602 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



The pictures n and 12 are taken under similar conditions to 9 and 10 but with the magnetic 

 poles reversed, moreover the tension is now 2900 2500, the pressure being 0.005 mni - 



The pictures 13 and 14 are taken during experiments where the hour-angle of the south pole was 

 255 and declination 19. The pictures are taken from places on the prolongation of the magnetic axis, 

 the tension was 2600 2400 volts and the pressure 0.006 mm. 



The pictures 15 and 16 are taken during similar conditions, but the magnetic poles are reversed 

 (the terrella re-magnetised) and the tension was 2700 2300, under a pressure of 0.007 mni - 



The magnetising current for the terrella was, as will have been understood, 20 amperes in all the 

 experiments, and the discharge-current about 25 milliamperes. 



It is also seen by the 4 last pictures how the luminous patch referred to takes different shapes in 

 different positions and encroaches upon the luminous ring. 



The most striking result of these experiments is that the polar spiral of light always forms itself, 

 in surprisingly nearly the same manner around the magnetic poles without regard to whether the position 

 of the magnetic axis is altered at all in relation to the central line between the terrella and cathode. The 

 difference between the spiral round a magnetic north pole and the spiral round a magnetic south pole 

 is easily recognised, as the spiral seen from above a north pole winds itself in the direction of the hands 

 of a clock, while the spiral over a south pole winds the opposite way. 



On the other hand, the position of the polar luminous patch is more sensitive to changes in the 

 position of the magnetic axis, as the light patch with such alterations had changed place and shape to a 

 certain degree. 



When we apply the results described above to the earth, we would expect to find that similar 

 spirals of precipitation to those here depicted formed around the magnetic poles or perhaps nearest 

 around the points in which the magnetic axis of the earth intersects the earth's surface (see p. 58 of 

 this work, Section I, and STORMER'S Memoir in Arch, de Geneve, I.e. 17). 



These spirals of precipitation must in the course of the daily rotation of the earth, swing round 

 the true poles of the earth, while they, however, always retain their direction in relation to the line of 

 direction to the sun, and their position in relation to the magnetic poles. 



As we have seen before, the north pole spiral can, as regards the earth, with some degree of 

 resemblance be' compared with a circle of from 40 to 45 spheric diameter and with the centre in a point 

 with latitude 78 20' N, longitude 71 u'w. (New year 1903) which was the northern point of intersec- 

 tion with the axis just mentioned. If the corpuscle rays from the sun happen to be specially flexible, 

 the spherical diameter can be less than 40. 



It is obvious what ample opportunity is here afforded for testing the correctness of our theories 

 The theoretic positions of both the precipitatons which occasion polar magnetic storms and the preci- 

 pitations which occasion auroral arcs, are, as may be seen, hereby ascertained by a simple construc- 

 tion, after which it is merely necessary to observe the hour and place. 



We get a theoretical daily and annual motion in these phenomena, by which the theory can be 

 controlled. 



Owing to the relation of the auroral spiral to the direction towards the sun, the spiral will, when 

 compared with a fixed point of observation, appear to turn with the sun, in addition to also periodically 

 shifting in relation to the spot in other ways. 



A thorough study of these questions will be made and the results be made known in the second 

 volume of this work. By that time other new experiments will be made as to the correct size of the 

 polar ring of precipitation (45 angular diameter), and the situation of this at the various positions of 

 the terrella will be determined with the utmost possible precision. 





