l'6 L. VEGARD. IÇ23. No. 8. M.-N. Kl. 



measured. If our view is right, however, such aurorae would show us 

 where Uie neutral atmosphere begins. 



Th^ aurorae, however, also show colour changes of another type. 

 Separate auroral streamers or ray-bundles forming part of a drapery or 

 drapery-shaped arc may turn into another colour usually red or bluish-red. 

 The colouring is in this case not restricted to the bottom edge and is not 

 kept for a conciderable time, but it extends to almost any part of the 

 streamer, and some times the whole ray bundle may turn red. 



The red streamers appear in between the ordinary green-yellow ones; 

 usually they only last for a fraction of a second. Other red coloured 

 bundles shoot in, and the distribution of colours and thus the whole picture 

 will undergo very rapid changes. 



To explain this kind of colour variation, I have in previous papers 

 introduced the hypothesis that usually only a fraction of the rays, that 

 enter the atmosphere gets absorbed, so that most of the rays return into 

 space. Remembering that the spectrum from a gas bombarded by cathode- 

 rays depends on the velosity of these rays, we may expect to get a varia- 

 tion of colour according as a greater or smaller fraction of th3 rays are 

 completely absorbed. 



Some of the colour changes may no doubt be caused in this way ; but 

 if the upper layer of the atmosphere is positively charged, there will also 

 be another possibility for colour changes to take into consideration. 



If a very intense rey-bundle (consisting of negative electrons) pass through 

 the positively charged layer, and if it keeps its position for some short time, 

 the gas on its way may for a moment become partly neutralized and the 

 A'^-spectrum may partly turn into the ordinary type corresponding to neutral 

 Nitrogen and which gives the reed colour. 



It is of interest to notice that the latter explanation of colour changes 

 would require negatively charged cosmic rays. 



Physical Institute, University, Christiania March i, 1923. 



Printed October 5th 1923. 



