178 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



If we seek a simple explanation of the fields formed during this second section of the storm, we 

 find that it is only necessary to assume a further development of the systems that were supposed to 

 have produced the fields during the first section. We saw, that the system on the midnight-side had a 

 westward motion, and the conditions at Dyrafjord may be considered as produced by a system similar 

 to that assumed in the first section of the storm, that is to say by rays that descended upon the day- 

 side and were deflected, perhaps in a manner resembling that shown in fig. 50 b on p. 105. 



Here, too, the same difficulties present themselves as on several previous occasions. At Tiflis, for 

 instance, we find positive values of P v , at any rate at first in Charts V and VI; and we are therefore 

 compelled to assume that, as already mentioned, perturbing forces also appear in lower latitudes, possibly 

 produced by systems similar to those producing the cyclo-median storms. We cannot, however, go 

 into this subject, as the fields do not furnish us with any reliable information concerning these systems. 

 In any case, the perturbation clearly shows the great variableness of the storm in the region about the 

 auroral zone, a condition which plainly proves that during this storm the current must come compara- 

 tively near the earth. 



The third section. 



The field is given in two charts, IX and X, for the hours I7 1 ' 30 and 17'' 45 respectively. 

 The form of the field is the same, on the whole, as during the first period. The chief difference is that 

 hardly any disturbance is now noticeable at Dehra Dun and Batavia. The conditions at the Norwegian 

 stations also are the same. At Matotchkin Schar the current-arrow is in the act of swinging round to 

 the opposite quarter counter-clockwise; and at 17'' 30" its direction is SSE. There is no current-arrow for 

 this station on Chart-X, the magnetogram-paper having been changed at that hour. The curves show, 

 however, that the force ends by being directed northwards along the magnetic meridian. It thus seems 

 reasonable to assume that all through the intermediate storm; the effect of this system, which we find 

 before and after, has been perceptible. 



Upon the whole we recognise in the current the characteristic feature of these perturbations, 

 namely, greatly varying local conditions in the arctic regions, while in lower latitudes they vary less 

 rapidly with time and place. We conclude from this that the perturbation there must be due to a distant 

 system. 



There is another circumstance connected with this perturbation, that may be worth noticing. If we 

 look at the //-curve in the district from Stonyhurst to Pola during the intermediate storm, we notice 

 three types of curves. The first of these is formed at the stations Stonyhurst, Kew, and Val Joyeux, 

 the second at Wilhelmshaven and Potsdam, and the third at Munich and Pola. The curves of the first 

 and second types both have a marked undulating form; while in the 3rd type there is a single, uni- 

 formly-directed deflection. This last condition is also found at Asiatic stations. 



In accordance with the undulating form in the first two types, there is a more pronounced turning 

 of the current-arrow. In this there is possibly a resemblance to the previously-described polar elemen- 

 tary storms. There, too, the turning of the current-arrow was most pronounced at the stations whose 

 curves were classed under the first two types, and less pronounced in southern latitudes ; and the cause 

 would then be sought for in a movement of the current-system that produced the effect. I have already 

 drawn attention to this circumstance in my report "Expedition Norve'gienne 1899 1900," pp. 32 & 33. 



