194 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



bardment of neon in a neon sign by electrically propelled ions. The 

 solar particles consist mostly of free protons and electrons, and are 

 therefore electrically charged. As they approach the earth they en- 

 ter its magnetic field which guides them in and concentrates them 

 in the auroral zones around the north and south magnetic poles. Their 

 collisions with air molecules excite the molecules to luminescence, 

 which we see as the aurora. This is the normal state of affairs. When 

 some disturbance on the sun sends out corpuscles at higher speed, they 

 are less easily controlled by the terrestrial magnetic field, and strike 

 the ionosphere in lower latitudes. Thus occasionally the aurora 

 borealis will be seen from the latitude of Washington. Wlien that 

 happens you can be quite sure that a vigorous magnetic storm is in 

 progress. 



The exact ionospheric mechanism responsible for magnetic storms 

 is still a matter of debate. However, we can say this much. Just as 

 the earth's field exerts forces on the incoming solar corpuscles, equal 

 and opposite forces are exerted on the field. This is an example of a 

 very fundamental physical law which is so strikingly used in rocket 

 propulsion. Next time you swing a cat by the tail, note that the cat 

 pulls on you as hard as you pull on him. It is the same thing. The 

 earth's magnetic field is quite elastic, and when the solar corpuscles 

 give it a shove it gets pushed out of shape. If you will picture the 

 earth's field as a vast framework of invisible elastic wires held to- 

 gether by the mysterious bonds that we call a field, it will not be too 

 difficult to see that a distortion of this framework at one point will 

 mean accommodating distortions throughout the whole, and at any 

 one place the direction and tension of the wires will be changed. You 

 would get a similar effect if you dropped a stone on a drumhead of 

 balloon rubber. The distortion is greatest near the stone, but the 

 shape of the drumhead is changed at every point. This distortion 

 in the earth's field is what we detect as a magnetic storm and, like 

 the rubber of the drumhead, it is different at different locations on 

 the earth although all locations are affected simultaneously. Since 

 the magnetic storms and abnormal aurorae are apparently caused by 

 the same solar particles, the two always occur together. I hope there 

 are no representatives of the Department of Terrestrial Magnetism 

 present tonight. If there are, I fear for my scientific scalp, after such 

 a simplification of a very complex matter. 



One other solar effect is worth our attention. Occasionally when 

 we have an especially intense disturbance in the ionosphere, it pro- 

 duces such powerful electric fields at ground level that wire communi- 

 cations fail. Transcontinental teletypes go completely crazy, and 

 the messages received are nothing but a meaningless jumble of letters, 

 which sometimes come through when no message is being sent at all. 



