On Eapicl Periodic Variations of Terrestrial Magnetism. 



49 



of our atmosphere may be regarded as almost an established fact 

 since the classical investigations of Schuster/^ Birkeland and 

 Störmer.'^ Now, according to Schuster and Bezold, there exists 

 in the upper atmosphere a definite system of currents whose 

 position is nearty fixed relative to the sun, wliich produces the 

 remarkable diurnal variation of the terrestrial magnetic field. 

 The first suggestion which naturally arises as to the cause of the 

 magnetic pulsations, is the fluctuation of this permanent system 

 of currents.^-' This seems the more plausible, if we remember the 

 very regular diurnal reiteration of the characteristic phenomena. 

 The material nearest in hand for testing this conjecture is the 

 daily variation of the azimuth of the liorizontal components of 

 the periodic disturbing field. Examination of Fig. 16 will show 

 that a similar daily variation could be joroduced, if a nearly 



circular zonal system*-* of 

 electric currents, fixed with 

 respect to the sun and having 

 its pole situated at a con- 

 siderable distance from the 

 earth's axis, undergoes some 

 periodic fluctuations in its 

 different parts. Referring to 

 Fig. 18, let JV be the earth's 

 astronomical pole, while P 

 is the pole of the zonal 

 current. Denoting the latitude 

 of the point of observation A by <p, and its longitude counted from 

 the meridian containing P, by X, the azimutlial angle « of the 



1) A. Schuster, Philosophical Transactions, 180 A, 1889, p. 467; v. Bezold, Gesammelte 

 Abh., p. 404. 



2) Birkeland, loc. cit.-, C. R., 147, p. 539. C. Störmer, Archiv for Math, og Naturvidenskab., 

 31, 1911, Nr. 11.; Archiv des Sc, phys. et nat., 24, 1907, pp. 5, 113, 221, 317 ; numerous papers 

 in C. R. 



3) The idea is not at all new, being expressed already by Eschenhagen, Birkeland, van 

 Bemmeln etc., though more or less vaguely. 



4) We do not necessarily mean a continuous system of current flowing in a given 

 direction at the same timr, but only that the directions of the different portions of the 

 current causing pulsations, when put together, form the 25oî'<îo7îs of a circular zone with a 

 given pole. 



