SOLAR INFLUENCE ON EARTH — EVANS 195 



Fortunately such spectacular disturbances are fairly rare and quite 

 short-lived. 



Because of the practical importance of ionospheric disturbances, 

 particularly to radio communication, many agencies of the Govern- 

 ment and industry are intensely interested in doing something about 

 them. A few years ago I could have remarked that you cannot con- 

 trol the ionosphere any more than you can control the weather, but 

 that would be an anachronism in this day of cloud-seeding experi- 

 ments. The next best thing to making the ionosphere behave itself 

 is to be able to predict its fits of misbehavior. This takes us directly 

 to the cause of all ionospheric disturbances, the sun. 



It is for this reason that the geophysics directorate of the Air 

 Force Cambridge Research Center has just built a modern solar ob- 

 servatory at Sunspot, N. Mex., on Sacramento Peak, at 9,000 feet alti- 

 tude. "We know a lot about the sun, but the more we learn, the more we 

 find that we ought to know and do not. In particular, we have learned 

 of a few isolated kinds of solar activities that are associated with 

 ionospheric storms, but so far no one has advanced an acceptable 

 theory of how these activities produce the ultraviolet radiation and 

 blasts of corpuscles that affect the earth. We are like the puppy who 

 observes that the baby's explorations in the clothes closet are often 

 followed by corporal punishment for Fido, an innocent bystander, 

 with no apparent causal connection. 



In order to save myself numerous small digressions later, I shall 

 indulge in a long one now to describe the general characteristics of the 

 sun. It is the central body of the solar system, at a distance of 93 mil- 

 lion miles from the earth. It contains 99.8 percent of the material of 

 the whole solar system, and to the outside observer the planets and 

 comets revolving around it would appear as interesting but inconse- 

 quential trifles in comparison. The sun's diameter is over a hundred 

 times that of the earth and its weight is 332,000 times as great. It is 

 composed entirely of gas, because the temperature of even its coolest 

 parts is so high that the most refractory materials are vaporized. Al- 

 though it contains oxygen, no combustion takes place because of the 

 same high temperature. Combustion is the chemical combination of 

 atoms of oxygen with other atoms. The hot atoms of the solar gas 

 move so fast and collide so violently that any two that happen to stick 

 together are instantly knocked apart. Hence the heat and light of the 

 sun are not due to any burning process. Instead, the source of energy 

 is a thermonuclear reaction in the deep interior, which converts 4 mil- 

 lion tons of the solar mass into energy every second. If this seems 

 like an alarming rate of expenditure of solar material, I can only advise 

 a relaxed attitude. The sun has enough expendable material to keep 

 going at the same rate for a number of billions of years. 



