PART I. ON MAGNETIC STORMS. CHAP. II. 



CONCERNING THE CAUSE OF THE PERTURBATION. 



36. The cause of the great magnetic disturbance at Dyrafjord, and subsequently at Axeleen also, 

 must mainly be sought in the effect of a horizontal current. This follows from the fact that the places 

 of the greatest effect are found for a long distance in the direction of the current- arrow, while in the 

 direction perpendicular to it, the effect very quickly diminishes. At i h 45"", for instance, the perturbing 

 force at Dyrafjord is 240 /, at Axeleen 193 y, and the direction about the same, reckoned from the 

 meridian of the place. At the same time, the strength at Kaafjord and Matotchkin Schar is respectively 

 39.6 / and 20.6 y, and the distance between Dyrafjord and Axeleen is 1809 kilometres, while between 

 Axeleen and Kaafjord it is only 896 kilometres (see fig. n). 



In the district between Dyrafjord and Axeleen we must assume a horizontal current, which ought to 

 flow fairly close to the earth for a long distance; for, owing to the rapid diminution in the effect out 

 towards the sides, the current must flow rather low in relation to the earth's dimensions. We shall 

 return to this later on. 



We may conclude from the vertical intensities that it must be a current above the earth's surface, 

 This is proved in the case of similar storms (see February roth and March 3ist, 1903), also by a 

 consideration of the earth-current curve; but this is unfortunately wanting for the day under discussion. 



With regard to the further course of the current, there are two possibilities that may be considered. 



(1) The entire current-system belongs to the earth. The current-lines are really lines where the 

 current flows upon the earth's surface, or rather at some height above it. 



(2) The current is maintained by a constant supply of electricity from without. The current 

 will consist principally of vertical portions. At some distance from the earth's surface, the current 

 from above will turn oft" and continue for some time in an almost horizontal direction, and then either 

 once more leave the earth, or become partially absorbed by its atmosphere. 



According to the first assumption, the total current-volume at Dyrafjord and Axeleen should be 

 squeezed together so that the greater part of it must pass through a comparatively small section, while 

 the electricity, both before and after, should be spread over a wider section. In this case the current- 

 lines drawn on fig. 40 would possess a physical reality, as there should actually be currents above the 

 earth, somewhat in the direction of the current-arrow, answering to these current-lines. 



It is true that systems of plane currents can always be arranged for a given field, which, from a 

 purely mathematical point of view, would be able to explain the field; but when we consider the physical 

 conditions for the formation of such a system, we meet with great difficulties, for it is not easy 

 to comprehend what terrestrial processes would be able to maintain a current with this peculiar form, 

 which moreover remains constant for several hours. 



In my report of the 2nd Aurora Expedition -- "Expedition Norvegienne de 1899 1900", etc. - 

 I assumed such a system of horizontal currents in order to explain the magnetic perturbations. But the 

 currents there are imagined as having come into existence mainly as a secondary effect of the electric 

 corpuscles from the sun drawn in out of space, and thus far come under the second of the possibilities 

 mentioned above. With observations from Pawlowsk, Copenhagen, Potsdam, Paris, Greenwich and 

 Toronto as a foundation, 1 have drawn up a chart of the ordinary current-directions at midnight, Green- 

 wich mean time, which is reproduced in fig. 45. It will be seen how well these current-directions fit 

 into the current-lines in the idealised diagram, fig. 40. 



There does not appear, however, to be any special reason why a current-system upon the earth 

 should maintain such fixed directions and such a motion. If this were only a single case, one might per- 

 haps regard it as a freak ot nature. Among all the phenomena that occur from time to time, some will 



