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Solar irradiance reference spectra for two solar active levels
Advances in Space Research 34 (2004) 256–261 www.elsevier.com/locate/asr
Solar irradiance reference spectra for two solar active levels G. Thuillier
a,*
, L. Floyd b, T.N. Woods c, R. Cebula d, E. Hilsenrath e, M. Herse a, D. Labs f a
c
Service d’Aeronomie du CNRS, 91370 Verrieres-le-Buisson, France b Interferometrics Inc., Chantilly, VA, USA Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA d Science Systems and Applications, Lanham, MD, USA e NASA Goddard Space Flight Center, Greenbelt, MD, USA f Landessternwarte, D69117 Heidelberg, Germany
Received 19 October 2002; received in revised form 26 December 2002; accepted 26 December 2002
Abstract Two new composite solar irradiance reference spectra extending from 0.1 to 2400 nm are constructed using recent space measurements for two distinct time periods during solar cycle 22. For wavelengths above Lyman a, data were gathered from the instruments placed aboard three space platforms, the ATmospheric Laboratory for Applications and Science (ATLAS), the Upper Atmosphere Research Satellite (UARS) and the EUropean Retrieval Carrier (EURECA). Below Lyman a, data from an instrument flown on rockets are used. The two spectra obtained at the time of ATLAS 1 and 3 missions cover about half of the total solar cycle amplitude as gauged by the Mg II and F10.7 solar indices. The accuracy of the spectra varies from 40% in the X-ray to about 3% in the UV–visible, and near IR ranges. A comparison between the total solar irradiance obtained through integration of the reference spectra and their observed values at the time of the ATLAS missions shows agreements of the order of 1%. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Solar irradiance reference spectra; Two solar activity levels
1. Need of a solar spectrum outside the earth atmosphere 1.1. Scientific rationale Accurate solar spectra are needed for several physical disciplines, including studies of the sun, planets, comets, and zodiacal light, the estimation of signal during the preparation of instruments, and material behavior in space. The solar spectrum characterizes the Sun’s radiative energy by wavelength. Accordingly, it also contains information about temperature and composition of the solar atmosphere. Within planetary atmospheres, important photodissociation, photoabsorption, and photoionization processes are wavelength dependent. Thus, the distribution of irradiant solar energy as a function of wavelength is required for the calculation *
Corresponding author. E-mail address: [email protected] (G. Thuillier).
and understanding of the properties (i.e., temperature, composition and dynamics) of planetary atmospheres. The Sun is indeed a variable star and its variation is manifested most strongly in the UV, EUV, and X-rays domains, wavelength ranges which are photochemically active in the Earth’s atmosphere. The solar spectral variability is a function of wavelength and generally increases toward shorter wavelengths. To first order, studies of the physical processes affecting Earth’s climate require the knowledge of the total solar irradiant energy. However, the existence of coupling processes between the stratosphere and the troposphere requires also knowledge of the UV spectrum and its variability. For both atmospheric and climate studies, an accurate solar spectrum is important, for example, in the case of catalytic reaction processes in Earth’s stratosphere. Furthermore, constituent species (e.g., molecular oxygen) have complex photoabsorption structures (Schumann–Runge bands) for which solar
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2002.12.004
G. Thuillier et al. / Advances in Space Research 34 (2004) 256–261
spectral resolution of better than one nanometer is required. Other uses of solar spectra include in the design of the instruments for observing the Sun itself or, for example, observing the Sun as a known radiant source through an intervening atmosphere in studies using occultation methods. In both cases, an estimate of the solar signal is required. Furthermore, for the preparation of instruments for space missions, the solar spectrum is needed for the prediction of materials behavior and the thermal conditions during instrument operation. 1.2. Specifications for a solar spectrum For the applications listed above, a reference solar spectrum should have the following characteristics: • absolute spectral irradiance with the best achievable accuracy; • spectral range from XUV to IR; • spectral sampling/resolution of 1 nm or better; • irradiances in a form as would be received at 1 AU; • spectra representing two distinct levels of solar activity (close to minimum and maximum), thus providing a range of irradiances modulated by the effects of solar activity.
2. Solar data No single instrument is able to measure the full spectral range from the X-ray to IR spectral range because optical elements and detectors have a more limited spectral response. Consequently, a composite spectrum must be constructed using several different spectra obtained from instruments using differing techniques. Data are gathered from instruments on space platforms allowing measurements without absorption by the Earth atmosphere. This is not only mandatory for EUV and UV, but also important in the visible and infrared range. Recent data are preferred because they are provided by instruments whose calibrations were more accurate and were generally better performed and documented. Further, the instruments participating to the UARS and ATLAS missions which will be detailed below, were intercompared before flight and some of them made simultaneous measurements. Solar EUV, UV and visible–infrared irradiance measurements are reviewed by Woods et al. (2004), Rottman et al. (2004), and Thuillier et al. (2004), respectively. Starting in the 1980s, several observations of UV to 400 nm have been made on board SpaceLab I (Labs et al., 1987), SpaceLab II (VanHoosier et al., 1988), the UARS and ATLAS missions. The UARS measurements are made by the Solar Ultraviolet Spectral Irradiance Monitor (SUSM) (Brueckner et al., 1993) and the SOLar STellar Irradiance Comparison Experiment (SOLSTICE)
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Table 1 Instruments, wavelength range, and space platform which are used to construct the two ATLAS spectra Instruments
Range (nm)
Platforms
Reference
EGS and XPS SOLSTICE SUSIM SSBUV SUSIM* SOLSPEC SOSP
0.5–Ly a
3 Rocket flights UARS UARS ATLAS ATLAS ATLAS EURECA
Woods et al. (1998)
Ly a–400 Ly a–400 175–400 Ly a–400 200–870 850–2500
Woods et al. (1996) Floyd et al. (1998) Cebula et al. (1996) VanHoosier (1996) Thuillier et al. (1998a,b) Thuillier et al. (2003)
The corresponding reference for each is indicated.
