ground-based helioseismology networks

Adv. Space Res. Vol. 11, No. 4, pp. (4)149—(4)158, 1991 0273—1177/91 $0.00 + .50Printedin Great Britain. 1991 COSPAR

GROUND-BASED HELIOSEISMOLOGY

NETWORKS*

FrankHill andJohnLeibacher

National Solar Observatory,National Optical AstronomyObservatories**,

Tucson, AZ 85726, U.S.A.

ABSTRACT

The diurnal rising and setting of the Sun severelycompromiseshelioseismologyfrom a singleground—basedobservatory.This periodic interruptioncreatessidelobesin powerspectraat multiplesof i/day (11.57p.Hz) centeredaroundeachsolar line, contaminatingthe spectraandhamperingmodeidentification andfrequencymeasurement.So far, threestrategieshavebeenusedto overcomethedifficulty — observingfrom thePolar regions,observingwith a network of stationsplacedaroundtheEarth,or observingfrom afully sunlit orbit in space.This paperreportson the statusof the networksthat areeithercurrently in operationor beingplanned.Theseincludethe Global OscillationNetworkGroup (GONG) project, the Birmingham network, the IRIS network of the University of Nice, theSCLERA network of theUniversity of Arizona, andthe ESTEC network. The scientific objectivesand instrumentationof thesenetworksarebriefly described. Therelationshipbetweennetworksandthe helioseismologyexperimentson theSOHOmissionaredescribed.

THE NEED FORCONTINUOUS DATA

Helioseismology,the studyof solaroscillations,is a powerful tool for probing the solar interior. Theoscillations are global and permeatethe entire Sun,thus their propertiesare sensitiveto the Sun’sinternal structure. Careful measurementsof the frequencies,amplitudes,and lifetimes of the oscilla-tions provideinformationon the internalsoundspeed,temperature,rotationrate,composition,opacity,convectiveflow patterns,andmagneticfield. Comparisonof the observedfrequencieswith theoreticalpredictionsderivedfrom solar modelshelpsto testpotentialexplanationsfor the observeddeficiencyof solarneutrinos.

The most usefulmeasurementsare the frequenciesof the oscillations as a function of their sphericalharmonic “quantumnumbers”— ~, the sphericalharmonicdegree,m, the azimuthaldegree,andn, theradialorder. It is relativelysimpleto computethesefrequenciesfrom theoreticalsolar models,andtheobservedfrequenciescan also be directly combined via inverse techniques to infer the depth

‘This presentationis basedlargely upon Hill /1/, and it should be regardedas merely updating that publication. It isincluded here for completenessof the collectionof discussionson the current stateof helioseismology.

‘Operatedby the Association of Universities for Researchin Astronomy, Inc. under cooperativeagreementwith theNationalScienceFoundation.

(4)149

(4)150 F. Hill and J. Leibacher

dependenceof physicalconditions. Frequenciesarealso the mosteasilymeasurableproperty of theoscillations. The temporallengthof the sequenceof observations,T, andthe precisionwith which thefrequenciesv can be determined,~v, are simply related: z~v= lIT. Thus longer time strings arealwaysthe goal of the observationalhelioseismologist.Unfortunatelyfor him or her, theEarthrotatesandtheSun setseveryday, placingperiodicgapsin the sequenceof observations.

This is especiallyunfortunatebecauseobservationaltime stringsof manydays,months,or evenyearsare essentialfor helioseismicdiscriminationbetweendifferentmodelsof internal solar structure. Asan example,thesolar surfacerotation periodvaries from about28 daysatthe equatorto 32 daysatthepole. The rotation changesthe frequencyof the modesby an amountproportionalto their rn-value(thenumberof wavelengthsaroundtheequatorof the coordinatesystem);theconstantof proportional-ity is approximatelythe inverseof the rotation period. In orderto resolvetheseshifts, onemusthavea frequencyresolutionof twicethe shift, implying a minimum observationaldurationof two rotationperiodsor about60 days. Evenlonger time strings areneededto investigatesolar cyclechangesthatmay occuroveran il-yearperiod.

