Adv. SpaceRes.Vol. 8. No. 7, pp. (7)81—(7)84, 1988 0273—1177/88$0.00 + .50Printed in GreatBritain. All rights reserved. Copyright© 1989 COSPAR

ABSOLUTE, EXTREME-ULTRAVIOLET,SOLARSPECTRALIRRADIANCE MONITOR(AESSIM)Martin C. E. Huber,* PeterL. Smith,** W. H. Parkinson,**M. Kühne*** andM.Kockt*ESASpaceScienceDepartment,ESTEC,Noordwijk, NL—2200AG,

TheNetherlands**Harvard~SmithsonianCenterfor Astrophysics,Cambridge,MA 02138,U.S.A.** *PTB Radiometrielabor,BESSY,D—1000Berlin, F.R.G.

t Institutfür Plasmaphysik,UniversitätHannover,D—3000Hannover1,F.R.G.

ABSTRACT

AESSIM, the Absolute, Extreme-Ultraviolet, Solar Spectral Irradiance Monitor, is designedto measure the absolute Solarspectral irradiance at extreme-ultraviolet (BUY) wavelengths. -The data are required for studiesof the processesthat occur inthe Earth’s upperatmosphereand for predictionsof atmosphericdrag on spacevehicles. AAtSSIMis comprisedof Sun-pointedspectrometersand newly-developed,secondarystandardsof spectral irradiancefor the BUY. Use of the in-orbit standardsources will eliminate the uncertainties causedby changes in spectrometerefficiency that have plagued all previousmeasurements of the Solar spectralEUV flux.

SCIENTIFIC MOTIVATION AND OBJECTIVES

Solar Extreme-Ultraviolet Radiation and the Earth’s Atmosphere

A major theme of upper atmospheric studies for the future will be formulation of a coupled global view that modelsthe entireatmosphere. In order to produce correct results, models that calculate global circulation, temperatures,and compositionalstructure of the upperatmosphere /1/ must have reliable input data. In particular, they require accurate, contemporaneousdata for the BUY radiation from the Sun, the principal sourceof energy for the upper atmosphere. Currently, the Solar EUVinputs are based on parameterizations of data that were obtained about 10 years ago /2/ and are extended by ‘proxy’ or‘surrogate’ indices /3,4,5/ to cover more recenttime periods. The resultant impreciseSolar flux dataleads to uncertainty anderror when attempting to model thermospheric and ionospheric conditions during specific periods.

on Satellites and ~p~çgVehicles

The density of the atmosphere at lower satellite altitudes changes as a result of the variations in Solar EUY output.Orbital lifetimes, the pointing of astronomicaltelescopes,the positions of navigation satellites, and the trajectoriesof spacevehicles that will operate in the thermosphere are influenced by the drag that results. Neutral atmospheredensity models thatdepend upon surrogate BUY flux data are used for spacecraftoperations. Several workshops /6,7/ have addressedthelimitations that result from the useof proxy datafor this purpose; regular and accurate measurementsof the Solar BUY fluxhave been recommended /4,5,8/.

AESSIM Data as Support for Astronomical Science

In the past, astronomical satelliteshave observed backscatteredBUV radiation from the atmospheresof the Earth /9/~andplanets /10/ and from the local interstellar medium /11/. Analysis of similar observationsin the future will be improved byhaving accurate Solar BUY flux data against which the backscattered radiation can be compared. In-orbit, radiometriccalibration of future BUY astronomical satellites, e.g. Lyman/FUSEand EUVE, using AESSIMBUY irradiance standards (seebelow) to create a ‘standard star,’ has been proposed /12/. Finally, for future space missions that will make BUYobservations of the Sun, c.g., the Solar and HelsosphericObseritatorij (SOHO), reliable solar irradiance data will provide afundamental check on the radiometric efficiency of the relevant optics of the satellite telescope-spectrometers.

OTHER MEASUREMENTS OF THE SOLAR EUV FLUX

There have been a number of reviews of Solar BUY flux measurements/13,14/. Recent studies have claimed thatuncertainties may exist in the data. Resolution of the discrepanciesis complicatedby the difficulty in determining the source

of changesin the measuredflux: either the Solar EUV outputcould have varied, the instrumentsensitivity could havechanged,or both could have occurred. However, even if all uncertaintiesin previouslymeasureddatacould be eliminated, they wouldnot be adequate: real-time, accurately-calibrated, EUV spectral irradiance data that can be correlated with othercontemporaneousmeasurementsare required for spacecraftoperations and for current and future researchactivities. Otherthan AESSIM, there is no current or planned instrumentationfor long term, calibrated measurementsof the Solar BUY flux.

