66 Prof. L. Vegard : Results of Northlight Investigations 



that the difference of height of the bottom edge corresponds 

 to a difference with regard to the penetrating power of the 

 rays. 



The smallest values of the height of the auroral light and 

 the sharpest lower limit (smallest value of Z x ) we should 

 get when all electric rays at a given height formed the 

 same angle with the magnetic lines of force. Then, apart 

 from scattering, all rays would turn round at the same 

 distance from the ground and the number of turns per unit 

 length along the streamer would for all rays be a maximum 

 just at the extreme bottom edge. The pictures 377, 378, 

 380, 381, 382, 383 of the work of Krogness and the author * 

 give instances of this type of aurorse with the maximum of 

 luminosity at the extreme bottom edge. 



As it is hardly to be expected that the angle will remain 

 the same for all the rays, we may expect that bundles every 

 now and then may arrive which have a somewhat different 

 direction from the main bulk, and these rays will turn round 

 at another height. In this way we can in a simple way 

 explain the phenomenon that ray bundles may be seen to 

 shoot down below the main bottom edge of a drapery-shaped 

 arc. This is a most characteristic feature of a drapery- 

 shaped arc and hardly to be accounted for in any other way. 



When the single rays of the ray bundles, which produce 

 an aurora, at a given distance above the lower limit form 

 different angles w r ith the magnetic lines of force, the various 

 rays will turn back into space or be absorbed at different 

 heights, and thus the bottom edge of the aurora will be less 

 sharp and the length of l l3 l 2 will be greater. 



In the ray-form we have the most extreme case of this 

 type of light distribution. 



Here — in the ray-form — we should have rays which turned 

 back into space or were absorbed at the most various 

 heights, and although the pressure increases rapidly down- 

 wards the density of electric rays diminishes rapidly, 

 because the rays gradually are brought to turn back to 

 space or brought to absorption through the effect of the 

 magnetic field. 



And thus we may understand that the ray-form very often 

 has no sharp bottom edge, but that the intensity gradually fades 

 away downwards so that it is difficult to see exactly where the 

 luminosity ceases. 



We can also understand the fact that the ray-form on an 

 average gives a greater value for the height of the bottom edge 



* Loc. cit. 