IL .~3000 3100 3200 3300 3400 3200 3000 3100 3200 3300 3400 3500

FIGURE 1. Left The 3000 to 3500 j.tHz bandof a simulatedsolar spectrumas might be obtainedfrom continuousobservationscoveringoneyear. Right The sameportion of the simulatedspectrumas mightbe obtainedat a singlemid-latitudesite experiencingthe day-nightcycleandtypical weatherpatterns.

Fourieranalysisshowsthat whentwo functionsaremultiplied togetherin the temporal domain,theirtransformsareconvolvedin the frequencydomain. For helioseismology,the two functionsare thetimeseriesof solardataandatime seriesof onesor zeroesrepresentingthetimesof data(ones)or nodata(zeroes)called the window function. Thus,in the frequencydomainweobtain thereal solarspec-trum convolvedwith thespectrumof the window. Thewindow function hasaregulardiurnal periodicstructure,thus its spectrumconsistsof a strongpeakat a frequencyof 1/day,or 11.57jiHz, alongwithits harmonicsatintegralmultiplesof the fundamental1/day frequency. Thesepeaksarecalleddiurnalsidelobes;the fundamentaldiurnal sidelobetypically hasan amplitudethat is 60% of theamplitudeofthe zerofrequencycomponentof the window spectrum. Since the solarspectrumand the windowspectrumarebothnearlysetsof discretedelta functions,theconvolution of the window spectrumwiththe solar spectrumresultsin a replicaof the window spectrumappearingsymmetricallyaroundeverysolar spectralline, as shownin Figure 1. As can be seen,therearemany instanceswhereasolarlineoverliesa diurnal sidelobe,greatly compromisingmodeidentification and frequencymeasurements.The first 60 j.tHz of thepowerspectrumof anactual window function observedatTucsonis shown inFigure2. The fundamentalandthefirst four harmonicsof the diurnal sidelobesareclearly visible, asis anoisebackground.

Severaldataprocessingschemesto eliminatethe effects of the diurnal sidelobeshavebeeninvesti-gated. Radioastronomershavelong facedthe problemof unwantedsidelobes,andhavedevelopedaniterative peaksubtractionalgorithm known as CLEAN. It hasbeenusedto searchfor solargravity

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FIGURE 2. The first 60 p.Hz of the power spectrumof the an actual window function observedatTucson. The powerhasbeennormalizedby the power in the zero-frequencyor DC componentandthe logarithm taken. The fundamentaland first four harmonicsof the diurnal sidelobesare clearlyvisible. it is thereductionof thesesidelobesthatis thegoalof thenetworkconcept.

modes12/ andto removethe sidelobesfrom p-modespectra/3,4/. In the contextof helioseismology,the CLEAN algorithmcan misidentify asidelobeas a solarfrequency. Anothermethodrearrangesthedatato fill in the daily gaps,but this reducesthe frequencyresolution of the spectrum/5/. Lineariterative deconvolutiontechniquestendto increasethe noisein the spectra,while nonlinearspectraldeconvolution/6/assumesthatthe actualsignal is bandlimited andthatpoweroutsidethebandcomesfrom the gaps. This assumptionis not met by the solaroscillations17/. Maximum entropyhasbeenusedto extrapolatedatainto gaps/8/ andthis methodhasbeenstudiedin the contextof helioseismol-ogy /9/. With this technique,solar spectracanbe reliably recoveredif thefilling factor,or duty cycle,is greaterthan0.8, andthe signal-to-noiseratio is larger than100. Finally, Bayesianprobability tech-niqueshavebeenusedto combineall availablea priori information along with the observedpowerspectra to extract and identify the modes/10,11/. This method has also proven to be capableofmisidentifyingmodes. One mustdrawthe conclusionthat, so far, no techniqueis availablethat willallow the reliableelimination of the diurnal sidelobesfrom spectraobtainedat a single ground-basedsite. We must look to otherobservingstrategiesto eliminatethe daily gaps as muchas possibledur-ing theobservingprocessitself.

