138 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



of course produce positive values of P,. But it seems difficult to imagine that its effect may be traced 

 as far off as at Tiflis. 



It seems more natural to explain the conditions by rays that come rather near to the earth in lower- 

 latitudes, as in the cyclo-median perturbations. The considerable strength of the current-arrows in Europe, 

 as shown on Chart I, seems to point in this direction, although the increased strength may possibly be 

 chiefly due to the fact that the two polar systems are here acting in about the same direction. 



On account of the quiet character of the deflections, and the small perturbing forces, these currents 

 must nevertheless lie fairly high; and it is possible that they are connected with one of the polar 

 systems, probably that in America. 



The field of force during the second storm is shown on Charts III, IV, V and VI, for the 

 hours 23 h o m , 23 h 15, 23'' 30, and 23'' 52.5 m , respectively. 



Chart III shows the conditions at the beginning of the second storm. It is only at Dyrafiord that 

 the perturbing force has reached any magnitude. The arrows for the European stations represent a very 

 curious field of force; but as they are small, the determination is somewhat uncertain, owing to the 

 inaccuracy in the determination of the normal line. 



The field in Charts IV and V shows very distinctly the form that is typical for the polar elemen- 

 tary storms. 



At Dyrafjord, the force is exceedingly great, and is directed westwards along the auroral zone. 

 The storm-centre, which is presumably situated very near Dyrafjord, is now about 145 east of the sun. 

 The field to the south exhibits a well-marked area of convergence. There is probably, however, not 

 only precipitation round Dyrafjord; but it also seems as if there were local currents round the other 

 Norwegian stations, as the force there is also comparatively strong. 



At 23'' 52. 5 m , Chart VI, the strength of the field is considerably less. At Dyrafjord the direction 

 of the arrow is different, being now south. . 



We notice a peculiar circumstance, namely, that with the turning of the arrow at Dyrafjord, the 

 whole field turns. 



The arrow at Kaafjord, and at the more southerly European stations from Kew to Tiflis, indicates 

 an area of convergence. Judging from the shape of the field, the centre of this area should be about 

 Pawlowsk ; and in fact we find that at this moment the force there equals 0. 



We thus see that the conditions in more southern latitudes are in very close connection with those 

 round the auroral zone. This circumstance, as we have said, may be explained in a very simple way, 

 the perturbations in low latitudes, in these cases, being assumed to be produced by the action, at a great 

 distance, of the systems that are necessary to the production of the perturbations about the auroral zone. 



In all the elementary polar storms described, it will generally have been remarked (i) that all the 

 current-arrows in lower latitudes turn clockwise during the perturbation, and (2) that in the same lati- 

 tudes, the simultaneous current-arrows turn clockwise, if one moves from eastern to western stations. 

 These assertions I have already made in my earlier work, 'Expedition Norvegienne de 1899 1900', etc., 

 pp. 32 & 33. 



In this earlier work, I assumed that these assertions were explained by a current-system like that 

 in fig. 45, and by the fact that this current-system, starting in the polar regions, was there deflected 

 westwards during the perturbation. We have here maintained a somewhat different view, as, instead of 

 the horizontal current-system, we have supposed a system that, idealised, consists of two vertical 

 branches connected by a horizontal portion, and that this current-system has a district of precipitation 

 in the polar regions, with its principal axis along the auroral zone. The current-line system (see Art. 

 34 and fig. 40) is however even now similar to the formerly assumed real current-sytem. The turning 



