the visible solar spectral irradiance from 350 to 850 nm as measured by the solspec spectrometer...

THE VISIBLE SOLAR SPECTRAL IRRADIANCE FROM 350 TO 850 nmAS MEASURED BY THE SOLSPEC SPECTROMETER DURING THE

ATLAS I MISSION �

GERARD THUILLIER1, MICHEL HERSE1, PAUL C. SIMON2, DIETRICH LABS3,HOLGER MANDEL3, DIDIER GILLOTAY 2 and THOMAS FOUJOLS1

1Service d’Aeronomie du CNRS, F91371 Verrieres le Buisson, France2Institut d’Aeronomie Spatiale de Belgique, 3 avenue Circulaire B1180 Bruxelles, Belgique

3Landessternwarte, Konigstuhl 12, D69117 Heidelberg, Germany

(Received 18 November 1996; accepted 21 April 1997)

Abstract. The SOLSPEC instrument has been built to carry out solar spectral irradiance measurementsfrom 200 to 3000 nm. It consists of three spectrometers designed to measure the solar spectralirradiance in ultraviolet, visible, and infrared domains. It flew with the ATLAS I mission in March1992. This paper is dedicated to the visible part of the solar spectrum. Comparisons with recent dataare shown and differences below 450 nm are discussed.

1. Introduction

The visible part of the solar spectrum is accessible by ground observations. Butthe atmospheric absorptions must be taken into account to derive the absolutespectral irradiance. This part of the solar spectrum has many interests rangingfrom solar physics to climatology and planetary physics. In particular, the solarspectrum presents absorption bands in the visible domain due to several importantminor constituents of the Earth’s atmosphere such as ozone, nitrogen, and carboncompounds, water vapour, etc.

The recent history of the solar irradiance measurements appears as follows.In a first period, data were collected by Labs and Neckel (1962 and successive

papers), Peytureaux (1968) from the ground, and Arvesen, Griffin, and Pearson(1969) and Thekaekara (1974) from airplanes at an altitude of about 12 km. Labsand Neckel’s solar spectral irradiance was deduced from measurements at theJungfraujoch (3600 m altitude) at the centre of the solar disk and corrected forcentre-to-limb variations and atmospheric transmission. In general, measurementsare carried out from high-altitude observatories or airplanes to minimize the cor-rections due to atmospheric absorption. However, the main absorptions occurringin the stratosphere are still to be corrected. A review of the data available at thattime was made by Pierce and Allen (1977). Significant discrepancies were found(up to 10%) especially in the UV part of the measurements where the absorptionsare the most important.

� Paper presented at the SOLERS22 International Workshop, held at the National Solar Observat-ory, Sacramento Peak, Sunspot, New Mexico, USA, June 17–21, 1996.

Solar Physics177: 41–61, 1998.c 1998Kluwer Academic Publishers. Printed in Belgium.

42 G. THUILLIER ET AL.

Shaw (1982) has obtained solar irradiances in 10 wavelength bands using inter-ference filters at the Mauna Loa Observatory to measure absolute values and theirvariabilities. For the period of observations, the variability was found smaller than0.5%, while the absolute values were found in agreement within 2% with the Labsand Neckel (1968) spectrum, except at 415.6, 460.0, and 850.5 nm.

The original measurements of Labs and Neckel (1962) have been revised withan improved accuracy. In the visible region 350 to 900 nm, the solar spectralirradiance was re-evaluated by Neckel and Labs (1984) taking into account newcentre-to-limb variation and high-resolution Fourier transform spectra obtained atthe National Solar Observatory of Kitt Peak. The solar irradiance values listedin their publication at 1 nm interval from 330 to 630 nm, and at 2 nm intervalfrom 630 to 870 nm, are currently used as a reference in photochemical modelling,climatology or whatever.

More recently, Wehrli (1992) obtained several measurements by the same tech-nique from balloons and rockets. Lockwood, Tug, and White (1992) providedsolar irradiance using measurements calibrated againstVegaas a stellar irradiancestandard in the range 329.5–850.0 nm from measurements on board the Convair990 at an altitude of 11 km; but they did not correct the telluric absorptions incertain wavelength domains. Solar irradiance measurements were obtained fromthe ground by Burlov-Vasiljev, Gurtovenko, and Matvejev (1995) correspondingto 5 nm resolution, after making a running mean of their radiance measurementsat 1 nm resolution.

