PART I. ON MAGNETIC STORMS. CHAP. II. 



137 



THE PERTURBATIONS OF THE 26th DECEMBER, 1902. 



(PI. XII). 



48. The perturbations to which we have especially turned our attention are two successive, brief, 

 well-defined storms, that are particularly powerful at our Norwegian stations, more especially Dyrafjord 

 and Matotchkin Schar. 



The first of these two well-characterised polar storms is especially powerful at Matotchkin Schar, 

 where PI attains a value of 248 y. At Axeleen there is a perturbation that is quite distinct in all three 

 components. At Kaafjord there is simultaneously a very distinct perturbation, but one that is very small 

 both in D and H, whereas in V it is considerably stronger. At Dyrafjord, the curve shows clearly 

 that this brief polar storm occurs simultaneously with a more lengthy perturbation. Its effect, on the 

 whole, at Dyrafjord, is contrary to that of the longer storm. A decomposition of the perturbing force 

 may here be effected. 



The same conditions, although less marked, are found on the continent of Europe, where the 

 //-curve shows a faint, but long perturbation. There too, the course of the intermediate perturbation is 

 the reverse of that of the longer storm; but as the former is much more powerful, it will predominate 

 during the time in which it occurs. 



The second storm is especially powerful between 22'' 30 and 24''. It also occurs in the north as 

 a characteristic polar elementary storm, which is particularly powerful at Dyrafjord. This is in accord- 

 ance with the fact that it appears later. 



At the stations in lower latitudes, we notice in the case of both storms simultaneous but compara- 

 tively slight perturbations; and the effect becomes weaker with an approach to the equator. At Sitka, 

 the perturbation is only of the same magnitude, and has the same course, as in the rest of America. 



According to this, it is natural to consider these two storms as two successive polar elementary 

 storms, in which the storm-centre is situated somewhat differently. This will be still more apparent on 

 a closer examination of the field of force. 



The field of force during the first storm is shown on Charts I and II, for the hours 2o h 45, and 

 2i h respectively. 



The form of the field is in the main the same in both cases, as also the relative strength. This 

 clearly indicates that the system in question is one that on the whole preserves its form and its posi- 

 tion, and only varies in strength. The arrows at Axeleen and Matotchkin Schar form exceptions in this 

 respect, the force at these stations being almost as great at 21 h as at 2o h 45. This does not neces- 

 sarily, however, alter our view of the conditions; for, owing to the local character of the perturbations 

 in these regions, very slight movements of the system may here have a great effect, and thus the force 

 at one place may very well have its greatest value at a time other than that at which the system as a 

 whole is strongest. 



The form of the field is that typical of the polar elementary storms. The storm-centre is situated 

 in the region north-east of Matotchkin Schar, and the area of convergence in north-eastern Russia. The 

 current-arrow about the centre is as usual directed WSW. There is an area of divergence in America, 

 which seems to belong to another storm-system, this being also confirmed by the arrows at Dyrafjord. 



As regards the vertical intensities, we find at Pawlowsk a perturbing force directed upwards, just 

 as we should have expected. At Wilhelmshaven, Pola and Tiflis, on the contrary, we find positive 

 values for P v . The deflections, it is true, are only slight, but still are sufficiently distinct. They cannot 

 be due to the system that we have assumed to be at our easterly stations, as that system can produce 

 only negative values of P, in the area of convergence. 



It is difficult to decide what forces here play a part. The system that produced the area of diver- 

 gence in America, may indeed possibly be supposed to exert an influence here too; and this would also 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 18 



