172 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



Table. XXV (continued). 



THE PERTURBATION OF THE 15th FEBRUARY, 1903. 



(PI. XIX). 



'* f, 



57. This perturbation appears on an otherwise very quiet day. It is of fairly long duration, 

 commencing at about 2 p. m. Greenwich mean time, and lasting about 4^2 hours. It is nevertheless 

 very well defined, and in most cases the normal line can be easily determined, as the conditions before 

 and after are rather normal. In this respect, however, the conditions in North America present some 

 difficulty, as the normal line commences at the moment when the curve shows a marked curvature owing 

 to the diurnal variation; and it appears that, even assuming that conditions are quiet, the form of the 

 curve is not repeated exactly from day to day. 



We have drawn up a table for this perturbation, giving the times of its commencement and ter- 

 mination and of the P 1 maximum, as also the value of the last-named. It appears, as regards the 

 European stations in particular, that the perturbation does not begin and cease simultaneously in D and 

 H; and we have therefore determined these times separately. 



We see that in Central and Southern Europe the perturbation begins almost two hours sooner 

 in H than in D, and ends about half an hour earlier in D than in H; but as a set-off, it is on the 

 whole very strong in D as long as it lasts. We further see from the table that on the whole the 

 maximum occurs almost simultaneously everywhere, somewhere about i6 h 40. It should be remarked, 

 however, that the time of the maximum cannot be exactly determined, as the maximal point is not 

 sharply defined. 



Axeleen, Sitka and Tiflis form exceptions in this respect. Axeleen, as the curve shows, has no 

 well-defined maximum; but the force is maintained, with occasional violent oscillations, in great strenght 

 from i6 h 15 until I7 h 30. Before the great storm, however, there is a fairly well defined, but much 

 slighter perturbation. Its course is almost similar to that of the first perturbation appearing at Sitka; it 

 occurs at about I4 h , and has its maximum at about I4 h 40. 



At Sitka, the impression given by the curve is that of two almost separate perturbations, each 

 with its well-defined maximum. The first last from I4 h io m to i6 h io m , and the second from i6 h io m 

 to about i8 h , the peculiarity here being that, in contrast to the other parts of the world, the first part 

 is the more powerful of the two. 



At Kaafjord the conditions on the whole are similar to those farther south in Europe, with the 

 exception that the conditions in D and H are interchanged, the perturbation in H at Kaafjord almost 

 corresponding with that in D farther south. During the first part, from 13^ 45 m to I5 h 35 m , it is a 



