THE UPPER ATMOSPHERE DOBSON 195 



years by the observations of radio waves. It would be impossible 

 to send wireless signals to distant parts of the earth if the wireless 

 waves were not bent round to follow the earth's surface. The fact 

 that wireless waves are bent round the earth is due to the existence 

 of the electrically conducting region at a great height, which bends 

 thege radio waves downward in much the same manner that the 

 upper warm region bends the sound waves. If a very short signal 

 be sent out from a transmitter, a neighboring receiver will receive 

 the signal by the direct path along the ground and shortly after- 

 ward may receive another signal by means of waves which have 

 traveled straight up into the atmosphere and have been reflected 

 back again by the upper conducting region. Since we know how 

 fast wireless waves travel, it is possible to measure to what height 

 they have been before being reflected back again. 



The conductivity of the upper atmosphere is due to the presence 

 of free electrons, formed, as we have said, by ultraviolet light from 

 the sun. The concentration of these electrons can also be found 

 because it requires more electrons to reflect back a short-wave-length 

 signal than a long-wave-length signal. If a series of tests are made 

 in which the wave length of the signal is gradually reduced, we 

 find that at first the height to which the signal goes before reflection, 

 slowly increases. This is because it has to go somewhat higher be- 

 fore it reaches a place where there are enough electrons to reflect 

 it, showing that the concentration of electrons increases with height. 

 But as the wave length of the signal is still further reduced, some- 

 thing new is found, for, instead of the signal being reflected a little 

 higher, it is now reflected at a very much greater height and no 

 signals are reflected from the intermediate height. This shows that 

 there are two regions where the concentration of electrons is high. 

 The first is at a height of about 100 to 150 kilometers and the second 

 from 200 to 400 kilometers, according to conditions. 



Such experiments with radio waves show that the number of free 

 electrons in the upper atmosphere — and hence its conductivity — 

 varies greatly through the day. During the night the lower con- 

 ducting layer (100 to 150 kilometers) is largely absent, since the 

 electrons quickly attach themselves to air molecules and no more 

 are formed. 



About sunrise the number of electrons rapidly increases and re- 

 mains large through the day. As would be expected, the number of 

 electrons present during the daytime is greater in summer than in 

 winter. The daily variations of the number of free electrons in the 

 upper conducting region (200 to 400 kilometers) are less regular 

 than those of the lower region. During the night the free electrons 

 here also attach themselves to air molecules so that their number 



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