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(7)82 M. C. E. Hubereta!.

THE AESSIM INSTRUMENT

Becausethe radiometricsensitivity of BUY spectrometerschangesduring their use,previous measurementsof the absoluteSolarBUY flux have required periodic recalibrating rocket flights and post-flight interpolations, extrapolations, and other adjustmentsto the data that have turned out to be problematic in the long run. AESSIM will take a different approach: it will include, (i)several secondary standards of irradiance and, (ii) spectrometersthat will comparethe spectralirradiance of the Sun to that

from the standard sources (see Figure 1). AESSIM will not have a telescope; the spectrometer alit will form a ‘pinhole camera’image of the Solar disk on the grating.

The AESSIM Spectrometers

AESSIM will contain two, ‘twin’ spectrometers. Normal-incidence and grazing-incidencespectrometersare required to cover thedesired wavelength range, while ‘primary’ and ‘secondary’ spectrometers are desired to minimise degradation caused byexposure to Solar radiation. All spectrometersare f/70, which results in small aberrations and allows for both pointinguncertainties of ±3arc mm and some unused area at the edgesof the gratings. Other parametersare given in Table 1.

The twin, AESSIMnormal incidence spectrometers (NIS) are direct descendantsof the successfulone in the Harvard EUVSpectroheliometer (S-055) on Skylab /15/. The sensitivity of the NIS has been estimated from the information given for forS-055 /16/: for a flux of io~photons cm

2 sec’ typical of Solar BUY lines /2/) onto a 250 tim-diameter spectrometeraperture, approximately 500 counts sec4 would be detected throughout the NIS band.

For EUV wavelengths less than 50 nm, toroidal-grating, grazing-incidence spectrometersprovide very high throughput andmedium spectral resolution with a simple, single-rotation design /17/. The AESSIMtoroidal grating spectrometer (TGS)will be equippedwith long-wavelength ‘pass’ filters to eliminate second-and higher-order radiation. The grating reflectionefficiency for the TGS should be at least 0.05 to 0.2, depending upon the wavelength /18/, and theCEM quantum efficiencyshould be 0.08 to more than 0.4 /19/. Taking minimum valuesand considering a 250~gtmdiameter aperture, the count rateswill be 2 x 10’ counts sec~1for 10’ photons cm2 sec’ incident on the TGS.

SLITMECHANISM -

SCTRQ,/ttCiOENcE

0 Nop~0 ~fCTp~,4NCiDfNcf

0

POINTINGIRRADIANCE MECHANISMCALIBRATIONLAMPS ELECTRONICS

Figure 1. AESSIM ConceptualDiagram

AESSIM Calibration ~

A small, rugged, standardof spectral irradiance for EUV wavelengths has been developedat the Physikalisch-TechnischeBundesanstalt (PTB) in Berlin /20,21/. Somestudiesof this hollow cathode ‘lamp’ have been made especially for the AESSIMapplication (see Figure 2): (1) the lamp has been shown to operate stably at 10 watts, the approximate power level that wouldbe used for AESSIM; (ii) the emissionconsists of a large number of resonancelines of neutral or singly- or doubly-ionized,rare-gasatoms; (iii) about 25 lines, roughly evenly separated in the wavelength range20 to 130 nm, are unblended and suitablefor calibration; (iv) the lines are comparable in intensity to Solar EUV lines; and, (v) gas consumption is 1 litre (STP) hour-’.The output of an early version of the lamp /20/ decreasedby about 5 percent after 50 hours of operationat 800 Watts becausematerial sputtered from the cathodeblocked the optical aperture. The AESSIM version of the lamp will operate at much lowerpower levels and will incorporate additional baffles and differential pumping orifices that protect the optical aperture /21/.At AESSIMpower levels, the lamp outputs should be unchangedfor periods of up to hundreds of hours. If each AESSIMprimary spectrometer were recalibrated approximately every two weeks——in a operation that would require about one hour(including lamp warm-up time)— —50 litres of gas (at one atmospherepressure) would be required per year.

