9 6 



RIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



Current-Lines at Midnight. 



Fig. 45- 



assume strange forms. But this is not an isolated case; as the entire treatment of these great polar storms 

 will show, we shall always, in them, find again the same direction for the current about the Norwegian 

 stations. We know, however, no circumstances connected with the earth itself and its immediate sur- 

 roundings, that are sufficient to explain why one direction should so persistently predominate. A current 

 such as this, moreover, which is a surface-current, would have to keep in the higher strata of the earth's 

 atmosphere. It would have to be a corpuscular current in a medium in which these corpuscles can freely 

 move out to the sides. The direction of the current would thereby be compelled to conform to the 

 laws for the deflection of such currents in the terrestrial-magnetic field. But with an acquaintance with 

 the laws for these movements, it is immediately evident that quite different forms would then be produced. 



If such plane currents were possible at all, one would have to assume that the corpuscles, on account 

 of some properties belonging to the upper strata of the atmosphere, would be obliged to move within a 

 spherical shell situated at some distance above the earth's surface; for if the electric rays are at all 

 pliable, they will in the main follow the lines of force, and from the polar regions these issue quite 

 vertically. The rays might either go out into space, or back to the south pole of the earth. If the 

 rays were very stiff, they would certainly for a time be able to keep approximately horizontal, but would 

 at last have to run out into space, so that no entire circle of the above-mentioned kind would be formed. 



Those rays, moreover, that move approximately horizontally at the poles, would have to turn 

 off to the same side; or, in other words, on the northern hemisphere there would only be positive vor- 

 tices, or areas of divergence for the perturbing force. But, as we see, we also have areas of con- 

 vergence of a very simple form. 



This brings us to the necessity of considering more closely the second possibility, namely, that 

 the current is fed by a fairly constant supply from without, lasting for several hours. The supply 

 must then, in the first place, be given in the regions in which the perturbation is strongest; and the 

 strong perturbations in the north ought to be a direct effect of the descending current, which acts as 



