A study of Table 10 seems to indicate that, at Cape Adare, the main body of the 

 aurora did uot pass over the station during the evening maximum, but remained to 

 the north of the station, the aurora approaching the station only rarely at this time, 

 but occasionally moving over into the minimum sector. Cape Evans being closer to 

 the magnetic axis of the earth was affected even more rarely, so the secondary maximum 

 is merely indicated, probably corresponding to periods of great brilliancy of aurora. 

 (As will be seen later, magnetic disturbances at the station and brilliant aurorse are 

 most likely to coincide in the afternoon, not at times when the aurora is most likely 

 to be overhead, or closest to, the station.) 



Further indication of the striking daily period is given by the recurrences of 

 aurorae at the same hour on successive days, sometimes even of the same form. 



Qualitatively, it seems that the auroral phenomena are well explained on the theory 

 due to the successive labours of Professors Birkeland, Stormer and Vegard. On this 

 theory, the aurora is caused by charged particles emanating from the sun, their paths 

 being deflected by the earth's magnetic field, causing them to fall chiefly in an 

 " auroral belt " distant about 20 from the point where the earth's magnetic axis 

 cuts the earth's surface. At any instant, the auroral belt* should take the form of 

 a spiral directed towards this point. Due to the earth's rotation about its axis, this 

 spiral sweeps above the earth, with " centre " approximately fixed over the point where 

 the magnetic axis cuts the earth's surface. A brilliant exposition of the theory has been 

 given by Professor Vegard,f who, at the same time, presents evidence for his contention 

 that the solar particles causing the aurora are positively charged and similar to the 

 alpha particles (charged helium atoms) which are due to radio-active disintegration.! 



In any case, there can be little doubt that the particles causing the Aurora 

 Australis are the same as those to which the Aurora Borealis owes its origin and that, 

 therefore, the height at which the particles lose their charge and cause the most intense 

 ionisation will probably be the same in the South as in North Polar regions. If we 

 assume the same height (85 to 160 km. from the earth's surface, on the average), it 

 will be evident that the same aurora can be seen both from Cape Adare and from 

 Cape Evans. 



From the observations of only two stations, it is clear that little evidence can be 

 adduced for or against a theory based on many years' experiment and observation 

 in northern latitudes. Of the evidence which can be offered, the data relating to the 



* The auroral belt being defined as an area between two curves whose centre is above the point 

 \\IHTC the magnetic axis cuts the earth. Aurorae will be seen by observers outside this belt, but 

 most frequently by an observer in this belt and distant about 20 from the magnetic axis. At any moment 

 the aurora is spread in space along a broad curve, which represents the sum of all the spiral paths of 

 particles projected with different velocities, or particles which have travelled from the sun by different 

 paths. This broad curve can, for convenience, be called the " auroral spiral." 



t L. Vegard " Bericht iibcr die neuercn Untersuchuiigen am Nordlicht." ' Jahr. d. Radioactivitilt 

 ii!i<l Blectronik,' XIV, 4, Dec., l'J17. 



I Ye^iril has later abandoned his contention that the rays are positively charged alpha rays 

 ' Geofysiske Publikationer,' Vol. I, No. 1. 



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