608 B1RKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



When we mention above an angle to the east, we mean an angle whose projection on the latter 

 plane falls to the east. 



Rays of group A which have intersected the magnetic equator must, according to the theory and 

 in conformity with the observations made by Mr. SIRKS, be supposed to form auroral coronae situated 

 some degrees lower than the magnetic zenith. We refer to the form of the precipitation on the west 

 side of the screens, fig. 212, Nos. i, 4 and 7, and fig. 213, Nos. 4 and 8. 



Rays of group B, on the contrary, would be expected to create auroral coronae situated higher 

 than the magnetic zenith. 



Thus we see that the theory gives reasons explaining that the different observations vary as to 

 the situation of the auroral coronse, as stated by BRAVAIS and CARLHEIM GYLLENSKIOLD. We shall return 

 to this important question in Volume II. 



Another question which we shall soon deal with is this: Can we suppose that the cosmic rays 

 which produce the luminous auroral rays can return to space, or are they at once absorbed by the 

 atmosphere? 



We will suppose that they are at once absorbed, because if the cosmic rays should return, then 

 this must take place in and from the foot-point of the auroral ray nearest the earth. But as H.Q 

 for the cosmic rays is between i and 10 millions, the lowest value that the radius of curvature can have- 

 namely when the ray moves perpendicularly to the lines of force above the magnetic poles of the earth- 

 will be between 15 and 150 kilometres. The thickness of the aurora at the foot-point should then be be- 

 tween 30 and 300 kilometres. Now we know that even in the aurora which approaches to within a 

 couple of kilometres^), or very close, to the surface of the earth, the rays have a proportionally small angular 

 diameter at the foot-point. GYLLENSKISLD states the value to be between 10' and 3 (1. c., page 132). 



It must consequently be considered as certain that the cosmic rays which come vertically towards 

 the earth in such way as to form auroral rays, are entirely absorbed by the atmosphere. 



Let us now see to what our experimental results will lead us, when they are applied to the 

 auroral curtain formed by the auroral rays. 



The cosmic rays approach the earth in the same manner as our cathode rays approach the terrella. 

 We must now suppose that the auroral rays are formed by just such distinct, proportionally small groups 

 of cosmic rays, which successively detach themselves from a larger bundle of rays after having passed 

 through the magnetic equator, n, (n -+- 1), ( -f- 2), (n + 3), etc. times. 



It is relatively easy, from our experiments with the terrella, to calculate, in some measure, the 

 difference of time which in this manner should correspond to the entrance into the atmosphere of the 

 "' and (n -\- p)' h auroral rays at the moment when the auroral curtain is formed, provided that the velocity 

 of the cosmic rays be known. This will be done later on in Volume II, but even now we may form 

 an idea to the effect that we shall be led to results which are not in contradiction with the experience 

 which we have now acquired. 



Supposing that H . Q is between i and 10 millions, and that the velocity of the cosmic rays is equal 

 to that of light, we can conclude from the experiments that it is only a question of a fraction of a 

 second between the formation of one auroral ray and the next one. 



We proceed in the same manner as regards the so-called luminous waves which pass through an 

 auroral curtain. If the original bundle of rays from the sun suddenly increases or decreases, this increase 

 or decrease will be shown successively through the rays, one after another. If the rays produce pre- 

 cipitation corresponding to that found on the night side of our terrella, the wave will move from 

 west to east; if the precipitation corresponds to the so-called positive precipitation, the wave should go 



(') ADAM PAULSEN: Aurores boreales observes a Godthaab 1882 1883, pages 8 and 13. Copenhagen, 1893. 



