14 



L. VECiARD. M.-N. Kl. 



Streamers, which gave an explanation to the fact that the auroræ having 

 long ray-streamers usually appear at the time when the magnetic storm is 

 most intense. 



I have also pointed out that the auroræ are carried down to lower 

 latitudes through the effect of the perturbing magnetic fields, and thus we 

 should expect that the chance for the formation of long streamers should 

 be greater at lower latitudes. 



In this way we should be able to explain that long rays in comparison 

 to other forms were more frequent at lower latitudes. It is, however, 

 not only true that ar aurora at lower latitudes has a greater tendency to 

 form long ray streamers, but the considerable material of height measurements 

 obtained up to the present time seems to show that the maxinuini height 

 obtainable is greater at lower latitudes than near the auroral zone. 



In the material of 434 pairs of photographs from the Haldde Obser- 

 vatory^ no ray was found reaching a height greater than 303 km. and 

 Størmer's^ measurements from the same locality have given a similar result. 

 At Kristiania, however, StøRxMEr^ finds much higher values for the upper 

 limit. Rays reaching heights of 500 km. are not infrequent, and he has 

 measured heights as great as 750 km. 



If the distribution of matter in the auroral region was independent of 

 the latitude, we schould expect that also near the auroral zone the con- 

 ditions might sometimes be favourable for the formation of the very long 

 ra3's. If such ver3' long rays uevei- occur at these localities, it would indicate 

 that the height at which the density of the atmosphere falls below a certain 

 value necessary for giving noticeable light emission, should increase towards 

 lower latitudes. 



As already mentioned, such a variation of the height of the atmosphere 

 might be due to the variation of the solar action, but I think that also the 

 magnetic field of the earth may act in a similar way. The electrified 

 particles will during their upwards motion have a tendency to follow the 

 magnetic lines of force, and thus the electrified "dust" may be driven 

 towards the plane of the magnetic equator of the earth. 



Birkeland has made a most interesting experiment which forms an 

 analogy to this process. When an electric discharge is passing through a 

 discharge tube with a magnetised sphere as a cathode, the discharge is 

 almost entirely restricted to the region near the magnetic equator. The 

 motion of the electrified corpuscles are on both sides directed towards the 

 magnetic equator through the effect of the magnetic lines of force. At 

 the plane of the magnetic equator the particles wnll accumulate and work 

 their way outwards perpendicular to the magnetic lines of force — driven 

 bv the electric field. 



' Geophvs. Publ. Vol. I, nr. i. 



^ .,' ., „ I. „ 5. 



3 C. Størmer, C. R. t. 176, p. 109, 1923. 



