Adv. Space Res. Vol. 11, No.5. pp. (5)77—(5)80,1991 0273—1177/91$0.00÷.50Printedin GreatBritain. All rightsreserved. Copyright© 1991COSPAR

PLASMA DIAGNOSTICSWITH THE SOLAR-ABRAGG CRYSTAL SPECTROMETER

J. L. Culhane,*E. Hiei,** R. D. Bentley,* C. M. Brown,***G. A. Doschek,*** U. Feldman,*** J. Lang~andT. Watanabe**

*Mullard SpaceScienceLaboratory, University CollegeLondon, London, U.K.**National AstronomicalObservatoryof Japan, Mitaka, Tokyo 181, Japan***E. 0. Hulburt Centerfor SpaceResearch,Washington,U.S.A.

~RutherfordAppletonLaboratory, Chilton, Didcot, U.K.

ABSTRACT

A BraggCrystal spectrometeris oneof theinstrumentson theSolar-Amission.Usingbentcrystals,thespectrometerwill observetheresonanceline complexesof theH-like Fe XXVI, andHe-likeFe XXV andCaXIX ions with asensitivity5 to 10 timesthatof theSMM Bent CrystalSpectrometer.It will alsostudy thelowertemperaturelines ofHe-likeS XV. Theimprovedsensitivitywill allowobservationsmuchearlierin theimpulsivephaseof flaresthanhaspreviouslybeenpossible.The new observationsshould help to answerquestionsaboutplasmaheatinganddynamics.As well asproviding informationon line profilesandshifts,theselectedspectrallines will alsoprovideelectrontemperatureandemissionmeasureestimatesoverarangefrom 5 to 50MK. Theonboardmicroprocessorwill permitspectralresolutionto betradedagainsttimeresolutionduringanobservation.

INTRODUCTION

TheSolar-Amission,whichwill belaunchedby theJapaneseInstitutefor SpaceandAstronauticalScience(ISAS) inAugust1991, camesarangeof instrumentsdesignedto studythehighenergyaspectsof SolarFlares.Oneof these,the BraggCrystal Spectrometer(BCS),will beusedto studyplasmaheatinganddynamicsduringtheimpulsivephaseof solarflares.High spectralandtemporalresolution,aswell ashighsensitivity,arenecessaryto achievethisobjective.TheSolar-A BCS hasbeendesignedto provideapproachingtentimes greatersensitivitythanwasavailablefrom theinstrumentsflown on P78-i [1], SMM [2] andHinotori [3]. It will employfixed bentcrystalsandhavecomparablespectralresolutionto the SMM BCS.Thetemporalresolutionwill beaboutis. In additionto studyingplasmaheatinganddynamics,theBCS will routinelyestablishplasmatemperaturesandsoft X-ray light curvesforsolarflares. Specialemphasiswill beplacedon elementabundancechangesandonthesuperhotcomponentof flareplasma[4]. Comparisonof thehigh timeresolutionspectrawith the3 arcsec.imagesfromtheSoft X-raytelescopeis expectedto yield particularlyinterestingresults.In this paperthe designandoperationof the spectrometerisdescribedandtheprincipalscientificobjectivesaresummarized.

THESOLAR-A BRAGG CRYSTAL SPECFROMEThR

Conventionalbraggspectrometersscanin wavelengthby rotatingaflatcrystalso thatarangeof angularpositions(9)

convertsto arangeof wavelengths(X) accordingto Bragg’sLaw nX= 2dSin 9. In bentcrystalspectrometers,thefixedrangeof angles(Oi to 02)convertsto afixed rangeof wavelengths(X1 to X2, seeFig. 1). TheSolar-ABraggCrystalSpectrometer(BCS) containsfour bentcrystalsthatcovernarrowselectedwavelengthsrangesof diagnosticimportance.Theserangesincludetheresonancelines of theHe-like speciesFe XXV, CaXIX andS XV togetherwith theLyman-aline ofH-likeFeXXVL Thespectrometeris madeup of twounits, eachcontainingtwo channels.Eachunit is separatelymountedon the spacecraft.Fig. 2 showsthetwo spectrometersmountedeithersideof thespacecraftcentrepanel.Theabsenceof acollimatorreducessizeandweightandwill enableflaresto beobservedoverall thedisk. Thecrystalsaregermaniumandarelargerthanthoseflownon previousmissions.TheGe (220)crystalswereusedon both the SMM BCS andP78-i spectrometer,whileaGe (111)crystalwasusedon theSMMFCS.Foreachcrystal,thethermalDopplerwidth of theprincipal spectralline is muchlargerthanthecrystalrockingcurvewidth.Theincreasein the BCS sensitivityoverthatof previouslyflownspectrometersis dueto acombinationof theincreasedcrystalsize,theparticularcrystalreflectivities,reducedthermalfilter absorption,lackof spectrometerdataaccumulationdeadtimeintervals,andtheabsenceof acollimator.