(Rottman et al., 1993). The UARS observations began in October 1991 and continue to this day. The three ATLAS missions occurred in March 1992, April 1993, and November 1994. Simultaneous observations were made by three ATLAS spectrometers, a second SUSIM* instrument (VanHoosier, 1996), similar to the UARS instrument (Floyd et al., 1998, 2002), the Shuttle Solar Backscatter Ultraviolet (SSBUV) (Cebula et al., 1996) and the SOLar SPECtrum instrument (SOLSPEC) (Thuillier et al., 1997, 1998a,b). After each mission, these instruments were retrieved and recalibrated. The Solar Spectrum (SOSP) instrument, a twin of SOLSPEC, made measurements aboard the EURECA platform from September 1992 to January 1993. XUV and EUV data were obtained by rocket flights made during the 1992–1994 time frame (Woods et al., 1998). Table 1 shows the data that were used to construct the solar reference spectrum.
3. Construction of the spectrum 3.1. Principles Measurements are inevitably beset by two types of uncertainties, systematic and random. Uncertainty analysis performed on the data listed in Table 1 shows that the systematic uncertainties are dominant and their principal origin is the instrument calibration. The UARS data from the SOLSTICE and SUSIM instruments were combined in a single spectrum (Woods et al., 1996); the same was made with SSBUV, SOLSPEC, and SUSIM* aboard ATLAS (Cebula et al., 1996). The two mean resulting spectra were compared, showing an agreement to better than 0.14% for the mean and 1% for RMS differences (Cebula et al., 1996). This level of agreement is indeed smaller than the difference between any two individual spectra participating in this comparison. As the participating spectra have independent design and calibration, it may be possible that the individual systematic experimental uncertainties may
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compensate one another. This principle, applied earlier, has been adopted for the construction of the two spectra shown here. In order to meet the variability requirements, we have considered the ATLAS 1 and 3 periods which correspond to moderately high and low levels of solar activity, respectively. This article presents the key points in the construction of these two spectra. The details and the justifications are given in Thuillier et al. (2004). 3.2. The data sets 3.2.1. The XUV and ELJV (0.5–120 nm) domain Woods et al. (1998) performed rocket flights in the time frame 1992–1994, and obtained solar irradiance data in the XUV–EUV range. Similar observations were previously made aboard the Atmospheric Explorer-E (AE-E) spacecraft, whose main contribution here is the measurement of solar spectral irradiance variability. The rocket data and the XUV–EUV variability from AE-E were used to produce a solar irradiance spectrum at solar minimum and the solar variability applicable to the cycle 22 (Woods and Rottman, 2002). The results of this study are used to generate the spectral solar irradiance below Ly a calculated for the conditions of the ATLAS missions 1 and 3 based on the F10.7 indices given in Table 3. 3.2.2. Far UV (120–200 nm) The mean of the SOLTICE and SUSIM simultaneous observations are calculated for each of the ATLAS 1 and 3 periods. 3.2.3. Near UV (200–400 nm) SOLSTICE and SUSM aboard UARS, and SOLSPEC, SSBUV, and SUSIM* aboard ATLAS 1 and 3 are combined into a single spectrum. Spectral resolutions, slit functions and wavelength scales differ slightly which may cause errors near deep Fraunhofer lines. We attempted to adjust the five wavelength scales within their quoted accuracy to a single scale. No unique solution was found. Consequently, the mean of the five spectra is obtained by linear interpolation to the wavelength scale of the spectrum having the smallest sampling interval. A comparison with the UARS mean spectra shows at 5 nm resolution an agreement better than 0.5% for the mean and a RMS difference of 2% for the wavelength range 200–400 nm.
Larger changes may be seen in the core of certain Fraunhofer lines (Livingston, 1992), but these are not detectable at 1 nm resolution. To reduce the observational uncertainties as well as those stemming from instrumental photometric calibrations, we have used the mean of the three spectra. This only limits the uncertainties of statistical origin (e.g., low counting in the visible spectrometer wings in the calibration measurements), but not the systematic uncertainties that affect several instruments in common, e.g., as that due to the pyrometer used with the blackbody (Mandel et al., 1998) for the absolute calibration. The resulting spectrum has a resolution of 1 nm which must be joined with the 200–400 nm range spectrum having a resolution of 0.25 nm. In order to produce a spectrum not having a resolution jump at 400 nm, we have used the model spectrum of Kurucz and Bell (1995) degraded to 0.5 nm resolution after a photometric normalization to correspond with the mean SOLSPEC spectrum. The choice of 0.5 nm allows to produce a Ca II line profile at 393.4 nm by degrading the model spectrum of Kurucz and Bell (1995), similar to the observed profile. 3.2.5. IR (870–2400 nm) SOSP, an instrument similar to SOLSPEC, was operated from September 1992 to February 1993 aboard the EURECA platform. Better thermal regulation (16 0.5 C) during the mission produced IR measurements of better quality than during the ATLAS missions. The IR spectrometer and the corresponding data processing are described by Thuillier et al. (2003). The SOSP spectrum has been compared to Labs and Neckel (1968), Colina et al. (1996), and Kurucz and Bell (1995) solar continua models. The SOSP spectral irradiance is slightly greater than the other three irradiance spectra (4% between 1500 and 2000 nm and 3% at 2400 nm). Fox et al. (2004) have generated a synthetic solar spectral irradiance model based on radiative transfer calculations. The resulting solar continuum is closer to the SOSP measurements than either Labs and Neckel (1968) or Kurucz and Bell (1995). As the SOSP spectrometer slit function is 20 nm, no Fraunhofer lines are present in the observations. To ensure continuity with the rest of the composite spectra, we have installed the Fraunhofer lines of the Kurucz and Bell model spectrum using the same method as was done in the visible range.
