386 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



THE PERTURBATIONS OF THE 1st FEBRUARY, 1883. 



(PL XXVII). 



88. Here, as on the preceding term day, we can separate a first comparatively slight pertur 

 bation, appearing, more or less isolated, at about n h , from the subsequent powerful storms. Only at 

 two stations, Fort Rae and Godthaab, do we find, during the first period, rather powerful perturbing 

 forces. We find, moreover, distinct deflections at several other stations, but these in the first place are 

 considerably weaker, and in the next, of longer duration, than at the two stations just mentioned; and 

 the character of the deflections does not seem to indicate that they have any very close connection 

 with one another. 



This first perturbation is particulary distinct at Fort Rae. At io h 55 m the deflections suddenly 

 increase to a maximum, and then again decrease rather rapidly. The perturbing forces are negative in 

 the horizontal intensity, and directed eastwards in the declination; and the current-arrow, as will be seen 

 from Chart I, is directed westwards along the auroral zone at the time when the deflections are strong. 

 There is thus, certainly, negative polar precipitation in the neighbourhood of this station; and at Godthaab 

 too, a negative polar storm seems to be acting. 



If we look more carefully at the chart, there appear to be signs of positive forces at Ssagastyr, 

 and possibly a positive system has formed on the afternoon-side, but if so, it is not very clearly developed. 

 In this respect, however, we have not sufficient data to go upon. 



After this precursor of the subsequent powerful storms, there follows an interval in which no very 

 great forces appear. Soon, however, new storms begin, which rapidly develope until they attain con- 

 siderable strength, and form the principal systems of that day. 



The storms in this period will naturally be divided into two sections, 



(1) those that occur between i4 h 3o m and ig h 45, and 



(2) the storms from I9 h 45 m until the end of the period. 



Such a division of the phenomena will of course be imperfect, and may appear somewhat artificial, 

 since we have constantly found, that one system developes from another; but it is done for practical 

 reasons, in order, if possible to obtain a clearer general view of the conditions. 



At about 14'' 3O m , some more or less powerful deflections begin at Kingua Fjord in the horizontal 

 intensity and declination simultaneously, their direction indicating the presence of a negative polar storm. 

 This can apparently be traced farther, over Fort Conger, where there is at the same time a distinct 

 deflection in the declination-curve; and judging from the conditions of this curve at Cape Thordsen, this 

 system is also at work there. There appears to be a weaker positive storm in the vicinity of Jan Mayen. 



This perturbation, however, is of brief duration, and its field of operations is comparatively restricted. 

 In the course of about an hour, it is practically over. At about i6 h , on the other hand, powerful storms 

 begin to develope at all the stations round. 



The deflections at Kingua Fjord increase most rapidly to a considerable amplitude, and attain their 

 highest value as early as 17'', after which they remain more or less powerful in declination, while PI, 

 decreases fairly evenly, reaching its normal condition again at about 2o h . This negative system of pre- 

 cipitation apparent at Kingua Fjord, now extends as a great system westwards. It is felt at all the arctic 

 stations, more strongly, indeed, than anything else at the time when the deflections are greatest; for 

 here too, there occurs simultaneously a positive system of precipitation, which to some extent counter- 

 acts the negative. 



The distribution of force round the auroral zone is here, too, exactly similar to that found during 

 the earlier storms. At Kingua Fjord, Fort Rae, Uglaamie, Ssagastyr, Cape Thordsen, and possibly Fort 

 Conger, it is almost exclusively the negative system of precipitation that acts; at the other polar stations, 

 the positive system also asserts itself more or less strongly. 



