180 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



At 100 km, the threshold of several important regions is reached ; 

 at this height the pressure is only about 10"^ of that at ground level. 

 The temperature, having passed through two peaks and a trough, 

 ranging from 180° K. at 80 km. to 270° K. at 50 km., is here about 

 240° K. The composition of the air, as far as the major constituents 

 are concerned, is still much the same as at ground level ; about four- 

 fifths consists of molecular nitrogen and most of the rest of molecular 

 oxygen. 



At a slightly greater altitude an important change rapidly sets in — 

 the oxygen becomes predominantly atomic. This produces consider- 

 able changes in the absorbing power of the oxygen for solar radiation, 

 and also makes possible certain light-emission processes which are 

 important in determining the color of the night air-glow. 



220 r 

 200 

 180- 

 160- 



J 140 



120- 



F2 Ionized 

 layer 



F, Ionized 

 layer 



]E Ionized 

 layer 

 Oxygen transition 



region "1 D Ionized 



. -J -I region 



J Sodium layer 



Dynamo 

 currents 



Meteor 

 n destruction 

 level 



Aurora 

 and air- 

 glow 



Ozone layer 

 • Everest 



Stratospliere 

 Troposphere 



2, 100 



V 



•5 80 

 < 60 



40 



20 







Figure 1. — Schematic representation of the high atmosphere exhibiting the main regions 

 of interest. (Note that the Fi and Fj ionized layers are shown separately. They are so 

 only in daytime,, merging at night.) 



Another technically important ionized layer is located between the 

 100 km. and 120 km. levels. It is a relatively thin (5-6 km.) one, 

 known as the E layer. Its electron concentration is considerably 

 higher than that of the D region and the absorption very much less, 

 so that the reflection of radio waves from the E layer plays an im- 

 portant part in long-distance radio propagation. 



It seems probable that the main atmospheric currents that are re- 

 sponsible for the so-called steady lunar and solar magnetic variations 

 flow in, or just below, the E layer. These currents result from the 



