268 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION igO2 1903. 



Current.Arrows for the llth October, 1902; Chart XV at 23 h 45 m . 



Fig. 124. 



CONCERNING THE CAUSE OF THE PERTURBATIONS. 

 POSITIVE AND NEGATIVE POLAR STORMS. 



69. In describing the preceding perturbations, we have discussed more or less fully the various 

 systems that might be supposed to be the cause of the various fields of perturbation. The results of 

 these reflections, as regards the polar storms, may be summarised as follows: that on the night-side, 

 and to some extent also, in very high latitudes, on the day-side (Axeleen), powerful perturbations will 

 as a rule be formed, with current-arrows directed westwards in the area of precipitation ; and that on the 

 day-side, only a few degrees farther south, fields of precipitation will often be formed, with eastward- 

 pointing current-arrows. There is a continual recurrence of conditions such as these, but they are often 

 indistinct, a fact which may probably be accounted for by the small number of polar stations from which 

 we have received registerings. 



We have already touched upon the question as to how these systems may be supposed to be 

 formed; and we will therefore here only refer the reader to Article 36, especially pp. 105 and 106, and 

 fig. 50 a & b. From the experiment represented in fig. 38 b, there is every reason to suppose that 

 not only the rays that descend on one side of the screen in low latitudes, but also some, at any rate, 

 of those that descend in the polar zone of the terrella, are rays that curve round somewhat in the 

 manner shown in fig. 39, in the equatorial plane, for rays answering to values of y between 0.5 and 

 0.9, and in fig. 50 b. In the experiment shown in fig. 47 b, there is a precipitatation at the top and 