4. Normalization 3.2.4. Visible (400–870 nm) SOLSPEC aboard the three ATLASmissions observed the 400–870 nm wavelength domain. Thuillier et al. (1998a,b, 2004) have reported the consistency of these measurements. The solar variability above 400 nm is very small as predicted by Fontenla et al. (1999), who estimated a change of 0.1% in the solar continuum.
After merging the XUV–EUV, the UV–visible and IR spectra up to 2400 nm, we obtain the two composite ATLAS spectra. At these same times, the total solar irradiance (TSI) was also directly and independently measured (e.g., Crommelynck et al., 1996). Several spectra such as Smith and Gottlieb (1974), Labs and
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Table 2 Selected integrated spectral and total solar irradiances in energy units (W m2 ) for Kurucz and Bell (KB, 1995) and the two ATLAS spectra Spectra
SI < 2397.5
SI > 2397.5
I2397.5
TSI
TSI*
%
KB (1995) ATLAS 1 ATLAS 3
1316.79 1330.28 1330.12
51.46 – –
59.97 61.33 61.33
1368.1 1367.7 1366.7
– 1382.92 1382.74
– 1.11 1.17
Energies below and above 2397.5 nm are given in the second and third columns, respectively. The solar spectral irradiance at 2397.5 nm in units of mW m2 nm1 is shown in the fourth column; these two ATLAS spectral irradiances are identical per construction. TSI’s from KB (1995), and as observed by Fr€ ohlich and Lean (1998) are shown in the fifth column. Column six displays the reconstructed TSI’s (TSI*) from the two composite ATLAS spectra complemented by the KB (1995) model. The last column provides the percentage of difference between the reconstructed TSI and the value corresponding to the ATLAS 1 and 3 periods.
Neckel (1968) and Kurucz and Bell (1995) imply, through direct spectral integration, unique TSI values. Considering that the radiometers measuring TSI have a higher accuracy (0.1%) than do the spectrometers (quoted as about 2% at best), we normalize the two ATLAS composite spectra to the observed TSI at the time of the measurements. Although the EIJV was not observed at that time, its contribution to the TSI is negligible. The IR portion is assumed to be constant based on theoretical considerations (Fontenla et al., 1999). To achieve the desired normalization, the spectral distribution above 2400 nm is needed. As it was not measured, we instead use the model spectrum of Kurucz and Bell (1995) and assume its continuity with the 870– 2400 nm interval (see above). The longest wavelength given in the two spectra is 2397.5 nm. Table 2 provides the details of the energy below and above 2397.5 nm as well as the spectral irradiance at that wavelength for the two ATLAS spectra and the model of Kurucz and Bell (1995). For example, the TSI value derived from the ATLAS 1 initial composite spectrum is the sum of the measured irradiance below 2397.5 nm (1330.28 W m2 ) and the estimated irradiance above 2397.5 nm using the model spectrum of Kurucz and Bell (1995) (51.46 W m2 61.33/59.97). We obtain 1382.9 W m2 to be compared to the 1367.7 W m2 TSI value observed by Fr€ ohlich and Lean (1998) at the ATLAS 1 period. The percentage of difference is then derived. A similar calculation is made for ATLAS 3. The percentage differences between integrated TSI and the measured TSI are similar despite changes in solar activity between the ATLAS 1 and 3 periods. Due to the solar activity change between ATLAS 1 and ATLAS 3, the percentage should be greater for ATLAS 3 than for ATLAS 1 given the identical spectrum used for both above 400 nm. The two composite spectra are now adjusted by the corresponding percentages, which, we note, are smaller than the uncertainties of the spectral measurements. The visible–IR range provides the largest energetic contribution to the TSI; it is provided by two single instruments (SOLSPEC in the visible and SOSP in the IR) which were calibrated with the same equipment (blackbody and pyrometer). This gives rise to a systematic
Fig. 1. The reference spectrum for ATLAS 1 in logarithmic coordinates showing data at short wavelengths, and the quasi linear logarithmic irradiance above 500 nm. The normalization has been implemented in this spectrum.
uncertainty that likely contributes to the 1% difference. The ATLAS 1 and ATLAS 3 spectra are obtained by applying the 1.11% and 1.17% differences to all wavelengths. ATLAS 1 spectrum is displayed in Fig. 1 in logarithmic scale allowing to show the spectral irradiance from XUV to IR.
5. Characteristics of the composite spectra The above spectra correspond to the solar conditions listed in Table 3. This table shows the difference in solar activity between the two ATLAS time periods; the variation experienced between the ATLAS 1 and ATLAS 3 time Table 3 Sunspot number (monthly mean, Rz), the daily F10.7, and the 81-day smoothed hF10.7i for the ATLAS 1 and 3 missions on March 29, 1992 and November 11, 1994, respectively Mission
Rz
F10.7
hF10.7i
ATLAS 1 ATLAS 3
121 20
192 77.5
171 83.5
The monthly mean sunspots number during cycle 22 reaches a maximum of 200 and a minimum of 1. This is why the ATLAS 1 and 3 missions have covered about half of the activity of that cycle.