From the ground, the technique of measurements is based on Bouguer’s lawwhich applies to monochromatic radiation. The correction of the atmosphericabsorption certainly the most important source of errors, since the method assumesno absorbent time variation from sunrise to sunset, which is not the case for manyminor atmospheric constituents (Brasseur and Solomon, 1984). From orbit, nocorrections due to atmospheric absorption are needed, but other sources of errorsexist due to the space environment, and in particular the possible aging of the opticalelements generated by short-wavelength photons (EUV and below), particles, andreactions with atomic oxygen.

From space, few measurements exist. Only the near-visible domain is coveredup to 410 nm (Cebulaet al., 1996; Woodset al., 1996).

2. Instruments, Observations, and Data Processing

2.1. INSTRUMENT

The measurements have been made by the SOLSPEC instrument, which is com-posed of three distinct spectrometers named UV, VIS, and IR. Each spectrometeris made of a double monochromator, using holographic gratings. The six gratingsare mounted on the same mechanical shaft and rotated by using a stepping motor.

VISIBLE SOLAR SPECTRAL IRRADIANCE FROM 350 TO 850 nm 43

In the visible domain, an elementary step corresponds to about 0.1 nm. For solarobservations, the sampling is made every 10 elementary steps. The three opticalschematics are similar, but detectors differ due to the specific wavelength domainof each spectrometer. For the visible domain, the detector is a photomultiplier tubecooled 20�C below the instrument temperature by using a Peltier effect system.The VIS spectrometer observes in eleven minutes, obtaining the solar irradiancebetween 340 and 850 nm in increments of 1 nm with a spectral resolution of about1 nm. Spectral bandwidth and wavelength increment are slightly varying as a func-tion of wavelength. More details are given by Labset al.(1987) and Thuillieret al.(1981, 1996). The latter is also providing the solar irradiance between 200 and350 nm.

2.2. OBSERVATIONS

ATLAS I was launched from Kennedy Space Flight Center on 24 March 1992 bythe Space Shuttle Atlantis (STS 45) and the mission ended on 2 April.

Solar data were gathered during the four periods when the shuttle was orientedtowards the Sun. After several hours of solar observations, the instrument exper-ienced unexpected high temperatures leading to an increase in the VIS detectordark current as well as, sometimes, unstable measurements in some particularwavelength domains. The spectra recorded in these particular circumstances havebeen eliminated.

The pre- and post-flight calibrations allow one to compare the instrumentresponsivity after solar measurements in orbit. A degradation of 1 to 2% of theinstrument responsivity was found with respect to the first spectrum.

2.3. INSTRUMENT CALIBRATION

2.3.1. Wavelength CalibrationThe wavelength calibration is made, as for the UV spectrometer, using sourcesdelivering lines of known wavelength to determine the dispersion law. They are Krand Ne lamps and a laser.

For these measurements, the gratings are rotated by increment of one elementarystep (� 0:1 nm). Scanning the slit function using 50 elementary steps, allows todefine its centre to a fraction of step. This number is then associated to the linewavelength used for that measurement. This is done with all available lines. Asfor the UV spectrometer, the dispersion law is parabolic, and the correspondingcoefficients are derived by a least-squares process. As already explained, althoughmeasurements are carried out in air, the dispersion law is also usable in vacuum.

The dispersion law for the VIS spectrometer is

� = A0+A1N +A2N2 ;

whereA0 = 235:02 nm,A1 = 0:11104 nm step�1 andA2 = �1:682� 10�6 nmstep�2.


Table ISOLSPEC visible spectrometer characteristics

VIS

Spectral range 340–850 nm

Grating manufacturer Jobin–YvonDiameter 30 mmRuled area diam. 17 mmRuling frequency 1281 mm�1

Curvature radius 100 mmFocal length 99.42 mm

Bandpass at 389nm: 1.4 nm707 nm: 1.2 nm

Increment rste 1 nm

Entrance slit Suprasil ISurface (mm2) 0:16� 1

Detector PMTType EMR 641 EWindow glass 9741Photocathode TrialkaliCooling �T = �20�C

2.3.2. Absolute Irradiance Calibration

2.3.2.1. General description. The absolute calibration of the instrument has beenperformed at the Heidelberg Observatory using a black body described by Grolland Neuer (1972). Its cavity is in pure graphite and is heated to about 2930 Kfor the VIS spectrometer calibration. The black-body temperature is read by useof a pyrometer calibrated by the Physikalisch-Technische Bundesanstalt (PTB) ofBerlin (Germany). The descriptions of the black body and the pyrometer have beengiven by Labset al. (1987) and Thuillieret al. (1996) and are not repeated here.

