100 BIRKEI.AND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



At great distances from the polar regions, e. g. in the south of Europe, only the mean magnetic 

 effect of the precipitation in those regions will make itself felt. 



The question that now presents itself for closer consideration is, Will a galvanic current such as 

 this give rise to a field such as we have found for the storm now under discussion? 



By the aid of the elementary law for the effects of electric currents, it will be easy to see that 

 such will be the case. 



At great distances it will be mainly the two long vertical parts of the current that will be of 

 decisive effect. In the vicinity of the storm-centre, the effect on P, of the vertical parts will be opposite to 

 that of the horizontal part; but as the latter lies nearest the earth, it will predominate in these regions. 

 If, however, we come out along the transverse axis of the system, we shall reach a point at which the 

 horizontal component will equal 0, and farther out its direction will be reversed. 



As approximately the long vertical portions of the current are a necessity for the appearance of 

 these polar storms in the auroral zone, and as it is they which should especially give rise to the 

 universal part of the perturbation, this explains in a simple manner the fact that the polar storms are 

 always accompanied by perturbations in lower latitudes. It also gives an explanation of a circumstance 

 which is especially distinct in this perturbation, namely, that the variations in the field with time are 

 called forth by the motion of a field with a constant form. 



This current-system further explains the following typical properties of the polar storms: 



(1) That during the storm the curves for the arctic stations undergo great and sudden changes 

 with time and place, in accordance with our supposition that the current in these regions really comes 

 near the earth. 



(2) That the curves in lower latitudes, during the great polar elementary storms, exhibit a very 

 even course, that the form of the curve may be preserved over comparatively large regions of the earth, 

 and that the transitions take place very gradually. The explanation of this is simple, namely that the magnetic 

 disturbances are the effect of a comparatively distant system. The variations that will appear in certain 

 parts of the current-system, and which give to the curves their very jagged character around the storm- 

 centre, are not observable at great distances, as we then only get the average effect outwards of that 

 which takes place within the current-space. 



(3) It explains the peculiarity which these elementary polar storms exhibit, in appearing with such 

 comparatively great strength around the auroral zone, while we find, as a rule, that southwards the strength 

 suddenly drops to a small fraction of what it is at the centre. 



(4) It explains an exceedingly characteristic quality of the magnetic storms, namely, that it is 

 only around the storm-centre that the vertical component of the perturbing force has a magnitude of 

 the same order as the horizontal component; while in lower latitudes, it will, as a rule, even during 

 the greatest storms, be only just perceptible with the apparatuses generally employed. Its value in 

 Central Europe seldom exceeds 8y. The only place where P e may have a greater value in relation to 

 Pj (see Art. 14) is near the points of convergence and divergence, where P^ equals 0. 



It is easy to see that our current-system must give rise to a condition such as this. In the 

 neighbourhood of the storm-centre, the effect will be mainly determined by the horizontal part. If we 

 consider the effect of this portion of the current out, for instance, along the transverse axis, the direc- 

 tion of the magnetic force, which was horizontal immediately beneath the current, gradually becomes 

 more vertical. At the two points, one on each side of the principal axis, in which the tangential plane 

 through the horizontal current touches the surface of the earth, the force will be exactly perpendicular 

 to that surface, and thus the horizontal component = 0. 



Farther along the transverse axis, the effect in the horizontal plane will be the reverse of those 

 previously found, and P t , as those points are passed, turns round to the opposite direction. 