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correspondence to actual spectra at other times requires further study.
Table 4 Sampling (s) and resolution (r) of the reference spectra Ranges
s (nm)
r (nm)
XUV–EUV Ly a–400 nm 400–2400 nm
1 0.05 0.2–0.6
1 025 0.5
From 400 to 2400 nm, the sampling intervals increase monotonically.
periods is about 50% of the total variation of the solar cycle 22. The solar activity ATLAS 1/ATLAS 3 is taken into account by the normalization on different TSI, but also in the UV domain which contains a change of energy below 400 nm equal to 0.2 W m2 , representing roughly 20% of the TSI variation. This percentage is in agreement with the estimate of Lean et al. (1997), who give for the contribution of the spectral domain below 400 nm to the TSI to be about 30% of the amount from solar maximum to solar minimum. The sampling intervals and resolution vary with respect to wavelength because of the composite nature of the two spectra. They are given in Table 4. The different data incorporated in the composite spectra carry along their own accuracies. The XUV and EUV accuracy is estimated to 40% and 30%, respectively (Woods and Rottman, 2002). From Ly a to 200 nm, the accuracy is quoted to 3.5%. From 200 to 2400 nm, the accuracy is also of this order. However, at the merging wavelength between different datasets, it may degrade to 4%. A more complete analysis including comparisons with earlier solar reference spectra is given by Thuillier et al. (2003, 2004).
6. Conclusion Using the most recent space-based data, we have built two solar spectra corresponding to moderately high and moderately low solar activity conditions as experienced during the ATLAS 1 and 3 periods, representing about half of the overall solar cycle 22 variability. These composite spectra which extend from XUV to 2400 nm, are made by assembling data from different instruments that were part of the UARS and ATLAS missions. The accuracy of these spectra is wavelength dependent, varying from 40% in the XUV, 30% in the EUV, and to about 3% above 200 nm. The solar activity which decreased from ATLAS 1 to ATLAS 3, is taken into account by the measured TSI at these times, and the specific energy variation below 400 nm is included in these two spectra. Extending the two composite spectra above 2400 nm, has made possible the calculation of the corresponding TSI which was found to be 1% above observations, an amount below the quoted accuracy of the spectral data. Since the spectra presented here were derived from solar cycle 22 measurements alone, their
Acknowledgements Each participants have provided the necessary data to build these spectra. Data handling was carried out by Georges Azria from Service d’Aeronomie. R. Cebula, L. Floyd and T.N. Woods were supported by NASA contracts NAS1-98106, S-44772-G, and NAGS-12194, respectively. References Brueckner, G.E., Edlow, K.L., Floyd, L.E., Lean, J.L., VanHoosier, M.E. The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) experiment on board the Upper Atmosphere Research Satellite (UARS). J. Geophys. Res. 98, 10695–10711, 1993. Cebula, R.P., Thuillier, G., Vanhoosier, M.E., Hilsenrath, E., Herse, M., Simon, P.C. Observations of the solar irradiance in the 200– 350 nm interval during the ATLAS 1 mission: A comparison among three sets of measurements – SSBUV, SOLSPEC, and SUSM. Geophys. Res. Lett. 23, 2289–2292, 1996. Colina, L., Bohlin, R.C., Castelli, F. The 0.12–2.5 jam absolute flux distribution of the sun for comparison with solar analog stars. Astrophys. J. 112, 307, 1996. Crommelynck, D., Fichot, A., Domingo, V., Lee, R. SOLCON solar constant observations from the ATLAS missions. Geophys. Res. Lett. 23, 2293–2295, 1996. Floyd, L.E., Reiser, P.A., Crane, P.C., Herring, L.C., Prinz, D.K., Brueckner, G.E. Solar Cycle 22 UV Spectral Irradiance Variability: Current Measurements by SUSM UARS. Solar Phys. 177, 79–87, 1998. Floyd, L.E., Prinz, D.K., Crane, P.C., Herring, L.C. Solar UV Irradiance Variation during cycles 22 and 23. Adv. Space Res. 29, 1957–1962, 2002. Fontenla, J., White, O.R., Fox, P.A., Avrett, E.H., Kurucz, R.L. Calculation of solar irradiances. I. Synthesis of the solar spectrum. Astrophys. J. 518, 480–499, 1999. Fox, P.A., Fontenla, J.M., White, O.R. Solar irradiance variability – comparison of models and observations. Adv. Space Res., this issue, 2004, doi:10.1016/j.asr.2003.08.054. Fr€ ohlich, C., Lean, J. The Sun’s total irradiance’ cycles, trends, and related climate change uncertainties since 1976. Geophys. Res. Lett. 25, 4377–4380, 1998. Kurucz, R., Bell, B. Smithonian Astrophys. Obs., CD rom # 23, 1995. Labs, D., Neckel, H. The radiation of the solar photosphere from 2000 to 100 lm. Z. Astrophys. 69, 1–73, 1968. A Labs, D., Neckel, H., Simon, P.C., Thuillier, G. Ultraviolet solar irradiance measurement from 200 to 358 nm during Spacelab 1 mission. Sol. Phys. 107, 203–219, 1987. Lean, J., Rottman, G.J., Kyle, H.L., Woods, T.N., Hickey, J.R., Puga, L.C. Detection and parametrization of variation in solar mid-and near-ultraviolet radiation 200–400 nm. J. Geophys. Res. 102, 29939–29956, 1997. Livingston, W.C. Observations of solar irradiance variations at visible wavelengths, in: Donnelly R.F. (Ed.), Proceedings of the Workshop on the Solar Electromagnetic Radiation Study for Solar Cycle, vol. 22, pp. 11–19, 1992. Mandel, H., Labs, D., Thuiller, G., et al. Calibration of the SOLSPEC spectrometer to measure the solar irradiance from space. Metrologia 35, 697–700, 1998.