STRATEGIESFOROBTAINING CONTINUOUS DATA

Therearethreebasicstrategiesthatenablehelioseismologiststo obtainnearlycontinuousdata. Thesestrategiesare observingat the polar regions,observingfrom space,or observingfrom a network ofground-basedstations.All of the strategiesface environmentalandlogistical challenges,and producelargedataflows if they involve observationswith moderateto high spatialresolution.

The Sun staysabovethehorizon for six monthsat a time atthe polarregions,thus observationsfromtheseregionsarecapableof providing morethanthe 14 continuoushoursavailablefrom lower latitudesingle sites. In practice,the presenceof acontinentalland massat theSouth poleandthe associatedinfrastructuremakesit thepoleof choicefor observationalastronomy. Severalhelioseismologyexper-iments havebeencarried out at the Southpole, including non-imaging observations/12/, diametermeasurements/13/, andimagedobservations/14/. Thereare threeadvantagesto a Southpoleexperi-ment. First, sinceonly a single instrumentis involved, thereis no needto mergesimultaneousdata

(4)152 F. Hill andJ. Leibacher

sets. Second,thecost of the experimentis substantiallylower than that of the other two strategies.Finally, it is possibleto recoverfrom instrumentalfailure,although it is rathermoredifficult thanatsitesat lower latitudes. The disadvantagesare twofold: First, the weatherpatternsat the Southpoleplacea limit on the uninterruptedduration of the observationsof about sevendays, far short of thesixty neededto cleanly separatemodeswith adjacentvaluesof m. Secondly,the presenceof theEarth’satmospheredegradesthespatialresponseof the solarpowerspectra,especiallyat highervaluesof t /15/. The logistical challengeConsists of the remotenessof the site and the harshnessof theenvironment.

A spaceplatform can be placedin a fully sunlit orbit aboutthe Earthor aroundaLagrangianpoint.Various studieshavebeenmadeof space-bornehelioseismicobservations/16,17/andthe EuropeanSpaceAgency will be launchingthe SOHOmission in the mid 1990s/18,19/. Thereare two advan-tagesto the space-borneobservations:First, thereis naturally no degradationof the observationsfromatmosphericseeing. Secondly,no datamergingis requiredfor asingle instrument. Threedisadvan-tagesexistFirst, thereis little chanceforrecoveryfrom an instrumentalfailure,especiallyfor aspace-craft in orbit aroundthe L1 point. Second,telemetrybandwidthlimitations renderit impossibletotransmit the entire dataset from a high-resolutionimaging experiment,thus requiringon-boarddataprocessingand forcing aselectionof dataparameters.Telemetry groundstationsare typically three-sitenetworkswhicharevulnerableto single-siteinstrumentalfailures. Furthermore,theymaynot pro-vide 24-hourcoveragein all seasons.Finally, a spaceexperimentis monetarily the mostexpensivemethod of obtaining nearly continuousdata. The lifetime and hencethe frequencyresolution of aspaceexperimentis limited by theexpendablesuppliessuchaspropellant,andthe logisticalchallengesconsistof placing thevehiclein orbit, andcopingwith theradiationandthermalenvironment.

