PART I. ON MAGNETIC STORMS. CHAP. II. gcr 



(3) The conditions before and after are comparatively quiet. 



(4) The oscillations at the polar stations, especially the more southern ones, run a simple course. 

 At the poles, they are often characterised by a simple increase to a maximum, and decrease to zero. 

 We may sometimes, even at the northern stations, have to some extent an undulating form, answering 

 to a slow turning of the perturbing force. 



It follows from this, that these perturbations must be well-defined, and thus afford an opportunity 

 for an exact determination of the perturbing force. 



THE TYPICAL FIELD FOR THE POLAR ELEMENTARY STORMS. 



34. It proves as the aggregate treatment of these elementary types of perturbations shows 

 that the same field of force is repeated almost exactly from perturbation to perturbation. It will there- 

 fore be most convenient for its description, to note, even at this point, its typical form, in order 

 thereby to avoid too many repetitions. We shall then keep principally to the horizontal perturbing 

 force, and the field that it forms upon the earth's surface. 



In the auroral zone we have very great perturbing force, and we will call the regions about those 

 places where the perturbation is strongest, the perturbation-centre or storm-centre. If we imagine our- 

 selves moving along the surface of the earth, so as always to follow the direction of the horizontal 

 component of the perturbing force, we should be moving along some curve or other upon the earth, 

 which we will call a line of force. 



Supposing we were to move in such a way as always to advance in the direction of the current- 

 arrows, we should get another set of curves, which we will call current-lines. The one set of curves 

 will intersect the other at right angles. 



We will now suppose that we project these two sets of curves upon the earth's surface, upon a 

 plane by some kind of zenithal projection, which at the same time is conform, and in such a way that 

 the plane of projection is tangent to the earth in the storm-centre. The two sets of curves will thus 

 be projected orthogonally. 



If we imagine this done for the field of the various polar elementary storms, we shall obtain a 

 system of lines, which, in the main, is of the form represented in figure 40 (p. 86). The continuous 

 lines are the lines of force, the broken lines are the current-lines. C is the projection of the 

 storm-centre, and the figure is symmetrical round it, as also on both sides of two axes, A and B, at 

 right angles to one another. The former we will call the principal axis of the system, the latter the 

 transverse axis. On the transverse axis, and symmetrical as regards the principal axis, are two points, 

 from one of which the lines of force issue, while in the other they terminate. We will call the point 

 from which they issue the point of divergence, and that to which they converge the point of conver- 

 gence. The immediate surroundings of these points we will call respectively the field of divergence and 

 the field of convergence. We find that the current-lines in these two fields form respectively positive 

 and negative vortices. The field of force has some formal resemblance to the field induced by two 

 opposite poles; but this resemblance disappears when we consider the strength of the force. At the two 

 points in which the lines of force here meet, the horizontal force equals 0. In the neighbourhood of 

 these points we have a neutral area. The perturbing force, then, should stand, at these points, perpen- 

 dicular to the surface of the earth. 



With regard to the vertical component, it may generally be said that except in the regions, 

 nearest to the centre, it is exceedingly small in proportion to the horizontal. It is only in the points of 

 divergence and convergence that P v will predominate, athough it is generally comparatively small. 



In order to obtain an idea of the conditions for P v , we will consider the values along the trans- 

 verse axis B. In the centre, C, P, will equal 0. Starting from this point, P, will rapidly rise to a 



