Some Experimental Techniques of Space Research — Measurements of Solar Electromagnetic Radiation

Abstract

Whilst the corpuscular emission of the sun is undoubtedly of great importance in the ionosphere, principally under disturbed conditions but also under quiet conditions (see for example Chapter 39 by A. P. Willmore), the major influence of the sun occurs as a result of its emission of electromagnetic radiation. Also for the study of the sun itself this is still the most powerful tool available. For wavelengths below 3000 Å, atmospheric absorption is significant throughout the ultraviolet and X-ray spectrum down to the normal shortwave limit of the solar spectrum near 5 Å and far beyond. The radiation between 2000 Å and 3000 Å is mostly absorbed in dissociating ozone, and can be observed above the ozone layer at about 70 km. The radiation between 1000 Å and 2000 Å is absorbed in dissociating molecular oxygen near 100 km, and so can be observed at heights above 130 km. Radiation of shorter wavelengths is absorbed in ionizing various atomic constituents at altitudes which increase with decreasing wavelength to the He II resonance line at 304 Å, which must be observed from altitudes above 230 km. At still shorter wavelengths the radiation is more penetrating, until at 2–3 Å an altitude of 80 km is sufficient.