1923- N<^- 8- THE ATROR.AL SPFCTRIM AND TIIK ATMOSniFRF. 



will — as it were — make the molecules lighter. In this way we can 

 understand that Nitrogen in the auroral region can be destributed as if it 

 was one of the very lightest gases. 



Light gases like Hydrogen and Helium when they get ionised may 

 have their weight so much reduced that they would fly away from the 

 earth; and thus we may explain the absence of Hydrogen and Helium 

 layers which have earlier been assumed to exist on the top of our 

 atmosphere. 



If the upper strata of the atmosphere to a great extent consist of 

 positively charged A^-molecules, we can also understand that under this 

 condition the bombardment with electric rays may produce other A'-lines 

 than we observe in our laboratory experiments, for it only means that we 

 have not been able to produce artificialy in the observation chamber a 

 quantity of Nitrogen containing a sufficiently large persentage of positive ions. 



I also think that this hypothesis of an eleatrically charged upper layer 

 gives a very simple explamation to the results found by Lindema.nn and 

 DoBSox from their investigations of the meteors; for the greater density 

 which they want will be produced by the electric charge of the upper 

 layers, and we need not take our refuge to the very improbable hypothesis 

 of the high temperature which they suppose to exist above 10 km. 



The highly ionised state of the atmosphere must be restricted to a 

 certain layer, and as long as the cosmic rays are absorbed in this layer 

 we get the ordinary auroral spectrum shoving the typical green line. As 

 the typical grenish-yellow aurorae may have their bottom edges so far 

 down as 100 km. above the ground, this would mean that the positive 

 aver should go down to a height which is not greater than 100 km. 



If the penetrative power of the rays could be great enough to enable 

 them to pass into the neutral atmosphere the Nitrogen spectrum would 

 turn into the more ordinary type and the green auroral line should 

 disappear. 



As a change of spectrum must be accompanied with a change of colour, 

 this hypothesis of an electrically charged upper layer gives us new possi- 

 bilities for explaining the marvellous changes of colour which the aurorae 

 may display. 



Thus I think that the peculiar colour of a drapery-shaped arc, which 

 I observed at Bossekop 1912, may be simply explained in this way. From 

 the upper limit to a distance of a few km. from the bottom edge the arc 

 had the ordinary greenish-yellow colour, but at a certain height the colour 

 turned into dark red. This red bottom edge was found simultaneously 

 all along the arc, which extended across the sky from ENE to WNW, and 

 was observed at least 8 minutes. 



It would be of great interest to measure the height of such forms to 

 see if they come lower down than the ordinary green arcs; b-it these 

 phenomena are very rare, and the height of such arcs has not yet been 



Vid.-Selsk. Skrifter. I. M.-N. KJ. 1923. No. 8. 2 



