Solar radiometry: Spectral irradiance measurements
Post on 21-Jun-2016
Adv. Space Res. Vol.2, No.4, pp.177183, 1983 0273l177/83/04017707$03.50/OPrinted in Great Britain. All rights reserved. Copyright COSPAR
SOLAR RADIOMETRY: SPECTRALIRRADIANCE MEASUREMENTS
G. E. Brueckner
E. 0. Hulburt Center for Space Research, NavalResearchLaboratory, Washington, D.C. 20373, U.S.A.
The present measurement accuracy of the solar spectral irradiance is insufficient to derivethe real longterm solar spectral irradiance variability at all wavelengths. Possibleerror sources are discussed. A series of new second generation solar irradiance photometersare now under construction which should considerably improve these measurements. At thesame time, efforts are made to improve the absolute DV calibration methods to derive aunified UV radiation scale.
Solar physics and geophysics have shown a renewed interest in solar spectral irradiancemeasurements from space. Einstein and TilE observations have shown that other G5 starshave a different energy distribution of their Xray and ultraviolet spectrum, whichvaries widely from star to star. Therefore, their chromospheres and coronae are dif-ferent from those of the sun.
It has been speculate~that the photosphere of the sun may be variable over timescales ofthe solar cycle, centuries and the evolutionary time of the sun in the HR diagram.
The energy balance of the earth is determined by the solar output. Small changes in photospheric radiation may result in large climatic changes on the earth. Solar ultravioletradiation governs the energy balance of the stratosphere, mesosphere and thermosphere.Solar radiometry from space has distinct advantages: It extends the accessible wave-length ranges into the ultraviolet and infrared and offers a large improvement of themeasurement accuracy.
Although many solar spectral irradiance measurements have been carried Out from soundingrockets and satellites, crucial questions have yet to be answered: What is the absolutesolar spectral irradiance in the ultraviolet and its variation during tiniescales of asolar rotation and a solar cycle? Is there a variation of the suns photospheric tenperatule over a solar cycle or even longer time periods?
SOLAR CYCLE VARIABILITY AND THE PRESENT STATE OF MEASUREMENT ACCURACY
Fig. 1 shows selected measurement discrepancies of the solar ultraviolet irradiance com-piled by Simon [11. The discrepancies refer to different published values which wereobtained within time intervals of not more than two years, to a large extent the 11yearcycle variations are therefore excluded. I~wever, the discrepancies include real solarshortterm variations like the strong 27day modulations. High quality measurements existonly in the spectral range above 300 me. They are carried out from the ground, where itis easy to compare standards directly with solar intensities. Fig. 1 also shows as crossesthe predicted variability of the sun . These values are based on direct measurement ofthe intensity contrast plage vs. quiet sun. The variability model makes the assumptionthat plages are the sole source of enhanced solar ultraviolet radiation and that thisradiation is emitted from the same plage area which is seen in Ca II H and K. The Cooket al. model does not include possible contributions from an enhancement of the quiet sunor from very small local areas which are not included in the Ca II plage index. White andLivingston  have observed the radiance of small areas of the quiet sun at sun center inthe emission core of the Ca II lines over a long period. They do not find any enhancement.Although the Cook et al. model predicts the minimum of solar variability over an 11yearcycle, the values should not be too far from the real variability. It can be seen thatthe UV variability has Its largest values below 152 me (Si triplet edge) and decreases
178 C. E. Brueckner
+ PREDICTED II YEAR SOLAR VARIABILITY PRESENT MEASUREMENT DISCREPANCIES
% +.IOOr~ +i.
I I I120 160 200 240 280 320 360 400 em
Fig. 1. Predicted 11year solar variability (+) and present measurementdiscrepancies (o).
10 I1000 2000 3000 4000
Fig. 2. Spectral irradiance of the sun, the SURP and a deuterium lamp.
Solar Radiometry: Spectral Irradiance 179
rapidly at wavelength longer than the Al edge at 208 nm. The given values above 208 meare estimates, because the measurement accuracy of the plage contrast is insufficient.The value at 400 tim is based on recent measurement of total solar irradiance fluctuations. Fig. 1 clearly shows that a decisive improvement of the measurement accuracy isneeded to improve our knowledge of the solar UV irradiance variability. For120 rim < A < 160 rim the precision must be improved to at least 2% to draw any conclusionabout the longterm solar DV variability. The wavelength regime 210 < A < 300 nmrequires a precision of batter than a fraction of 1 percent. None of the present spacesolar UV irradiance measurements are even close to the required precision. It is difficultto assess the contribution of many error sources to the uncertainties of the solar DVirradiance measurements. Fig. 1 shows a dramatic improvement of measurement accuracy atwavelength A > 340 nm, which is due to the fact that these measurements were carried outfrom the ground by direct comparison with irradiance standards. It is realized that thelack of insitu irradiance standards for space measurements is their largest deficiency.The alignment of a spectrograph, the efficiency of photomultipliers and the transmissionof the optics remain highly questionable either after the launch vibration, the zerogoperation of the instrument or the long time exposure to solar WI radiation during satelliteflights.
