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ANNUAL REPORT SMITHSONIAN INSTITUTION,, 1942 



these layers electrically conducting. These conclusions drawn many 

 years ago from magnetic observations became more important after 

 the invention of wireless telegraphy, because by them we may explain 

 why wireless waves are bent around the earth along these same con- 

 ducting layers. 



Figure 7.— Total distribution of auroral forms and of lower limits for auroral 



heights. 



The range of diurnal variation varies with magnetic character. 

 Statistical studies, which tend to smooth the violent irregular fluctua- 

 tions, have led to the identification of three main features of a mag- 

 netic storm. These are: (1) An everywhere similar but unequally 

 large general source according to universal time; (2) a superposed 

 diurnal movement according to local time, differing characteristically 

 from the ordinary diurnal variation on quiet days; and (3) an after- 

 effect, which gradually subsides in the recovery during the days after 

 a storm. 



