On Rapid Periodic Variations of Terrestrial Magnetism. 7]^ 



luminosity correspond to the traces of tlie horizontal eleclric 

 current, as Birkeland supposed,'- they remind us strongly of 

 the liypothetical parallel cm'rents considered ahove. It must 

 be noticed that according to the calculation cited, the progressive 

 motion must ])e S to N instead of N to S, in order to explain 

 the observed phase relation of X- and Z-components. It miglit. 

 however, be surmised that the motion of the current in tlie 

 upper atmosphere is in many respects of opposite directions in 

 the lower and liigher latitudes, as is suggested by Schuster- 

 Bezold's system of currents. A similar observation in a con- 

 sidérai >ly high latitude will be in any case very desirable, 

 especially witli a reliable instrument for tlie vertical component. 



27. As to tlie remarkable dependency of the periods of the 

 most frequent waves on the hours of day, we may suggest first 

 of all that it probably has some relation with the difference in 

 the temperature as well as in the state of ionization of the upper 

 atmosphere. According to Prof. Nagaoka,'^ the tliickness of 

 tlie conducting layer in the atmosphere must also be decidedly 

 greater during the day than in the night. 



If the cause of the periodic fluctuatie)ii of the atmospheric 

 current is to be sought in the purely horizontal oscillation of the 

 atmosphere, the dependenc}^ of its |)eriods on the hours of day 

 is rather difficult to explain, even if we suppose the atmosphere 

 consisting of different layers with remarkably different composition 

 and temperatures. 



Another, and probabl}' the last possibility is that in the 

 upper atmosphere there may occur a vertical longitudinal wave 

 (.)f limited extent, the period of which may sensibly depend on 

 the effects of the solar radiations. That the vertical vibration 

 of the atmosphere is possible, and may have a definite natural 

 period depending on the sound velocity, has been fully 



1) According to the recent investigations of Stornier, the auroral band is produceil by 

 the vertical inflow of positive corpuscles. If this be actually the c isr, t!ie idea must be 

 abandoned. 



2) H. Nagaoka, Proc. Tokyo Math.-Phy.«. Soc, 7, I'Jll, p. lû;î : Eevue générale des 

 Sciences pures et appliquées, 26, p. 570. 



