UIRKELAND. THE NORWEGIAN AURORA POLARIS 



Current-Arrows for the 23rd October, 1902, at 



EXPEDITION, 1902 1903. 



19h 16m and 22h 22.5m. 



Fig. 36. 



CONCERNING THE CAUSE OF THE POSITIVE EQUATORIAL PERTURBATION. 



31. The fact that this type of perturbation exhibits such great simplicity with regard to the distri- 

 bution of the force, and also that it shows such a tendency to repeat itself from time to time, indicates 

 that these perturbations might have a simple explanation. 



As already remarked in the introduction, it will always be possible, in a purely formal manner, to 

 satisfy the properties of the field in several ways. It is our intention here to mention some of the 

 possibilities that might perhaps explain these perturbations, and we will in the first place find out what 

 magnetic systems might be assumed to have produced the field. 



(1) We cannot assume a variation in the terrestrial-magnetic field itself, which would explain the 

 field about the equator; for as we go north, the perturbing force is no longer directed along the total 

 intensity. P is directed horizontally almost everywhere; in the south its direction is somewhat down- 

 wards, in the north often upwards. In the far north, moreover, P, (see p. 45) is no longer directed 

 along the magnetic meridian. 



(2) As we shall subsequently see, current systems will undoubtedly appear in the polar regions 

 during a series of polar perturbations. It might then be reasonable to try whether this equatorial 



