174 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



There are fairly powerful perturbations all this time at the Norwegian stations. We also receive 

 a distinct impression that a perturbation commences during the time in which the great deviation takes 

 place in more southern latitudes. The conditions before and after the intermediate storms, however, are 

 somewhat different. Before it, both at Axeleen and Kaafjord, there is apparently a comparatively inde- 

 pendent system occuring simultaneously, with a course similar to that of the first powerful perturbation 

 at Sitka, which has its maximum at i5 b . 



It must thus be assumed that these are in the main polar perturbations; but the conditions are not 

 simple, indicating, as they do, both in the arctic regions and in lower latitudes, that there are a number 

 of systems acting to some extent simultaneously. This then is not an elementary storm, but must be 

 classed among the simplest compound storms. 



According to the above, we may consider it beyond a doubt that during the time from i6 h to 

 i7 h 30, we have the effect of an intermediate perturbation with a field of force of its own, the 

 latter differing considerably, especially in Europe and Asia, from the field before and after. 



We have worked out a plate for this perturbation from I4 h to i8 h , showing the perturbing forces 

 at one place at various times (fig. 81). 



On considering the conditions in Europe and Asia, we get a direct impression that in the above- 

 mentioned period the effect apparent is that of an independent system. 



is* 16* IT* is* * is* ie* n* is* / n* is* is* n* is 



Fig. 81. 



