PART I. ON MAGNETIC STORMS. CHAP. III. 209 



THE PERTURBATIONS OF THE 27th & 28th OCTOBER, 1902. 



(PI. IV). 



61. Throughout the first half of October, there was calm as far as our arctic stations were con- 

 cerned. About the 24th, however, a violent storm takes place, lasting from about 5 hours before mid- 

 night Gr. M. T. until 4 hours after. During the succeeding days, perturbations of more or less strength 

 occur, beginning late in the evening and attaining their highest development at about midnight. As day 

 advances, there is once more calm, but the storm returns again before midnight. This condition of 

 things contjnues, and culminates m the violent storms about the 3ist. From some of the stations there 

 is included a characteristic equatorial perturbation, occurring on the 2gth and soth. This perturbation 

 is already described Art. 54. 



The time occupied by the perturbations of the 2yth and 28th October is from 14'' on the 27th 

 until about i u on the 28th, the curve for this period being shown on Plate IV. 



At the arctic stations, the character of the conditions is that of two separate storms, one of which 

 occurs early in the afternoon, with its maximum about 16''. This is fairly powerful at Axeleen, while 

 at the other Norwegian stations it is comparatively less so. The other storm is at its height at about 

 22 h to 23'', and is a well-defined, fairly powerful perturbation, lasting about three hours. 



In southern latitudes, the direct impression of the conditions of this perturbation is to some extent 

 quite different. We will take, for instance, the condition at Tifiis, a station that occupies an intermediate 

 position between the polar and the equatorial regions, and where we are therefore likely to find con- 

 ditions that are characteristic of both. Here the perturbations last much longer. Even earlier than noon, 

 there are perturbations indicating the presence of a perturbing force directed northwards. At about 13'' 

 the force turns round, the perturbation appearing also distinctly in declination, where it is directed east- 

 wards. With the exception of one intermediate storm, this state of affairs lasts until 20'' 24. The 

 interruption lasts from 15'' 24 to i6 h 54, and thus coincides in time with the already-mentioned 

 perturbation in the north. The same thing is found at Dehra Dun and Batavia, but there the perturba- 

 tion is chiefly in H. 



Finally, from 21'' 40 until about midnight there is a perturbation that occurs simultaneously, and 

 is in connection with the perturbation round the Norwegian stations. It is most powerful at our Nor- 

 wegian stations, but in southern latitudes it is much less than the perturbation that occurred earlier. 

 In this way, the treatment of the perturbation falls naturally into two sections, the first from I4 h to 

 2o h 30"", and the second from 2i h 40 until about midnight. 



THE DISTRIBUTION OF FORCE. 

 62. The first section. I4 h 2o h 30. 



The perturbation during this period is especially worthy of remark from its being particularly 

 powerful at the equator, in the regions about Dehra Dun and Batavia. 



While these comparatively powerful perturbations are taking place at the equator, there are also 

 storms round the auroral zone. We see, on the other hand, that the effect in America increases 

 towards Sitka, where there are two distinct maxima during this period. One of them coincides with the 

 already-mentioned intermediate storm and occurs between is h 30 and iy b i5 m . This is preceded by 

 a powerful perturbation lasting from I3 h to I4 h 45"". 



From this it would appear that this part of the perturbation shows, to some extent at any rate, 

 the effect of polar systems, which this time seem to keep, in some measure, fairly near the regions to 

 the north of Sitka. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 27 



