Absolute, extreme-ultraviolet, solar spectral irradiance monitor (AESSIM)

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<ul><li><p>Adv. Space Res. Vol. 8. No. 7, pp. (7)81(7)84, 1988 02731177/88 $0.00 + .50Printed in Great Britain. All rights reserved. Copyright 1989 COSPAR</p><p>ABSOLUTE, EXTREME-ULTRAVIOLET,SOLAR SPECTRAL IRRADIANCE MONITOR(AESSIM)Martin C. E. Huber,* Peter L. Smith,** W. H. Parkinson,**M. Khne*** andM. Kockt*ESA Space Science Department, ESTEC, Noordwijk, NL2200 AG,The Netherlands**Harvard~SmithsonianCenter for Astrophysics, Cambridge, MA 02138, U.S.A.** *PTB Radiometrielabor, BESSY, D1000 Berlin, F.R. G.t Institutfr Plasmaphysik, Universitt Hannover, D3000 Hannover 1,F.R. G.</p><p>ABSTRACT</p><p>AESSIM, the Absolute, Extreme-Ultraviolet, Solar Spectral Irradiance Monitor, is designed to measure the absolute Solarspectral irradiance at extreme-ultraviolet (BUY) wavelengths. -The data are required for studies of the processes that occur inthe Earths upper atmosphere and for predictions of atmospheric drag on space vehicles. AAtSSIM is comprised of Sun-pointedspectrometers and newly-developed, secondary standards of spectral irradiance for the BUY. Use of the in-orbit standardsources will eliminate the uncertainties caused by changes in spectrometer efficiency that have plagued all previousmeasurements of the Solar spectral EUV flux.</p><p>SCIENTIFIC MOTIVATION AND OBJECTIVES</p><p>Solar Extreme-Ultraviolet Radiation and the Earths Atmosphere</p><p>A major theme of upper atmospheric studies for the future will be formulation of a coupled global view that models the entireatmosphere. In order to produce correct results, models that calculate global circulation, temperatures, and compositionalstructure of the upper atmosphere /1/ must have reliable input data. In particular, they require accurate, contemporaneousdata for the BUY radiation from the Sun, the principal source of 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 orsurrogate indices /3,4,5/ to cover more recent time periods. The resultant imprecise Solar flux data leads to uncertainty anderror when attempting to model thermospheric and ionospheric conditions during specific periods.</p><p>on Satellites and ~p~g Vehicles</p><p>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 astronomical telescopes, the positions of navigation satellites, and the trajectories of spacevehicles that will operate in the thermosphere are influenced by the drag that results. Neutral atmosphere density models thatdepend upon surrogate BUY flux data are used for spacecraft operations. Several workshops /6,7/ have addressed thelimitations that result from the use of proxy data for this purpose; regular and accurate measurements of the Solar BUY fluxhave been recommended /4,5,8/.</p><p>AESSIM Data as Support for Astronomical Science</p><p>In the past, astronomical satellites have observed backscattered BUV radiation from the atmospheres of the Earth /9/~andplanets /10/ and from the local interstellar medium /11/. Analysis of similar observations in 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/FUSE and EUVE, using AESSIM BUY 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 Helsospheric Obseritatorij (SOHO), reliable solar irradiance data will provide afundamental check on the radiometric efficiency of the relevant optics of the satellite telescope-spectrometers.</p><p>OTHER MEASUREMENTS OF THE SOLAR EUV FLUX</p><p>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 discrepancies is complicated by the difficulty in determining the sourceof changes in the measured flux: either the Solar EUV output could have varied, the instrument sensitivity could have changed,or both could have occurred. However, even if all uncertainties in previously measured data could be eliminated, they wouldnot be adequate: real-time, accurately-calibrated, EUV spectral irradiance data that can be correlated with othercontemporaneous measurements are required for spacecraft operations and for current and future research activities. Otherthan AESSIM, there is no current or planned instrumentation for long term, calibrated measurements of the Solar BUY flux.</p><p>(7)81</p></li><li><p>(7)82 M. C. E. Huber eta!.</p><p>THE AESSIM INSTRUMENT</p><p>Because the radiometric sensitivity of BUY spectrometers changes during their use, previous measurements of the absolute SolarBUY 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) spectrometers that will compare the spectral irradiance of the Sun to thatfrom the standard sources (see Figure 1). AESSIM will not have a telescope; the spectrometer alit will form a pinhole cameraimage of the Solar disk on the grating.