PART I. ON MAGNETIC STORMS. CHAP. II. 145 



THE PERTURBATION OF THE 6th OCTOBER, 1902. 



(PI. I). 



50. This perturbation appears quite suddenly upon an otherwise very calm day. As far as one can 

 decide from the magnetograms, it makes its appearance simultaneously in all parts of the area over 

 which it is observable. Only at Axeleen, and to some extent at the other Norwegian stations, has the 

 perturbation a somewhat peculiar character. At the other stations at which it is noticeable, its course 

 is as follows. 



It makes its appearance at 14'' i3.5 m simultaneously in both D and H. The deflection increases 

 suddenly, and about 5 minutes later reaches its maximum, this also occurring simultaneously in the two 

 curves. The deflection thereupon decreases in both, first rather suddenly, afterwards more slowly, until 

 about I4 h 48"", when no deflection is observable. 



It will be seen from the copies of the magnetograms, that the geographical distribution of the 

 perturbation is within fairly sharply-defined limits. The effect is greatest in Europe, especially at the 

 more westerly stations up to and including Wilhelmshaven and Pola; but even at Pawlowsk, where the 

 perturbation is distinctly perceptible, it is only slight. If we compare simultaneous perturbing forces 

 in Pawlowsk and Wilhelmshaven, we see that at the latter station they are about four times as great as 

 at the former. At Tiflis the perturbation is only just perceptible. 



At Dehra Dun, Zi-ka-wei, Batavia and Christchurch, the //-curve, as the perturbation makes its 

 appearance, gives a little leap, which means that H receives a small, and as it appears, permanent 

 increase. These stations are marked (o) on the chart, as no definite perturbing force can be taken out. 



At the three American stations, Toronto, Baldwin and Cheltenham, the perturbation runs nearly 

 the same course as in Europe, except that the deviation in declination is to the opposite side. From these 

 stations the effect diminishes greatly westwards. At Sitka it is almost, and at Honolulu quite, imperceptible. 



At our Norwegian stations it appears as follows. At Kaafjord it is distinctly noticed, but its course 

 is somewhat different, especially as regards the latter half. At Matotchkin Schar a disturbance is notice- 

 able, but no measurable deflection. On Axeleen there is simultaneously a perturbation of about the 

 same duration and strength as on the continent of Europe; it takes place on the whole within the same 

 period, but its course is different. On the other hand it is of about the same magnitude as the pertur- 

 bation in the south-west of Europe, or perhaps a little smaller. 



From Dyrafjord we unfortunately have no observations; but it seems likely, judging from the course 

 of the current-lines as shown by the charts, that this station would have been the most important. 



THE FIELD OF FORCE. 



51. During the perturbation the form of the field is maintained unaltered, the strength alone varying. 

 We have therefore found it sufficient to work out two charts, namely, for the hours I4 h 22.5 and I4 h 30. 



We have made the calculation, however, for several hours, and these will be found in Table XXII. 

 With a view to increased accuracy, we have had all the curves enlarged photographically to five times 

 their original size. 



Fig. 64 shows these enlarged copies of curves from Wilhelmshaven. 



In the area from which we have observations, the greatest effect is in the south-west of Europe, 

 and the east of North America. It occurs, as we see, upon the day side. The current-arrow indicates 

 very distinctly a negative vortex, which should go round the North Atlantic Ocean; in reality we have 

 an area of convergence for the perturbing force. Whether the vortex is closed, whether in 



Birkeland. The Norwegian Aurora Polaris Expedition, 1903 1903. 



