1915- No. 6. A TEST OF THE THEORIES OF AURORAL CURTAINS, ETC. 5 



It is possible, too, that cinematograph-films are now to be had so 

 sensitive that the phenomenon can simply be taken photographically on a 

 running film. 



I have asked the director of the Haldde Observator}' to take up this 

 important problem for investigation as soon as possible. 



Polar Magnetic Sfoniis. Supported by the above-mentioned terrella- 

 experiments, one is led to assert boldh- the existence of new, powerful, 

 electromagnetic phenomena in the sky and in the earth, phenomena which I 

 suppose ma\' easily be discovered by observations. There must, indeed, 

 be some powerful ekntetitaty waves of a period of about one-fifth of a 

 second in the earth's magnetism and in earth-currents. In the above- 

 mentioned terrella-experiments, the luminous ring in the auroral zone 

 consisted of a countless succession of secondary precipitations overlapping 

 one another in such a manner that the luminous ring appeared to be 

 continuous. 



Now that we have applied the results of this observation to our con- 

 ception of auroral curtains, it becomes a logical necessit}- that this same 

 result should be applied to the great kindred precipitation of cosmic rays 

 that especiallv manifest themselves in the form of polar magnetic storms. 



We must assume, then, that the precipitation that produces a polar 

 magnetic storm is formed of co-ordinated, distinct, comparatively small 

 groups of cosmic rays more or less parallel with the magnetic lines of 

 force, these groups having successively detached themselves from a larger 

 bundle of ras's. 



There is however an essential difference, on which great stress must 

 be laid, between those rays which form auroral curtains and those which 

 collectivelv give rise to polar magnetic storms. The former penetrate so 

 far down into the atmosphere that we must necessarily assume them to be 

 absorbed into it; while the latter no doubt as a rule penetrate such a little 

 way, that, owing to terrestrial magnetism, they turn back — turn right 

 round in an involute curve, and pass out again into space, to continue for 

 a certain time in certains paths in the vicinit\- of the earth. 



The point to be emphasised here is that when a precipitation 

 such as produces polar magnetic storms, begins to be formed, there also 

 appear a number of more or less parallel columns of rays that dart down, 

 one b}' one, towards the earth at intervals of about one-fifth of a second; 

 but they occur so far above the atmosphere that they do not seem like 

 the rays in auroral draperies. We must therefore, at the beginning of 

 polar magnetic storms, or in sudden variations in such storms, as also 



