SOLAR INFLUENCE ON EARTH — EVANS 193 



be the source of these. Finally this was confirmed by observation of 

 disturbances on the sun and corresponding disturbances in the iono- 

 sphere. Having got this far we have the main outlines of the 

 activity without ever having seen it. As I have described it, it 

 appears much less complicated than it actually was. The scientific 

 method rarely runs smoothly. A theory is advanced. Investigators 

 think it over and give it up, and think some more and give it up some 

 more, until some key discovery convinces them that it is a good idea, 

 or quite impossible. Then they are ready for the next step. 



In the case of the ionosphere, theory has received brilliant observa- 

 tional confirmation in the last few years. Scientists of the Naval Re- 

 search Laboratory and of the University of Colorado have sent rockets 

 into the ionosphere from the Wliite Sands Proving Grounds. They 

 unequivocally recorded the ultraviolet radiation from the sun for the 

 first time. A year or so earlier the incoming corpuscles were also ob- 

 served directly at the Yerkes Observatory and at Cornell by spectro- 

 scopic means, and their velocities were found to vary from a few hun- 

 dred to more than 2,000 miles/sec. 



I think that by now you have anticipated me in seeing how the 

 ionosphere is disturbed by the sun. Its existence is due to a steady 

 stream of ultraviolet quanta and corpuscles from the sun. The in- 

 tensity of this stream frequently varies violently. These variations 

 cause changes in the density of ions at different heights in the iono- 

 sphere. To go back to our analogy, the height and mesh of the 

 ionospheric screen is drastically altered, and detectable effects imme- 

 diately follow. Wlien the ionosphere is pushed downward into the 

 denser regions of the atmosphere it becomes clogged and absorbs all 

 radio signals, and we have fading. 



Radio fading, however, is by no means the only result of abnormal 

 ionospheric conditions. I have discussed it in detail because it prob- 

 ably affects more people than any of the others. The others are equally 

 interesting, and usually result from some other kind of change than 

 a depression of the ionospheric net into the gummy denser air of the 

 lower levels. 



The magnetic field of the earth is often affected, quite independently 

 of radio fading. The compass needle points a trifle off its normal di- 

 rection, and the strength of the field increases or decreases slightly. 

 This is referred to as a magnetic storm. Without delicate magnetom- 

 eters we would be quite unaware of the most violent magnetic storms 

 because they stimulate none of our five senses. One manifestation 

 of a magnetic storm, however, is often beautifully visible. This is 

 the aurora. Ordinarily the aurora is confined to a narrow zone around 

 the earth in the polar regions. It is a glow high in the ionosphere 

 produced by corpuscular bombardment from the sun, like the bom- 



