1923- No. 8. THE ALRORAL SPECTRUM AND THK ATMrmpHFRF. 



13 



Nitrogen, in order to give the most prominent of all lines in the auroral 

 spectrum. Being a light gas, however, Geocoronium schould soon nredominate, 

 and we should expect that any trace of a N -Spectrum should disappear at 

 a height of. say, 140 km. 



This conclusion is independent of the assumption we make about the 

 temperature of the upper strate of the atmosphere for if the temperature 

 is high the pressure of X at a height of 100 km. will be greater and for 

 Geocoronium we had to assume a correspondingly great pressure to make 

 its green line predominant at this altitude. Somewhat higher up in the 

 atmosphere the lighter gas (Geocoronium) would take the lead. 



If, then, we were able to show that in the auroral spectrum the Nitrogen 

 lines are maintained at a height greater than 150 km., I think we were 

 justified in concluding that also the green line (5578) must be due to 

 Nitrogen. Observations are now in progress for testing this point. 



From a physical point of view, however, it is very unlikelv that a 

 new gas Geocoronium should exist, because there is no place for it in the 

 periodic system. 



I therefore think the best procedure would be to try to interpret the 

 results of our spectral analysis without introducing the possibility of a new 

 type of matter. The introduction of Geocoronium is merelv an easv wav 

 of getting out of the difficulty. 



But if -u'c giir tip Geocororiiimi, it seems as if the green fine shouUi 

 have to be aseribed to Xitrogcn, ami Xitrogeu should be the predominant 

 gas up to the very limit of the atmosphere. 



Here, however, we meet a difficulty of another nature. From measure- 

 ments of the height and position of the aurorae, we know that some times 

 the upper end of an auroral ray can reach a height of 5 — 600 km., and 

 at this height its light intensity- is of the same order of magnitude as 

 further down towards its bottom edge. 



In previous publications' I have dealt with the variation of light 

 intensity along the ray streamers. The increase of intensity upwards can 

 be explained by assuming that a greater part of the cosmic rays when 

 they enter the atmosphere form great angles with the magnetic lines of 

 force. In this way we may explain that intensity variations along the 

 streamers may accur, but in order to get a noticeable intensity it is of 

 course necessary that the pressure of the atmosphere does not fall below 

 a certain limit. 



From the table W we see, that the Nitrogen pressure decreases rapidly 

 upwards. Assuming a temperature of 220'' absolute above 10 km. the 



' L. Vegard and O. Krogxess : The Position in Space of the Aurora Polaris, p. 1 49. 

 L. Vegard : Phil. Mag. XLII, p. 59. 



