The solar electromagnetic radiation environment

Abstract

Observations of the spectral distribution of solar electromagnetic radiation have been made from wavelengths shorter than 1 Å (10 −8cm) to wavelengths longer than 100 m (10 4cm) by many different techniques, each of which is applicable over only a small part of the total range. Most of the energy emitted by the sun (98 per cent) lies between 3000 and 40,000 Å. A large fraction of the solar radiation falling between these wavelength limits manages to penetrate the Earth's atmosphere, so that it can be studied from ground level. By observing the amount of solar energy reaching the ground at many times during the day, an extrapolation can be made that gives a reasonably accurate value of the energy received at the top of the atmosphere—the solar constant. No radiation from the Sun at wavelengths below approximately 2900 Å has been detected at ground level, and very little radiation in the i.r. at wavelengths greater than 30–40,000 Å can come through the atmosphere. Therefore, observations in these spectral regions must be made from rockets or satellites. Balloons and certain aircraft can also be used to advantage, especially in the i.r. In addition to the usual difficulties involved in the carrying out of space experiments, there are those due to the lack of adequate laboratory standards and the variability of the Sun in the extreme u.v. and X-ray region. Nevertheless, the uncertainties in information about the spectral distribution of solar radiant flux are smaller than the departures from the solar irradiance curve of the flux from the best high energy solar simulators available.