PART II. POLAR MAGNETIC PHENOMENA AND TERRELI.A EXPERIMENTS. CHAP. II. 445 



of force also comes out quite distinctly. At Uccle, for instance, is seen the powerful positive vertical 

 component that is characteristic of the area of divergence. 



It will be seen that the positive polar storms may be explained as effects of a current-system of 

 the same form as that which we assumed as the cause of the negative polar storms, if we assume that 

 the current flows in the opposite direction. In tig. 50, p. 105, we have given a diagrammatic representation 

 of a system of rays, of which the effect in the main will be equivalent to the current-system that we have 

 employed, and which possibly, on the whole, will be more like the actual positive polar current-system. 



These two principal systems, the negative and the positive polar perturbation systems, rarely occur 

 quite alone. As a rule they occur simultaneously, but in different districts. It appears that they always, 

 on the whole, are grouped in the same manner in relation to the sun, and in the following manner: 



On the morning and night sides of the globe, there is always a powerful, negative polar system 

 of precipitation, generally fairly extensive, in which the principal axis of the system falls, as a rule along 

 the auroral zone. This negative system continues westwards on to the afternoon side, but here the 

 principal axis of the system turns northwards to the districts north of the auroral zone, and it looks as 

 if the system also as a rule would be continued westwards until it joined the negative system on the 

 morning side. What the form as a whole, of the system of precipitation would be, cannot, however, be 

 determined; but it is conceivable that it is more or less analogous to the spiral luminous figures that are 

 reproduced in fig. 140 on p. 327. The positive polar system developes along the auroral zone, most 

 strongly in the southern part of the zone. It may sometimes be of very considerable extent, but as a 

 rule is much smaller than the negative system. In this way there will be a boundary-station in the 

 auroral zone, as a rule upon the evening side, which will be situated between the positive and negative 

 systems. Thus, while at the stations on the afternoon side in the auroral zone, the positive storm Is 

 the principal phenomenon, and on the night side the negative, and the perturbations here occur with 

 great distinctness and with well-defined deflections in a positive or negative direction, as the case may 

 be, at this boundary-station now one system, now the other, will prevail, causing the deflections in hori- 

 zontal intensity to be at one time positive, at another negative. 



We have a very clear example of this circumstance in the perturbation of the i5th January, 1883 

 (Chart V, p. 336). 



While in the district to the west of Little Karmakul, i. e. at Bossekop, etc., effects of a positive 

 polar storm are apparent all the time, and to the east, at Ssagastyr and Uglaamie the effects are ex- 

 clusively those of a negative polar storm, the current-arrow here oscillates backwards and forwards, is 

 at first, i8 h 25 m , very small, but increases rapidly with direction easterly, I9 h 5 m , then turns, and at the 

 last point of time, I9 h 25, is a powerful westward-pointing current-arrow. 



At a station situated on the afternoon side a little north of the auroral zone, the northern negative 

 system and the southern positive system will counteract one another horizontally, but co-operate in 

 vertical intensity. Powerful perturbing forces, therefore, are very often found in vertical intensity. The 

 current-arrows for the horizontal perturbing forces there now point in one direction, now in another, 

 and are sometimes exceedingly small. In Jan Mayen, we constantly find this condition very marked (see 

 Charts V VII for the I5th January, 1883, pp. 336 & 337; Charts V X for the isth July, 1883, pp. 

 381383; Charts V VII for the 15* October, 1882, pp. 421 & 422). 



This division of the negative and positive systems of precipitation will always appear in a more or 

 less complete form whenever polar storms occur. 



This area of perturbation will thus, as a whole, be moved westwards in the course of the pertur- 

 bation, in a manner such as would be found if the systems of precipitation formed systems closely con- 

 nected with the sun. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 57 



