SOLAR INFLUENCE ON EARTH — EVANS 197 



We can draw a further conclusion. The low-level ionization pro- 

 duced by flare radiation appears without any serious changes in the 

 normal ionosphere above 60 miles. In other words, the normal iono- 

 sphere, which is completely opaque to the ultraviolet of the normal 

 sun, fails to absorb the quanta from the flare. They penetrate it quite 

 freely and are absorbed in the process of producing ions at the 40-mile 

 level. Hence there must be something different about flare quanta. 

 The difference can only be a difference in wavelength of the radiation. 

 If we were talking about visible light we would simply call it a dif- 

 ference in color, and I am going to refer to this wavelength difference 

 as a difference in ultraviolet color. 



The atmosphere of the earth contains many different kinds of mole- 

 cules, and the percentage abundance of the different kinds varies with 

 height above the ground. Each kind is an efficient absorber of some 

 particular ultraviolet color. It absorbs this color and becomes ionized 

 in doing so. The ultraviolet radiation from the normal sun encoun- 

 ters molecules of a kind that absorb its particular color at levels above 

 60 miles. They are not the kind of molecules that absorb radiation 

 of the different ultraviolet color emitted by flares, however. There- 

 fore this radiation penetrates to a lower level where it finds molecules 

 of another kind, which forthwith absorb it and produce ions. Just 

 which kinds of molecules and which colors are involved is still rather 

 uncertain. 



In addition to terrific bursts of ultra\aolet quanta, flares emit equally 

 impressive showers of corpuscles, consisting largely of free protons 

 and electrons. They travel along at a rate of 1,000 miles/sec, and 

 arrive at the earth about a day after the flare outburst. As they 

 impinge on the earth's magnetic field they twist it slightly out of 

 shape, and some of them are guided down into the ionosphere. We 

 then have a lively magnetic storm, and aurorae appear in lower-than- 

 normal latitudes. 



Although the most spectacular of solar disturbances, flares are not 

 the only ones to affect the ionosphere. The sunspots themselves appear 

 to emit corpuscles that induce magnetic storms. At least we blame 

 the sunspots because the magnetic storms tend to occur a couple of 

 days after a large spot has rotated past the center of the solar disk. 

 However, we have to be careful here, since we cannot actually see the 

 corpuscles leaving the spots. A sunspot is only the most visible feature 

 of a much broader disturbance which we simply call an active solar^ 

 region. It is the fever thermometer which indicates a deep-seated dis- 

 order. Refined observations show the presence of flares, active prom- 

 inences (which I shall describe presently), regions of intense bright- 

 ness in the immediately overlying corona, and brighter-than-normal 

 patches on the solar surface known as faculae and plages. All these 

 features share in the 11-year sunspot cycle. Wliether it is the sunspot 