2.3.2.2. Procedure of measurement.The difficulties encountered during the UVspectrometer calibration were due to the low emissivity of the black body at shortwavelengths and the loss of responsivity by each spectrometer in the blue and rededges of its wavelength range. This last cause remains for the VIS spectrometercalibration enhanced by the detector dark current value. We have chosen to run theblack body as follows:

(i) The black body is heated to a temperature of about 2930 K, allowing theduration of the cavity to be as long as twelve hours.


(ii) Signal integration is made using enough single measurements such thatthe total is always greater than 104 counts. In this case, the statistical precision isalways better than 1% at each grating step.

At each step, the detector dark current and black-body temperature are recorded.

2.3.2.3. Detector linearity. For calibration, the instrument may record somehundreds of counts per second in particular wavelength domains; while observingthe Sun, the count number may reach 200 000 counts per second. This is whythe photomultiplier tube and its associated electronics are operating in conditionswhere the linearity of the counting has to be characterised. The dead time,� , of theelectronics was measured in the laboratory using neutral density filters.

We use the following equation for linearity correction due to statistical losses.

Cc = Cm=(1� �Cm) ;

whereCm is the measured count number andCc the corrected count number with� = (3:6� 0:1)� 10�7.

2.4. DATA PROCESSING

2.4.1. Processing of the Calibration MeasurementsCalibration data are processed as already explained by Thuillieret al. (1996).Special care is dedicated to the dark current subtraction due to its value as comparedto the signal in the blue and red edges of the spectrometer. The responsivity is nota continuous function of wavelength due to the filters which are set in front of thedetector as a function of grating step number. This avoids great count numbers persecond in certain wavelength domains.

2.4.2. Processing of the Solar MeasurementsThe first step consists of selecting spectra obtained in reliable conditions. Therejection criteria are:

– Excessive detector dark current as occurring in the South Atlantic Anomalyor when the instrument is overheated after several hours of exposure to the Sun.

– Partial absorption by the Earth’s atmosphere, which occurs when the line ofsight (instrument to Sun) has a tangent height smaller than 100 km.

– Incomplete spectrum.– Unstable pointing conditions.– Wavelength scale instability.The above criteria have been applied to 81 spectra recorded during the mission.

They were averaged, and the ratio of each spectrum to the mean was calculated.For most of them, this ratio is very close to 1. However, some still show a typicalincreasing or decreasing ratio near deep Fraunhofer lines (Ca, Fe, and H lines). Inthat case, a given Fraunhofer line is shifted by typically 10 steps with respect tothe position it had during the previous recordings. The few cases of this nature that


were found were eliminated. When occurring, this phenomenon was observed onboth UV and VIS spectrometers. For the selection of the UV spectra, the MgII lineat 280 nm was used. As the UV and VIS spectrometers share the same mechanicaldrive, the rejected UV spectra were also rejected in the VIS selection process. Thisphenomenon was identified as due to an offset of the scanning occurring during thespectrum recording. These spectra were either eliminated or rescaled in wavelengthwhen possible. The overall wavelength consistency setting of the selected spectrais 0.05 nm. This value is also confirmed by use of the internal He spectral lamp.

The instrument responsivity is used to convert the mean spectrum (counts persecond at a given wavelength) to absolute solar irradiance at the mean distanceSun-to-Earth for the date of the measurements. Afterwards, it is normalized at1 AU. During the observations, the instrument-to-Sun distance changes with timedue to the Shuttle movement around the Earth. This change has not been consideredin the data reduction because it is negligible with respect to the other sources oferrors.

3. Results and Comparisons

3.1. RESULTS

From the 81 spectra recorded during the solar periods of observation, 39 spectraremained after applying the criteria described above. These were obtained duringday 85, 86, 89, and 91 in the year 1992, due to the better thermal conditionsencountered at the beginning of each period of observations. We made no correctionfor aging since in the visible domain, this effect is much smaller than in the UV.

Variability in the visible domain is very small as compared to what is observedin the UV. Livingston (1992) provided observed variabilities at the wavelength ofsome Fraunhofer lines as a function of the 11-year solar cycle and solar rotation.The greatest figure is 13% for the Ca K line centre (0.1 nm index). Taking intoaccount the SOLSPEC bandwidth and the time interval of our observations, thiseffect is not detectable. Consequently, a mean spectrum of the selected spectra hasbeen built without corrections.

The mean square dispersion of each spectrum with respect to this mean issligthly variable with wavelength. Their values for four wavelengths are given atthe top of Table III. These small numbers are due to the high count rate of theraw measurements and to the coherence in wavelength of the selected spectra. Themean spectrum corresponds to a solar activity level represented by a solar flux at10.7 cm equal to 186 units.