G. Thuillier et al. / Advances in Space Research 34 (2004) 256–261 Rottman, G.J., Woods, T.N., Sparn, T.P. Solar Stellar Irradiance Comparison Experiment: instrument design and operation. J. Geophys. Res. 98, 10667–10677, 1993. Rottman, G., Floyd, L., Viereck, R. Measurement of solar ultraviolet irradiance, in: Pap, J., Fox, P., Frohlich, C., Hudson, H.S., Kuhn, J., McCormack, J., North, G., Sprigg, W., Wu, S.T. (Eds.), Solar Variability and its Effect on the Earth’s Atmosphere and Climate System, AGU Monograph Series. American Geophysical Union, Washington DC, 141, 2004. Smith, E.V.P., Gottlieb, D.M. Solar Flux and its Variations. Space Sci. Rev 16, 771–802, 1974. Thuillier, G., Herse, M., Simon, P.C., Labs, D., Mandel, H., Gillotay, D. Observation of the UV solar spectral irradiance between 200 and 360 nm during the ATLAS I mission by the SOLSPEC spectrometer. Solar Phys. 171, 283–302, 1997. Thuillier, G., Herse, M., Simon, P.C., Labs, D., Mandel, H., Gillotay, D. Observation of the visible solar spectral irradiance between 350 and 850 nm during the ATLAS I mission by the SOLSPEC spectrometer. Solar Phys. 177, 41–61, 1998a. Thuillier, G., Herse, M., Simon, P.C., Labs, D., Mandel, H., Gillotay, D. Observation of the solar spectral irradiance from 200 to 870 nm during the ATLAS 1 and 2 Missions by the SOLSPEC Spectrometer. Metrologia 35, 675–689, 1998b. Thuillier, G., Herse, M., Labs, D., et al. The solar spectral irradiance from 200 to 2400 nm as measured by the SOLSPEC spectrometer from the ATLAS and EURECA missions. Solar Phys. 214, 1–22, 2003. Thuillier, G., Floyd, L., Woods, T.N., Cebula, R., Hilsenrath, E., Herse, M., Labs D. Solar irradiance reference spectra, in: Pap, J., Fox, P., Frohlich, C., Hudson, H.S., Kuhn, J., McCormack, J., North, G., Sprigg, W., Wu, S.T. (Eds.), Solar Variability and its Effect on the Earth’s Atmosphere and Climate System, AGU
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Solar irradiance reference spectra for two solar active levels G. Thuillier
a,*
, L. Floyd b, T.N. Woods c, R. Cebula d, E. Hilsenrath e, M. Herse a, D. Labs f a
c
Service d’Aeronomie du CNRS, 91370 Verrieres-le-Buisson, France b Interferometrics Inc., Chantilly, VA, USA Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA d Science Systems and Applications, Lanham, MD, USA e NASA Goddard Space Flight Center, Greenbelt, MD, USA f Landessternwarte, D69117 Heidelberg, Germany
Received 19 October 2002; received in revised form 26 December 2002; accepted 26 December 2002
Abstract Two new composite solar irradiance reference spectra extending from 0.1 to 2400 nm are constructed using recent space measurements for two distinct time periods during solar cycle 22. For wavelengths above Lyman a, data were gathered from the instruments placed aboard three space platforms, the ATmospheric Laboratory for Applications and Science (ATLAS), the Upper Atmosphere Research Satellite (UARS) and the EUropean Retrieval Carrier (EURECA). Below Lyman a, data from an instrument flown on rockets are used. The two spectra obtained at the time of ATLAS 1 and 3 missions cover about half of the total solar cycle amplitude as gauged by the Mg II and F10.7 solar indices. The accuracy of the spectra varies from 40% in the X-ray to about 3% in the UV–visible, and near IR ranges. A comparison between the total solar irradiance obtained through integration of the reference spectra and their observed values at the time of the ATLAS missions shows agreements of the order of 1%. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Solar irradiance reference spectra; Two solar activity levels
1. Need of a solar spectrum outside the earth atmosphere 1.1. Scientific rationale Accurate solar spectra are needed for several physical disciplines, including studies of the sun, planets, comets, and zodiacal light, the estimation of signal during the preparation of instruments, and material behavior in space. The solar spectrum characterizes the Sun’s radiative energy by wavelength. Accordingly, it also contains information about temperature and composition of the solar atmosphere. Within planetary atmospheres, important photodissociation, photoabsorption, and photoionization processes are wavelength dependent. Thus, the distribution of irradiant solar energy as a function of wavelength is required for the calculation *
Corresponding author. E-mail address: [email protected] (G. Thuillier).
