Solar infra-red spectroscopy from high-altitude aircraft

Abstract

The infra-red solar spectrum has been the subject of a fairly continuous study since the first elementary observations of Sir John Herschel in 1832. Figure 12 reproduces the solar spectrum recorded by Langley & Abbott in 1900, and this early record serves as an example to show that much of the sun’s incident energy fails to reach the earth. The main absorbing constituents of the atmosphere are water vapour and CO 2 , and these are the cause of the deep bands in the infra-red spectrum. In recent years other rearr gases, such as O 3 , HDO, CO, CH 4 and N 2 O, have been identified through their characteristic absorption bands. It is interesting to note that the HDO absorption band at 3·67 μ , first reported by Gebbie, Harding, Hilsum & Roberts in 1949, is clearly recorded in the Langley & Abbott spectrum, although with deuterium unknown it was impossible for them to identify it. The rarer constituents of the atmosphere have interested a number of experi­menters in more recent years and, for example, Shaw, Chapman, Howard & Oxholm (1951) have identified some 800 lines of atmphoseric origin in the small region between 3·0 and 5·2 μ . However, at ground level measurements are only possible where some solar energy reaches the earth. In order to make observations in the regions normally obscured it is necessary to reduce the amount of water vapour and carbon dioxide in the path by going to high altitude. The percentage CO 2 content of the atmosphere is approximately constant, and hence the amount of CO 2 between the sun and an observer will be reduced to one-half in going to 18000 ft. and to about one-tenth in going to 50000 ft. The water-vapour content falls off much more rapidly and measurements can be made in the 2·5 to 3·5 μ band by going to only 30000 ft. Migeotte & Neven (1952 a, b ) have made an attempt to overcome the effects of the denser lower atmosphere by making observations from the summit of the Jungfraujoch at a height of almost 12000 ft. By carrying a spectrometer in a modern aircraft it is possible to make detailed observations from heights greater than 50000 ft. A program of high-altitude spectroscopy is being undertaken jointly by the Gassiot Committee of the Royal Society and the Royal Aircraft Establishment, Farnborough. The purpose of the program is to record the solar spectrum out into the far infra-red from a Canberra aircraft flying at these heights.