The visible spectrum is presented in Figure 1. Table II gives the solar irradianceas a function of wavelength at 1 AU.


Table IIThe SOLSPEC solar irradiance between 350 and 850 nm. This table is made of four columns. In eachcolumn, the left number is the wavelength in vacuum in nm and the right number is the solar irradiancein mW m�2 nm�1 at 1 AU.


Table IIContinued.


Figure 1. Absolute solar spectral irradiance from the SOLSPEC/ATLAS 1 spectrometer displayedtogether with the spectrum of Neckel and Labs (1984) measured at the Jungfraujoch.

3.2. ERROR ANALYSIS

The error analysis concerning the UV part of the solar spectrum measured by theSOLSPEC instrument, was based on a 2� error estimate. This has been kept forthe visible spectrum for the coherence of our results.

3.2.1. Errors in the Absolute Calibration

(a) Pyrometer calibration error. The pyrometer is not an absolute instrumentand it is calibrated regularly by the Physikalisch-Technische Bundesanstalt (PTB)of Berlin (Germany). This calibration consists of measuring its output when it isobserving a source at a known temperature. The uncertainty of its calibration allowsone to calculate the maximum error of the black-body irradiance. It is found todecrease with increasing wavelength (Table IIIb).

(b) Black-body temperature reading error. The black-body temperature isobtained from the pyrometer output. For a constant temperature, the statisticalfluctuation (1�) of the reading is 20 mV. This corresponds to 1 K. Using Planck’slaw, we calculate the 2� error around 2900 K. It is 0.9% at 370 nm and 0.4% at850 nm.


Table IIIa–dSOLSPEC identified sources of errors and their estimates (%) as a function ofwavelength

(a) r.m.s. of the selected spectra

Wavelength (nm) 370 500 650 850Variance (%) 1.15 0.92 1.04 3.16

(b) Sources of error in calibration (2�)

Wavelength (nm) 370 500 650 850(a) Pyrometer calibration 1.5 1.1 0.9 0.7(b) Pyrometer reading 0.9 0.7 0.5 0.4(c) Wavelength 0.22 0.12 0.1 0.1(d) Distance 0.1 0.1 0.1 0.1(e) Aperture 0.13 0.13 0.13 0.13(f) Misalignment 0.2 0.2 0.2 0.2(g) Air transmission � 0 � 0 � 0 � 0(h) Photon noise and dark current 3.8 1.1 0.9 2.2(i) Instrument function 0.1 0.1 0.9 0

Sub-total (1) 4.1 1.8 1.4 2.4

(c) Sources of error in solar measurements (1�) for an individual spectrum

Wavelength (nm) 370 500 650 850(a) Wavelength shift 0.03 0.0 0.0 0.0(b) Photon noise and dark current 0.45 0.23 0.3 1.1(c) Pointing 0.5 0.5 0.5 0.5

Sub-total (2) 0.7 0.63 1.2

(d) Sources of error in solar measurements (2�)

Wavelength (nm) 370 500 650 850(a) Wavelength shift 0.06 0.0 0.0 0.0(b) Photon noise and dark current 0.18 0.1 0.1 0.4(c) Pointing 1.0 1.0 1.0 1.0

Sub-total (3) 1.1 1.0 1.0 1.1

Total (1+3) 4.3 2.1 1.7 2.7

(c) Error in wavelength knowledge. The instrument has to be set at a well-known wavelength during calibration as well as during the solar measurements.The driving mechanism of the gratings was designed for a high reproductivity in


its positioning. The absolute position is deduced from line measurements using thehollow cathode lamp and several external sources. The 1� error is 0.05 nm. Fora 2900 K black body, this figure generates an error varying with wavelength. It is0.22% at 370 nm and decreases to 0.08% at 850 nm (Table IIIb).

(d) Error in the black-body-to-instrument distance.The spectrometer pre-slit,which is the first optical surface, constitutes the reference instrument plane. Itsdistance from the black body is adjusted by the use of a metallic rod. It is measuredbefore and after each calibration run, while the black body is kept operating withan accuracy of 0.1 mm for a total distance of 850 mm. Consequently, the 2� erroris small (<0.1%).

(e) Black-body aperture diameter error. The surface are of the aperture is49.901 mm2. This value is quoted by the manufacturer to have a precision of 0.2%.As the aperture, made of brass, is cooled to 20�C, there is no correction for thermalexpansion. The diffraction phenomenon is negligible as the aperture diameteris 10 000 times the maximum detectable wavelength of the visible spectrometer(850 nm).

