3 o8 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



(2) The Perturbation of the loth February, 1903. 



77. The current-arrows for Axeleen and Kaafjord remain in one direction for a considerable time, 

 and are almost perpendicular to the arc of the great circle between the two stations. It also appears 

 from the outer field that the storm-centre of the current-system is in the neighbourhood of Axeleen and 

 Kaafjord. We have therefore calculated the strength and altitude of the current at several hours at 

 which the conditions are approximately those mentioned in the introduction. 



In this case we employ P, for Kaafjord. The vertical component for Axeleen we shall use as a 

 check. This quantity, if our assumption is correct, will be determined by the formula 



P " _ P " 

 ~P ' 



In the following table, the calculation has been made for four different hours. 

 It may here be remarked that both in this and the succeeding Tables, the units of length and 

 current-strength are respectively a kilometre and an ampere. 



TABLE XLIV. 



V. 

 The table shows that at the first three of the hours mentioned the current would be at the same 



height about 420 km. ; and this is the more strange as the separate quantities in the formulae 

 differ considerably. 



The values for d seem to indicate that up to 23'* 45 m the current is moving towards Axeleen. 

 While moving thus, the current, on an average, would keep at about the same height above the surface 

 of the earth. 



A comparison between the calculated and the observed values for P,", will show that the cal- 

 culated vertical components on the whole are too large; the observed values are only about two thirds 

 of the calculated. A result such as this is just what might be expected. Our calculations presuppose 

 that the transverse section of the current is very small in proportion to the distance between Kaafjord 

 and Axeleen; but considering the cosmic constitution of the current, this is not very probable. 



We could make the calculation here also, assuming both the total forces to be given. The result 

 will be found in the following table. 



TABLE XLV. 



From this it appears that the two calculated current-strengths are not quite alike, but the difference 

 is not greater than would be expected. The mean gives values that agree very closely with those 

 previously found. The height found is somewhat greater in the last case. It will easily be perceived 

 that if the current is spread over a larger section, we shall find the height somewhat too great. 



