144 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



THE CYCLO-MEDIAN STORMS. 



49. The idea that seems to have gained most adherents regarding the nature of the currents that 

 should produce the great magnetic perturbations is that the magnetic storms should be conditioned, so to 

 speak, by electric cyclones, wandering over the earth's surface. 



This view is upheld very positively by Ad. Schmidt. We will here give a brief extract from his 

 previously-mentioned well-known paper, "Ueber die Ursache der magnetischen Sturme" ( l ). 



"The most characteristic thing of all, however, is the continual change that prevails in all these 

 respects. Surprising similarity is followed in the course of a few minutes by a complete difference or 

 a decided contrast ; a great deflection in one curve answers to a scarcely perceptible jag or bend in the 

 other, while soon after in the one calm ensues, and in the other the liveliest motion. 



"These well-known properties of magnetic storms, as especially the longer and more intense distur- 

 bances have aptly been called, point unmistakably to prevailing local occurrences as the likeliest cause of 

 these phenomena -- occurrences of varying strength and extent, which, appearing now here, now there, 

 perhaps also simultaneously at different places, probably exert a magnetic influence over the whole earth 

 at the same moment, and attain an intense influence, but for the most part only over a more or less 

 limited area". 



This characterisation of the perturbation-conditions during great magnetic storms will do sufficiently 

 well as far as the arctic regions are concerned. As regards lower latitudes, on the other hand, our 

 impression of the conditions is very often as nearly as possible the contrary. There, at any rate during 

 the great storms, the circumstance that attracts most attention is the similarity that the perturbation pre- 

 sents at the various places. As a rule, for instance, the curve for the entire district, Stonyhurst to 

 Pola and Wilhelmshaven to San Fernando, exhibits in the main the same form. The conditions at Tiflis 

 also, often constitute a transition form to those at Dehra Dun. The difference in the forms of curve 

 often only depends upon a gradual turning of the field. 



In conformity with this, our view of the great magnetic storms will be quite a different one, since 

 we assume that the storm is often only of a local nature in the regions around the auroral zone, while 

 the simultaneous perturbations in lower latitudes are probably, as we have seen in the treatment of the 

 polar elementary storms, due to the effect of distant systems. It appears, however,, that there is a class 

 of perturbations that are due to current-systems which appear in lower latitudes at a height above the 

 earth that is small in proportion to the earth's dimensions. These systems, however, seldom seem to 

 appear with any great strength, at any rate not in 190203. Whether, by following up the perturba- 

 tions in their smallest details, we should often find a component that must be due to current-systems of 

 a local character, is a question that we cannot here go into ; but it seems probable that when we come 

 to the very small perturbations, we shall find much to be of a local character. This follows indeed 

 from the fact that there are almost always more or less alternating earth-currents, and also, on account 

 of the current-systems during the great storms, and simultaneously with them, currents must be induced 

 in the earth, and this will give the perturbations in lower latitudes a local component. 



In the whole of our material, we have not found more than one considerable perturbation that in 

 its entirety must be due to systems that come near to the earth in lower latitudes. This was on the 

 6th October, 1902. 



It appears, however, so clearly and distinctly on an otherwise calm day, that its properties can be 

 all the more carefully studied; and it can also be traced over a considerable area. There is always a 

 possibility that such systems may also to some extent co-operate with the polar storms. 



(!) Meteorologischc Zeitschrift, September, 1899. 



