PART II. POLAR MAGNETIC PHENOMENA AND TERRELLA EXPERIMENTS. CHAP. II. 447 



A positive polar storm in the auroral zone will, as we have said, always, or at any rate generally, 

 be accompanied by a negative polar storm in rather higher latitudes. Some such idea as the following 

 might then seem probable as the explanation of this circumstance: 



We know, according to the theory, that corpuscular rays that move in the earth's magnetic field 

 will approach the polar regions in paths that twist spirally about the magnetic lines of force. If 

 the rays possess great magnetic stiffness, the radius of these spirals will be comparatively great. If we 

 assume that such ray-spirals exist on the afternoon side of the earth, and that they lie close 

 together somewhat in the manner shown in fig. 188, the connection with the southern positive system 

 and the northern negative system, as regards the polar regions, can be ex- 

 plained quite simply. For rays of a stiffness answering to HQ = 7 X Io6 > we 

 find in the polar regions, where //= about 0,5, Q = 1,4 X io 7 cm. The dia- 

 meter of these spirals must then be 280 km. The principal features of the 

 field in southern latitudes can probably also be brought out as effects of a 



Fig. 1 88. 



spiral system such as this. Judging from our experiments, of which the results 



are clear enough, an explanation such as this, is not entirely satisfactory, and would at any rate have to 

 be considerably modified. According to those experiments, the rays in the positive and negative storm- 

 centres seem inclined to behave in a manner similar to that shown diagrammatically in fig. 50. The 

 above explanation cannot, at any rate, be applied to the night storms, in which there is only a negative 

 storm in the south. 



We thought of showing two more types of perturbations, namely, the negative equatorial storm and 

 the cyclo-median storm. We have, however, only a few examples of these among our observations. 



The negative equatorial storms are most powerful in the region of the equator, where the per- 

 turbing forces in horizontal intensity are negative. Their area of perturbation may be explained as the effect 

 of a current-system of which the greater part is situated more or less in the magnetic equator, as the 

 experiment shown in fig. 37 shows, and where the rays have a movement similar to those that are 

 calculated in the magnetic equatorial plane for an elementary magnet answering to values of y that are 

 - i (see fig. 39, p. 82). In order that these rays shall come comparatively near to the earth, their 

 stiffness must be comparatively great. More flexible rays would be deflected like the rays in the equa- 

 torial plane for y~^> i, and thus glance by the other in the opposite direction. We believe we have 

 effects of rays such as these in the positive equatorial storms. The rays that we believe should produce 

 the negative equatorial storms must therefore be assumed to go round the earth and to be magnetically 

 more inflexible than those that produce the positive equatorial storms. In accordance with this, it is only 

 during very powerful storms that we have found these negative perturbation-areas. The fact that the 

 active rays during specially powerful perturbations have a greater magnetic stiffness than those in the 

 less powerful storms, is also indicated by the circumstance that has just been touched upon, namely, that 

 the storm-centres during the latter seem to move southwards. In particularly violent magnetic storms, 

 it is well known that the auroral zone moves southwards, so that polar aurora can be observed even in 

 very low latitudes. The simultaneously-occurring magnetic storms have also, in lower latitudes, a completely 

 polar character, which indicates that the acting current-systems come in to the immediate vicinity of the 

 place. But if the corpuscular rays come in towards the earth in such low latitudes, their stiffness must be 

 considerable. These circumstances will be explained fully in Chapter IV. The forces that occur in the 

 negative equatorial storms are also considerable greater than those found in the positive. Among our 

 observations, we have found only examples of negative equatorial storms, which occur simultaneously with 

 polar storms, and it is perhaps doubtful whether this type of perturbation on the whole can occur alone. 

 We have not sufficient material, however, for the formation of any well-founded opinion on the matter. 



