Adv.SpaceRes.Vol. 13,No. 1, pp. (1.)255—(1)259 1993 0273-1177/93$15.00Printed in GreatBritain. All rightsreerved. ‘ Copyright © 1992 COSPAR

COMPARATIVE SOLAREUV FLUX FORTHE SANMARCO ASSI

W. K. Tobiska,*,t S. Chakrabarti,*G. Schmidtke** andH. Doll***

* SpaceSciencesLaboratory, University ofCalifornia, Berkeley,CA 94720,U. S.A.

** Fraunhofer-Institurfur PhysikalischeMeJ3technik,HeidenhofstraJ3e8, D-7800Freiburg, Germany‘I’,~ Physikalisch—TechnischeStudienGmbH, Leinenweberstrafie16, D-7800Freiburg,Germanyt Presentlyat JPL~MS264-765,4800Oak GroveDr, Pasadena,CA 91109,U. S.A.

ABSTRACT

The Airgiow and Solar SpectrometerInstrument(ASSI) on the San Marco D/L satellite hasmeas-ured solarextremeultraviolet irradiances. The data arecurrently being releasedfor analysis. As apreliminary step in evaluatingthis importantdataset,modeled solar irradiancesfrom 4 to 105 nmare presentedfor comparisonto the San Marco data. The comparableflux for March—December1988 is obtained from a revisedand extendedempirical solar extremeultraviolet model derivedfrom OSO 1, OSO 3, OSO 4, OSO 6, AEROS A, and AE—E satelliteand six rocketflight datasets.Model wavelength intervals having the highest photon flux include the 15—35 and 85—105-nmranges. Solar rotational featuresareprominent on several occasionsin the model time series. Auseful exampleis the modeled integratedflux between30—31 nm which includes the Sixi (30.3-nm) and Hen (30.4-nm) irradiance. The modeled flux in this 1-nm range showsbothan absolute22% increasefrom beginning to end of mission and a solar rotational variability with a typicalpeak-to-valleyratio of 14%. Theabsolutemodeledirradiancein this wavelengthinterval is 9 x i09photonscm~2s_I ±35%.

INTRODUCTION

The SanMarco D/L satellite, launchedon 25 March 1988 into an equatorialorbit rangingfrom 190to 610 km in altitude, carried the Airglow Solar SpectrometerInstrument(ASSI) in its scientificpayload.The observationsof the solarEUV by this instrumentfor over 8 monthswasthe first timein 8 years that suchmeasurementshadbeencontinuouslymade. Previoussatelliteobservationsofsolar EUV werecarried out by the OSO,AEROS, and AE seriessatellites for intervals between1962—1980. There havebeenno continuoussolar EUV measurementssincethe end of the AE—Esatelliteobservationson 30 December1980. Donnelly /1/ has calledthis paucity of data the “EUVhole”; thereare no new satelliteobservationsscheduledin the nearfuture.

The ASSI measuredbothsolar andairgiow irradiancefrom 20—700 nm and thereforehad to meetconflicting requirementsin spectral ranges, resolutions, pointing accuracy, and fields of view.Schmidtkeet al. /2/ describe the ASSI in detail. For the solarEUV observations<130 nm, ASSIhad a spectralresolutionof approximately1 nm, independentsolar-pointingcontrol which allowedthe instrumentto collect solar irradianceduring the semiannual23°changein the sun’s inclinationrelative to the orbit planeand satellite spin axis (parallel to the Earth’sspin axis), anda bias vol-tage mechanismcombinedwith a variableentranceslit to extendthe intensity-sensitivityrangeoverseveralordersof magnitude. The solarEUV spectral rangewas 20—110 nm.

During the San Marco D/L mission,two solarEUV rocketsmademeasurementsin bothbroadbandintegratedflux and 1-nm spectralresolutionas discussedby Ogawaet al. /3/ andWoodsandRott-man /4/. Their measurementsprovide importantconstraintson theabsolutemagnitudeof the solarEUV. Ogawa et a!. /3/ describe the 5.0—57.5-nmtotal integratedflux measuredon 24 October1988 by the University of Southern California (USC) rocket with a silicon photodiode detectorwhich had an uncertaintyof ±14%.Woods and Rottman /4/ describethe 30-100-nmirradiance

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(1)256 W. K. Tobiskaet at.

observedwith 1-nm resolution by the Laboratory for Atmosphericand SpacePhysics (LASP)rocket on 10 November 1988 with a spectrographhaving a wavelength-dependentuncertaintyof±5—35%and an overall averageuncertaintyof ±13%.The preflight EUV calibration of ASS! hasreceivedpreliminary validation by theLASP rocketdata.

