84 Prof. L.Vegard: Results of Nortlilight Investigations 



the distribution of matter in the upper strata of the 

 atmosphere*. 



On the Change of Colour of the Aurora?. 



A most curious and most characteristic feature of an 

 auroral display is the variation of the colour, which means 

 a corresponding change in the spectrum of the light emitted. 

 Ordinarily the colour is greenish-yellow, sometimes bluish- 

 white, and very often an aurora shows reddish-blue veins 

 in between the ordinary greenish-yellow. Every now and 

 then greater parts of the aurorse may be dark red_, almost 

 with the colour of blood. Sometimes the red colour only 

 appears near the bottom edge, while the upper part has the 

 ordinary greenish-yellow colour. A most brilliant aurora 

 of this kind of colouring was observed simultaneously by 

 Krogness at Haldde and by the author at Bossekop on 

 October 11, 1912. A magnificent drapery-shaped arc which 

 was extended in the usual direction across the sky appeared 

 with a dark-red bottom edge while the upper part had the 



* It might also be of interest to regard the penetrating power of 

 various positive rays from another point of view. We will suppose that 

 all the positive rays have acquired their kinetic energy by passing the 

 same potential (V s ) of the sun, or that 



± _y 2 = ne\ s , which gives 

 2 N 



V 3/2 



And the range in Nitrogen is .r = 2 - 8 .10" "' 



For a given potential V« the penetrating power decreases both with 

 increase of charge and with increase of mass, and the hydrogen nucleus 

 gives the greatest penetration. 



A hydrogen ray could not, as we saw, have a velocity greater 

 than about 2'5 . 10 s cm./sec, which gives for the sun's "potential 

 V*= 31,000 volts: 



The corresponding range in N a is 0156 cm. A helium nucleus 

 when falling through the same potential should have a range of 

 00055 cm., and a calcium atom with a single elementary charge a 

 range of 0-0025 cm. The corresponding heights found from the 

 air equivalents of nitrogen in the atmosphere are: 118 km. for H, 

 124-5 km. for He, and 129 km. for Ca. 



We see that the variation of the height is not very great, but the 



mv 



quantity «=__ increases. For two rays falling through the same 



Lai - 2 = A /Tr'" 1 ? which show that 

 «! 'y/ M 1 w 2 



potential - 2 = * / ^f\ which show that the quantity a for Ca would 



be 6-3 times as large as for hydrogen, and the Ca-rays could not 

 produce the thin streamers in the way here assumed. 



