138 BAUER— SOLAR ACTIVITY AND [April 23, 



an electric field of one volt per centimeter (the average potential 

 gradient found from atmospheric electricity observations on the 

 surface), the velocity of the ions would at the height of about 75 

 kilometers or 47 miles be such that a complete circuit of the earth 

 could be made in 3;?4 minutes. Preliminary calculations show that 

 at that height the existent electromotive force may be on the same 

 order as actually assumed in the calculation. The height of 75 km. 

 is about the average of that to which polar lights are seen. We 

 thus place the electric currents, which may produce our magnetic 

 disturbances, at a height where we know, from polar lights, electric 

 currents actually exist. 



Should the currents get lower down, then since their velocity 

 varies inversely with atmospheric pressure, they will travel more 

 slowly and the time required for a complete circuit of the earth 

 is correspondingly increased. Their actual efifect on a magnetic 

 needle is, however, increased as they get nearer the surface. Hence 

 we may say that the nearer a current gets to the earth's surface, the 

 greater, in general, the disturbance effects and the slower the rate 

 of propagation — this is in accordance, as we have seen, with actual 

 observation. It seems probable that the reason for the remarkably 

 large effects experienced during the magnetic disturbance of Sep- 

 tember 25, 1909, must be ascribed chiefly to the fact that the cur- 

 rents generated succeeded in getting closer to the earth than for 

 the average magnetic storm. 



It is thus seen that on the basis of the ionic theory of magnetic 

 disturbances it is possible not only to explain, in a perfectly natural 

 manner, why magnetic storms do not begin at absolutely the same 

 instant of time over the earth, but also to account for the direction 

 of propagation and the reason for the possible different rates of 

 progression. Thus far, however, the examination has applied ex- 

 clusively to the sudden beginnings of disturbances or to the simple 

 disturbances characterized by Birkeland as " equatorial perturba- 

 tions," which our analysis has shown to be chiefly due to a simple, 

 uniform magnetic or electric system superposed upon the existing 

 field. It seems probable that for this class of disturbances, the 

 electric currents producing them are farthest away from us, i. e., 

 they are in the stratum of the atmosphere where their velocity is 