(f) Misalignment. The instrument and the black-body optical axis may suffer froma misalignment. An autocollimation is made by reflecting a laser beam through theempty black-body cavity mounting on the spectrometer pre-slit. The resulting 2�

error is less than 10 arc min. The instrument relative responsivity in the field of viewwas measured in orbit during the ‘criss-cross’ manoeuvers. Because of a diffuserplaced in front of the entrance slit, a maximum of 1% per degree was found. Sincethe black-body aperture is seen from the instrument under a solid angle close tothat of the Sun, it is possible to estimate the error induced by the misalignment.For a 2� error, this corresponds to about 0.2%.

(g)Air transmission. During the black-body calibration, the instrument is flushedwith dry nitrogen. The optical path length of 85 cm between the black body and theentrance diffuser is in air. This distance is too short to give a noticeable absorptionin the visible range.

(h) Dark current and photon noise. These two errors have to be studied togetherdue to the way the calibration is made in order to increase the accuracy in thespectral ranges where the calibration signal is weak. For the visible detector, thedark current is not negligible with respect to the signal generated by the blackbody, as the extended red photocathode cannot be cooled due to the impossibilityto operate at ground the Peltier effect system and its associated heat pipe. However,in order to reduce the dark current, the instrument is installed on a plate cooledby running water. At the Heidelberg Observatory, its temperature is about 13�C.The value of the dark current during black-body calibration is of the order of 500


counts per single measurement (800 ms integration time). It has been verified thatthe distribution of the dark current is gaussian. In order to improve the statistics, thedark current is summed during 80 s. This results in a 1� accuracy of 0.5% for thedark current mean value. As the temperature of the instrument and, consequently,the photocathode temperature is slowly varying, dark current measurements aremade regularly.

It has been also verified that for a constant signal the statistical distribution of thenumber of counts is gaussian as the signal is usually large. To improve the statistics,the number of counts is summed up to 10 000 counts for the signal generated bythe black body. Then, the statistical precision is 1% at 1�. The operation is madeas follows: at a given wavelength, the number of counts (including dark current) ismeasured, and the dark current is subtracted. Then, the duration of summation isdeduced. There are two cases:

(a) The black-body signal is less than the dark current.This is the situation at both edges of the spectral range of the visible spectromet-

er. For example at 370 nm, the black-body signal is about 200 counts per elementarymeasurement (800 ms). In order to have a 1% accuracy of the signal, 10 000 countsare summed using fifty measurements. This requires 50 s since the sampling rate isone measurement per second. During the same time, the accumulated counts of thedark current are 500� 50= 25 000 counts and the r.m.s. fluctuation of the signalis added to the r.m.s. fluctuation of the dark current. The resulting r.m.s. fluctuationof the measured effective signal is 1.9%.

(b) The black-body signal is larger than the dark current.In the largest part of the visible spectral range, the signal is at least 2000 counts

per single measurement and the statistics are dominated by the signal photon noise.As the measurement is made typically during about 20 s, the statistical fluctuationof the signal is 0.5% at 1� for 2000 counts per single measurement. In this case,the contribution of the dark current fluctuation is negligible.

(i) Instrument spectral slit function. Each measurement results in the convolutionof the source spectrum by the instrumental slit function. The calibration coefficientsare determined by observing the black-body irradiance. After observing the Sun,these coefficients are used to convert the counts in absolute irradiance. An erroris generated because of the different spectral distribution of the Sun and the blackbody, due in particular to the different gradients of intensity met in calibration andin solar measurements. For the case of the solar continuum, the error is smaller than0.1% at 370 nm. Greater errors may be generated in the presence of Fraunhoferlines. In this case, the magnitude of the error depends on the shape of the Fraunhoferlines and the instrument slit function. When comparing solar spectra as in Figure 1,agreement is generally found for the solar continuum, whereas within regionsof strong Fraunhofer lines, two different instruments generally measure slightlydifferent line depths (for example, the CaII lines at 393 and 396 nm). However,other causes of discrepancy exist which will be discussed in Section 3.3.


3.2.2. Errors during the Solar Observations

(a) Wavelength change. The wavelength scale is checked in flight by using thehollow cathode lamp. The consistency of the wavelength scale of each spectrumused in the mean spectrum calculated above is 0.05 nm. For the solar continuumestimated with the Planck function and an equivalent temperature of 5500 K, theerror is 0.06% at 370 nm and negligible at greater wavelengths. However, largererrors can occur nearby deep Fraunhofer lines.

The Space Shuttle velocity in orbit generates a Doppler shift which can reach0.016 nm at 650 nm. The corresponding error is negligible.