Since there were no long-term solar EUV measurementsmadein conjunction with the San Maitodataset,the instrumentdegradationcalibration is madeby analyzingthe in-flight calibration modeandby intercomparingthe overlappingchannelsfrom the short wavelengthsthroughthe visible. Inthe latter wavelengthregion (above300 nm) the sun itself is usedas a standardlight sourceduringthe mission. Modeled time seriesEUV irradianceswill be comparedto the observedirradiancetime seriesin order to validate the instrumentdegradationcurve. Tobiska /5/ describesthe solarEUV irradiancemodel which was revisedand extendedfrom the SERF2solar EUV flux model.Themodel, which producesEUV irradiancesat 1 AU by discreteline andwavelengthinterval, usesground- and space-basedmeasurementsof solar chromosphericand coronal emissionsto producemodeledchromosphericand coronal BUY flux, respectively. Prior to the model and data com-parison, an estimatedrange of flux values and some anticipatedfeatures in the observedEUVdatasetarepresentedhere.

MODEL EUV DISCUSSION

The solar EUV flux model used in this study/5/ was developedas a revisedand extendedmodelfrom SERF2describedby TobiskaandBarth /6/. The empiricalmodel is derivedfrom six satellitedatasets,including OSO 1 (28.4 and30.4 nm), OSO 3 (25.6,28.4,and 30.4 nm), OSO4 (30.4nm),OSO 6 (30.4 and 58.4 nm), AEROS A (28.4,30.4,and 58.4 nm), andAE—E (14.0-105.0nm), andsix rockets,including threeionizationcell or silicon photodiodeinstruments,two spectrographs,andone spectrometer.The rocket measurementsdescribedby Ogawa et a!. /3/ and Woodsand Rott-man /4/ are incorporatedinto this model development. Improvementsfrom the SERF2 modelrelevantto the SanMarcomissionincludea moreaccuratereproductionof solar rotationalvariationat all wavelengthsand less uncertaintyin the absolutemagnitudeof several important solar EUVlines. Thesechangescontributed to a simpler model formulation basedupon a multiple linearregressionalgorithm. The modelcan beexpressedas anEUV irradiance,I, at a wavelength,?~,ona givendate,t

I(X,t)=a0(?)+a1(?)F1(t)+ . . +a,~(?t)F,~(t) (1)

with n = 4 independenttermsand whereF5(t) is one of four proxy datasetsusedto modeleitherchromosphericor coronal irradiances. The four proxy datasetsare H Lymana, He I 10,830 Aequivalentwidth scaledto Lymana, F 10.7 daily values,andF 10.7 81-dayrunning meanvalues. Thea5 coefficientsderivedin the modeldevelopmentaredetailedby Tobiska/5/.

The modelwas run from 1 March (88060 in year-dayformat) through 25 December1988 (88360),with the results shown in Figure 1. The irradiancesare binned in 5-nm intervalsbetween2 and105 nm andare slightly smoothedfor displaypurposes. Severalfeaturesare apparentfrom Figure1. The 15—35-nmandthe 85—105-nmintervalscontributethe largestphoton fluxesat the top of theEarth’satmospherefor all levels of solar activity. March throughDecember1988 was during therising phaseof solar cycle 22 and the absolutevalues of the irradiancesgenerally increasein themodel. Finally, the effects of solar rotation, which give a 27-day periodicity with phaseshifts tothe modeledflux, arenoticeable.

An exampleof model flux was selectedto provide estimatesfor the San Marco D period. TheASSI B /IV spectrometerchannel17 hada centerwavelength,?.(~3~),of 30.4 nm at the exit angle,~

3o,with resolution of approximately 1 nm. Correspondingto this, the modeledflux time seriesofthe Sixi (30.3 nm) and Hell (30.4 nm) were summedfor the March—December1988 timeframe.Thesecombinedwavelengthswere selectedfor two major reasons. First, from a solar perspective,the 30.4-nmline is representativeof solarEUV variability on the timescaleof the San Marco mis-sion and therehavebeennumerousmeasurementsof this wavelengthas describedabove. Thesetwo lines providenearlyall the irradiancein the 30-31-nmspectral range. Second,from an aero-nomical perspective,the 30.4-nmwavelengthprovidesa substantialfraction of the terrestrial ther-mosphericheating and photoelectronproduction in the ionosphericF-region during all levels ofsolar activity. The photoabsorptionand photoionizationcross sectionsof the major neutral ther-mosphericconstituentshaverepresentativevaluesat this wavelength.

