PART I. ON MAGNETIC STORMS. CHAP. III. 



193 



are relatively very small, both in declination and in horizontal intensity, while they remain very powerful 

 in V. The oscillations in H and D, however, immediately become stronger again. 



At Kaafjord the storm becomes powerful at ig h 5, with a deflection that is particularly marked in 

 declination. It does not become really great in H until 19'' 22. At about 21'' 41 the conditions are 

 quiet for a time, after which there is only a very slight perturbation; and at 22 h 40 comparative calm 

 has supervened. In all the three curves the deflections are uniform in direction all the time, and towards 

 the side that is typical for these powerful polar storms. The deflection in the F-curve is particularly marked. 



At Axeleen we also get an impression that the storm makes its appearance while other disturbances 

 are taking place. The actuel storm begins here very decidedly at 19'' j m . It suddenly increases, and 

 ten minutes later it is at its height. Right on to 21 u o m , it continues very violent; but from that time 

 until its close at 22'' 33"" there is only a small perturbation. 



At Matotchkin Schar the powerful storm is of longer duration than at the other stations. In H it 

 sets in with considerable strength as early as i8 h 37, and in the D-curve at i8 h 58. The perturba- 

 tion principally affects the //-curve, where it lasts until 22'' 2i m . Considering the violence of the storm, 

 the oscillations in the Z>-curve are very small and variable. What is especially remarkable is that the 

 perturbation throughout has so little effect upon V. It does, it is true, generally decrease V; but the 

 oscillations are not great and sometimes to the opposite side of the mean line. 



The oscillations at the Norwegian stations, with the exception of those in declination at Dyrafjord, 

 which are deflected towards the west, have the directions characteristic of those storms, which occur 

 before midnight at the Norwegian stations, and are powerful and of short duration. 



(b) The conditions in southern latitudes. 



Simultaneously with the storm in the north, a powerful perturbation is noticed on the continent of 

 Europe. It is especially powerful after 19'* 5, and increases in the course of a few minutes to a 

 maximum, which occurs at ig h 18. At 2o h 34 it is once more comparatively slight, and at 22'' 48 

 it ceases in declination, although it still continues for a long time in H. 



At Potsdam, and still more at Pawlowsk, there is a well-defined perturbation in V. The deflection 

 is always in one direction, and answers to a diminution of V. 



At Munich a small deviation from the normal is just perceptible. Here, too, V becomes less. 



At Pola there is a greater effect in V, and principally on the opposite side. 



The conditions at Tiflis form the transition to those at Dehra Dun and the Asiatic district. On 

 the one side they very much resemble those farther north in Europe; but on the other hand, the varia- 

 tion in the //-curve at Tiflis exhibits a close correspondance to the variations in the district between 

 Dehra Dun, Zi-ka-wei, and Batavia, which exactly correspond with these in the storm in the auroral 

 zone. We notice, for instance, the sudden great change that took place in H about 19'' 5, indicating that 

 the polar storm at the Norwegian stations makes its appearance at this hour. We here find conditions 

 that justify a decomposition of the perturbing force. We will in the first place remark that there are 

 variations in //, which in the main closely correspond with simultaneous variations in the perturbation- 

 conditions at the Norwegian stations. We find, for instance, at 19'* 6, a sudden change in the //-curve, 

 H having risen, in the course of twenty minutes, from a value that is 14 y below the normal, to its 

 highest value, which is 28 y above the normal. The oscillation then decreases a little in strength, and 

 then once more increases, attaining a new maximum at 2o h io m . The perturbation then gradually de- 

 creases, and about 2o h 40"', the //-curve coincides with the normal line. In the course of an hour, the 

 horizontal intensity has become almost normal, and continues to decrease, remaining below the normal 

 until far into the night. There is, as we see, an oscillation which actually accompanies more or less 

 simultaneously the storm in the north; and in order to bring out the conditions that belong to these 



Birkeland. The Norwegian Aurora Polaris Expedition. 19021903. 25 