A network of identical observingstationsredundantlydistributed in longitude reducesthe diurnalsidelobesas well as the impactof weatherandinstrumentalfailures. The optimalnumberof sitesfora networkis six, distributedin longitude so that atleast two stationsarepotentially observingat alltimes. Models predict that sucha network should provide observationswith a duty cycle of about93% 120/in goodagreementwith observations/21/. With a duty cycle this high, andassumingthattheinstrumentis well-designedandbuilt with asignal-to-noiseratio greaterthan100, thentheremain-ing gapsshouldbe easily filled usingthe maximum entropymethod. Theadvantagesof anetwork aretwofold: First, unlimited frequencyresolution is availablein principle. Secondly,it is possible torecoverfrom instrumentalfailures. There are two disadvantages:The existenceof multiple sitesobservingsimultaneouslyrequiresthatthe databemerged,aproblemfor whichthereis currentlylittlepreviousexperience.Secondly,theobservationsaredegradedby seeingandtransparencygradientsinthe Earth’s atmosphere.The logistical challengeconsistsof manufacturinga numberof identicalinstrumentsandengineeringthem to withstandalmostthe entirerangeof climatesfoundon the Earth.In addition,the internationalaspectprovidesthe opportunity to samplethe customsof many nations.Theexpenseof an imaging networkis moderate,beingsubstantiallylessthana spaceexperiment,butmore thana Southpoleexperiment.

SUMMARY OF CURRENT AND FUTURE NETWORKS

Four major networksareeither currently in partial operation,or beingplanned.None of thesenet-works have yet reachedfull operationalstatusof six or more sites. In addition, therehavebeenanumberof two-site networkswith limited lifetimes.

Historically, theoriginal helioseismologynetworkis the BirminghamSolar SeismologyNetwork,withheadquartersatthe University of Birminghamin the United Kingdom /22/. It was proposedin 1975~andfundedfor afull six stationsin 1987. Thefirst stationwasinstalledatIz~fla,Tenerifein the Span-ishCanaryIslandsoff the westerncoastof Africa in 1975. A secondstationwas installedin 1981 atHaleakala,Hawaii,a third automaticstationbeganoperationsin 1985 atCarnarvon,WesternAustralia,andthefourth stationwasinstalledin January1990at Sutherland,SouthAfrica. A siteatLasCampa-nas, Chile should be installed late in 1990. One additional site will be selectedand installedat afuture time. The BirminghamNetwork instrumentationconsistsof a non-imagingpotassiumoptical


resonancespectrometer.Sincethe experimentis non-imaging,the observationsaresensitiveonly tooscillationswith 0 �~� 3. Scientific objectivesof the experimenthaveincludedthemeasurementofline widths/23/, solar internal structure/24/, rotationalsplitting /25/, g modes/26,27/solarcycle fie-quencychanges/28/ andtheidentificationof modesin thelow-e portionof the solarspectrum/29/.

A secondnon-imagingnetwork is being developedand administeredat the Université de Nice inFrance. This network is knownas IRIS (Installationd’un RéseauInternationalde SismologieSolaireor InternationalResearchon the Interior of the Sun) /30/. This network was proposedin 1983 andfundedfor sevenstationsin 1984. A prototypeinstrumentwas operatedatLa Silla,Chile from May1986 to the Springof 1987. Five instrumentsarecurrentlyoperatingat Stanford,California (opera-tions beganin 1987),Kumbel, Uzbekistan,U.S.S.R.(1988), l’Oukaimeden,Morocco(1988),La Silla,(1990) andIzIfla, Spain (1990). The final two instrumentsshouldbe installed in 1991, and Lear-month,WesternAustralia, andHaleakalaare the prospectivesites. Theoperationof the IRIS networkis scheduledto continuefor onefull solarcycleof 11 years,endingin 2001. The IRIS instrumentissimilar to the Birminghamdevice,but usessodiuminsteadof potassium/31/. The full-disk observa-tions aresensitiveto oscillationswith 0 � e � 3. TheNice grouphasbeenactive with asingle instru-mentfor sometime. Scientificgoalsof the IRIS projectincludesolar cyclefrequencychanges132/,gmodes/33/,andfrequencyandamplitudemeasurements/12/.

