u8 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



In declination there is a brief, well-defined, powerful perturbation, which takes place at the time 

 when the storm about the auroral zone is at its height. In this case it lasts from o h 12 to i 1 ' i6 m . 

 In reality this only means that the perturbing force has turned. A similar condition was observed on 

 the I5th December. These two perturbations on the whole resemble one another in a striking degree, 

 a circumstance that is undoubtedly connected with the fact that they both occur at about the same time 

 of day. 



THE DISTRIBUTION OF FORCE. 



43. In the above table the time of the maximum of the horizontal perturbing force is given as 

 the value of P t (max.) at that time. 



The maximum occurs, strangely enough, earliest at the European mainland stations, where it is 

 very distinct and well defined. At Tiflis and the Asiatic stations, the force remains for some time almost 

 constant in magnitude. At o h 39 the maximum occurs at the three American stations; and last of all 

 it occurs at the northern stations, together with Honolulu and the Asiatic stations. 



The earlier occurrence of the maximum on the continent of Europe and in North America than at 

 the source itself round the auroral zone, is a peculiar circumstance that, regarded superficially, might 

 lead to the belief that the phenomena in the arctic regions were separate from those in more south-lying 

 districts. We shall return to this subject later. 



The maximal force, as we see, is strongest at Dyrafjord, where it attains the rather unusually large 

 value of 546 y. The table clearly shows that the force increases with proximity to the district about 

 this station, independently of the direction of its approach. 



After the arctic district, the force is greatest at Toronto, where it attains the comparatively large 

 value, 65.5 y. On the whole, this perturbation is stronger at Toronto and the two stations in the United 

 States than at the European stations, as will best be seen from the charts. 



Next to Toronto comes Wilhelmshaven, which thus, on this occasion also, occupies a comparatively 

 prominent place, a circumstance to be accounted for by local conditions (see the zoth February, 1903). 



The perturbing forces are calculated for a series of times, given in the table, and are synopti- 

 cally represented by a number of charts. As the reasons which led us, on the 15th December, to our 

 assumption of the current-system, are also present in this case, we will, in describing each separate chart, 

 compare the field of force with our current-system. 