X-raysreachthe spectrometerthroughaperturesin the spacecraftwall coveredby thin aluminizedKapton filmswhich actasheatshields.Eachspectrometerhasthreefeetwith preciselylocatedandsizedscrewholesto enableaccuratemounting.An opticallylocateddrill templateis usedto positionthematingholesin thecentrepanelin thecorrectrelationshipto the spacecraftZ-axis.Theopticalalignmentprocedureensuresthat theBCS opticalaxisis co-alignedwith the spacecraftZ-axiswith aprecisionof±20 arcsec.As thecentrepanelis specifiedto beflat to within.i?~4nr~~minnrnviqic~n it mn~-1efrw chin,,r,~no~lr,ri.,rr ,~t~en,h1vr,~, timi~e the flrauo 2noleQ The ~letion o~lt fre. the

(5)78 J. L. Cuihaneetal.

available,it wasdecidedto constructthedetectorbody from stainlesssteel.A smalladditional weightsavingwasachievedby makingeachdetectordouble,with differentspectrometerchannelscoveredin eachhalfof thedetector.Multiple anodeshavebeenusedto combatproblemscausedby the unfavorableaspectratio of the detector.Aschematicdiagramof thedetectoris givenin Figure3. Theonedimensionalpositionreadoutsystem- achargeratio

doubleconductingwedgecathode,hasachievedapositionresolutionof betterthan250~.tm(FWHM). Thedetectoroperateswith aXenon/Argon/CO2gasmixture at 1.1 atm..

Eachofthespectrometerunitshasapairof bentcrystalswhoseoutputsareregisteredin a“double”detector.Anodewiresprovideeventpulseswhichidentifytheoutputof thepositionencodingcircuitswith thecorrectcrystal.Onlythoseanodeeventswhich fall within asinglechannel“window” areacceptedasvalid.This ensuresarejectionfactorfor GeFluorescenceeventsof about500:1.A schematicdiagramof thedataprocessingsystemis givenin figure4.For eachdetector,the two wedge-and-wedge(cathode)signalsareprocessedby two flash analogue-to-digitalconverters(ADC’s). In both cases,theeight most significantbits of the digitizedwedgesignals,A andB, areconvertedin theposition encoderto wavelengthbin addressesby therelationAI(A+B). This is accomplishedbyusingtheA andB valuesto accessalookup tableheldin ROM. Only onesuchtableis neededfor both detectorssincethelookupoperationrequiresanintervalapproximately10 timeslessthantheanalogueeventprocessingtime.Usingthe wavelengthbin addressproducedby the positionencoder,eacheventis integratedin the accumulatorwhichcontainsdoublebufferseachof 4x256wavelengthbins (16bits deep).Theaccumulatorsweresizedto allowfor 256bins in eachchannelalthoughonly 128 bins will normallybe usedfor theCaXIX andS XV channels.Inorderto optimizethenumberof wavelengthbins used,before aneventis depositedin the accumulatorit is re-groupedinto a“smaller” numberof bins throughtheuseof adatagrouper. Severalgroupingplansare alwaysselectable.After thedatahavebeenaccumulatedfor anintegrationperiodcontrolledby the accumulatortimer, theaccumulatorbuffers aretoggledandthedatais transferredfrom theaccumulatorto ahardwaredatacompressor.Here,theaccumulatednumberof eventsfor eachwavelengthbin is reducedto aneight bit valuebeforebeingstoredin the dataqueue.The dataqueueis a 384 kbyte randomaccessmemorywhich is controlled by the BCSmicroprocessor.Dataaretransferredto thespacecraftdigital processingunit in 256 byteblocks ataratesetby thespacecraftsystem.Thepropertiesof theinstrumentaresummarizedin TableL

