PART I. ON MAGNETIC STORMS. CHAP. II. 129 



fjord very slight. We should notice this circumstance with regard to the vertical components. On the 

 whole, this perturbation is in accordance with the usual equatorial perturbations, and to these we may 

 refer for the explanation of its cause. 



THE POLAR STORM. 



46. The polar storm, as the curves show, is very well defined and brief. It is especially worthy of 

 notice that the deflections, which, in the Central European field, are particularly powerful in the declina- 

 tion, keep to one direction all the time. Even at the arctic stations, the deflections, both in H and in 

 D, are nearly uniform in direction, Dyrafjord alone having an oscillation in declination. In the Table 

 XIX will be found the times of the commencement and termination of the polar storm, as also the time 

 of the maximum of the horizontal component, and the value of the latter at the moment. Since, as we 

 have said, an equatorial perturbation appears in advance of, and presumably simultaneously with, the polar 

 storm, it would seem difficult to decide when the polar storm commences and terminates. In the northern 

 regions, however, the polar storm will make its appearance with such strength, that the effect of the 

 equatorial perturbation will be comparatively minimal. At the arctic stations, we have therefore taken 

 the times when the great storm commences and ceases. As regards the southern districts of Europe, 

 \\t- are aided by the circumstance that the polar storm appears mainly in the declination, while the 

 previous storm has kept principally to the horizontal intensity. In the United States and Honolulu, on the 

 other hand, they both appear in H, but there the effect of the polar storm is marked as a decided undulation. 



The position of the normal line for Sitka was somewhat difficult to determine, and there is there- 

 fore also some difficulty in accurately determining the commencement of the perturbation. At the Asiatic 

 stations, both the perturbations appear in H, so that neither beginning nor end can be determined to 

 any advantage. 



It will be seen that the perturbing force on the whole diminishes with increasing distance from the 

 region of the Norwegian stations. Wilhelmshaven, as usual, comes out of its order in the series, being 

 before Stonyhurst, and with a very much greater maximal force. At most of the stations, the storm 

 lasted, as we see, for about z l /z hours. 



We find, as usual, that the perturbation appears first at Bossekop, then at Dyrafjord, and then at 

 Axeleen. In the central and southern districts of Europe, the maximum occurs at about 22'' io m ; in the 

 United States and at Honolulu it is later about 22 h 40. The maximum on the whole is not well 

 defined, but the force remains for a fairly long time almost constant. This even applies to the arctic 

 stations, and we have therefore set no definite point of time here. 



It appears from the Table, as also from direct observation of the copies of the curves, that the 

 perturbations at all the places are connected with one another, as they appear simultaneously, and their 

 course is somewhat similar. We find again, moreover, a very characteristic feature of these polar storms, 

 namely, that whereas the perturbation in the arctic districts changes very much from one time to an- 

 other, and from one place to another, the conditions in lower latitudes vary more slowly with time and 

 place. This must necessarily lead to the assumption that the perturbation in lower latitudes must be due 

 to the same cause as that in the arctic districts. The perturbation in southern latitudes can, moreover, 

 only be the distant effect of the same current-systems that come nearer to the earth about the auroral zone. 



The circumstances are represented on Charts IV VII, for the hours 22 h o m , 22 h 15, 22 h 30, 

 and 23*" o ln . 



On the whole, the distribution of force remains constant all the time. There is the same system 

 of lines of force, the intensity alone varying. 



This time also, however, the force in Central and Southern Europe makes a distinct, though very 

 slight, turn clockwise. The field is of the same typical form as that of the polar elementary storms 

 already described. 



Birkeland. The Norwegian Aurora Polaris Expedition, 19021903. 17 



