Solar Spectra: Air Mass Zero

ABOUT THE STANDARD SPECTRA

• 2000 ASTM Standard Extraterrestrial Spectrum Reference E-490-00

  In 2000, the American Society for Testing and Materials developed an AM 0 reference spectrum (ASTM E-490) for use by the aerospace community. That ASTM E490 Air Mass Zero solar spectral irradiance is based on data from satellites, space shuttle missions, high-altitude aircraft, rocket soundings, ground-based solar telescopes, and modeled spectral irradiance. The integrated spectral irradiance has been made to conform to the value of the solar constant accepted by the space community; which is 1366.1 W/m².

  In the 0.1195 to 0.41 m range, the values are averages of two different instruments on the Upper Atmosphere Research Satellite (UARS), the Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) and the SOLar STellar Irradiance Comparison Experiment (SOLSTICE), reported by Woods et al. These data were obtained in April 1993 during a period of moderate solar activity, and were scaled by a factor of 0.96843 to match the Neckel and Labs data over the 0.33 to 0.41 m range. In the 0.41 to 0.825 m range, the values are from the McMath Solar Telescope at Kitt Peak, Arizona, as reported by Neckel and Labs. In the 0.825 to 4.0 m range, the values are from the high-resolution solar atlas computed by Kurucz. These data were smoothed to 2-and 20-nm wavelength resolution and scaled by a factor of 1.00085 to match the Neckel and Labs data at 0.825 m. In the 4.0 to 1000 m range, the values are from the logarithmic irradiance versus wavelength fits reported by Smith and Gottlieb. These data were scaled by a factor of 0.99437 to match the Kurucz data at 4.0 m. The composite spectral irradiance data were then scaled by a factor of 0.99745 to force the integrated total irradiance equal to the solar constant.

  ASTM E-490-00 can be ordered on-line here.

  
  

• 1985 Wehrli Standard Extraterrestrial Solar Irradiance Spectrum

  The WMO/WRDC Wehrli Air Mass Zero (AM 0) solar spectral irradiance curve has often been cited and used as the extraterrestrial solar spectal irradiance distribution. This spectral distribution was constructed in 1985, based on the following references:

  • Wehrli, C. "Extraterrestrial Solar Spectrum", Publication no. 615, Physikalisch-Meteorologisches Observatorium + World Radiation Center (PMO/WRC) Davos Dorf, Switzerland, July 1985.

  • Neckel, H. and D. Labs "Improved Data of Solar Spectral Irradiance from 0.33 to 1.25 um", Solar Physics, Vol 74, 1981.

  Wehrli cites four sources for the data used to construct the WMO Extraterrestrial (ETR) spectral distribution. A preliminary version of this spectral distribution is integrated into the NREL Simple Spectral Model SPCTRAL2; and is also the ETR used in generating the ASTM Reference Spectra for terrestrial applications, namely E-891 and E892, which have been combined into ASTM G-159.

  (NOTE: Previous versions of reference spectra, ASTM E-891, E-892, and G-159 have been superceded by ASTM G-173.)

  
  

• A Comparison of ASTM E-490 and Wehrli 1985

  The Excel spreadsheet E490_00a_AM0.xls displays a plot of the two standard spectra, ASTM E490 and Wehrli 1985, showing very small differences.

OTHER (NONSTANDARD) AM0 SPECTRA

There are other, nonstandard AM0 (extraterrestrial, or ETR) solar spectra that have been derived since 1980. Several of these spectra are available as components in the MODTRAN and SMARTS models for atmospheric spectral transmission of sunlight.

DOWNLOAD THE STANDARD SPECTRA

• 2000 ASTM Standard Extraterrestrial Spectrum Reference E-490-00   (119.5 - 1,000,000.0 nm)

• 1985 Wehrli Standard Extraterrestrial Solar Irradiance Spectrum   (199.5 - 10,075.0 nm)

REFERENCES

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