Absolute.EUV SpectralIrradianceMonitor (7)83

Table 1. AESSIM SpectrometerParameters

Normal Incidence Spectrometer Toroidal Grating Spectrometer

f/number 70 sameinput arm length 500 mm 230 mm

output arm length 500 mm 518 mmgrating ruling 1800 mm’ 1800 mm’

diffraction angle 3~ 150~wavelength range 29 nm to 135 nm 5 nm to 54 am

grating dimensions 8 mm x 8 mm 50 mm x 3.3 mmentrance slits 250, and 750 pm diameter, 100 pm and 250 pm diameter,

and 100 pm x 1.5 mm and 100 pin x 1.5 mmexit slit 100 pm x 1.5 mm 230 pm x 1.5 mm

reciprocal dispersion 1.1 urn rnm’ 0.35 nm mrn~’ (nominal)angular dispersion 185 arcsecurn

4 735 arcsecnm~resolution (nominal) 0.3 urn (with 250 pm slit) 0.1 urngrating drive speed 0.75 sec for any step within range same

Figure 2. Hollow Cathode Lamp output1000

Irradiance in unitsof 10’ photonscm2sec~’]~ 100 . as a function of wavelength [in nm] of the

lines in the hollow cathode when operated at~ 10 watts and viewed through a 1.2 mm

E ~ , iN apertureat a distance of 129 mm. Thea a 8 vertical lines in the region 100 to 130 nm

U) 1 . U indicated the wavelengthsof strong lines of• - Xe and Kr that are expectedto be suitable

a AL for calibration purposesbut which have not

C) .1 • He yet been studied at the PTB/BESSY= a Ne :- radiometry laboratory. These hollow

• Kr cathode irradiauce values have been.01 a extrapolated,using the observedfact that the• Ar line intensities are roughly proportional to

.00 1 I the input power, from laboratorydatataken0 20 L~0 60 80 100 120 at 800 Watts.

WAVELENGTH mm]

DISCUSSION

Illumination of AESSIM Gratings

There are a number of second-order, spatial effectsthat will influence the radiometricfidelity of AESSIM: (1) the EUV Sun isspatially non-uniform; (ii) the AESSIM gratingswill probably be spatially non-uniform in reflectivity; (in) the apparentsize ofthe Sun varies throughout the year as the Earth-Sundistance varies; and (iv) because the Sun and the hollow cathodelamps are at different distancesfrom the entranceslit plane, they illuminate the gratingsslightly differently. The net impactof these effects is expected to be small, but will be quantitatively assessedby equipping one of the calibrationlamps with a

smaller apertureso that it illuminates only a small portion of the grating and by using the pointing mechanismto movethe spectrometersso that the grating reflectivity can be mapped.

Absorption and Emission

At minimum SpaceStation altitudes of 350 km, the overhead column density of atomic oxygen will vary significantly withSolar activity but will be of the order of 10” cm2 /22/. The peak of the absorption cross section is 13.5x10” cm2at “62 tim /23/. Thus, there will be 1 to 2 percentabsorptionof theSolar BUY flux when the Sun is viewed within ±45~ofthe zenith. This absorptioncan be modelled and correctionsapplied to the AESSIMdata. Absorption at BUV wavelengths bymaterial outgassedfrom any platformon which AESSIMmight be mountedwill be negligible /24/ and natural BUY emissionsat satellite altitudes will be orders of magnitude weaker than the Solar flux /25/.

ç~)~g~in Radiometric Sensitivity

The detection sensitivity of S-055, which included a telescopemirror that wasexposedto the Solar flux at full intensitythroughout the mission, decreasedby abouta factorof 3 during its 250-daymission/16/. With AESSIM,no opticalcomponentwill seethe full Solar flux and the duty cycle will be 10 percentor less, so the efficiency changescan be estimated tobe of the order of one percent per week. Recalibratiosiusing the on-board standardswill be performed approximately

every two weeks.

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ACKNOWLEDGMENTS

The authors thank R. H. Munro, P. Muller, R. G. Roble, G. A. Victor, and W. J. Wagnerfor their contributions and criticalcomments, and the Ball Aerospace SystemsGroup of Boulder, 00, for scientific and technical support. This work wassupportedin part by NASA Grant NSG-7176 to Harvard University, by a NATO Grant for International Collaboration inResearch,and by the Harvard College Observatory.

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