SOLAR INFLUENCE ON EARTH — EVANS 191 



The ionosphere gets its name and its electrical conductivity from 

 the fact that it contains an appreciable fraction of ions and free elec- 

 trons. As many as one molecule in every 10,000 may be ionized. It 

 is actually the free electrons that reflect long radio ^Yaves. The process 

 of reflection is too complicated to explain here, but I will ask you to 

 endure a crude analogy. Think of the electrons in the ionosphere as 

 an ionospheric screen, like a window screen. The fineness of the mesh 

 of our screen is proportional to the number of electrons per cubic 

 inch in the ionosphere. Wlien radio waves hit the screen, they are 

 caught and thrown back if they are larger than the mesh, but if they 

 are smaller they pass through between the electrons, so to speak, to 

 outer space. Thus the long waves of the broadcast band are re- 

 flected, and can be received over long distances around the curve of 

 the earth, while the short TV waves are not reflected, but penetrate 

 through the ionospheric screen, for the benefit of inhabitants of arti- 

 ficial earth satellites and the moon. The ionospheric screen has one 

 other property that is directly responsible for radio fading. Under 

 normal conditions there is good reflection from it when it is in the 

 high atmosphere where the electrons have room to shake themselves 

 without too much interference from neighboring molecules. But if 

 for some reason the screen is pushed down to low levels where there 

 may be a hundred times as many molecules per cubic inch, the screen 

 gets clogged and the radio waves are neither reflected nor trans- 

 mitted. They are absorbed, and distant stations fade out. The energy 

 of the radio waves stops right there and goes into warming the air. 

 I am sure this fact is not generally known in the halls of Congress. If 

 it were, and our legislators realized that under special conditions they 

 could warm the air of the upper atmosphere while warming that in 

 their immediate vicinity, the condition of the ionosphere would be fixed 

 by law, since any presidential veto would surely be overridden. 



While the fading of radio signals is the most familiar solar effect, 

 whether we recognize it as such or not, there are others which I shall 

 discuss presently. The noteworthy fact is that all of them can be 

 traced to disturbances in the ionosphere, and we may justly conclude 

 that the primary solar terrestrial effect from which all observable 

 effects stem, is the influence of the sun on the ionosphere. Now, just 

 what is this influence ? 



The thing that counts in the ionosphere is the concentration of ions — 

 the number of ions per cubic inch — at various heights above the 

 ground. Normal concentrations vary from about 1 to 3 million ions 

 per cubic inch at different heights. 



I have already mentioned that ions are formed when air molecules 

 are subjected to sufficiently large jolts. In ordinary terms, the re- 

 quired jolt is very small. In climbing an inch up the wall a fly 

 expends enough energy to ionize a thousand billion molecules. But 