(b) Photon noise and dark-current fluctuation. The instrument temperatureincreases when the instrument is facing the Sun during the observations. Con-sequently, a dark-current measurement is made between each spectrum. The darkcurrent has reached 1400 counts for a single measurement at the end of the solarobservations period. However the selected solar spectra have a low dark currentaround 200 counts per second. The total duration of a dark-current measurement is80 s, leading to an r.m.s. error of 0.8% of its mean value. To take into account a pos-sible variation of the dark current with time, a linear interpolation is made betweenthe dark current measurements performed before and after each solar spectrum.The time interval of 11 min between these two measurements has allowed a linearinterpolation to be adequate for estimating the dark current as a function of time,i.e., at each grating step.

The signal from the Sun is by far larger than the one obtained when calibratingwith the black body. The statistical fluctuation of the dark current is negligible, aswell as the corresponding error on its mean value.

The photon noise is wavelength dependent. For an individual spectrum, it isaround 1% at the ends of the visible spectrometer range. Some typical values at 1�

are shown in Table IIIc.

(c) Pointing error. In orbit, the Space Shuttle manoeuvers ‘criss-cross’ to allowone to determine the position of the instrument optical axis. These manoeuvers areimportant to verify that the pointing error effect is small, as the Sun stays within acone of angle less than 1 deg around the instrument optical axis.

(d) Other errors. Other errors may exist. For example, the width of the electroniccounting gate does not vary in principle, because it is driven by a crystal clock, andalso because the level of the pulse discriminator, which could modify the linearitycorrection, is constant because the thermal range is about the same at ground andin flight. These sources of errors give a negligible contribution to the error budget.

All errors (a) to (c) contribute to make a spectrum slightly different from anyother one among the selected spectra. The observed r.m.s. deviation of the selectedspectra is given in Table IIIa. The r.m.s. deviation is estimated as a function of


Table IVCharacteristics of several spectra in the visible domain

Authors Domains Resolution Increment(nm) (nm) (nm)

Arvesen, Griffin, and Pearson (1969) 300–340 0.4340–380 from 0.2380–620 0.1 0.1620–660 to 0.2660–700 0.3 0.4700–1000 2

Thekaekara (1974) 225–610 0.44 at 350 5610–1000 1.1 at 500 10

Neckel and Labs (1984) 330.5–629.5 1631–869 0.01 2872.5–1247.5 5

Lockwoodet al. (1992) 329.5–549.5 0.6 0.4549.5–848.5 1.2 0.4

Burlov-Vasiljevet al. (1995) 332.5–667.5 5 5Cebulaet al. (1996) 190–410 1.1 0.15

wavelength and is given in Table IIIc as subtotal (2). Its value is close to whatis measured from the selected spectra (Table IIIa), and varies accordingly, butpresents a smaller value by about 0.4% except at 850 nm. This suggests that theremay exist another cause of perturbation, for example the dark current subtraction,particularly in region of weak signal.

Table IIId shows all contributing errors to the mean spectrum. The total uncer-tainty at 370 nm is 4.3% and reduces to 1.7% at 650 nm, but degrades to 2.7% at850 nm. The largest sources of errors are the pyrometer calibration, the weaknessof the signal during calibration measurements at both ends of the spectral rangeand after filter changes, and finally the effect of pointing during flight.

3.3. COMPARISONS

3.3.1. Data SetsData in the visible domain are scarce compared to what is now available inUV, including the measurements carried out by the UARS and ATLAS missions.Table IV lists several solar irradiance data sets which concern our spectral domain.For several spectra, data have been obtained at shorter and/or longer wavelengthsthan the visible. For these cases, Table IV refers only to data which match ourspectral range.


A synthetic spectrum from 200 nm to 7�m has been produced by the WorldRadiation Centre. It is a compilation by Wehrli (1992) of published spectra using,in particular for the visible part, the spectrum of Neckel and Labs (1984). This iswhy the WRC spectrum will not be considered in the comparisons, because theywill be directly performed with the Neckel and Labs spectrum.

The Lockwood, Tug and White (1992) spectrum obtained at the Lowell Obser-vatory contains several telluric absorptions from H2O and molecular oxygen whichhave not been corrected, following the authors. This is why we have not carriedout the comparison above 600 nm.

3.3.2. Method of ComparisonTo make the comparison between different spectra, they can be superposed onthe same figure. However, below a 5% difference, such values are not easilyread. Making the ratio provides more pertinent information, but generates largeoscillations of the ratio which are not always relevant to the absolute irradiancedifference. For example, instruments of different slit functions and/or having aslight wavelength scale shift, but both observing in the same absolute scale, maygenerate data which present large oscillations of their ratio due only to a relativewavelength scale shift or a different slit function or a combination of both.

