SOLAR ACTIVITY — SPENCER JONES 233 



diurnal variation shows a striking similarity to the curve of sunspot 

 frequency, the diurnal range being from 60 to 100 percent greater at 

 sunspot maximum than at sunspot minimum. When, however, the 

 individual values of the sunspot areas and of the diurnal range in 

 declination are compared, the similarity is much less apparent, as will 

 be seen from the unsmoothed curve. The diurnal changes of the earth's 

 magnetism are not therefore to be attributed directly to the spots 

 themselves, though they must be due to some effect originating in the 

 sun. 



The daily motion of a suspended magnet needle is such that the 

 needle points to the east of its mean position in the morning, moves 

 westward during the day, and returns during the night. Some effect 

 emanating from the sun and reaching the earth without sensible delay 

 therefore appears to be involved. The passage of a large spot across 

 the central meridian of the sun is often followed after a day or two 

 by a magnetic storm, which suggests that an effect emanating from 

 the sun reaches the earth with a time lag of one or two days. The 

 central meridian passage of a large spot is not invariably followed, 

 however, by a magnetic storm and, on the other hand, a storm may 

 occur when there is no spot on the sun. 



The ultraviolet light emitted by the sun causes photoionization (or 

 splitting of molecules and atoms into free electrons and ions) . There 

 are two principal ionized layers : the lower, called the E-layer, is at an 

 average height of about 120 Idlometers ; the upper, called the F-layer, 

 has an average height of about 250 kilometers. During the daytime 

 the F-layer frequently splits into two overlapping layers, referred 

 to as the Fi- and Fg-layers ; the Fg-layer is the higher, having an aver- 

 age height of about 350 kilometers, and is very diffuse. There is also 

 a third layer, Imown as the D-layer, which has an average height of 

 about 80 kilometers. The precise nature of the reactions taking place 

 in these layers is not fully understood, though the photoionization by 

 ultraviolet light from the sun of molecules of oxygen and nitrogen 

 undoubtedly plays an important role. These ionized layers collec- 

 tively form what is termed the ionosphere. Radio waves are able to 

 travel round the curved surface of the earth because they are reflected 

 by one or another of the ionized layers and prevented from escaping 

 into space. Long waves, of wavelength from about 600 meters upward, 

 are reflected principally by the D-layer, medium waves of w^avelengths 

 between about 200 and 600 meters principally by the E-layer, while the 

 short waves of wavelengths from about 10 to 200 meters are reflected 

 by the F-layer. 



For each layer there is a critical frequency, depending upon the 

 electron concentration in the layer, beyond which the radio waves are 

 no longer reflected but penetrate the layer. Thus waves of medium 

 frequency are able to penetrate the D-layer but are reflected by the 



