PART I. ON MAGNETIC STORMS. CHAP. III. 



177 



two systems, as the field in low latitudes, as described under the first section, is supposed to continue 

 through this period also. 



In the intermediate storm, the form of the field in America will be very much as before, the effect 

 of the force there being rather slight as compared with that in Europe. The perturbing forces also, 

 which appear during the intermediate storm, and are conditioned by it, form an area of divergence in 

 this district. An endeavour has been made to separate the field of force of the intermediate storm in 

 the district of Europe and Asia from the total field. The result of the decomposition is given in Charts 

 V, VI, & VII. This has not been done in Chart VIII, but the effect of the intermediate storm is still 

 distinct. This field has the following course. The current-arrow passes through Europe in a SSE 

 direction, and turns eastwards through India. We here have a distinctly-marked area of convergence, 

 lying much farther west than in the previous field. The neutral field should be in the region about 

 the river Obi or perhaps somewhat farther to the east. 



This accords well with the conditions at the Norwegian stations. At the north-easterly stations, 

 Axeleen and Matotchkin Schar, the storm is very violent; and this fact, together with the rapid alter- 

 nation with time and place, in the curves, shows that the current system must have approached those 

 stations. Even at Kaafjord we find conditions quite different to those at the two stations named, the 

 force at the former being much smaller, and its direction very different. 



The current-arrows at Axeleen and Matotchkin Schar on Chart V, for i6 b 30, are somewhat 

 different in direction, that at Axeleen being WNW, and that at Matotchkin Schar WSW. On the follow- 

 ing charts, they have become almost parallel, a fact which points decidedly to a westward movement of 

 the current-system along the auroral zone. This condition is rather unusual, for the ordinary polar 

 elementary storms that we have treated up to the present, and which have had their centre between 

 Dyrafjord and Axeleen, move eastwards (see I5th December, 1902). This storm, however, occurs ear- 

 lier than the above mentioned; and we shall find from the material from 1882 83 that this is to be 

 regarded as the normal condition at this time of day. In southern latitudes the corresponding movement 

 in perturbations such as that of the I5th December, is a turning of the force clockwise. This time we 

 should have expected a turning in the oposite direction, and on looking at three charts in succession, 

 we do find a slight counter-clockwise turning in Central and Southern Europe. 



At Matotchkin Schar, during the intermediate storm, the balance makes a distinct deflection in one 

 direction, such as would imply a vertical component directed upwards. The centre of the current -system 

 should therefore lie almost to the north of this station. At Axeleen the balance oscillates up and down 

 about its mean position. The force is at first directed upwards, then downwards. If this effect is mainly 

 due to the system under consideration, it would mean that the greater part of the current-system at first 

 lay somewhat to the north, and afterwards somewhat to the south, of this station. In accordance with 

 this, P t is generally more powerful at Axeleen than at Matotchkin Schar. The total force at the latter 

 station, however, ,is somewhat smaller than the force that is due to the intermediate storm, as the two 

 systems probably counteract one another. 



At Kaafjord and Dyrafjord the perturbation is much weaker, P t attaining at both places at the 

 most about 140 y. The direction of P t is particularly worthy of notice. At Dyrafjord the direction of 

 the current-arrow all the time is ENE along the auroral zone, that is to say exactly the reverse of the 

 arrow at the two north-eastern stations. At Kaafjord it has an intermediate direction. At first it is south- 

 east in direction, and thus has a tendency to be regulated by the conditions at Dyrafjord. It changes 

 afterwards to SSW, more in accordance with the conditions at Matotchkin Schar. But on the whole 

 the conditions at Kaafjord form the transition to the conditions farther south. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 



