288 BIRKELAND. THF NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



At Honolulu the conditions resemble those at Dehra Dun, the horizontal intensity remaining below 

 the normal until far into the morning of the 2yth. We cannot say when it became normal, as we have- 

 no magnetogram for the 24-hours following. It appears that the position of the curve at the conclusion 

 of the magnetogram received is a little too low, and the normal line is therefore here put a trifle low. 



THE FIELD OF FORCE. 



73. The perturbation-conditions, as already mentioned, appear to some extent to be those of a 

 long storm interrupted by powerful intermediate storms. 



The decomposition of these phenomena, however, is somewhat difficult of accomplishment; and \\v 

 have therefore, as in the case of the preceding perturbation, calculated only the total perturbing force. 

 We then obtain at each place only the aggregate effect of all the simultaneously-acting forces; and it is 

 therefore probable that the characteristic peculiarities of the polar fields will be most apparent at the 

 times when the polar storms are most powerful, unless the other systems, equatorial or otherwise, that 

 might be supposed to be acting, were at the same time correspondingly increased. 



If we look at the various fields that occur, we find an exact resemblance to the fields in those 

 perturbations that occurred about midnight Gr. M. T. All the systems exhibit the peculiar fields that 

 characterise the polar storms, namely an area of convergence and an area of divergence. The first of 

 these comes out clearly on all the charts. Its position varies indeed, but only slightly ; and it remains, 

 throughout the series of charts, in the district Europe and Asia. This indicates that the negative system 

 of precipitation extends very far in a direction east and west along the auroral zone on the night-side 

 of the globe, a circumstance that we have frequently met with in previous storms. 



The area of divergence is often very faint and indistinct, for instance in the first three charts, in 

 which the current-arrows in America are very small. In Europe, however, at these hours, there is a 

 more or less distinct indication of its existence. In Chart II, for instance, the current-arrows in the 

 west of Europe seem to be turning westwards, while those at the eastern stations turn in the opposite 

 direction. In the subsequent charts, the perturbing forces in America attain to considerable dimensions, 

 and the area of divergence also comes out distinctly there. 



The arrow at Sitka, which throughout is directed westwards along the auroral zone, seems to 

 indicate that the influence of the polar precipitation which produces the negative polar storm in Europe 

 and Asia, also has some effect at that place. It might indeed be imagined that the positive storm also 

 would predominate at Sitka, so that the current-arrow there would belong to the area of divergence ; but 

 this does not seem very probable, as in that case the positive field of precipitation would need to have 

 a disproportionately high northerly position. 



With regard to the vertical intensity we find that there are exceedingly distinct negative values of 

 P, in the area of convergence, especially at Pawlowsk and Ekaterinburg, near which the point of 

 convergence, or rather the neutral district, appears to lie. This district, according to the charts, 

 seems to be situated in the north-east of Europe or the north-west of Asia. Here the vertical arrows 

 are comparatively powerful all the time, while the horizontal component of the perturbing force is often 

 exceedingly small, a condition of affairs that we should expect to find in the vicinity of the point of 

 convergence. As, therefore, this is very clearly shown by the vertical intensity curve for Ekaterinburg, 

 we have placed on the charts current-arrows for the hours 22 h and 23 h , as well as for intermediate 

 times, although the values interpolated between the entire hours will often be very uncertain, especially 

 when the perturbing force is small. A similar course has been followed with respect to Irkutsk; for the 

 field, as already mentioned, does not appear to vary much as time passes, and the uncertainty of the 

 interpolated values is therefore smaller. 



