234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 5 



E-layer ; those of higher frequency are able to penerate both the D- 

 and E-layers but are reflected by the F-layer. Waves of very high 

 frequency pass through all three layers. The critical frequencies 

 beyond which reflection no longer occurs are subject to marked solar 

 control. They depend not only on the daily and seasonal variations in 

 the sun's altitude but also vary in sympathy with the frequency of 

 sunspots. The dependence upon sunspot frequency implies a varia- 

 tion through the sunspot cycle in the degree of ionization in each layer 

 and therefore also in the intensity of the radiation from the sun in the 

 far ultraviolet, which is responsible for the photoionization ; this 

 radiation must be much more intense at sunspot maximum than at 

 minimum, in order to account for the observed variation through the 

 sunspot cycle in the critical frequencies. 



The diurnal variations in the earth's magnetic field are associated 

 with this ionization. For simplification we need consider only the 

 lower and more important layer. It has a diurnal motion, due in part 

 to its heating by the sun and consequent expansion and in part to the 

 tidal action of the sun on the atmosphere. The layer, because of its 

 strong ionization, is electrically conducting and behaves somewhat 

 like a metallic conductor. Its diurnal motion in the earth's magnetic 

 field therefore causes, by induction, an electric-current system, which 

 may amount to many thousands of amperes, to circulate in it. This 

 current system can be regarded as fixed with respect to the direction 

 earth-sun, relative to which the earth itself is rotating. The magnetic 

 effect of the current system at any point of the earth's surface therefore 

 varies as the earth rotates, producing the diurnal variations in the 

 earth's magnetic field. The correlation between the mean amplitude 

 of the diurnal variation and the mean sunspot frequency is further 

 evidence of a variation through the sunspot cycle in the emission of 

 ultraviolet light by the sun. At sunspot maximum the emission in the 

 far ultraviolet is of the order of 60 percent greater than at sunspot 

 minimum. The close correlation between sunspot frequency and the 

 rate of emission of ultraviolet light must be due to some miderlying 

 cause to which sunspots themselves are directly related. 



Carrington had noted that a moderate but very marked magnetic 

 disturbance had occurred at the time of the bright flare which he had 

 observed and that it had been followed some 17 hours later by a great 

 magnetic storm. With true scientific caution, though he suspected 

 that these phenomena were related, he remarked that "one swallow 

 does not make a summer." This same sequence of events has, however, 

 been found to follow many intense flares in recent years. Carrington's 

 observation of a flare in integrated light is almost unique; it is ex- 

 tremely rare for a flare to be so intense that it can be seen by ordinary 

 visual observation of the sun. The flares are best observed in the light 