Another combined large source of error may be found in the primary calibration standards,the transfer standards and the method of applying these standards to the instrument cali-bration. Two different calibration methods have been applied: use of absolutely calibrateddetectors (diodes) or use of transfer standard light sources. The former method is basedon a relative pyrometric calibration of the diodes with an absolute calibration of thepyrometer in the visible. The very low UV signals introduce errors into this calibrationmethod particularly even in the presence of small amounts of straylight. The second step ofthis calibration method requires the comparison of an absolute calibrated amount of incomingmonochromatic light with the response of the flight instrument. Again these measurementsare extremely sensitive to different amounts of straylight at the front and aft end of theflight experiment.
Using transfer standard light sources one is confronted with two major error sources. First,because of the different (often opposite) shape of the intensity vs. wavelength relation ofthe light sources and the sun, there is a different response of the flight instrument tostraylight which will affect the calibration at all wavelengths. Second, the large intensitydifference between the calibration source and the sun is bound to introduce errors, unlessan extremely careful linearity check of the flight instrument has been carried out (Fig. 2).In addition, dark current changes from ground to space, as well as photomultiplieramplification fluctuations, may contribute to the discrepancies at low intensity values.If a synchrotron light source is used a careful evaluation of the instrument polarizationis needed.
NEW EXPERIMENTS FOR THE NEXT DECADE
In order to eliminate these uncertainties it is required to improve the calibration methodsand to build into the flight experiments many checking and tracking devices. A newgeneration of solar ultraviolet irradiance experiments is now under construction. TheSolar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment of the Naval ResearchLaboratory  consists of two double dispersion spectrometers, seven detectors and adeuterium lamp which is used as an inflight calibration source. Fig. 3 shows the opticalarrangement. The double dispersion Tandem Wadsworth Mount without an intermediate slithas been selected to reduce straylight, to make the experiment as insensitive as possibleto alignment changes, and minimize the number of optical elements which are degrading as aresult of contamination. The optical elements have been arranged out of plane to allow aproper light baffling between the entrance slit and the detector. Two of those spectro-meters are coupled together. A deuterium lamp used as an inflight tracking source can bemoved in front of either spectrometer. One spectrometer is used for the solar observations,the other only to check the absolute output of the deuterium lamp. This allows to measurethe sensitivity of the primary spectrometer even in the presence of intensity fluctuationsof the deuterium lamp. Deuterium lamps are not stable. Measurements of their absoluteoutput show fluctuations up 15%; in the case of a very good lamp 32. However, it hasbeen found that their relative spectral distribution stays within 12over a short periodof time. It is therefore possible to use them as an inflight calibration source andnormalize their output with the solar radiation at longer wavelength (A > 350 nm) wherethe solar variability is small compared with the variability at shorter wavelength(A < 208 nm). The throughput of the calibration spectrometer which is not subject tosolar radiation must be assumed to be constant. The instrument uses photomultipliers aswell as diodes, the latter ones because of their better stability record. (For details ofthe performance characteristics see Table 1).
180 C. E. Brueckner
TOP VIEW DETECTOR~WHEEL
GRATING #1 ,~ RANGEPIVOT POINT.jc
G #2IMAGE PLANESID~~_/
SEALED CANISTER\D2 LAMP
__. .~ ~ CALIBRATION J...~~I SPECTROMETER
IELECT~~SIFig. 3. Schematics of the Solar Ultraviolet Spectral Irradiance Monitor(SUSIM) experiment (OSSl, SL2, EOM, UARS).
- - _____ LEVEL3
GRIND S1 LEVEL I
M3 (15 IS) O~-O~IGRIND ~
SUN LEVEL i
DETECTOR I / /(IivsI 3) ,iY.7/ /~~(25e25) I Iiis
Fig. 4. Schematics of the Thullier et al. solar spectral irradianceexperiment (SL1, EON).
Solar Radiometry: Spectral Irradiance 181
Extensive calibrations of this instrument have been carried Out at the SURF synchrotronfacility as well as with absolutely calibrated deuterium lamps. Discrepancies between thesynchrotron and the deuterium lamp calibration up to 20% have been found which requirefurther investigations. For the variability measurements, the SURF facility is planned asa reference standard over the next decade.