SolarEIJV Flux (1)257

iO

iO

2.0><10 -iO

~ 1.O:~.c1O 9

.~.~

-~

-

-~.~ .~~.Fig. 1. Solar EUV model flux from 1 March through 25 December1988 between1.8and 105.0 nm in 5-nm bins. Values havebeen smoothedwith a 3-bin running meanalgorithm to reducenoisein the display. The rise of cycle 22 combined with phase-shifted solar rotational variationsare evident in the figure. The largest fluxes, in unitsof photonscm2~ are in the 15—35and85—105 nm intervals.

2.10x1010

1.86x1010 - . -

1.62x1010 - -

USC (estimate)S 1.38x1010 - -

~ 1.14x1010 - ~. -

I 9.OOX

~ 6.60x1O~- ILASP -

4.20x109 - -

1.80x109 I I I

60 90 120 150 180 210 240 270 300 330 360

Day of 1988

Fig. 2. The time seriesof two lines, Sixi (303.31A) and Hen (303.78A), summedduring the period of 1 March (88060)through25 December1988 (88360). Seetext fordiscussionof model values comparedto two rocketdata points, the associateduncer-tainties,andthe characteristicsof the modeledtime series.

Figure 2 showsthe modeledtime serieswhich was heavily weighted by the absolutevaluesfromthe AE—E dataset. The uncertaintyin the AE—E data which were usedfor thesewavelengthswas±35%and is shown in the dottedline error bars on day 185. Superimposedon Figure 2 are therocket-observedSixI plus Hell irradiancesfrom the USC (88298) andLASP (88315)flights. TheUSC observation,which had an uncertaintyof ±14%,denotedby the solid error bars, was per-formedby a silicon photodiodedetectorwhich integratedthe entire 5.0-57.5-nmrange. To obtainline emissionvalues,a first order approximationwas madeof the combinedSixI andHell lines forthe USC flight (F

107 = 168) by using the Air Force GeophysicsLaboratory(AFGL) 14 August1979 rocket flight spectrumdescribedby Van Tassel et a!. 17/. The latter rocket flew during

(1)258 W. K. Tobiskaetat.

similar levels of solar activity (F 10.7 = 154). The AFGL spectrumwas first scaledin the 5.0-57.5-nm range to the USC-observedlevel; the scaled Sixi and He~lines were extractedand thensummed to provide the data point on Figure 2 marked“USC (estimate).” The error bars (dottedlines) were thenexpandedto ±55%to accountfor uncertaintyin the extractionscheme. TheLASPobservationof the 1-nm interval containing thesetwo lines (when F 10.7 = 148) hadan uncertaintyof ±18%(solid error bar). Woods /private communication,1990/ noted that this consideredthequestionof scatteredlight but did not accountfor error resultingfrom applyinga Gaussianfit to theline(s) to obtain the publishedvalue. The WoodsandRottman/4/ publisheddatasetdid not includea listing for the Sixi line at 30.3 nm. When modeling the Hell line, Tobiska /5/ increasedtheLASPmeasurementuncertaintyto ±68%(denotedby the dottederrorbar) to encompass2a values.During the San Marco period, the uncertaintiesin the model daily values (Figure 2) overlap thedatapoint uncertainties.

The Hell (30.4-nm) line provedto be one of the most difficult lines to model in the entire EUVspectrum. Therewere 11 distinct datasets,which were often not consistentwith one another. Mostother lines and intervals had better agreementamongthe fewer datasets. However, in the SanMarco time period, if the AE—E uncertainty is used for the model, then it is consistentwith thesmaller errorbarsof boththe LASPandUSC-estimatedmeasurements.

If the model valuesare assumedto be a reasonableestimateof the Sixi plus Hell irradianceduringthe SanMarco period, then the following featurescan be noted. There is an increasein the aver-ageabsolutevalueof the flux from 1 March through25 December1988. If a linear representationis madeof the model values in the form y(t) = a + bx(t), wherex(t) are day numbers,x(0) is 1March, andy(t) is the model valueon a date, then a = 7.7x i0~andb = 7.2 x 106. This allowsoneto empirically estimatethe ratio of flux on 30 Novemberto 1 April as

J(30.4nm,88335) — 1 22 (2)

1(30.4nm,88092) —

i.e., there is a 22% averageflux increaseover the missionduration. Additionally, solar rotationalmaxima areclearly seenon the dates88105, 88132, 88160,88185,and 88244. A typical peak-to-valley ratio in a 27-day solar rotational feature is betweenthe maximumon 88244and the preced-ing minimum on 88233 (ratio = 1.14). In both the Hell AE—E dataand modelvalues,the typicalpeak-to-valleyratio during therise of cycle 21 was 1.11 to 1.15.

