84 ISIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



THE POLAR ELEMENTARY STORMS. 



33. One cannot look long at the curves for the registered magnetic elements without observing a 

 regularity in a number of details, especially in the behaviour of the great storms. This, strange to 

 say, is not least apparent at the stations round about the auroral zone, and especially in the storms 

 that have occurred at our Norwegian stations during the period in which the magnetic conditions have 

 been observed by us. In the first place, it appears that the great majority of storms of short duration 

 are at their height at our stations at about midnight by local time; and when they make their appearance 

 at that time, it is found that they nearly always cause oscillations in the same direction for the horizontal in- 

 tensity and declination. We further find that the direction of the oscillation in the vertical curve, espe- 

 cially in the case of Axel Island and of Kaafjord, is also repeated time after time. We get a direct 

 impression that, notwithstanding little accidental circumstances, the magnetic storms, in their formation 

 and course, are controlled by very limited conditions, and that these conditions are pre-eminently fulfilled 

 in very limited areas in the polar regions. This impression is opposed to the theory upheld by Ad. 

 SCHMIDT ( l ) and other terrestrial-magnetists that the magnetic storms are produced by free cyclonic 

 electric current-systems. 



In the well-known paper mentioned below, Professor SCHMIDT says: 



"Electric currents have hitherto principally been accepted as the cause of perturbations, either currents 

 in the ground or in the air, especially in the upper, probably better conducting strata of the atmosphere. 

 Although no great clearness prevails as to the physical conditions under which such currents may occur, 

 yet we shall venture to maintain this hypothesis, notwithstanding the objections raised against it by 

 BIGELOW, the rather that no doubt can any longer exist as to the reference of the diurnal variation to 

 such currents. Regarded from this point of view, these centres of action can hardly be anything else 

 but current-phenomena that stand out with a certain distinctness from the current-system of the whole 

 earth, on account of their intensity and individual limitation, in fact wandering current- vortices that, in 

 the simplicity of the elementary perturbation, we may also expect as the normal, like the cyclones and 

 anti-cyclones of the atmosphere". 



The violent storms in the north are always accompanied by simultaneous perturbations, that are 

 observable right to the equator; and as a rule we shall find, by direct study of the curves, that in 

 general the effect becomes slighter towards the equator. 



The important question now presents itself: In what way are the perturbations in southern lati- 

 tudes connected with the perturbations in the north? Is there any simple connection at all? 



In order to throw light upon these questions, we have made a careful investigation of a number 

 of very simple storms. At the outset it is only natural to suppose that when we have a perturbation 

 that runs the simplest possible course, this phenomenon will be particularly well adapted for throwing 

 light upon the laws of the perturbation. 



The next section will deal with a number of simple polar storms such as this, which we have 

 picked out and called polar elementary storms. These, independently of any hypothesis, can be charac- 

 terised as follows: 



(1) They are comparatively strong at the poles. The simultaneously perturbing forces, even as 

 far north as the 6oth parallel, have already sunk to about a tenth of their strength in the auroral zone. 



(2) They are of short duration, frequently lasting not more than two or three hours. 



(') Ueber die Ursache der magnetischen Sturme. Meteorologische Zeitschrift, Sept., 1899. 



