190 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



but once in a while the signal fades out and all you can get are local 

 stations within 100 miles or so. While parents of teenage youngsters 

 may regard this as a blessing, the pilot of a transatlantic airplane is 

 apt to disagree. Whether we like it or not, this fading-out of distant 

 radio stations is traceable to the sun. What happens is something 

 like this : 



Radio waves are like light. If not disturbed they go in straight 

 lines, and if nothing interfered, we could not receive them any farther 

 away than we can see. In fact, the short high-frequency waves of 

 television have this property, which is the reason a given television 

 station can serve only a small area within 25 or 30 miles. The longer 

 waves of the broadcast band however, can be transmitted to the other 

 side of the world. To get there, they have to travel around the curve 

 of the earth in anything but a straight line-of-sight path. This is 

 possible because the ionosphere serves as a highly reflecting mirror to 

 long radio waves, which bounce back and forth between it and the 

 ground and are thus conducted around the curve of the earth. 



That word "ionosphere" is very important in any explanation of 

 solar terrestrial effects. In broadest terms, the ionosphere is that 

 portion of our upper atmosphere which is capable of conducting 

 electricity. Its normal lower limit is some 60 miles above the ground, 

 where the density of the air is less than a millionth of that at ground 

 level, and it extends upward to about 130 miles. 



The atmosphere at all heights is composed of myriads of tiny par- 

 ticles known as molecules. There are so many of them that when 

 you take a good deep breath you can be quite sure of inhaling a couple 

 of hundred that were breathed out by Patrick Henry in uttering the 

 words "Give me Liberty or give me death," and have since been thor- 

 oughly mixed with the whole atmosphere of the earth. The thought 

 would perhaps be a little unappetizing if Patrick Henry had not 

 used that breath so effectively. Its significance is simply that there 

 are about 200 times as many molecules in a single breath as there are 

 lungfuls of air in the whole of the earth's atmosphere. What I am 

 trying to say is that the molecules of the atmosphere are tiny and 

 numerous. Furthermore they are complicated. They are made up 

 usually of two atoms of oxygen or nitrogen, held together by electric 

 bonds. AVlien one of these molecules is given a sufficient jolt it comes 

 apart. Sometimes the two atoms are separated. Wlien this happens 

 one of the atoms may steal an electron from the other, and both then 

 have an electric charge. Sometimes the jolt merely removes an elec- 

 tron without otherwise disturbing the molecule. Either of these 

 processes is called ionization. The particles resulting from ionization 

 are known as ions and free electrons. Their important property is 

 that they carry a small electric charge and therefore can be pushed 

 around by electric forces, unlike normal molecules. 