Measurementsof thesolardiameterhavealsobeenusedto searchfor solaroscillations/34/. Althoughtheresultsof suchstudiesarecontroversial/35/, anetwork of instrumentsto obtain the measurementsis being planned. The SCLERA (Santa Catalina Laboratory for Experimental Relativity andAstrometry)network is beingdevelopedat theUniversity of Arizona in Tucson. Two sitesatTucson,Arizona and YunnanObservatory,Kunming, China, havebeenselected,andnegotiationshavebeenconcludedfor a third site at Kislovodsk in the U.S.S.R. Other sites will be determinedata futuretime. The instrumentationwill comprisea combinationof diametermeasurementsandbroad-bandphotometryof the centralportionof the disk. Thescientificobjectivesof thegrouphaveincludedthegravitationalquadrupolemoment/36/, the 160-minuteoscillation/37/, g modes/38/, internal rotation/39/,andtheneutrinoproblem/40/.

The final major network is perhapsthe mostambitious. The Global Oscillation Network Group(GONG)project is planningto placesix imaging instrumentsaroundthe world in 1993 /41,42/. Thenetworkwas proposedin 1984 andfundedin 1985. Guidedby models/20/, a sitesurveyis underwayto select the six sites from fifteen candidates/21,43/. The candidatesitesareCerro Tololo andLasCampanas,Chile; IzlEla; l’Oukaimeden;Riyadh,SaudiArabia;Udaipur,India; Urumqi, China Lear-month; Haleakala,MaunaKea andMaunaLoa, Hawaii; andML Wilson, Big Bear,YumaandTuc-son, U.S.A. Thesitesurveyhasbeenunderwaysince1985,andsite selectionis currentlyplannedfor1991. Deploymentof the networkis now scheduledfor 1993,with observationsstartinglate in 1993andendingthreeyearslater.The instrumentis underdevelopmentandcomprisesaFouriertachometerdesignusinga Michelson interferometerand a Ly(~tprefilter /44/. The detectornominally will be a256 x 256 CCD, and the instrumentwill featurelasercalibration,automaticoperation,andExabytecartridgedatastorage.Theamountof dataproducedby the experimentwill total approximatelythreeterabytes,necessitatingcarefulplanningof the datareductionsystem/45,46/. The project is open toall helioseismologists,andso its scientific objectivescover the entire rangeof helioseismology.Withthe currentdetectorconfiguration,oscillationswith 0 <1 � 150will be observable. Scientific interestsof the projectstaff includethe internal solar rotation /47,48, 49, 50/, solarcycle frequencychanges/14/, andlarge-scaleflow mapping/51, 52/.

In addition to thesefour major helioseismologynetworks,at leastsix limited-duration two-sitenet-workshaveeither existedor areplanned. TheESTEC-sponsoredSolarLuminosity OscillationTele-scope(SLOT) project installed an instrumentatb~nain 1984,and anotherat Observatoriode SanPedroMktir, Baja California, Mexico in 1987 /53/. This two-site network usedafour-channelpho-tometerto observethe oscillationsin intensityandcorrelatethem with Doppler measurements.Thisprovidesinformation on the adiabaticbehaviorof the solaratmosphere.Observationswith this net-work haveconcluded. Another two-site network has recently begun to obtain non-imagedsolar


observationswith a magneto-opticalfilter /54/. The two Sitesare at ML Wilson, California, U.S.A.,and at the University “La Sapienza”,Rome, Italy. This network begancoordinatedobservationsinSeptemberof 1988. A two-site network of imagedobservationswith a video-magnetographwasattemptedby workersatthe CaliforniaInstitute of TechnologyandtheUniversityof SouthernCalifor-nia. The two siteswereat Mt. Wilson, California,andat Tel Aviv, Israel. Instrumentaldifferencesgreatlycomplicatedthe datamergingproblemandthe attemptwasabandoned.A possibletwo-stationnetworkwhich would employ imaging magneto-opticalfilters at moderatelyhigh spatialresolutionisnow understudyat theUniversity of SouthernCalifornia. One of the siteswouldbe atML Wilsonwith the secondin either the Crimeaor nearRome, Italy. This network would obtain ground-baseddatawith spatialresolutionsimilar to the SOI experimenton SOHO. A differentapproachto networkdesignis beingundertakenby theUniversity of Hawaii. This two-site network hasits siteslocatedataboutthe samelongitude,but at ratherdifferent latitudes— AlaskaandHawaii. This strategycom-binestheredundancyof anetwork with the continuoussunlightof the polarregions. Theinstrumenta-tion consistsof anarrow-bandfilter andahigh-resolutionCCD camera,similar to theNSOSouthPoleHigh-DegreeHeioseismograph.Finally, IZMIRAN, (Moscow) is planningatwo stationnetwork toinvestigateintensityoscillationsin collaborationwith SCLERA.

