PART I. ON MAGNETIC STORMS. CHAP. It. Iig 



Chart. I. Time 2j h ij m . 



The field of perturbation here shows itself to be of the typical form that is always to be found 

 during the polar elementary storms. The principal axis of the system falls, as shown by the chart, 

 along the auroral zone; and the storm-centre seems to lie a little nearer to Matotchkin Schar than to 

 the other Norwegian stations, though its position cannot be given more exactly. The rest of Europe 

 is in the vicinity of the system's area of convergence. Judging from the force at Pawlowsk, the point 

 of convergence itself should be situated a little to the north of that place. In America we again find 

 the usual directions for the current-arrows, namely, west at the three more easterly stations, and north- 

 west at Sitka. 



Chart. II. Time 2j h jo m . 



The conditions are not essentially different from those of the preceding chart. The principal 

 axis of the system is more conspicuous in the forces at the Norwegian stations, where they are now 

 more or less of the same strength. It still lies along the auroral zone between Kaafjord and Axel- 

 een, and a little to the north of Dyrafjord and Matotchkin Schar, judging from the vertical intensities. 

 In the southern European stations, the forces are more or less uniform in direction with those to be 

 found on Chart I, except that at Pawlowsk there is a slight turn clockwise. The point of convergence 

 still lies a little to the north of the last-named station. 



Chart III. Time a/ 1 45. 



The storm-centre seems to have moved eastwards, the force at Dyrafjord being considerably smaller 

 than before. At the same time the forces at the southern stations in Europe have turned considerably, 

 clockwise. 



Chart IV. Time 24* o m . 



The forces have diminished considerably everywhere, as the close of the perturbation is now ap- 

 proaching. At the southern European stations, the turning is continued in the same direction as before, 

 so that the current-arrow is now directed distinctly southwards. In other respects, the form of the field 

 is in all essentials the same as before. 



CONCERNING THE CAUSE OF THE PERTURBATION. 



39. By reasoning as in the case of the perturbation of the I5th December, 1902, we here too arrive 

 at the conclusion that the perturbation at the four arctic stations is mainly due to the effect of a hori- 

 zontal current-system, which keeps fairly close to the surface of the earth in the area over which the 

 storm is most violent. In this case therefore, it should be mainly a horizontal current from Matotchkin 

 Schar to Dyrafjord. As it is more or less horizontal in this district, the direction of the current must 

 in a large measure coincide with that of the current-arrows drawn on the chart. It follows from the 

 vertical components, that the main volume of the current must flow north of Matotchkin Schar, passing 

 in a WSW direction between Kaafjord and Axeleen, and on to the north of Dyrafjord. This is in the 

 main the same course as that taken by the current on the i5th December. 



We should mention, in this connection, that the earth-currents during this perturbation have been 

 very beautifully registered (see Part III, Earth-Currents). This is most fortunate, as this perturbation is 

 so simple in its course, increasing to a maximum and decreasing to zero. The earth-current, on the 

 other hand, as the curve shows, takes the following course. While the magnetic storm is increasing to 

 its maximum, the current flows in the same direction, increasing to a maximum and decreasing to zero; 

 during the second part of the perturbation, while decreasing, the direction of the earth-current is reversed, 



Birkeland, The Norwegian Aurora Polaris Expedition, 1902 1903. 15 



