Solar energy outside the earth's atmosphere.

Extensive measurements have been made in different countries and by many research groups to determine the solar constant and the spectral distribution of solar energy. Earlier determinations were made from ground-based observations. The extrapolation to zero air mass under the highly variable conditions of the atmosphere led to large uncertainties in the proposed values. In recent years renewed interest in this topic has arisen due to the direct effect of solar radiation on thermal balance of spacecraft, electrical output of solar panels, and other satellite-related problems. Measurements of the solar constant have been made by total irradiance detectors from high altitude research aircraft, balloons, and space probes. Their data have produced converging evidence that the value of the solar constant commonly accepted in earlier years, 2·00 cal cm−2 min−1, was too high. Spectral irradiance measurements at high altitude were made using a variety of monochromators and filter radiometers. The results of all recent high altitude measurements have been critically examined by an ad hoc committee on “Solar Electromagnetic Radiation.” The committee has proposed standard values for engineering use for the solar constant and the solar spectrum. These values will be presented and also the reasons for recommending their acceptance. Detailed comparisons have been made between the revised spectral irradiance values and those which had been widely accepted in earlier years. New values have been computed for frequently-cited astrophysical quantities which are derived from the solar constant and the solar spectrum. The spectrum has been extended to 10A in the X-ray range and to 10 m in the microwave range. The revised value of the solar constant is 1·94 cal cm−2 min−1. The proposed standard solar spectrum shows significant differences from other curves throughout the spectral range.