Data on total and spectral solar irradiance: reply to comments Ann T. Mecherikunnel

NASA Goddard Space Flight Center, Greenbelt, Mary­land 20771. Received 25 February 1984. This Letter is in response to Frohlich's comments1 on

Mecherikunnel et al. 2 The authors2 have given a brief survey of the data on the solar constant and its spectral distribution available at the time of this writing (see p. 1355, paragraph 2 and Table I for recent solar constant measurements from space; paragraph 3 and 4 and Fig. 1 for a comparison of the spectral irradiance data reported by Neckel and Labs3 and NASA/ASTM4). Recently Neckel and Labs5 pubhshed a modified version of the solar spectral irradiance data by in­corporating telluric line corrections. The authors2 are quite aware of developments in the field.

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The spectral irradiance data at ground level reported in the paper2 are computed with accurate spectral water vapor transmission data for the 700-1100-nm wavelength interval; and the computed values are in good agreement with experi­mental observations. The solar constant and its spectral distribution value used in the computations are based on a data set accepted by the American National Standards In­stitute (ANSI), the American Society for Testing Materials (ASTM), and the design criteria for NASA Space Vehicles.4,6-9

The solar constant and its spectral distribution values were derived from aircraft measurements (altitude 11.8 km) made in 1967. The solar constant value is 1353 ± 21 W m - 2 with an uncertainty of ±5% associated with it. In the 0.3-2.2-μm wavelength range the spectral irradiance values were based on four instruments. The agreement between the instruments was within the estimated errors of each.9 Results from these experiments were augmented by an extensive set of filter radiometer measurements obtained by the Ep-pley—Jet Propulsion Laboratory team.6 The estimated error in the spectral irradiance value is ±5%.9 The radiometric calibration was based on the NBS 1964 spectral irradiance standards. In 1973 NBS set up new spectral irradiance standards for the 250-1600-nm spectral region. Generally the 1964 scale is within its stated tolerance (3-8%) throughout the UV and visible region but begins to differ from the 1973 scale with wavelength increasing beyond 1000 nm.10 Inves­tigations of this discrepancy were in progress when Thekae-kara passed away. A few representative values for the cor­rection factors to be applied to the 1964 values to convert it to the 1973 standard are as follows: 250 nm 0.9342; 300 nm 0.9577; 400 nm 0.9803; 500 nm 0.9988; 550 nm 1.000; 600 nm 0.9957; 700 nm 0.9643, 800 nm 0.9703; 1000 nm 0.9672, 1500 nm 0.9262; 1600 nm 0.9140.

Neckel and Labs3,5 data were computed from the radiance measurements at the center of the solar disk made from a mountain station (altitude 3.6 km) in the early 1960s. The uncertainties associated with ground measurements and the problems of extrapolating ground measurements to zero air mass values are widely discussed in the literature. In addition to this there are uncertainties associated with center-to-limb darkening functions needed in the conversion of radiances measured at the center of the solar disk to solar spectral ir-radiance. Moreover, very little is known about the solar ir-radiance variability by the presence of active features such as sunspots and faculae on the solar photosphere.

Neckel and Labs solar spectral data and the standard values of solar spectral irradiance used in our computations were obtained by different measurement techniques, calibration procedures, and data processing methods.

It is not my intention to comment on the absolute accuracy or merits and demerits of any set of measurements. However, the following Hnes quoted from Neckel and Labs5 (pp. 241-242) should throw light on the reasons for using NASA/ASTM values in our computations.2 However, restrictions are set by telluric lines, which occur throughout the largest part of the spectrum. Again we declined to evaluate the telluric line blocking from the FTS spectra themselves {compare Section 4), but tried to profit from the relevant data which are available in the literature. For this reason we can give re­liable spectral averages only for the wavelength region below 8700 Å; beyond that wavelength the continuum data and a lumped solar line absorption have to serve as a substitute.'

Again the current status of the solar constant may be summarized in the following lines quoted from Neckel and Labs5 (pp. 249-250): A summation of all the smallest and

1 January 1985 / Vol. 24, No. 1 / APPLIED OPTICS 9

all the largest values yields lower and upper limit for the solar constant: 1.369 and 1.378 kWm-2. {From the pre­liminary reduction published in 1981 we had obtained 1.368 and 1.377 kWm-2.) Recent quotations of the solar constant, which are based on measurements made from balloons, rockets or satellites by means of radiometers recording the total solar flux, range from 1.3655 {Brusa, 1983) to 1.3727 k W m - 2 {Hickey et al., 1982). Willson {1982) gives 1.3682 kWm - 2 as the mean value obtained on the Solar Maximum Mission. A compilation given by Frohlich {1983)—see also Eddy and Foukal {1983)—seems to favor a value slightly above 1.367 kWm-2. This is 0.44 percent lower than the value proposed by him in 1977, which is 1.373 kWm-2. So it is again demonstrated, that the —unavoidable—average systematic error in our radiation data can not exceed 1 per­cent, but is very likely—by chance—much smaller.

The current status of the solar spectral irradiance data only shows the need to determine the solar constant and its spectral distribution simultaneously from space. Development of calibration techniques, detectors, the availability of space platforms where the atmospheric effects can be completely eliminated, and the possibility of recalibration of the instru­ment after the measurements make it ideal for accurate de­termination of the solar spectral irradiance.

References 1. C. Frohlich, "Data on Total and Spectral Solar Irradiance Com­

ments," Appl. Opt. 22, 3928 (1983). 2. A. T. Mecherikunnel, J. A. Gatlin, and J. C. Richmond, "Data on

Total and Spectral Solar Irradiance," Appl. Opt. 22, 1354 (1983).

3. H. Neckel and D. Labs, "Improved Data on Solar Spectral Irra-diance," Sol. Phys. 74, 231 (1981).

4. Anon, "Standard Specification for Solar Constant and Air Mass Zero Solar Spectral Irradiance ASTM Standard," E490-73A (1974), Annual Book of ASTM Standards, Part 41 (ASTM, Philadelphia, 1974).

5. H. Neckel and D. Labs, "The Solar Radiation Between 3300 and 12,500 Å," Sol. Phys. 90, 205 (1984).

6. A. J. Drummond and M. P. Thekaekara, Eds., The Extraterres­trial Solar Spectrum (Inst. Environmental Sciences, Mt. Pros­pect, I11. 1973).

7. M. P. Thekaekara, "Solar Irradiance: Total and Spectral and its Possible Variations," Appl. Opt. 15, 1915 (1976).

8. Anon, "Solar Electromagnetic Radiation," NASA SP-8005. 9. M. P. Thekaekara, R. Kruger, and C. H. Duncan, "Solar Irra-

diance Measurements from a Research Aircraft," Appl. Opt. 8, 1370 (1969).

10. W. E. Schneider, Optronic Laboratories, Orlando, Fla. 32809; private communication.