PART II. POLAR MAC.XETIC PHKXOMKX A AMI TKKRKI.I.A KXI'KRIMKNTS. CHAP. III. 



549 



eld, will have perturbing forces of the same order of magnitude. Thus if we had a storm-centre at 

 tyrafjord, we should find that this storm would produce about the same strength of field in south 

 iurope as at a place on the opposite side of the magnetic axis and with angular distance equal to that 

 f Dyrafjord. 



Table CV gives the ratio (/) of the strength of the field near the centre to that in southern 

 lurope (San Fernando and Munich) for a number of polar storms. 



TABLE CV. 



Storm 



Remarks 



Dec. 15 . 

 Keb. 10 . 

 March 31 . 

 22 . 

 Dec. ->6 . 

 Feb. 15 . 



Oct. 27 



23 



12 



17 

 I I 



n 



12 



13 



13 



Calculated from the fields at i h and i' 1 15"' p. 90 



table XVI 

 field at 



oli 45 



23" 



Table XXXII 



107 

 123 



'35 

 ] 43 



184 

 205 



- ' - 



Mean 



14.6 



The quantity /. varies between 23 and n. The larger value corresponds to an elementary storm when 

 he ratio is taken for the storm-centre and a point near the transverse axis of the field. If the principal 

 ixis is turned more towards the south, as on the 22nd of March, it will have the effect of making -/. 

 .mailer; for, at equal distances from the centre, the forces will be greatest along the principal axis, 

 "iirther, the ratio /. is smaller for compound than for elementary storms, which is easily understood if 

 vc take into account the local character of storms near the centre. 



The effect produced at our stations by storms on the opposite side of the magnetic axis will be 

 smallest for those stations which have the greatest angular distance from that axis. In view of the results 

 expressed in the table, we can put 



For Matotchkin-Schar and Kaafjord . . . / <^ 20 

 Axeloen and Dyrafjord x <C 15 



The greatest storminess in the disturbed region for the period of our observations is known only 

 or that part of the auroral zone extending from Dyrafjord to Matotchkin-Schar; but from the treatment 

 if storms from the polar year 1882 1883, we have seen that storms occur with about equal strength 

 ind frequency all round the auroral zone. 



Let |S)' represent the diurnal maximum of total storminess. This quantity is very nearly equal to 

 .he maximum perturbing force of the average storm. For the three-month period considered we obtain 



For Matotchkin-Schar . . . (S) m = 30 y 



Kaafjord = 20 



Axeloen = 43 >, 



Dyrafjord = 29 



Mean 30.5 y 



At the time when one of the Norwegian stations has calmness, we can assume the maximum strength 

 jf the average storm on the opposite side of the pole to be at least 30 /. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 



70 



