PART I. ON MAGNETIC STORMS. CHAP. IV. 313 



We may thus take it for granted that a kinetic energy answering to io 9 horse-power during power- 

 ful storms, will not be too high for the corpuscular current. 



This is calculated, however, on the supposition that the corpuscular current moved parallel with 

 the surface of the earth in the auroral zone. 



The matter, however, as we have shown at the conclusion of Article 36, is not so simple. In 

 order to know what kinetic energy should be ascribed to a corpuscular current that had the observed 

 magnetic effect upon the earth, we should need to have a complete mathematical solution of the manner 

 in which the rays from the sun would distribute themselves round the earth. Up to the present, indeed, 

 St0rmer has found the possible paths of the rays by numerical quadrature, and he may perhaps in time 

 succeed in finding a more complete solution, from which the above-mentioned magnetic effect might be 

 calculated. We will even now, however, make an attempt, by an estimate, to find out whether it is 

 possible that the corpuscular current which the sun emits from a sun-spot is so large as to indicate a 

 disintegration on the sun, which might account for the solar radiation of heat and light. 



Let me say at the outset that in making certain, for the time being, purely computational assump- 

 tions, which yet may subsequently, at any rate in their aggregate effect on the result, prove to be more 

 or less correct, we come directly upon a value of the development of heat by disintegration per square 

 centimetre of the sun's surface, that is very near that which is deduced from the solar constant. 



These assumptions, or estimates, are as follows. 



In the first place it is assumed that the corpuscles issue at right angles to the sun's surface, and 

 that their density decreases inversely as the square of their distance from the sun. 



In the second place it is assumed that as the corpuscles do not move parallel with the earth's sur- 

 face, but come in towards the earth more or less as shown in fig. 50 a & b, their kinetic energy is much 

 greater than calculated for the district between Kaafjord and Axeleen during the storms under considera- 

 tion; we assume 100 times greater. 



In the third place we take it for granted that the quantity of rays that are drawn in towards the 

 polar regions of the earth, is not nearly so great as the quantity of rays that would have come into 

 contact with the earth if the latter had been non-magnetic. This we conclude from our terrella-experi- 

 ments. We there see distinctly that the more strongly the terrella is magnetised, the narrower does the 

 zone become, where the rays come in towards the terrella. And we see by the illumination that fewer 

 and fewer come in. 



If our terrella were to be magnetised so powerfully that the conditions corresponded with those on 

 the earth, it would have to be immeasurably more magnetic than it is possible to make it (see "Expedi- 

 tion Norvegienne de 18991900", etc. p. 40). 



We now assume that 100 times as many rays would fall upon the earth if it were non-magnetic, 

 as actually do so in the auroral zone. 



By these assumptions we thus arrive at the fact that a corpuscular current, of which the energy 

 amounts to io 13 horse-power, would have come into contact with the earth, if the latter had been non- 

 magnetic. 



The last factor is perhaps rather large. On the other hand we have disregarded the fact that 

 only a portion of the rays that are eventually formed by the disintegration in the sun, succeed in 

 forcing their way into space; most of them will be absorbed into the solar atmosphere. Only the most 

 penetrating, most inflexible rays escape into space and reach the earth. If this were also taken into 

 consideration, it would perhaps compensate in the result for the possibly too high estimate of the above- 

 mentioned factor 100. 



We found, then, that we could put the energy of the rays that would come into contact with the 

 earth, if the latter were non-magnetic, at io 13 horse-power. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1903 1903. 



