3io 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



This shows that the strength of the universal disturbances that accompany the storms in the north, 

 stand in about the same relation to one another as the strength of the currents which we assume to be 

 the direct cause of those storms. This accords well with our assumption; for if it be assumed that 

 during these polar storms the form of the current-system is more or less the same, the force at 

 corresponding places in the outer field would be proportional to the strength of the current. 



In connection with these calculations of current-strengths, I will here refer to the current-strengths 

 that I found for the stations Bossekop and Jan Mayen, given in my former report, "Expedition Nor- 

 vegienne de 1899 1900" etc., pp. 27 & 28. They agree well with those now found, as they vary 

 between 317,000 and 983,000 amperes. 



(3) The Perturbation oj the 22nd March, 1903. 



78. In this perturbation, as already mentioned, the storm-centre is in the neighbourhood of Axeleen 

 and Bossekop, whither the station at Kaafjord had now been moved (see p. 10). The current-arrows 

 for these stations are similar in direction, and are almost at right angles to the great circle between 

 them. The vertical components are very large and in contrary directions. It would thus appear that the 

 conditions are such as to justify a more elaborate calculation of the strength of the current, according 

 to the methods previously given. 



In the calculation on this occasion, we shall consider the total force for Axeleen as known, as 

 also Pj for Bossekop. At the latter station the patch of light for the balance has moved off the paper, 

 so that there, during the time at which the storm is most powerful, we only know the lower limit of 

 this quantity. 



In the table below are given the most important values that enter into the formulae, as also the 

 values found for P t ' , h and ;'. 



TABLE XL VII. 





The height on this occasion is not great. The strength of the current, on the other hand, is 

 fairly great, amounting to i l / 3 million amperes. If we compare the calculated values of P,' with those 

 observed, we also on this occasion, at 22 h o m , find that the calculated value is too high. As regards 

 the subsequent hours we can say nothing decided; probably they also are too high. For the explanation 

 of these conditions, the reader is referred to the perturbation of the roth February. 



(4) The Perturbations of the 2jth & 28th October, 1902. 



79. In the storms that occur just before midnight on these two days, there are, as we said when 

 discussing them, circumstances which justify a calculation of the strength and altitude of the horizontal 

 portion of the current. The results of this calculation are given in the table below. 



TABLE XLVIII. 



