600 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



while I consider the fainter collection of rays that cross one another in the second line of intersec- 

 tion to be the primary cause of the nocturnal zodiacal light-phenomenon known by the name of "Gegenschein" 



EXPERIMENTS FOR THE DETERMINATION OF THE SITUATION OF THE POLAR ZONE 

 OF PRECIPITATION IN VARIOUS POSITIONS OF THE MAGNETIC AXIS. 



117. The experiments are made with the object of obtaining more detailed material for judging 

 whether the situation of the zone of precipitation on the terrella in the various instances can serve 

 a guide for understanding the occurrence of the auroral draperies in the polar regions, and the situation 

 of those polar precipitations which give rise to magnetic storms on the earth. 



We shall first go through the different conditions under which the pictures i to 16 of fig. 220 are t 



Nos. i and 2 are taken in the course of an experiment, in which the discharge-current was 26 

 milliamperes, the tension 2500 to 2300, and the magnetising current 20 amperes. The pressure 

 0.008 mm. The south pole of the terrella lies in the plane of the horizon through the centre, with an 

 hour-angle of 290. The photographs are taken from places with hour-angles of 290 and 110, situated 

 on the prolongation of the magnetic axis. The pictures 3 and 4 are taken under the same experimental 

 conditions, only that the hour-angle of the south pole is 250, and the photographs are again taken from 

 places on the prolongation of the magnetic axis with hour-angles of 250 and 70. We see at once from 

 these 4 pictures, how the so frequently mentioned luminous patch is round and lies within the ring o 

 light when the magnetic pole turns towards the cathode (i and 4), while the patch is drawn out and 

 merges with the ring of light in the positions 2 and 3, in which the pole turns away from the cathodi 



One thing in connection with this patch of light is particularly deserving of attention, that is, that 

 the rays which cause it are rays that have gone the shortest way from the cathode to the terrella. I 

 figs. 200 and 219 the rays which form these polar patches will be seen, showing themselves in the rarefied 

 gas, like two luminous horns, as we repeatedly have mentioned. 



This circumstance is of importance when we imagine the conditions transferred to the earth. 1 

 sudden flare-up or eruption of corpuscle-rays take place in the sun, these would make themselves felt 

 on earth first by a precipitation corresponding to the above-mentioned polar patch of light. 



Stations on the day side of the earth which happen to be near this first precipitation, will therefore 

 receive from it a first impulse announcing a coming magnetic storm. 



When then, an instant later, the polar precipitation on the night side of the earth or the equatorial 

 ray-formations are produced, it may appear as if there was a noticeable difference in time at the different 

 stations on the earth for the commencement of the one and identical magnetic storm. In reality there 

 are several impulses which act in places very locally. I believe that perhaps some observations 

 that have been made when magnetic storms were commencing, can be explained by the view here set forth. 



The pictures 5 and 6 are again taken from places on the prolongation of the magnetic axis, bi 

 the south Dole is now given a declination of 19, and the hour-angle is 270. The conditions of I 

 experiment are the same as before, the tension, however, being 2500 volts and the pressure 0.006 mm. 



In the pictures 5, 9, u and 15, it will be seen that the phosphorescent coating on the terrella 

 has a defect uppermost by the luminous ring. Something like a shadow appears there which has nothing 

 to do with the precipitation. 



The pictures 7 and 8 are taken under exactly the same conditions as 5 and 6, but with the magneto 

 poles reversed. 



The pictures 9 and 10 are taken under similar conditions as before, but the tension is 2700 to 

 2500 volts and the pressure in the discharge-box 0.006 mm. 



The magnetic axis is turned, so that the north pole has an hour-angle of 285 and declination 19 

 No. 9 is taken straight out from the north pole, and No. 10 out from the south pole from places on th 

 prolongation of the magnetic axis. 