</p><p>The AESSIM Spectrometers</p><p>AESSIM will contain two, twin spectrometers. Normal-incidence and grazing-incidence spectrometers are required to cover thedesired wavelength range, while primary and secondary spectrometers are desired to minimise degradation caused byexposure to Solar radiation. All spectrometers are f/70, which results in small aberrations and allows for both pointinguncertainties of 3arc mm and some unused area at the edges of the gratings. Other parameters are given in Table 1.</p><p>The twin, AESSIM normal incidence spectrometers (NIS) are direct descendants of the successful one 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 cm2 sec typical of Solar BUY lines /2/) onto a 250 tim-diameter spectrometeraperture, approximately 500 counts sec4 would be detected throughout the NIS band.</p><p>For EUV wavelengths less than 50 nm, toroidal-grating, grazing-incidence spectrometers provide very high throughput andmedium spectral resolution with a simple, single-rotation design /17/. The AESSIM toroidal grating spectrometer (TGS)will be equipped with 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 the CEM quantum efficiencyshould be 0.08 to more than 0.4 /19/. Taking minimum values and considering a 250~gtmdiameter aperture, the count rateswill be 2 x 10 counts sec~1 for 10 photons cm2 sec incident on the TGS.</p><p>SLITMECHANISM -</p><p>SCTRQ,/ttCiOENcE</p><p>0 Nop~0 ~fCTp~,4NCiDfNcf</p><p>0POINTING</p><p>IRRADIANCE MECHANISMCALIBRATIONLAMPS ELECTRONICS</p><p>Figure 1. AESSIM Conceptual Diagram</p><p>AESSIM Calibration ~</p><p>A small, rugged, standard of spectral irradiance for EUV wavelengths has been developed at the Physikalisch-TechnischeBundesanstalt (PTB) in Berlin /20,21/. Some studies of 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 emission consists of a large number of resonance lines of neutral or singly- or doubly-ionized,rare-gas atoms; (iii) about 25 lines, roughly evenly separated in the wavelength range 20 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/ decreased by about 5 percent after 50 hours of operation at 800 Watts becausematerial sputtered from the cathode blocked 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 AESSIM power levels, the lamp outputs should be unchanged for 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 atmosphere pressure) would be required per year.</p></li><li><p>Absolute. EUV Spectral Irradiance Monitor (7)83</p><p>Table 1. AESSIM Spectrometer Parameters</p><p>Normal Incidence Spectrometer Toroidal Grating Spectrometer</p><p>f/number 70 sameinput arm length 500 mm 230 mm</p><p>output arm length 500 mm 518 mmgrating ruling 1800 mm 1800 mm</p><p>diffraction angle 3~ 150~wavelength range 29 nm to 135 nm 5 nm to 54 am</p><p>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,</p><p>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</p><p>reciprocal dispersion 1.1 urn rnm 0.35 nm mrn~ (nominal)angular dispersion 185 arcsec urn</p><p>4 735 arcsec nm~resolution (nominal) 0.3 urn (with 250 pm slit) 0.1 urngrating drive speed 0.75 sec for any step within range same</p><p>Figure 2. Hollow Cathode Lamp output1000</p><p>Irradiance in units of 10 photons cm2 sec~]~ 100 . as a function of wavelength [in nm] of the</p><p>lines in the hollow cathode when operated at~ 10 watts and viewed through a 1.2 mm</p><p>E ~ , iN aperture at a distance of 129 mm. Thea a 8 vertical lines in the region 100 to 130 nm</p><p>U) 1 . U indicated the wavelengths of strong lines of - Xe and Kr that are expected to be suitable</p><p>a AL for calibration purposes but which have notC) .1 He yet been studied at the PTB/BESSY= a Ne :- radiometry laboratory. These hollow</p><p> Kr cathode irradiauce values have been.01 a extrapolated, using the observed fact that the</p><p> Ar line intensities are roughly proportional to.00 1 I the input power, from laboratory data taken</p><p>0 20 L~0 60 80 100 120 at 800 Watts.