The Nature of the Upper Atmosphere' 



By H. S. W. Massey 



Department of Physics 

 University College, London 



Three yita-l functions are performed by the atmosphere — thermal 

 insulation, removal of lethal ultraviolet light from solar radiation, 

 and the volatilization of meteors. The ultraviolet absorption takes 

 place mainly at an altitude between 20 and 40 km., where there is a 

 concentration of ozone. The volatilization of meteors, on the other 

 hand, takes place at about 100 km. The thermal effects of the atmos- 

 phere depend on its properties over a wide range of altitude. This 

 is because a number of processes are involved — absorption of the 

 short-wave solar radiation, which takes place at great heights, its 

 degradation to long wavelength heat radiation, and the subsequent 

 effects of these heat waves. 



There are a number of other important upper atmospheric phe- 

 nomena, and it is perhaps best to start by summarizing them as they 

 would be encountered in the course of ascent from the ground (fig. 1). 

 After one passes through the troposphere, the stratosphere is reached 

 at a height of about 12 km. At about the same altitude the primary 

 cosmic rays, consisting mainly of very energetic protons as well as 

 other nuclei, are strongly absorbed, producing the bewildering mul- 

 titude of secondary particles the study of which forms one of the main 

 branches of fundamental research in physics. The next region of 

 importance is the ozone layer. At 70 km. an unexpected constituent 

 becomes apparent, namely sodium. Although present in only minute 

 concentration this sodium makes an important contribution to air- 

 glow effects. Some 10-15 km. higher there are encountered for the 

 first time regions in which there is an appreciable concentration of 

 ions and electrons; that is to say, the concentration is large enough 

 to influence the propagation of radio waves. This ionized region, 

 known as the D region, can be a source of radio fadeouts at times, 

 especially during periods of intense solar activity, when the ioniza- 

 tion in the region is much enhanced. 



' Reprinted by permission from Endeavour, vol. 13, No. 50, April 1954. 



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