1910] on Magnetic Storms. 785 



emanating from the sun to reach the earth's atmosphere at all, except 

 in a narro\y band round each magnetic pole. The earth's magnetic 

 field protects it from the approach of electrified particles, these 

 having to describe spirals round the lines of magnetic force. The 

 larger the mass and the greater the velocity of the particle, for a 

 given electrical charge, the nearer can it approach the earth in the 

 equatorial plane, and the larger is the radius of the zone surrounding 

 each magnetic pole within which the particle can actually reach the 

 earth. The B particles of radium, from their higher velocity, have 

 more penetrating power than ordinary cathode rays, and are in their 

 turn eclipsed by the a rays, whose lesser velocity is more than com- 

 pensated by their larger mass. According to Stormer, the greatest 

 angular distance from a magnetic pole at which average cathode rays 

 emanating from the sun can reach the earth is only 2° '4, while the 

 corresponding angular distances for /:> and a rays are respectively 

 4°-l and 12° -7. Xone of these distances is large enough to repre- 

 sent the band of maximum auroral frequency on the earth. 



The way in which cathode rays fail to reach the earth, when 

 emitted from the sun in a plane coincident with the earth's magnetic 

 equator, is illustrated by diagrams of Stormer's, of which a slide is 

 shown. 



§ 19. Undeterred by these mathematical results, Birkeland assumes 

 that a type of magnetic disturbance which he calls the " polar ele- 

 mentary " storm is due to cathode rays from the sun which get within 

 a few hundred kilometres of the earth's surface at considerable 

 distances from a magnetic pole. The paths of approach and retreat 

 are supposed to be radial and the connecting part horizontal, as shown 

 in the slide, which is a photograph of a drawing by Birkeland. These 

 " polar elementary " storms were o])served on a good many occasions 

 at four temporary observatories provided with magnetographs, which 

 Birkeland was able to set up and keep in action in Arctic regions 

 during the winter 1902-3. The characteristics of " polar elementary " 

 storms are their comparatively simple character and short duration, 

 and the fact that their amplitude — unlike that of Birkeland's " equa- 

 torial" storms— is much larger in the Arctic than elsewhere. These 

 storms have at least a general i-esemblance to a special type of dis- 

 turbance* of which I found numerous examples in the records of the 

 National Antarctic expedition of 1901-4. 



§ 20. Birkeland found that frequently, after an " equatorial " storm 

 had been in progress for some hours, one or a series of "polar elemen- 

 tary " storms intervened. He obtained copies of the curves taken at a 

 number of magnetic observatories on the days of the disturbances re- 

 corded by his Arctic stations, and he has reproduced these with his 

 own records in a most valuable series of plates published in his recent 



* National Antarctic Expedition, 1901-1904. Magnetic Observations, 

 p. 186. 



3 F 2 



