TERBESTRIAL MAGNETISM — HAZARD 253 



with the development of our knowledge of electrical discharges in 

 a vacuum. 



Birkeland, Arrhenius, and Nordmann agreed in considering the 

 auroral rays as a luminescence produced by the absorption of cathode 

 rays in the upper atmosphere, and attracted toward the earth's 

 magnetic poles. Birkeland, who devoted many years to the study of 

 auroras, first supposed that the cathode rays were emitted directly 

 from the sun, but later he advanced the modified theory that cathode 

 rays from the sun set up electric currents in the atmosphere which in 

 turn emit secondary cathode rays. He supported his theory by the 

 production of artificial auroras in the laboratory, about a magnetized 

 steel ball in a tube of rarefied air exposed to cathode rays. 



Paulsen in 190G, after calling attention to the difiiculties with ear- 

 lier theories, sought to exj^lain the aurora and magnetic storms by a 

 strong ionization of the upper layers of the atmosphere above the 

 zone of maximum frequency of the aurora. 



Stormer, who had worked with Birkeland in both his observational 

 and experimental studies of the aurora, knowing that the phe- 

 nomenon of the concentration of cathode rays toward a single mag- 

 netic pole had been mathematically treated by Poincare, thought it 

 might be w'orth while to determine mathematically the trajectories 

 of electric corpuscles coming from the sun into the magnetic field 

 of the earth, hoping to bring out the principal features of the aurora. 

 These studies began in 1903 and covered a period of about ten years. 

 Stormer simplified the problem at the outset by treating the earth 

 as a spherical magnet, and neglecting the relative motion of the 

 earth and sun, and then modified the results obtained for this sim- 

 ple case to correspond to the more complex conditions actually 

 existing. In this Avay he was able to develop paths for the cor- 

 puscles which seemed to fit the general features of the aurora as 

 it appears in nature and of the artificial aurora of Birkeland, and 

 thus tended to strengthen the corpuscular theory of its origin. 



The correlation of magnetic storms with sun spots, although very 

 satisfactory when based on yearly averages, leaves much to be desired 

 when individual cases are considered. Thus, severe magnetic storms 

 sometimes occur Avhen no large sun spots are visible and, on the other 

 hand, the appearance of a sun spot is not always accompanied by a 

 magnetic storm. To account for this. Maunder advanced the theory 

 that the solar activity which gives rise to magnetic disturbances on 

 the earth does not act equally in all directions but along narrow well- 

 defined streams, not necessarily truly radial ; that these streams arise 

 from active areas of limited extent; that these active areas are not 

 only the source of our magnetic disturbances but are also the seats 

 of the formation of sun spots; that these areas can be active both 

 before a spot has formed and after it has disappeared. The fact that 