NETWORKDESIGN AND PERFORMANCE— PREDICTEDAND ACTUAL

Themainadvantageof anetwork overasingle siteconsistsof thereductionof theday/nightcycleandthe impactof both weatherandinstrumentalfailures. In orderto maximizethis advantage,the loca-tion of the sitesmust be arrangedsuch thatat leasttwo, andpreferably morethan two, stationsareable to potentially observeat any time. If this conditionis not met, thenthe loss of the single stationcoveringa given band of UniversalTime will result in the immediatereappearanceof the diurnalsidelobesin thewindow powerspectrum. Themultiple-coveragerequirementconstrainsthechoiceoflocationsof the sites, but an even strongerconstraintis imposedby the distribution of land on theEarth. The multiple-coveragerequirement,along with an averageobservingday ata single site ofeight hours,resultsin apatternof six sites,spacedasevenly as possiblearoundthe globe. In ordertotake advantageof theseasonalweatherpatterns,the six sitesshouldideally alternatebetweenNorthernandSouthernhemispherelocations. Perusalof aglobeshowsthatthereis virtually no reasonablesitein the mid-Atlantic Ocean,forcing onelocation to bein themid-Pacific,namely,Hawaii. Oncethis isrealized,thenall otherlongitudebandsareset by geography:Onemust havesitesin Australia,mid-Asia,WesternAfrica, SouthAmerica,andthe WesternCoastof North America. Theseconstraintsaredemonstratedby Figure3, which showsthe locationsof the chosenor candidatesitesfor threeof themajornetworks(Birmingham, IRIS, andGONG) on amapof the world. Thereis considerableover-lap in the choice of sites. Further constraintsare imposedscientifically andlogistically. Existing,developedastronomicalobservatoriesareclearly advantageous,especiallyif they are solar and haveestablishedhelioseismologyprograms. The efficiency of communicationand shippingchannelsarealsoof concern.

A modelof network performancewas developedby Hill andNewkirk/20/. Thismodelusedasimplefour-parameterprobabilistic descriptionof the temporal distribution of cloud cover at a given site.Themost importantparametersof the modelare the fraction of cloudy time at a site in the summerandwinter. For the model, theseparameterswere estimatedfrom dimatologicalmaps. Suchmapscannotreflect micro-meteorologicalconditionsat individual Sites that may alter the parameteresti-mates. The resultsof the modelpredictedthat a well-chosensix-site network would achievea dutycycleof about93.5%.

The GONGproject hasbeenrunning a network of normal-incidencepyrheiometerssince 1985 tomeasurethe parametersand test the model, andto establishcommunication,shipping,and workingrelationshipswith the candidatesites/21/. Figure4 displaysthe window powerspectrumof oneofseveralpossiblesix-sitenetworksin the sitesurvey. Comparisonof Figure4 with Figure2 showsthattheamplitude of the fundamentaldaily sidelobehasbeenreducedby afactor of 400, while the com-binedamplitudesof the first five sidelobeshasdroppedby a factorof 200,andtheoverallbackgroundnoisepower in the first 60 LLHZ is reducedby a factor of 50. The longestsegmentof uninterrupted


LI~Birmingham

* ~ . ~

? ~ ,, K’ GONG and IRISGONG. IRIS and

FIGURE 3. A world map showingthe actual, planned,or candidatesitesfor the Birmingham,IRIS,and GONG networks. Note the considerableoverlapof the networks,especiallyin Hawaii andtheCanaryIslands. Thisoverlapis imposedby the distributionof landon theEarth.

observationsso far achievedby the GONGsite surveynetwork is about418 hours. The duty cycleobtainedby this six-sitenetwork is 94.64%,in good agreementwith the modelprediction. Thus,thenetwork hasfulfilled its designgoals,andis capableof producinghelioseismicdatathatis virtuallyfreeof contaminationby thediurnalsidelobes.