The solar spectral irradiance experiment  employs three independent double monochromators for three different wavelength ranges (see Table 1). Fig. 4 shows the opticalarrangement. The double monochromators have an intermediate slit, which should allow thecomplete suppression of straylight in connection with an appropriate selection of thedetectors cathode response. This experiment uses a set of deuterium lamps and tungstenribbon lamps for inflight tracking and calibration. It also has a builtin hollow cathodelamp to check its wavelength scale, The blackbody radiation of a gold point is used forits absolute calibration.
The University of Colorados ultraviolet solar spectral irradiance experiment  uses asingle monochromator (Fig. 5) which is a modified Wadsworth arrangement. (Performancedetails are listed in Table 1). Light passes through an entrance aperture on a planegrating. The spectrum is focussed by an offaxis parabola on two photomultipliers whichdistinguish different spectral ranges. This arrangement allows the exchange of a verysmall aperture for solar observation with a large aperture to measure the ultravioletradiation of early type stars and use of the stars as a tracking source. Although not verymuch is known about the ultraviolet variability of early type stars, it is possible toderive a mean average LIV intensity from many stars which are to be measured. It must beassumed that the grating does not suffer from differential contamination at its surface,which may be caused because sunlight illuminates only a very small portion of the grating.
-______ . ~ PM 1p __________ .___________ . . ~ PM 2
UNIVERSITY OF COLORADO UV SOLAR SPECTRALIRRADIANCE EXPERIMENT (UARS)
Fig. 5. Schematics of the University of Colorados spectral irradianceexperiment (UARS). Light is entering through a Solar Aperture (SOA),which is a small slit, or the Stellar Aperture (STA). The spectrometeris a plane grating (G) offaxis paraboloid (P) Wadsworth mount. Twophotomultipliers cover different wavelength ranges.
Table 1 also lists the performance characteristics of several other experiments which areeither flying at the present time or are scheduled for flight during the next decade.The lack of tracking sources makes these instruments unsuitable to measure the longtermsolar variability.
182 G. E. Brueckner
SIECIRALWAVELEUITII HESOLUIION IN-FLIOFIT N OF OPTIC
____________ RANGE (NM) (NM) SPECIROMETER DETECTORS CATHODES TRACKING SURFACES
SUSIM I TANDEM WADS-RRL 120-1100 0.1 WORTH DOUBLE 2 P.11. BI ALKALI(SHUTTLE) 5 MONOCHROMATOR 5 DIODES RBIE 02 LAMP (1) 11
SUSHI IINRL ~2 LAMPS (3)(UARS) TUNGSTEN (2)
02 LAMPS (2)THULLIER 160-365 1 DOUBLE P.M. CsT, TUNGSTEN (2)ET AL. 277-889 1 IIONOCIIROMATOR P.11. fRI ALKALI HOLLOW CATHODE(SHUTTLE) 805-3160 20 (DIFFUS) PBS CELL (A) CALlS. 1
120-190 0.12 Cs)UCO 110-310 0.25 SINGLE CSTE EARLY TYPE(UARS) 290-440 0.25 IIONOCIIROMATOR 3 P.M. B! ALKALI STARS 7
EBERI-FASTIEUCO 120-190 0.75 SINGLE MON. CsT(SME) 175-310 0.15 (DIFFUS) CiTE NONE 5
SBUV 160-1100 1 SI ALKALI NONE(OARS)
CZERNY-IIJRNER ~2 LAMPSIAS SINGLE MONO- Cslo TUNGSTEN-HALIO(HCIJI 120-360 0.0015 CIIRtIMATOR 2 P.M. CsTt HOLLOW CATHODE 5
There are many other experiments still carried out from sounding rockets and balloons. Themeasurement discrepancies as shown in Fig. 1 are derived mostly from sounding rocket andballoon experiments. It remains therefore questionable whether these experiments cancontribute anything to improve our knowledge of the longterm UV solar variability.
Fig. 6 shows the planned flight schedule of solar irradiance experiments which are listedin Table 1. It is obvious that an improvement of our knowledge of solar variability can beexpected beginning in 1988 when simultaneous flights of instruments are planned on a freeflying platform (UARS, flight duration 30 months) and on Shuttle flights. The Shuttleflights will have superior precision, because of the short duration and the postflightcalibration capability. However, they do not cover a full 27-day rotational solar period,therefore they provide only scattered points to derive the longterm solar variability.But the Shuttle flights will allow a precise calibration of the freeflying experimentswhich will yield all necessary information about the shorttern fluctuation.