yo BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



THE PERTURBATIONS OF THE 9th DECEMBER, 1902 



(PI. IX). 



28. These perturbations may be briefly characterised as follows. 



They begin with a lengthy perturbation, which is relatively weak, but is especially developed at 

 the equator, where it appears only in H, and on the whole exhibits all the properties that characterise 

 the positive equatorial perturbations. 



It commences quite suddenly, simultaneously all over the earth, at 5 h 40^6 m Greenwich mean time. 

 At the equator it appears only in H, and the deflection answers to an increase in H. In the vicinity 

 of the poles, this condition is altered, while at the same time the mean deflection becomes smaller. 

 From 5 1 ' 40 on to 13 h , the deflection in H is continued in the direction mentioned, but with varying 

 strength. The character of the curve is somewhat quieter than usual. At the Norwegian stations there 

 is a particularly strong and characteristic impulse at the commencement. At Matotchkin Schar, for 

 instance, it is partly of an undulating form, answering to a rapid turning round of the perturbing force. 

 Subsequently the perturbation at the three westernmost of the Norwegian stations is chiefly characterised 

 by small oscillations about the normal line, interrupted by smaller, sometimes brief, impulses of a more 

 local polar nature. Between 15 h and i8 h , the character of the perturbation-conditions is essentially 

 changed. It is this feature that we continually find repeated, namely, that when the equatorial storm 

 has lasted for some hours, polar systems appear. 



It is early apparent from the curves at our Norwegian stations, that we have to do chiefly with 

 polar storms during this period. The system, however, is of the very simplest kind. At Dyrafjord and 

 Kaafjord the deflections in D and H are in a direction opposite to that usual during storms that 

 commence on the midnight side. When we come to Matotchkin Schar, we get the deflection that 

 characterises the nocturnal perturbations. 



As this perturbation during several hours is of a typical equatorial character, we have preferred to 

 class it among such. Even the polar storm with which it concludes, is a phenomenon that often seems 

 allied to this equatorial type. 



THE FIELD OF FORCE. 

 (i). The Equatorial Part. 



29. The field during the period is given on two charts, Chart I for 6 h o m , and Chart II for g h o m . 

 This field is of the typical form for negative equatorial perturbations. It is most powerful on the 

 sun-side, and becomes weaker towards the poles. On Chart II , the arrows have a direction that indicates 

 that they are circling round the magnetic pole. 



Chart III represents the conditions at I2 h 15, and at 15 h . At the first-named hour, the perturba- 

 tion is still mainly equatorial in character. At Axeleen and Sitka, only small polar disturbances are 

 observable. At the second hour named, we are just at the transition to the polar field. 



(2). The Field during the Polar Storm. 



Charts IV, V and VI show the field as it appears, in the main, during the polar storm. 



Chart IV shows the field at two hours, namely, i6 h o m and i6 h 45 m . At the first of these, the 

 perturbation was especially noticeable in Europe and Asia, where it forms a considerable area of divergence. 

 At Dyraijord, Kaafjord and Matotchkin Schar, the force is now very small. It appears, from the form 

 of the field in southern latitudes^), that the storm-centre is situated to the east of our Norwegian 



(') See "Polar Elementary Storms". 