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FIGURE 4. The first 60 pflz of the window powerspectrumof a candidatesix-site GONGnetwork.Comparisonof this spectrumwith that in Figure 2 shows that the amplitude of the fundamentalsidelobehas beenreducedby a factor of 400, the combinedamplitudesof the fundamentaland firstfour hannonic2idelobeshasbeenreducedby a factor of 200, and the overall noisebackgroundhasbeenreducedby afactorof 50.

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NETWORKS AND SPACEPROJECTS

The combinationof both networksandspaceprojectsfor helioseismologywill result in moreusefulknowledgethaneitherapproachalone. Thetechniquesarecomplementaryin somerespects,andsimi-lar in others. Both aspectsareuseful: similar observationsenablecalibrationandvalidation of results,while dissimilarobservationscanbeusedto extendtherangeof data.

The SOHO mission will carry both imaging and non-imaging instrumentation. Non-imaginghelioseismologydatawill beprovidedby the Global OscillationsatLow Frequencies(GOLF) /55/ andVariability of solar IRradianceandGravity Oscillations(VIRGO) /56/ instrumentson boardSOHO,andhavealreadybeenobtainedby the SMM/ACRIM experiment/57/ andthe IPHIR experiment/58/.Thesenon-imagingspaceexperimentsprovidethe opportunity for comparisonwith resultsfrom theBirminghamandIRIS networks.

The imaging experimenton SOHO is known as the Solar Oscillations Imager (SOl) /59/ andcomprisesanothervariationon theFouriertachometertechniquebut usingtwo Michelsoninterferome-tersinsteadof one/60/. The SOIexperimentmostcloselycorrespondsto theGONGnetwork,but theSOI datawith a 1024 x 1024 detectorwill havemuchhigher spatial resolution. This will allow themeasurementof the impactof terrestrialatmosphericeffectson the GONG data. Carewill haveto betakenwith thetelemetryof the SOl data. Thebandwidthlimits do not allow completerecoveryof theentire SOl datastream,forcingthe developmentof on-boardprocessinganddataselection.In addition,the useof theDeepSpaceNetwork for thetelemetrymeansthatthe SOIexperimentis alsoa network,but comprisingonly threesites. While weatheris not as greata factor for radio communicationas foroptical astronomy,the DSN is vulnerableto instrumentaland schedulingproblems. In addition,theDSN hasan uncoveredgapduringtheNorthern Hemispherewinter season.The currentschedulingofthetwo projects(SOHOlaunchin 1995 with asix-yearlifetime; GONGobservationscovering1993 to1996)will providespatially-resolvedhelioseismologydataovera largefractionof thesolarcycle. Thedatafor thetwo experimentswill beanalyzedandarchivedtogether. Thus,theground-basednetworksandthespace-borneexperimentsform acomplementaryandsynergeticapproachto helioseismology.

SUMMARY AND CONCLUSION

The developmentof ground-basednetworksis a fundamentalaspectof current work in helioseismol-ogy. Thenetworksarean efficient andpracticalmethodof greatlyreducingthe impactof thediurnalrising andsetting of the Sun on thepowerspectraof solaroscillations. The useof networks,in con-certwith spaceexperiments,holdsthe promiseof greatlyincreasingourunderstandingof theinsideofthe Sun throughorder of magnitudeimprovementsin the quality of solaroscillation spectra. Whilethis increaseis of importancefor solarphysics,it alsobearson aspectsof both particlephysicsandcosmologythroughthe solar neutrino problem. Thus, investmentin helioseismologynetworkswillpay handsomescientific dividends.

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