UPPER ATMOSPHERE — MASSET 185 



tion of the oxygen above 100 km. and possibly of the nitrogen also at 

 higher levels. 



Figure 2 illustrates our present knowledge about the temperature 

 variation at different heights. Above 100 km. there is considerable 

 uncertainty due to uncertainty about the degree of dissociation of 

 nitrogen. No direct measurements are yet available at these great 

 heights. 



THE IONIZED LAYERS 



A great deal of research on the properties of the atmosphere in the 

 E and F layers has been carried out by radio methods. The mecha- 

 nism of long-distance propagation of radio waves is roughly as illus- 

 trated in figure 3, which shows a reflecting ionized layer. Further- 

 more, when radio waves of wavelength A enter an ionized layer nor- 

 mally they are totally reflected at a level where the electron concentra- 

 tion n attains the value 7rmc-/e'^X^, where e is the charge and m the mass 

 of an electron and c is the velocity of light. By measuring the time 

 delay between vertical transmission of a ray of given wavelength and 

 reception of the reflected wave, it is possible to determine the varia- 

 tion of n with height in a particular level until the observed wave- 

 length is that for which n attains its maximum value in the layer. 

 Further increase of wavelength then leads to complete penetration of 

 the layer. Rocket methods now make it possible to trace the variation 

 beyond this maximum. The method is to measure the local velocity of 

 a radio wave by observation of the Doppler effect, due to the velocity 

 of the rocket, on a radio signal transmitted from the rocket. The pro- 

 portional Doppler shift in frequency is given by v/V, where v is the 

 velocity component of the rocket in the direction of observation and V 

 the phase velocity of the radio wave transmitted from the rocket in 

 the atmosphere round the rocket. V is given by 





where c is the velocity of light (total reflection occurs effectively 

 where V-* " ) .' 



For a long time data of great accuracy have been obtained by radio 

 methods using many ground stations distributed over a wide range C'f 

 latitude; rocket observations, on the other hand, have hitherto been 

 limited. 



As mentioned earlier, radio-sounding methods have given informa- 

 tion about the temperature distribution and the tidal motions at great 

 heights. The latter observations depend on careful studies of fluctua- 



* This formula neglects the effect of the earth's magnetic field, which modifles 

 the situation considerably. 