In Figure 3, the model spectrumfor the date of the LASP rocket flight (88315) is shown from1.8—105.0nm at a resolutionof the SC21REFWspectrumdescribedby Hintereggeret a!. /8/. Thisspectrumhasapproximately1—2 A resolution. The He~,HLyman, andCI continuaare prominent,as arediscreteemissionlines throughoutthe spectrum. The total integratedflux for the 30.0—31.0-nm range is 1.0 x 1010 photonscm~2~ ±35%,while the 5.0-57.5-nmrange integratedflux is4.8 x 1010 photonscm~2s~.The uncertaintyin the latter integratedvalue is unquantified. How-ever, Tobiska /5/ shows that this value is well within the expectedrange for moderatesolaractivity.

CONCLUSIONS

A newly revisedand extendedsolar EUV flux modelhasbeenrun to estimatethe characteristicsofthe solar EUV which are observedby the San Marco D ASSI. Observationswere madebetweenMarch and December1988 and the model time seriescovers 1 March through25 December1988.The applicationof this studywill be to assistthe validation of the ASSIbackgroundcountcalibra-tion method.

Model resultsanticipatethat the ASSI observationsshould show a generalincreasein all levelsofsolar flux from the beginning to the endof the missionwith solar rotational featuressuperimposedupon the time series. The mostprominentpeaksin solar rotational activity in themodeloccuron 5datesof 88105, 88132, 88160,88185, and 88244. Particularly high photon countsshould be seenin the 15—35-nm and85—105-nm intervals. In the interval between30-31 nm, which includestheSixI (30.3-nm) and Hell (30.4-nm) lines, the absolute model value is 9 x i09 photons cm2s~±35%for the entire period with a valuenear 1.0 x 1010 on the date of the 10 November 1988LASP rocket flight. For this period during the rise of cycle 22, typical peak-to-valleyratios in

SolarEUV Flux (1)259

1011 I

Integrated flux 50—575 A = 4.82309e+lO ph cm s (or day 88315

1010

~ io~ II

1~ liii ~~ LI~/~ io~

io6 He I ctm H Lyman atm C I atm

10~ I i I

0 200 400 600 800 1000Wavelength (A)

Fig. 3. Model spectrumat the sameresolutionas the SC21REFWreferencespectrumdescribedby Hintereggeret a!. /8/ between1.8 and 105.0nm. The spectrumis createdby first scaling the SC21REFWvaluesin each 5 nm interval by theappropriateratio toachievethe modelvalues. Thediscretelines calculatedby the modelarethensuperim-poseduponthe scaledspectrum.

solar rotationalfeaturesarereasonablybetween1.11 and 1.15. Finally, theprobableabsoluteaver-

ageflux increasein this 1-nm interval from beginningto endof missionis approximately22%.

ACKNOWLEDGEMENTS

This researchwas supportedby the US Army ResearchOffice (USARO) and NATO through thefollowing grants:USARO DAALO3-89-K-0057andNATO RG 0511/87.

REFERENCES

1. R.F. Donnelly, in So!ar Radiative Output Variation, NCAR, Boulder, Proceedingsof a

Workshop,November9—11, 139—142 (1987).

2. G. Schmidtke,P. Seidi, andC. Wita, App!. Opt., 24,3206—3213(1985).

3. H.S. Ogawa, L. R. Canfield, D. McMullin, and D.L. Judge, J. Geophys.Res., 95,4291—4295(1990).

4. T.N. WoodsandG.J. Rottman,J. Geophys.Res.,95, 6227—6236(1990).

5. W.K. Tobiska,J. Atinos. Terr. Phys., 53, 1005—1018 (1991).

6. W. K. TobiskaandC. A. Barth, J. Geophys.Res., 95,8243—8251 (1990).

7. R.A. Van Tassel, W.J. McMahon, and L. Heroux, Environ. Res. Pap., 737,

AFGL—TR-81-O11,Air ForceGeophys.Lab., HanscomAir ForceBase,Mass.(1981).

8. H. B. Hinteregger,K. Fukui, and B. R. Gilson,Geophys.Res.Lett., 8, 1147—1150(1981).