and understanding of the properties (i.e., temperature, composition and dynamics) of planetary atmospheres. The Sun is indeed a variable star and its variation is manifested most strongly in the UV, EUV, and X-rays domains, wavelength ranges which are photochemically active in the Earth’s atmosphere. The solar spectral variability is a function of wavelength and generally increases toward shorter wavelengths. To first order, studies of the physical processes affecting Earth’s climate require the knowledge of the total solar irradiant energy. However, the existence of coupling processes between the stratosphere and the troposphere requires also knowledge of the UV spectrum and its variability. For both atmospheric and climate studies, an accurate solar spectrum is important, for example, in the case of catalytic reaction processes in Earth’s stratosphere. Furthermore, constituent species (e.g., molecular oxygen) have complex photoabsorption structures (Schumann–Runge bands) for which solar
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2002.12.004
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spectral resolution of better than one nanometer is required. Other uses of solar spectra include in the design of the instruments for observing the Sun itself or, for example, observing the Sun as a known radiant source through an intervening atmosphere in studies using occultation methods. In both cases, an estimate of the solar signal is required. Furthermore, for the preparation of instruments for space missions, the solar spectrum is needed for the prediction of materials behavior and the thermal conditions during instrument operation. 1.2. Specifications for a solar spectrum For the applications listed above, a reference solar spectrum should have the following characteristics: • absolute spectral irradiance with the best achievable accuracy; • spectral range from XUV to IR; • spectral sampling/resolution of 1 nm or better; • irradiances in a form as would be received at 1 AU; • spectra representing two distinct levels of solar activity (close to minimum and maximum), thus providing a range of irradiances modulated by the effects of solar activity.
2. Solar data No single instrument is able to measure the full spectral range from the X-ray to IR spectral range because optical elements and detectors have a more limited spectral response. Consequently, a composite spectrum must be constructed using several different spectra obtained from instruments using differing techniques. Data are gathered from instruments on space platforms allowing measurements without absorption by the Earth atmosphere. This is not only mandatory for EUV and UV, but also important in the visible and infrared range. Recent data are preferred because they are provided by instruments whose calibrations were more accurate and were generally better performed and documented. Further, the instruments participating to the UARS and ATLAS missions which will be detailed below, were intercompared before flight and some of them made simultaneous measurements. Solar EUV, UV and visible–infrared irradiance measurements are reviewed by Woods et al. (2004), Rottman et al. (2004), and Thuillier et al. (2004), respectively. Starting in the 1980s, several observations of UV to 400 nm have been made on board SpaceLab I (Labs et al., 1987), SpaceLab II (VanHoosier et al., 1988), the UARS and ATLAS missions. The UARS measurements are made by the Solar Ultraviolet Spectral Irradiance Monitor (SUSM) (Brueckner et al., 1993) and the SOLar STellar Irradiance Comparison Experiment (SOLSTICE)
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Table 1 Instruments, wavelength range, and space platform which are used to construct the two ATLAS spectra Instruments
Range (nm)
Platforms
Reference
EGS and XPS SOLSTICE SUSIM SSBUV SUSIM* SOLSPEC SOSP
0.5–Ly a
3 Rocket flights UARS UARS ATLAS ATLAS ATLAS EURECA
Woods et al. (1998)
Ly a–400 Ly a–400 175–400 Ly a–400 200–870 850–2500
Woods et al. (1996) Floyd et al. (1998) Cebula et al. (1996) VanHoosier (1996) Thuillier et al. (1998a,b) Thuillier et al. (2003)
The corresponding reference for each is indicated.
(Rottman et al., 1993). The UARS observations began in October 1991 and continue to this day. The three ATLAS missions occurred in March 1992, April 1993, and November 1994. Simultaneous observations were made by three ATLAS spectrometers, a second SUSIM* instrument (VanHoosier, 1996), similar to the UARS instrument (Floyd et al., 1998, 2002), the Shuttle Solar Backscatter Ultraviolet (SSBUV) (Cebula et al., 1996) and the SOLar SPECtrum instrument (SOLSPEC) (Thuillier et al., 1997, 1998a,b). After each mission, these instruments were retrieved and recalibrated. The Solar Spectrum (SOSP) instrument, a twin of SOLSPEC, made measurements aboard the EURECA platform from September 1992 to January 1993. XUV and EUV data were obtained by rocket flights made during the 1992–1994 time frame (Woods et al., 1998). Table 1 shows the data that were used to construct the solar reference spectrum.