Fe)O(V

F.xxa #~ -

BCS-A~ ~ Fe/Fe~ ~27°

POSITION SENS~1IVtCAThODEFIXED O~TECTOE

~~1~E~TOI

.r’ ‘k~—-.— — FIXEDIENT

F~E FIAT cnyssoj.

a) b) BCS-BCa/Sx

(Son) ~52°V

S)Th#4

COSTS

Figure1. (a) flat scanningand Figure2. Isometricviewsof(b) fixed bentcrystalspectrometers. thefour spectrometerchannels,

TABLE I. SpectrometerProperites

CHANNEL WAVELENGTH CRYSTAL RESOLUTION SENSISTIV1TYNO.(ION~) RANGE (A’) ROCKINGCURVE (mA.?J&~’) (SOLAR-A/SMM’)1 1.7637- GE220 0.22, 8000 9~(FEXXVI) 1.8044 (12”)2 1.8298- GE220 0.24, 7700(FEXXV) 1.8942 (12’)3 3.1631- GE220 0.39,8000 6~(CAXIX) 3.1912 (33”)4 5.0160- GE111 1.38, 3600 63*(SXV) 5.1143 (68”)

TheSolar-ABraggCrystalSpectrometer (5)79

Beryllium Window ~-Gas filling Stem

SupportBars

~.-~—DetectorTopAssembly

AnodeWires

WedgeConnectionWedgeConnection ~ FeedthroughFeedthrough “...~

Wedgeand-_ -~ - WedgeCathode

CalhodeScreen Front-endAnalogueElectronics

DetectorBottom Assembly ElectronicsEndosureCover

Figure3.An explodedview of thedoubleproportionalcounterdetector.

A~c

2 ad&esslines

BCS Data Processing~Grouperand Accumulator csntrctrogistetsandAccumulator TunercanbeloadedbyBemimoprscesssr

Figure4. A schematicdiagramof theon-boarddataprocessingsystem

theproposedspectralcoverageis indicatedon spectraobtainedwith Hinotori [4] andwith theSMM flat [5] andbent[6] crystalspectrometers.Theresonanceandall importantsatellitelines arecoveredfor Fe XXVI, Fe XXV andSXV. In thecaseof CaXIX thewavelengthcoveragehaseffectivelybeenrestrictedto theresonanceline andtoadjacentshort wavelengthcontinuum.This is to ensurethatthegreatestpossiblesensitivity canbe obtainedforstudiesof plasmadynamics.Thefollowing is asummaryof theindividualscientificobjectives.PlasmaDynamics:PreviousBraggspectrometershaveshownthatspectrallines aremuchbroaderthanwould beexpectedfromnormalthermalDopplerbroadeningattheimpulsivephaseof flares.The“turbulent” or non-thermalmotionsaretypically about160km r1 whenfirst observed,anddecreasemonotonicallyto about60 km S4 by thetime of maximumX-ray flux. During the decayphasethesemotions are60 km s1 or less.Theremay be acorrelationof non-thermallinewidth with associatedhardX-rayflux andthereis thepossibilitythatthebroadeningis non-randomon short time scales[7]. In addition to theapparentlyrandomnon-thermalmotions,a blue-shiftedcomponentof emissionis oftenobservedfor disk flares,with velocitiesthatcantypically extendup to about— 500kms~1andcanon occasionreachevenlargervalues,asindicatedby theextentof theDoppler-shifted“blue” wingsof spectrallines. The “average”upwardmotion, as indicatedby the centroidof an assumedsingle-componentGaussiandistribution, is about300 km si. The upward-movingplasmahasbeenattributedto “chromosphericevaporation”,i.e. the ablationof plasmaheatedby eithercoronalconductionfrontsorelectronbeamsthatoriginateinthecorona[8]. Recentworksuggeststhat thebroadenedline profilesarein factnotdueto turbulenceassuchbut totheobserverseeingablatedplasmamoving alongthecurvedtrajectoriesof atypical loop geometry.The Solar-ABCS, with its high sensitivity, will be ableto study the line broadeningandblue shifts from thevery ealiestindicationof enhancedemissionandwith muchimprovedtimeresolution.PlasmaHeatingduringtheImnulsivePhase:TheECS will beableto studythehinh-temneratureolasmathatexists

(5)80 J. L. Culhaneet a!.