We have studied these effects numerically for the case of three spectrometersobserving in the same absolute scale, but having different slit functions and offsetsof their wavelength scales. The three slit functions are symmetrical with respectto their central wavelength, have the same maximum transmission (set to 1) andhave the same equivalent width. The wavelength scan increment is 1 nm. The slitfunctions are:

– a rectangle of 1 nm at half height width (R),– a triangle of 1 nm at half height width (T ),– a gaussian of 0.94 nm at half height width in order to have the same equivalent

width as the two other slit functions (G).A numerical integration is made for each slit function from 350 to 900 nm, using

the high-resolution solar spectrum from Delbouille, Roland, and Neven (1973). TheG andT spectra are respectively divided by theR spectrum, taken as reference.Both exhibit the same features as a function of wavelength due to the presence ofstrong Fraunhofer lines. We can distinguish the following features:

– The ratioG=R or T=R are very close. Above 450 nm they are within�1%and within�5% below.

– Some Fraunhofer lines contribute more than 5%, such as the hydrogen Balmerlines, FeI, Mg I, Ca lines at 393, 396, 853, and 856 nm. For the first two Ca lines,30% peaks may be reached. These lines associated with a 1 nm spectrometer slitfunction are those which generate the greatest ratios.

– The peak number increase below 450 nm is due to the increasing density ofFraunhofer lines in that region.


When a running mean is applied to the above ratios, it results in a strongsmoothing. However, the features as listed above remain, but their amplitude issignificantly decreased. For example, the 30% peak generated by the Ca (396 nm)line is now reduced to 2.2% when a running mean over 5 nm is made. But, theFraunhofer line presence is still noticeable.

We have now carried out a calculation which takes into account a possiblewavelength scales shift of the two spectrometers which are compared. This gen-erates other oscillations of the calculated ratios. For a running mean over 5 nm, a0.1 nm shift has a negligible effect. For 0.2 nm, the oscillation reaches 1% below450 nm and is negligible above.

To compare spectra obtained from different spectrometers, we shall use therunning mean method, keeping in mind that the slit function difference and apossible wavelength scale shift of 0.2 nm in the presence of Fraunhofer linescannot generate more than 2% differences. This means that above this thresholdthe differences may be accounted by a real instrument calibration difference.

Table IV shows the instrument characteristics and different spectral resolutionsand increments. Furthermore, some minor wavelength scale shifts may exist amongthese data sets. Using the method of the ratios, these features are able to generatedifferences which are not relevant to absolute calibration, especially below 450 nm.

3.3.3. Analysis of the ComparisonsFigure 2 shows comparisons of different spectra with the SOLSPEC visible spec-trum using the method of the running mean over 5 nm. A ratio smaller than unityindicates that the SOLSPEC irradiance is greater than the irradiance spectrum usedin the comparison.

Figure 2(a) shows the comparison with the Lockwood, Tug, and White (1992)spectrum. As expected, the Fraunhofer lines generate strong variations below400 nm, but the mean stays around unity. However, the ratio is greater than unityfrom 400 to 600 nm, with a mean difference of 2%. This difference reaches itsmaximum (above 5%) between 410 and 440 nm. Above 600 nm, it is not use-ful to continue the comparison due to the presence of uncorrected strong telluricabsorption (H2O, O2).

Figure 2(b) shows the comparison with the Arvesen, Griffin, and Pearson (1969)spectrum. This spectrum exhibits similar features compared to the Lockwood, Tug,and White spectrum between 410 and 440 nm. Above, the ratio stays around unity.

Figure 2(c) shows the comparison with the Burlov-Vasiljev, Gurtovenko, andMatvejev (1995) spectrum. Below 400 nm, this spectrum behaves as the otherstwo. But, the above discrepancy observed between 410 and 440 nm is significantlyattenuated. The ratio oscillates around unity, indicating a mean difference close tozero.

Figure 2(d) shows the comparison with the Thekaekara (1972) spectrum. Thisspectrum has the advantage of covering a great wavelength domain from 115 nmto 1000�m with different wavelength increments. This explains why it is still used


Figure 2. Comparison between different spectra and the SOLSPEC/ATLAS 1 spectrum: ratio oftheir respective 5 nm running mean to SOLSPEC. a: Lockwood, Tug, and White, 1992; b: Arvesen,Griffin and Pearson, 1969; c: Burlov-Vasiljev, Gurtovenko, and Matvejev, 1995; d: Thekaekara, 1974;e: Neckel and Labs, 1984.

for certain applications. As expected, below 400 nm great oscillations are shownaround unity. Above this, the ratio stays below unity with a mean value of about5%. The ratio greater than unity between 410 and 440 nm has disappeared.