</p><p>WAVELENGTH mm]DISCUSSION</p><p>Illumination of AESSIM Gratings</p><p>There are a number of second-order, spatial effects that will influence the radiometric fidelity of AESSIM: (1) the EUV Sun isspatially non-uniform; (ii) the AESSIM gratings will probably be spatially non-uniform in reflectivity; (in) the apparent size ofthe Sun varies throughout the year as the Earth-Sun distance varies; and (iv) because the Sun and the hollow cathodelamps are at different distances from the entrance slit plane, they illuminate the gratings slightly differently. The net impactof these effects is expected to be small, but will be quantitatively assessed by equipping one of the calibration lamps with asmaller aperture so that it illuminates only a small portion of the grating and by using the pointing mechanism to movethe spectrometers so that the grating reflectivity can be mapped.</p><p>Absorption and Emission</p><p>At minimum Space Station 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 percent absorption of the Solar BUY flux when the Sun is viewed within 45~ofthe zenith. This absorption can be modelled and corrections applied to the AESSIM data. Absorption at BUV wavelengths bymaterial outgassed from any platform on which AESSIM might be mounted will be negligible /24/ and natural BUY emissionsat satellite altitudes will be orders of magnitude weaker than the Solar flux /25/.</p><p>~)~g~in Radiometric SensitivityThe detection sensitivity of S-055, which included a telescope mirror that was exposed to the Solar flux at full intensitythroughout the mission, decreased by about a factor of 3 during its 250-day mission /16/. With AESSIM, no optical componentwill see the full Solar flux and the duty cycle will be 10 percent or less, so the efficiency changes can be estimated tobe of the order of one percent per week. Recalibratiosi using the on-board standards will be performed approximatelyevery two weeks.</p></li><li><p>(7)84 M. C. E. Huberetal.</p><p>ACKNOWLEDGMENTS</p><p>The authors thank R. H. Munro, P. Muller, R. G. Roble, G. A. Victor, and W. J. Wagner for their contributions and criticalcomments, and the Ball Aerospace Systems Group of Boulder, 00, for scientific and technical support. This work wassupported in part by NASA Grant NSG-7176 to Harvard University, by a NATO Grant for International Collaboration inResearch, and by the Harvard College Observatory.</p><p>REFERENCES</p><p>1. R.G. Roble, E.C. Ridley, A.D. Richmond, and R.E. Dickenson, A Coupled Thermosphere/knosphers General CirculationModel, Geophys. Res. Lett., in press (1988); see also R.G. Roble, Key BUY Inputs to Upper Atmospheric Models, in/25/.</p><p>2. H.E. Hinteregger, K. Fukui, and B.G. Gilson, Observational, Reference and Model Data on Solar EUY, from Measurementson AE-E, Ceophys. Res. Lett. 8, 1147 (1981); see also H.E. Hinteregger, Solar EUV Full-Disk Flux Observations andReference-Data Compilations for 1974 - 1980 and Associated Model Representations, private communication (availablethrough SERFS-WITS /26/).</p><p>3. A.E. Hedin, Correlations Between Thermospheric Density and Temperature, Solar EUV Flux, and 10.7-cm Flux Variations, IGeophys. Res. 89, 828 (1984).</p><p>4. R.F. Donnelly, Solar X-Ray, BUy, and UV Flux, in /7/; see also, Gaps Between Solar UV and EUV Radiometry andAtmospheric &amp;iences, in /25/.</p><p>5. 0.R. White, Ground-Based Surrogates for UV and BUY Fluxes, in /25/.6. M.H. Davis, R.E. Smith, and DL. Johnson, eds., ~pp~ and Middle Atmospheric Density Modeling Requirements for</p><p>Spacecraft ~ and Operations, NASA Conference Publication 2460 (1987).7. F.A. Marcos, ed. Proceedings of the Workshop on Atmospheric Density and Aerodynamic ~g Models for Air Force</p><p>Operations, Air Force Geophysics Laboratory, Bedford, MA (1988).8. G.L. Withbroe, Report of the NASA Review Panel for Predictions of Solar Activity and its Effects on the Upper</p><p>Terrestrial Atmosphere, unpublished (1988).9. S. Chakrabarti, F. Paresce, S. Bowyer, and R. Kimble, The Extreme Ultraviolet Day Airglow, J. Geophys. Res. 88, 4898</p><p>(1983); see also F. Paresce, S. Chakrabarti, S. Bowyer, and ft. Kimble, The Extreme Ultraviolet Spectrum of Dayside andNightaide Aurora: 800-1400 A, J. Geophys. Res. 88, 4905 (1983).</p><p>10. B.R. Sandel et aL, Extreme Ultraviolet Observations from Voyager E Encounter with Satura Science 215, 548 (1982); see alsoR.V. Yelle, B.R. Sandel, D.E. Shemansky, and S. Kumar, Altitude Variation of BUY Emissions and Evidence for PhotonPrecipitation at Low Latitude, in the Saturnian Atmosphere, I. Geophys. Res. 91, 8756 (1986).</p><p>ii. E. Chassefihre, J.L. Bertaux, R. Lallement, and V.G. Kurt, Atomic Hydrogen and Helium Densities of the InterstellarMedium Measured in the Vicinity of the Sun, Astron. Astrophys. 160, 229 (1986).</p><p>12. M.C.E. Huber, M. Kflhne, H. Nussbaumer, and P.L. Smith, Standard Star, an...</p></li></ul>

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