and Nature of the Cosmic Electric Rays. 55 



position to the auroral zone give a lowering of the radiation 

 point, and the same is found at lower latitudes *. 



When we are going to interpret this result, we have to 

 remember that the magnetic lines of force of the normal 

 field of the earth are not straight lines, and we should have 

 to compare the direction of the radiation point with that of 

 the magnetic lines of force at the auroral region. 



From the magnetic potential of the earth we have calcu- 

 lated the change of direction of the magnetic lines of force 

 at a height of 200 km. from the ground. If (/ia ) are the 

 coordinates of the magnetic zenith at the surface of the earth, 

 (h f a') the coordinates of the magnetic zenith at a height of 

 200 km. relative to the same coordinate system, we find for 

 a place near Haldde 



h' ~h= -0° 1', 

 a' - a = -1°31'. 



Thus the curvature resulting from the ordinary potential 

 functions is too small to explain the lowering of the radiation 

 point relative to the magnetic zenith, and we have to consider 

 the following possibilities : — 



(1) The auroral streamers do not always follow the mag- 



netic lines of force. As we shall see later on from 

 the study of the luminosity distribution, however, the 

 cosmic rays probably have too large a defiectibilicy to 

 give nearly straight streamers which form an angle 

 with the lines of force. 



(2) The lowering effect is caused by some current system 



existing above the surface of the earth, which may 

 either be more or less permanent or at any rate exist 

 during auroral displays. As there is a lowering for 

 all the stations on the northern hemisphere, the 

 current systems which produce the lowering effect 

 are probably not of the type producing those polar 

 magnetic storms which have their centres near the 

 auroral zone, but are probably of the type which are 

 supposed to produce the "negative" equatorial mag- 

 netic disturbances t, or they may be produced by 

 those systems which are at any rate partly responsible 

 for the diurnal variation of the magnetic field of the 

 earth. In both cases we have to do with fairly 

 distant systems, which are concentrated near the 

 plane of the magnetic equator. 



* See Hermann Fritz, Das Polarlicht, p. 38 (Leipzig, 1881). 

 t See K. Birkeland, The Norwegian Aurora Polaris Expedition 

 1902-03: Parti, p. 62. 



