14 



KR. BIRKELAND. M.-N. Kl. 



If once upon a time H • q was equal to lo' C. G. S. for the rays in 

 the permanent ray-disc round the sun, which now manifests itself as 

 zodiacal light, the conditions of temperature on the earth may have been 

 quite different from what they are now, as we shall soon see. 



There is an important class of magnetic storms, however, which point 

 to the existance of much stiffer solar corpuscle-rays still than those men- 

 tioned above; I have called these storms cyclo-median storms. 



There is only one such storm among my materials from 1902 — 1903, 

 so that this class of perturbations has not yet been well st-udied, but 

 there occur many cyclo-median storms on the earth, and sometimes they 

 are of great intensity, though not especially in the polar regions. In the 

 materials of 1902 — 1903, there were chosen from all 27 observatories which 

 participated in the work, only the magnetic registrations which were taken 

 on dates when great magnetic storms occurred somewhere at my four 

 polar stations, so the reason why cycle-median storms are missing in the 

 material is easily explicable. 



It seems that the cyclo-median storms are due to a ray current-system 

 which appears in lower latitudes at a height above the earth which is 

 certainly great but small however in proportion to the earth's dimensions. 



By my terrella experiments I tried to find if such systems could be 

 demonstrated by using very stiff cathode rays of 10 000 volts and small 

 magnetisation on the terrella. In »A. P.« pp. 151 and 153 some photo- 

 graphs are reproduced from these experiments. 



It is of interest to remark that if we commence the experiments with 

 no magnetisation on the terrella and slowly increase it to the highest 

 obtainable magnetisation, the ray-precipitation will first withdraw from the 

 poles and gather on both sides of the equatorial regions, then disappear 

 round the equator, and the rays will form two »auroral zones«, which then 

 approach the poles again getting thinner and thinner, and in such a 

 manner that the number of rays striking the terrella is always very rapidly 

 decreasing. Later Professor Størmer interested himself in calculating tra- 

 jectories corresponding to these experiments supposing the earth to be an 

 elementary magnet and his results are quoted in »A. P.« pp. 151 to 160. 

 Størmer found that H . q for such rays, which could produce cyclonic 

 vortices such as those observed on the earth, had to be enormously great. 

 He found the value 93 millions. At that time, I did not combine any 

 physical meaning with such an enormous number for H ■ q. It was only 

 later when I found experimentally that H • q for rays precipitating in the 

 auroral zones was 3 millions that the idea struck me that we had to deal 

 with very stiff cathode rays, which I called helio-cathode rays, and calcu- 



