234 B1KKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, IQO2 1903. 



As we come southwards towards San Fernando, we find the arrow turning more to the west. We 

 receive the impression that the perturbation-conditions have moved westwards with the sun. This move- 

 ment seems to be continued, as the magnitude of the force in Central Europe, as compared with that 

 at Dehra Dun, is increasing, while the direction of the arrows becomes more southerly, that is to say, 

 the turning is counter-clockwise. 



On Chart XVIII, for 23'' 45, the force is decomposed, as we have endeavoured to take out the 

 force for the other powerful storm at Axeleen. This storm, which commences at about midnight, and 

 is powerful at the Norwegian stations, has also, as far as may be judged from our material, the out- 

 ward field that is characteristic of these storms. There is an area of convergence in the north-east of 

 Europe and the north-west of Asia. 



The last chart at i b shows the perturbations in Europe, including the Norwegian stations, to be 

 greatly diminished, while at Dehra Dun the perturbation still continues fairly powerful for a long time. 

 Throughout the next twenty-four hours, H has a value that is about 10 y below that of the preceding 

 calm days, notwithstanding that the curve on the following day is of a quiet character. As the mean 

 line has been drawn in relation to the calm days, this low value of H will affect the perturbing force, 

 and serve to increase its total amount. 



HOW THESE PERTURBATIONS MAY BE EXPLAINED 



67. In the above description we have pointed out the most important properties of this pertur- 

 bation. These we will now briefly recapitulate. 



(1) The perturbation is very violent at the Norwegian stations. The character of their curves is 

 very disturbed. The curve for Sitka for that day is of the same character. 



(2) The perturbation, in the eastern hemisphere especially, may be divided into one long, principal 

 storm, whose field, in its main forms, varies only very slowly, and two intermediate, powerful, but 

 briefer storms, that differ considerably from the first-named in the fields of force that they produce. 



We will first take the conditions during the long and more constant storm, beginning with that part 

 of it for which we also have material from the American stations and Honolulu. 



On account of the violent nature of the storms round the Norwegian stations, we must assume that 

 the systems come close to these places. There are thus great precipitations on the day-side, and the 

 current-arrow during the period is directed eastwards along the auroral zone. 



The effect in lower latitudes undoubtedly seems to some extent to be due to the direct influence 

 of these polar precipitations. The fact that the perturbations in this period are all more powerful in the 

 district of Dehra Dun and Batavia than in Europe, might make it natural to suppose that in addition to 

 the polar systems there are also systems that have their greatest effect in the equatorial regions. This 

 kind of storm we have already mentioned, and have referred them to the so-called negative equatorial 

 storms (p. 83). In this perturbation we have a typical example of such a storm. 



In North America the perturbation-conditions varied in a manner that was without parallel in the 

 eastern hemisphere. This, together with the great changes in the perturbation-conditions from place to 

 place, points to the conclusion that the perturbations here are due to systems that are relatively inde- 

 pendent as compared with that which occurs farther east; and on a closer investigation, it also appears 

 that the field is of the same form as that during the polar elementary storms that occur on the night- 

 side of the earth. From the great strength of the perturbation at Sitka as compared with Toronto and 

 Baldwin, we may conclude that the first-named station must be situated in the neighbourhood of the field 

 of precipitation. The current-arrow also remains constant, pointing westwards along the auroral zone. 

 It would appear that on this occasion these polar storms occur rather far south. If we were thus to 



