176 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



The curve on the whole is much more disturbed; and at i6 h I5 m the instruments oscillate so violently 

 that we were unable to determine any perturbing force. These great disturbances shows that we are 

 now in the vicinity of the current-systems; indeed there are indications of precipitation close to 

 the station. 



At Axeleen the arrow during this period is on the whole westward in direction. It oscillates back- 

 wards and forwards about this mean direction. 



The form of the field, as we have seen, remains unchanged during this period in medium latitudes; 

 in other words, the course of the lines of force is retained. The conditions, however, are not such as 

 can be explained by the assumption of the existence of a simple, stationary system with constant form, 

 that has only altered in strength in the course of that time. Were this the case, the relative distribution 

 of strenght would remain constant all the time. This is not so, however. Sitka, for instance, shows a 

 very marked maximum in the perturbing force during this period, a maximum that we have already 

 found at Axeleen, Kaafjord, and Pawlowsk, and of which there is an indication " in North America, but 

 which is not found in the south of Europe. 



The polar storm thus seems to be somewhat variable in character; but there appear on the whole 

 to be fields with the characteristic properties of the polar elementary storms. We find especially two 

 areas that are characteristic of the polar elementary storms, the area of divergence in Europe and 

 America, and the area of convergence in Asia. 



If we imagine these two to belong to the same system, and the transverse axis to be drawn in 

 that system, this axis would pass from a point in the vicinity of Iceland, right across the Pole, to the 

 district of east Siberia. If we imagine a plane passing through the sun and the magnetic axis of the 

 earth, the above-mentioned line will almost coincide with the line of intersection of this plane with the 

 earth. The point of divergence lies nearest to the sun, the point of convergence far from it; and the 

 field of force shows that as the negatively charged particles sweep down to the earth, they turn off to 

 the left, as viewed from the sun. 



It is difficult to imagine, however, that these are only the effects of a single field of precipitation. 

 It seems far more probable that the precipitation is concentrated about various areas, and that each of 

 these produces its characteristic field of precipitation in the north of Asia, which should produce the 

 area of convergence that we find. The direction of the current-arrows in this storm-centre must be 

 westerly. The current-arrow at Axeleen indicates, too, a continuation of this system, and thus seems to 

 confirm our assumption. But in addition to this system, we must assume a weaker one that should 

 produce the area of divergence in Europe and America, where the direction of the current-arrows in the 

 storm-centre is easterly, the centre being situated somewhat north of Dyrafjord. Whether we have further 

 to assume perturbing forces that act principally in lower latitudes, it is impossible to decide; and we 

 will therefore content ourselves with establishing the fact that these two fields of precipitation account, 

 in the main, for the fields before us. That we are justified in assuming two such systems is perhaps 

 not shown with sufficient clearness by the observations we here can bring forward ; but in the chapter on 

 the perturbations in 1882 83, we shall find that this is the view to be taken of the conditions. It 

 should be possible to account for the direction of the current-arrows in the centre of the weaker system 

 north of Dyrafjord by rays out of space that are drawn in the manner, shown in fig. 38 b. To make 

 the matter still more clear, we may refer the reader also to the second case in fig. 39, with values of y 

 about 0.7 and further to fig. 50 b. 



The second section, from i6 h 15 to I7 h 15. 



At most of the stations from which we have observations, the storm is at its height during this 

 period, and its pronounced polar character is now very marked. We here at least have the effect of 