PlasmaDia2nostics:TheSolar-ABCS will be ableto carryoutmanyof theplasmadiagnosticmeasurementspossiblewith previouslyflown spectrometers,but with the advantagesof greatersensitivity andtimeresolution.It will bepossibleto obtain accuratevaluesof temperatureandemissionmeasure,while comparisonwith imagesfrom theSXT andHXT instrumentsonSolar-Awill giveemitting volumes,fromwhichdensitiescanbededuced.Differentialemissionmeasureoverawide temperaturerangewill beobtainedby combininginformationfromthe SXT andBCSinstruments.SunerhotComoonent:TheFeXXVI channelwill beparticularlyusefulin studyingthesuperhotcomponent[9,10].Temperaturescanbeobtainedfor this componentfromFe XXV dielectronicsatellitelines andtheFe XXVI Ly~lines. In additionline profiles can bedeterminedandthereforethedynamicsof the superhotcomponentcan beinvestigated.Hinotori dataindicatethattheFeXXVI line profilesvaryduringflaresbut thespectralresolutionwasinadequatefor definitiveprofile measurement[4].DecayPhase:The BCS will provideplasmadiagnosticinformation aboutthethermal flare in its decayphase,asdescribedabove.In addition,for largelimb flares,it maybepossibleto observetheincreaseof heightof theemittingplasmafrom spatialdisplacementwhichtranslatesinto spectraldisplacementin theBCS since,in thedecayof verylargeflares,theemissionoriginatesfrompost-flareloopsabovetheoriginalsiteoftheflare.Finally themeasurementofline to continuumratioswill allow flare-to-flareabundancechanges[11] to bestudiedfor arangeofelements.

a) Fe)0OdI ‘ C) CaXD( I

II S0~3.17elA -

Gn22sSystal. O~~25.5’ Ge20005sIXI,C5526~

T~T ___________isa tao J316 ~.1T sie 5.19 5.20 521 3.20 5th 0.04

...4 1.20 l.6 I.? t.~ 155 I.SO 5 5.05 5.1 5.15 ba

Wavetength (A)

Figure5. Spectralrangescoveredby thefourth spectrometerchannelssuperimposedon solarflarespectraa) FeXXVI [4] b) FeXXV [2] c) CaXIX [6] d) SXV [5]

ACKNOWLEDGEMENTS

TheBent Crystal Spectrometerfor Solar-A is beingdevelopedandconstructedby groupsfrom Japan(ISAS andNAOJ), UK (MSSL andRAL) andUSA (E.O. HulburtCentre).We acknowledgethesupportof ISAS , theUKSERCandtheUS NRL.

REFERENCES

1. U. Feldman,G.A. Doschek,R.W. Kreplin, Ap~L,2~,885 (1982).2. L.W. Actonand23 otherauthors,SolarPhysics,~, 53 (1980).3. I. Kondoin Hinotori Symposiumon SolarFlares,ISAS,Tokyo.4. K. Tanaka,Publ. Astron. Soc.Japan,39, 1 (1987). -5. L.W. Actonand 16otherauthors,~ 24.4, L137 (1981).6. J.L. Culhaneand16 otherauthors,~ 24.4,L 141 (1981).7. - A. Fludra,J.Lemen,J. Jakimiec,R. Bentley,J. Sylwester,ApA, 3.44,991(1989).8. E. Antonucci and9 otherauthors,SolarPhys.,:z~,107 (1982).9. R.P. Lin, R.A. Schwartz,R.M. Pelling, K.C. Hurley, ~ 2~i,L 109 (1981).10. K. Tanaka,T.Wantanabe,K,Nishi, K. Akita, Astmphvs.J., 254,L 59 (1982).11. J. Sylwester,Artificial Satellites/SoacePhysics,~, 17 (1987).