56 Prof. L. Vegard : Results of Northliglit Investigations 



If this view with regard to the cause of the lowering: 

 effect is the right one, we should expect on the southern 

 hemisphere to find an effect of the opposite direction, or the 

 radiation point should be situated above the magnetic zenith. 



At present I am not in possession of observations to test 

 this consequence, but it is to be hoped that observations from 

 the Antarctic expeditions and from other stations on the 

 southern hemisphere may give us the means of putting this 

 consequence of our view to a test. 



§ 4. The Direction of Band-Forms. 



The determination of the direction of band-forms, arcs,, 

 drapery-shaped arcs and draperies, is a matter of interest 

 in connexion with the auroral theory and the question 

 with regard to the nature of the cosmic rays. Birkeland * 

 has shown by experiment that the electric rays will strike 

 a magnetized sphere in a band which has the form of a 

 spiral round the magnetic axis-point, and this result can 

 also be verified by calculations f. The reproduction of 

 a photograph of one of Birkeland'' s experiments is shown in 

 fig. 2 (PL II.). 



On the northern hemisphere negative rays would give a 

 spiral like fig. 2a (PI. II.), positive rays one like fig. 2 b. 



On the assumption that the band-forms may be regarded 

 as forming a part of such a spiral, the directions of the bands 

 would give us a means of determining the sign of the charge- 

 carried by the cosmic rays. 



In order to compare observations with the theoretical 

 results we must refer the directions to the magnetic axis- 

 point, in a way similar to that in which we usually refer 

 points to the astronomic pole, and we get "magnetic" co- 

 ordinates, such as magnetic longitude, latitude, hour angle 

 and azimuth. Knowing the coordinates (</> A ) of the mag- 

 netic axis-point, we can easily pass from astronomical to 

 magnetic coordinates. 



The direction of a band may be given by the azimuth of 

 its westward directed end. Now from Birkeland's spirals 

 we should conclude that the magnetic azimuth should not be 

 exactly 90°, but on the northern hemisphere it should be 

 greater or smaller than 90° according as the cosmic rays were 

 negative or positive. 



From the Haldde-material we determined the directions 



* See K. Birkeland, The Norwegian Aurora Polaris Expedition 

 1902-03 : vol. ii. second section, p. 600. 



t See C. Stormer, Arch, des Sciences Pliys. et Nat. Ge7ieve, 1917,. 

 p. 287. 