3. Construction of the spectrum 3.1. Principles Measurements are inevitably beset by two types of uncertainties, systematic and random. Uncertainty analysis performed on the data listed in Table 1 shows that the systematic uncertainties are dominant and their principal origin is the instrument calibration. The UARS data from the SOLSTICE and SUSIM instruments were combined in a single spectrum (Woods et al., 1996); the same was made with SSBUV, SOLSPEC, and SUSIM* aboard ATLAS (Cebula et al., 1996). The two mean resulting spectra were compared, showing an agreement to better than 0.14% for the mean and 1% for RMS differences (Cebula et al., 1996). This level of agreement is indeed smaller than the difference between any two individual spectra participating in this comparison. As the participating spectra have independent design and calibration, it may be possible that the individual systematic experimental uncertainties may
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compensate one another. This principle, applied earlier, has been adopted for the construction of the two spectra shown here. In order to meet the variability requirements, we have considered the ATLAS 1 and 3 periods which correspond to moderately high and low levels of solar activity, respectively. This article presents the key points in the construction of these two spectra. The details and the justifications are given in Thuillier et al. (2004). 3.2. The data sets 3.2.1. The XUV and ELJV (0.5–120 nm) domain Woods et al. (1998) performed rocket flights in the time frame 1992–1994, and obtained solar irradiance data in the XUV–EUV range. Similar observations were previously made aboard the Atmospheric Explorer-E (AE-E) spacecraft, whose main contribution here is the measurement of solar spectral irradiance variability. The rocket data and the XUV–EUV variability from AE-E were used to produce a solar irradiance spectrum at solar minimum and the solar variability applicable to the cycle 22 (Woods and Rottman, 2002). The results of this study are used to generate the spectral solar irradiance below Ly a calculated for the conditions of the ATLAS missions 1 and 3 based on the F10.7 indices given in Table 3. 3.2.2. Far UV (120–200 nm) The mean of the SOLTICE and SUSIM simultaneous observations are calculated for each of the ATLAS 1 and 3 periods. 3.2.3. Near UV (200–400 nm) SOLSTICE and SUSM aboard UARS, and SOLSPEC, SSBUV, and SUSIM* aboard ATLAS 1 and 3 are combined into a single spectrum. Spectral resolutions, slit functions and wavelength scales differ slightly which may cause errors near deep Fraunhofer lines. We attempted to adjust the five wavelength scales within their quoted accuracy to a single scale. No unique solution was found. Consequently, the mean of the five spectra is obtained by linear interpolation to the wavelength scale of the spectrum having the smallest sampling interval. A comparison with the UARS mean spectra shows at 5 nm resolution an agreement better than 0.5% for the mean and a RMS difference of 2% for the wavelength range 200–400 nm.
Larger changes may be seen in the core of certain Fraunhofer lines (Livingston, 1992), but these are not detectable at 1 nm resolution. To reduce the observational uncertainties as well as those stemming from instrumental photometric calibrations, we have used the mean of the three spectra. This only limits the uncertainties of statistical origin (e.g., low counting in the visible spectrometer wings in the calibration measurements), but not the systematic uncertainties that affect several instruments in common, e.g., as that due to the pyrometer used with the blackbody (Mandel et al., 1998) for the absolute calibration. The resulting spectrum has a resolution of 1 nm which must be joined with the 200–400 nm range spectrum having a resolution of 0.25 nm. In order to produce a spectrum not having a resolution jump at 400 nm, we have used the model spectrum of Kurucz and Bell (1995) degraded to 0.5 nm resolution after a photometric normalization to correspond with the mean SOLSPEC spectrum. The choice of 0.5 nm allows to produce a Ca II line profile at 393.4 nm by degrading the model spectrum of Kurucz and Bell (1995), similar to the observed profile. 3.2.5. IR (870–2400 nm) SOSP, an instrument similar to SOLSPEC, was operated from September 1992 to February 1993 aboard the EURECA platform. Better thermal regulation (16 0.5 C) during the mission produced IR measurements of better quality than during the ATLAS missions. The IR spectrometer and the corresponding data processing are described by Thuillier et al. (2003). The SOSP spectrum has been compared to Labs and Neckel (1968), Colina et al. (1996), and Kurucz and Bell (1995) solar continua models. The SOSP spectral irradiance is slightly greater than the other three irradiance spectra (4% between 1500 and 2000 nm and 3% at 2400 nm). Fox et al. (2004) have generated a synthetic solar spectral irradiance model based on radiative transfer calculations. The resulting solar continuum is closer to the SOSP measurements than either Labs and Neckel (1968) or Kurucz and Bell (1995). As the SOSP spectrometer slit function is 20 nm, no Fraunhofer lines are present in the observations. To ensure continuity with the rest of the composite spectra, we have installed the Fraunhofer lines of the Kurucz and Bell model spectrum using the same method as was done in the visible range.
4. Normalization 3.2.4. Visible (400–870 nm) SOLSPEC aboard the three ATLASmissions observed the 400–870 nm wavelength domain. Thuillier et al. (1998a,b, 2004) have reported the consistency of these measurements. The solar variability above 400 nm is very small as predicted by Fontenla et al. (1999), who estimated a change of 0.1% in the solar continuum.
After merging the XUV–EUV, the UV–visible and IR spectra up to 2400 nm, we obtain the two composite ATLAS spectra. At these same times, the total solar irradiance (TSI) was also directly and independently measured (e.g., Crommelynck et al., 1996). Several spectra such as Smith and Gottlieb (1974), Labs and
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Table 2 Selected integrated spectral and total solar irradiances in energy units (W m2 ) for Kurucz and Bell (KB, 1995) and the two ATLAS spectra Spectra
SI < 2397.5
SI > 2397.5
I2397.5
TSI
TSI*
%
KB (1995) ATLAS 1 ATLAS 3
1316.79 1330.28 1330.12
51.46 – –
59.97 61.33 61.33
1368.1 1367.7 1366.7
– 1382.92 1382.74
– 1.11 1.17
Energies below and above 2397.5 nm are given in the second and third columns, respectively. The solar spectral irradiance at 2397.5 nm in units of mW m2 nm1 is shown in the fourth column; these two ATLAS spectral irradiances are identical per construction. TSI’s from KB (1995), and as observed by Fr€ ohlich and Lean (1998) are shown in the fifth column. Column six displays the reconstructed TSI’s (TSI*) from the two composite ATLAS spectra complemented by the KB (1995) model. The last column provides the percentage of difference between the reconstructed TSI and the value corresponding to the ATLAS 1 and 3 periods.