Figure 3. Absolute solar spectral irradiance from the SSBUV and SOLSPEC spectrometers measuredduring the ATLAS 1 mission, in the 350–410 nm domain.

Figure 2(e) shows the comparison with the Neckel and Labs (1984) spectrum.The ratio of the two spectra exhibits oscillations due to the Fraunhofer lines, a localslope at 760 nm is induced by the SOLSPEC spectrometer filter change. Below500 nm, this figure shows a difference increasing towards the short wavelengths.This was pointed out originally by Peytureaux (1968) quoting this difference tobe about 8%. The same feature has been also measured by Shaw (1982) from theMauna Loa Observatory, reporting 4% at 416 nm and 460 nm. On board SpaceLab 2,the SUSIM spectrometer measured the solar irradiance up to 410 nm (VanHoosieret al., 1988). These data also showed greater values than those given by Neckel andLabs (1984) by some percent. The SSBUV data (Cebulaet al., 1996) obtained onboard ATLAS 1 are in close agreement with SOLSPEC visible spectrum as shown inFigure 3, both being obtained at the same time from orbit. However, the Fraunhoferlines are observed with different depths by the two instruments. Although they haveclose bandwidth (about 1 nm), the SSBUV spectral sampling is every 0.15 nm whileit is 1 nm for SOLSPEC. This explains why the Fraunhofer lines are differentlyvisualized in Figure 3. More recently, Burlov-Vasiljev, Gurtovenko, and Matvejev(1995) from ground-based measurements also found below 500 nm greater spectralirradiance than those of Neckel and Labs (1984). Above 500 nm, the agreement isvery good since the ratio is oscillating around unity.


Table VDifferences between several data sets and SOLSPEC spectrum for some wavelength domains

Wavelength domain � < 400 410< � < 440 � > 400 � > 400Mean (%) and r.m.s. (%) m � m � m � m �

Lockwoodet al., 1992 2 2 6.2 2.3 1.9 1.3 2.4 0.9Arvesenet al. , 1969 �2 4 4.4 2.8 0.7 2.5 0.8 2.7Burlov-Vasiljevet al., 1995 1.0 5.0 2.3 1.9 �0.8 1.5 �0.8 1.5Thekaekara, 1974 1.2 7.0 �0.3 3.2 �4.0 2.1 �4.3 2Neckel and Labs, 1984 �4.0 1.0 �2.7 2.2 �1.1 1.6 �0.8 1.5Cebulaet al., 1996 �3.3 2.5

Table V summarises the main agreement/disagreement in term of percentages(mean and RMS differences). Above 410 nm, the r.m.s. differences remain aroundtwo to three percent and decrease with wavelength due to the reduced number ofFraunhofer lines.

Regarding the mean values, most of the discrepancies occur below 440 nm.

4. Conclusion

The SOLSPEC instrument has measured the solar spectral irradiance from 350 to850 nm at 1 nm resolution during the ATLAS 1 mission. The instrument was calib-rated on the scale based on the PTB standard by the black body of the HeidelbergObservatory, run at 2930 K.

The absolute accuracy, based on a detailed analysis of the sources of errors,indicates a mean absolute uncertainty of 2 to 3%. This analysis shows that thelargest sources of errors are the pyrometer calibration, the weakness of the signalduring calibration measurements at both ends of the spectral range and after filterchanges, and finally the effect of pointing during flight. The infrared part of theSOLSPEC spectrum at 850 nm is the least accurate due to the weak signal in thatregion. Comparisons with other spectra show an agreement within 2 to 3%, takinginto account the presence of the Fraunhofer lines except below 450 nm, wherediscrepancies may reach five percent.

Acknowledgements

This investigation was supported by the Centre National d’Etudes Spatiales (France),the Centre National de la Recherche Scientifique (France), the Federal Office forScientific, Technical and Cultural Affairs (Belgium), and the Bundesministeriumfur Forschung und Technologie (Germany). The participating institutes are the Ser-vice d’Aeronomie du CNRS, the Institut d’Aeronomie Spatiale de Belgique and


the Landessternwarte of Heidelberg, which provided us with the black body forthe instrument calibration. The ATLAS I mission was conducted by NASA and theflight operations were under the Marshall Space Flight Center responsibility.

The solar spectrum as described in Table II is available in a numerical file onrequest at the following E-mail address: [email protected].

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the visible solar spectral irradiance from 350 to 850 nm as measured by the solspec spectrometer...

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