguouslydetect the main chargestatesof theseelements. Ultimately, this new understandingshould leadtothe origin of the HP effect, for which thereare now a numberof concurringexplanationsandmodels/39,40, 41, 42,43/. Among mostof theseauthors,thereis consensusthat the FIP effectoperatesin theupperchromosphcre,but Lemaire/43/ has put forwardthe view that it is due to a contaminationof thecoronawith asteroidalandcometarymatter. Therefore,thehigh-latitudedataof the dustexperimentonUlysses/44/ will also be importantin testingthis hypothesis,eventhoughthe demonstratedabsenceoflow-chargedions /32/alreadymakesit an unlikely possibility.Recently, the understandingof the HP effect hasbeenfurther complicatedby Mg/Ne abundanceratioobservationsin the coronausingEUV /45, 46/, which provedto be extremelyvariable andsensitivetothe magneticfield morphologyin the regionwhereit wasobserved.While a photosphericMg/Ne ratiowas observedin impulsive flares, i.e. compactfield regions,the sameratio was evenabovethe coronalandsolar wind value in polar plumesand diverging fields. This contradictsour finding that the FTPenrichmentis lower in coronalhole solarwind, if we takethe Mg/Ne ratio as an indicatorof FtPstepheight, just like the Mg/O ratio in Figure 4, sinceboth 0 and Ne are high-HPelements.Also, a strongvariability in the solarwind Ne abundanceis unlikely, as we havevery accuratemeasurementsof the

(6)70 R. von Steigerand3. Geiss

I Sotar WIndSolar EnergeticParticles

- — I Coronal Holes

~

Mg H

Si SC 0 Ar N. He

• I ~

5 10 15 20 25First Ionization Potential [eVI

Fig. 5. Abundanceratios of elementsin the (slow, in-ecliptic) solar wind, in the solarenergeticparticle population,and in the fast solarwind from coronalholes, vs. FTP. Thefirst two form a step function(outlinedby the dashedline) with a stepnear10 eV of a factorof 3—5, while this step is much less in coronalhole solarwind. Helium is further depletedrelativeto thehigh-FIPgroup in all cases.Datafrom /27/, exceptsolarwind C/O andMg/Ofrom this work; for S. seealso /38/.

He/Ne ratio from the foil collection techniqueon themoon,which gaveonly small, butdefinite variations

betweenall lunar landings/47/.

Wave ParticleInteractions

The ability of SWICS to obtain distribution functions of many minor ion charge statesallows us toaddressanotherquestionin solarwind dynamics: While in the lower coronadifferent ion speciesandevenelectronsare not very far from thermalequilibrium at temperaturesof 1—2 MK, this is no longertrue in the solarwind. There,the chargestatesremainunaltered,but thedistribution functionsof all ionsheavierthan protons haveapproximatelyuniform bulk speedsin excessof the proton speed,and theirkinetic temperaturesare typically massproportional ratherthan isothermal. (This picture is disturbedonly occasionally,at timesof exceptionallyhigh solarwind densityand low temperature.)Thismay beunderstoodas the result of interactionsof waveswith the solarwind plasma,but a quantitativetheoryfor this surprisingly systematicbehaviourof heavy ions in the solarwind hasnot yet beenworkedout(but see/48/). In Figure 6 we give thedistribution functionsof the relevant chargestatesof He, C, 0,Ne, andMg, on two dayswith widely differing solarwind kinetic temperatures,but constantsolarwindvelocity. Wehavealso checkedthat thesolarwind wasnotcollision dominated/49/on thesedays,whichwould causethedifferent speciesto thermalize.Both the position (i.e. the bulk velocity)and the width(i.e. thekinetic temperatures)of the distribution functions f~(v)of all chargestatesanalyzedare seentobe nearlyequal. Noteparticularlythat this is true forboth low- and high-FIPelements.Equal bulk speedsand massproportional temperatures(if they were low enough for the peaksto beseparated)havebeenobservedearlier, usingthe massper chargeinstrumentson ISEE-1 /50/and ISEE-3/51,52, 53/. SWICSnow increasesboththe accessiblerangeof kinetic temperaturesandthe numberofchargestatesfor which distribution functionscanbe obtained,from H up to Fe /54/, giving invaluableinformation on plasmaprocessesin the solarwind sourceregion, in the openinterplanetaryspace,andin theneighbourhoodof discontinuitiesandshocks.The detailedinformation we gainfrom the distribution functionsof the minorions will complementthewealth of information on those of the major constituents,H and He, accumulatedover the past threedecades,andparticularly thoseof the SWOOPSexperimenton Ulysses/55/. As is the casefor He, wefind that thedistribution functionsof the minor ionscanbe representedby a convectedMaxwelliandownto spectraldensitiesof theorder of onepercent,with a suprathermaltail on thehigh-energyside/6/.