Neckel (1968) and Kurucz and Bell (1995) imply, through direct spectral integration, unique TSI values. Considering that the radiometers measuring TSI have a higher accuracy (0.1%) than do the spectrometers (quoted as about 2% at best), we normalize the two ATLAS composite spectra to the observed TSI at the time of the measurements. Although the EIJV was not observed at that time, its contribution to the TSI is negligible. The IR portion is assumed to be constant based on theoretical considerations (Fontenla et al., 1999). To achieve the desired normalization, the spectral distribution above 2400 nm is needed. As it was not measured, we instead use the model spectrum of Kurucz and Bell (1995) and assume its continuity with the 870– 2400 nm interval (see above). The longest wavelength given in the two spectra is 2397.5 nm. Table 2 provides the details of the energy below and above 2397.5 nm as well as the spectral irradiance at that wavelength for the two ATLAS spectra and the model of Kurucz and Bell (1995). For example, the TSI value derived from the ATLAS 1 initial composite spectrum is the sum of the measured irradiance below 2397.5 nm (1330.28 W m2 ) and the estimated irradiance above 2397.5 nm using the model spectrum of Kurucz and Bell (1995) (51.46 W m2 61.33/59.97). We obtain 1382.9 W m2 to be compared to the 1367.7 W m2 TSI value observed by Fr€ ohlich and Lean (1998) at the ATLAS 1 period. The percentage of difference is then derived. A similar calculation is made for ATLAS 3. The percentage differences between integrated TSI and the measured TSI are similar despite changes in solar activity between the ATLAS 1 and 3 periods. Due to the solar activity change between ATLAS 1 and ATLAS 3, the percentage should be greater for ATLAS 3 than for ATLAS 1 given the identical spectrum used for both above 400 nm. The two composite spectra are now adjusted by the corresponding percentages, which, we note, are smaller than the uncertainties of the spectral measurements. The visible–IR range provides the largest energetic contribution to the TSI; it is provided by two single instruments (SOLSPEC in the visible and SOSP in the IR) which were calibrated with the same equipment (blackbody and pyrometer). This gives rise to a systematic
Fig. 1. The reference spectrum for ATLAS 1 in logarithmic coordinates showing data at short wavelengths, and the quasi linear logarithmic irradiance above 500 nm. The normalization has been implemented in this spectrum.
uncertainty that likely contributes to the 1% difference. The ATLAS 1 and ATLAS 3 spectra are obtained by applying the 1.11% and 1.17% differences to all wavelengths. ATLAS 1 spectrum is displayed in Fig. 1 in logarithmic scale allowing to show the spectral irradiance from XUV to IR.
5. Characteristics of the composite spectra The above spectra correspond to the solar conditions listed in Table 3. This table shows the difference in solar activity between the two ATLAS time periods; the variation experienced between the ATLAS 1 and ATLAS 3 time Table 3 Sunspot number (monthly mean, Rz), the daily F10.7, and the 81-day smoothed hF10.7i for the ATLAS 1 and 3 missions on March 29, 1992 and November 11, 1994, respectively Mission
Rz
F10.7
hF10.7i
ATLAS 1 ATLAS 3
121 20
192 77.5
171 83.5
The monthly mean sunspots number during cycle 22 reaches a maximum of 200 and a minimum of 1. This is why the ATLAS 1 and 3 missions have covered about half of the activity of that cycle.
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correspondence to actual spectra at other times requires further study.
Table 4 Sampling (s) and resolution (r) of the reference spectra Ranges
s (nm)
r (nm)
XUV–EUV Ly a–400 nm 400–2400 nm
1 0.05 0.2–0.6
1 025 0.5
From 400 to 2400 nm, the sampling intervals increase monotonically.
periods is about 50% of the total variation of the solar cycle 22. The solar activity ATLAS 1/ATLAS 3 is taken into account by the normalization on different TSI, but also in the UV domain which contains a change of energy below 400 nm equal to 0.2 W m2 , representing roughly 20% of the TSI variation. This percentage is in agreement with the estimate of Lean et al. (1997), who give for the contribution of the spectral domain below 400 nm to the TSI to be about 30% of the amount from solar maximum to solar minimum. The sampling intervals and resolution vary with respect to wavelength because of the composite nature of the two spectra. They are given in Table 4. The different data incorporated in the composite spectra carry along their own accuracies. The XUV and EUV accuracy is estimated to 40% and 30%, respectively (Woods and Rottman, 2002). From Ly a to 200 nm, the accuracy is quoted to 3.5%. From 200 to 2400 nm, the accuracy is also of this order. However, at the merging wavelength between different datasets, it may degrade to 4%. A more complete analysis including comparisons with earlier solar reference spectra is given by Thuillier et al. (2003, 2004).
6. Conclusion Using the most recent space-based data, we have built two solar spectra corresponding to moderately high and moderately low solar activity conditions as experienced during the ATLAS 1 and 3 periods, representing about half of the overall solar cycle 22 variability. These composite spectra which extend from XUV to 2400 nm, are made by assembling data from different instruments that were part of the UARS and ATLAS missions. The accuracy of these spectra is wavelength dependent, varying from 40% in the XUV, 30% in the EUV, and to about 3% above 200 nm. The solar activity which decreased from ATLAS 1 to ATLAS 3, is taken into account by the measured TSI at these times, and the specific energy variation below 400 nm is included in these two spectra. Extending the two composite spectra above 2400 nm, has made possible the calculation of the corresponding TSI which was found to be 1% above observations, an amount below the quoted accuracy of the spectral data. Since the spectra presented here were derived from solar cycle 22 measurements alone, their
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