164 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



Chart I shows the conditions at i8 h 52.5'" and 2o h 3O m on the 2gth October. 



At these hours, it is the polar systems that give the field its character. There is a polar system 

 in its centre presumably in the neighbourhood of Matotchkin Schar. The direction of the current-arrow 

 is westward along the auroral zone, indicating that the storm-centre is on the midnight side. In lower 

 latitudes there is an area of convergence. On the mainland of Europe, the field is turning counter-clock- 

 wise as in the polar regions. 



Chart II shows the conditions at i' 1 on the 3oth October. 



The field is now mainly conditioned by the equatorial perturbation, which at this hour is very 

 powerful. 



This is an example of a composite perturbation of the very simplest kind, in which there is the 

 simultaneous occurrence of a very simple equatorial perturbation, and a polar storm that also exhibits 

 very simple forms. 



THE PERTURBATION OF THE 25th DECEMBER, 1902. 



(PI. XI). 



55. It is a brief, but powerful and well-defined perturbation, particularly marked at the observa- 

 tories in North America, that has here attracted attention. It commences there at 3'' 14, increases 

 rapidly, and reaches a maximum at 3'' 21, after which it decreases more slowly, and at 3'' 5y m the con- 

 ditions are once more almost normal. 



We notice especially that the perturbation appears with much greater strength at Toronto than at 

 Baldwin and Cheltenham. At Toronto, the horizontal component of the perturbing force attains a value 

 f 45-3 y> ar >d at Baldwin and Cheltenham values of 23 and 25.4 y respectively. At Sitka the pertur- 

 bation is noticed distinctly, but it is very faint. The perturbation that, on account of its course, should 

 be connected with the above, there attains a strength of 7.5 y. 



During the time under consideration, perturbations occur all over the world. At our Norwegian 

 stations, there are storms of considerable magnitude, and elsewhere in Europe slight, but distinct per- 

 turbations. 



These perturbations, however, run an altogether different course from those in America. At Dyra- 

 fjord, there is a perturbation of medium strength, but of much longer duration than those in America; 

 it has considerable strength as early as about i h , and lasts almost until 5 h . There is moreover a fairly 

 powerful storm at about midnight. 



At Axeleen, die conditions resemble those at Dyrafjord, except that the course of the perturbation 

 differs still more in its conditions from those in America. At Dyrafjord, during the time in which the 

 short perturbation in America is taking place, we can notice a distinct variation in the form of the curve, 

 especially that for H, which almost coincides with that for the perturbation in America. At Axeleen, 

 on the contrary, nothing special is noticed. There the perturbation has at that time already passed its 

 maximum, which occurs at 2'' 32. At Axeleen also, there is a comparatively powerful perturbation at 

 about midnight, commencing later, namely at 23 h 45 on the 24th, and continuing fairly powerful right 

 on to 5 h on the 25th. 



The conditions at Kaafjord on this date are particularly interesting, in that during the time in which 

 powerful storms are occurring in the north, there are only very faint perturbations here, such as might 

 best be characterised as slightly disturbed conditions. We notice, however, a perturbation that appears 

 simultaneously with, and runs the same course as, the perturbation in America. Its strength is also 

 about the same, if anything a little less. 