SolarWind atHigh Latitudes (6)71

1EI7 ~ -‘---, 1E~7

S\MCSIUIySSeS ~_‘~ ~2’ SWICS/Ulysses ~ I1.2~Day 134, 1991 — c’ Day 187, 1991 ~

1EIO -B— C’ 1E6 —9— c~—,~•— C

4• —h-— c4f ~ o~ ~ H— o’

IE*5 —E~’--- o~ IE*5 VV~’ —~e-- ~‘•

I,8 ~I 1E~4 ~ 1E’4 / ‘ ~

~ ~ .~ I I ~-_~ ~~ ~200 400 600 800 1000 200 400 600 800 1000

Velocity 1km/si Velocity 1km/si

Fig. 6. Distribution functionsof He~,C~,C5~,C6~,06+, O~,08+, Ne8~,andMg’°~inthesolarwind on two dayswith widely differing kinetic temperatures(left: day 134, 1991;right: day 187, 1991). The bulk velocity of all chargestatesis observedto be the same,as is the width of the distribution functions in velocity space,i.e. the kinetic temperatureismassproportional.A Maxwellianwas fitted to the core of the He~’distribution functionontheright to emphasizethe presenceof a suprathermaltail. (Two dayswere chosenwhenthesolarwind speedwasconstant,sothe width of the distribution functions is purely thermal.)

In orderto get the maximumbenefit from the SWICSdata,it will benecessarythat minorionsbe treatedinsolarwind modelsand predictionsbemadenotonly regardingtheir chargestates,but alsotheir kineticproperties.

CONCLUSIONS

The compositionmeasurementsof SWICS on Ulysseshavealready substantiallyaddedto our under-standingof the solarwind during Ulysses’in-ecliptic journey. Observationsof earliermissionscould beconfirmedandextended.Specifically, the variability of the HPfractionationeffect wasconfirmedin thecaseof Mg/0 and Si/0. Also, the equality of flow speedand massproportionalityof kinetic temperatureamongminor ions wasconfirmedand extendedto a much largernumberof observableion species.

Ulyssesis now beginning to crossthe most active intermediatesolar latitudes, from where it will beobservingthe solarwind transientspossibly from right abovetheir foot point. It will thusmost likelysamplematerial from the sourcesite, and therebyprovide complementaryinformation to the abundancedatafound in the coronausingEUV measurements.

At very high heliolatitudes,Ulysseswill slowly crossthepolar coronalhole. The transitionto fast solarwind will shednew light on thesourceregionsof the slowsolarwind, which arenot properlyunderstoodto date. Furthermore,SWICS will providea largedatabaseof compositiondatain the high-speedsolarwind streams,and enableus to addressthe questionof the apparentlyweakerFIP effect in this type ofsolarwind.

Finally, we expectthat Ulysses’voyageto the unknownwill also confrontus with ... theunexpected.

ACKNOWLEDGEMENTS

Wethank 0. Gloeckler,A. Galvin, andF. Ipavich for many stimulatingdiscussionsandsuggestions,andP. Bochslerfor carefullyreadingthemanuscript.This work was supportedby the SwissNational ScienceFoundation.

(6)72 R. von Steigerand3. Geiss

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