BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



in a striking way corresponds to the auroral zone. The bright areas on the terrella usually show the 

 characteristic property of being restricted to the side turned away from the cathode or the source of 

 the radiation. 



The existence of a calm period is a mere consequence of the distribution of storm-centres on the 

 night and evening side, and the rapidity with which the magnetic effect diminishes with the distance from 

 storm-centre. As we have seen, both these properties were consequences of our radiation theory, which 

 will then also explain the calm period, which is especially well marked on the southern border of the 



9 



auroral zone. 



The existence of two types of polar storms, each restricted to its own time of day, is I think a matter 

 of the greatest interest for the question regarding the cause of magnetic storms. 



In order to find whether the P and N storminess could be explained from our theory, I have 

 tried by means of screens placed in various positions outside the terrella, to trace the direction of tin- 

 rays before they strike the terrella. In this connection it was of special interest to regard the direction 

 of motion of the horizontal component of the corpuscles just before they struck. 



It is of course difficult, not to say impossible, to reproduce in the limited space of a vacuum-tulir 

 exactly the conditions that govern the formation of magnetic storms. We have, however, been able to 

 show from the terrella experiments the existence of two types of precipitation which I think will show tin- 

 way in which the two types of storms are to be explained. 



In one type most strongly developed on the night side, the horizontal components of the vrloclti. 

 the corpuscles are turned towards the east; and at the same time, with a proper adjustment of rnagnrtki- 

 tion and stiffness of rays, we get precipitation on the evening side with the horizontal component of iimtimi 

 turned towards the west. 



I think these two types correspond respectively to the negative and positive polar storms. Thus 

 the typical distribution and direction of the two types of storms can be explained when we assume- the 

 polar disturbances to be a direct effect of electric radiation from the sun. 



The local storms with their centres to the north of the auroral zone, which had a great effect at 

 Axeloen, will, I think, be explained by our radiation theory, when we remember that the precipitations 

 approach the magnetic axis when the rays become softer, and we should merely have to assume that the 

 sun gives out rays of different stiffness. 



We found that the large storms usually had their centres in lower latitudes than the small storms. 

 This indicates that probably the rays given out by the very powerful sources in the sun, are stiffen 

 than those from the many small sources producing the small storms occurring between the great maxima. 



When the corpuscular currents strike the atmosphere, secondary processes may be called into play; 

 but if these secondary processes, of whatever kind they might be, are to produce magnetic effects of the 

 same order as the impinging rays, they must follow quickly after the primary action. For if secondary 

 effects of the same magnitude and frequency as the primary effects were present, and these secondary 

 effects could show up several hours after the sun agency had left the place, it would be difficult indeed 

 to explain the existence of a calm period and the great rapidity with which the negative storms erase 

 after midnight. 



We saw that the typical field of polar storms could be explained by a current-system coming in 

 from space corresponding to the precipitation of electric rays from the sun. This would strongly 

 support the view that by far the greater part of the disturbance effect observed is caused ilirecllv by the 

 currents of sun-radiation. 



