CHAPTER III. 



COMPOUND PERTURBATIONS. 



THE PERTURBATIONS OF THE 29th & 30th OCTOBER, 1902. 



(PI. VI). 



54. These storms consist of two principal phenomena, first appearing at the equator mainly as 

 a positiye equatorial perturbation, which commences suddenly at 16'' 52*". At what hour it ceases it 

 is difficult to say, as perturbations of another kind soon begin. The perturbation at the equator is 

 especially powerful at about i h 30 on the 3oth October. It seems to be directly apparent from the 

 curves that this is really an equatorial perturbation. Unfortunately there are no observations for this 

 date from Honolulu and several other stations, as the time was not given in my Circular (p. 38). 

 Simultaneously with this perturbation, there are powerful storms round the Norwegian stations, that at 

 Matotchkin Schar being particularly so, and of long duration. The positive equatorial perturbations ob- 

 served by us are alicays accompanied by polar storms. As a rule, the polar storms do not begin until 

 a little while after the equatorial ; but on this occasion they begin almost simultaneously, that at Matotch- 

 kin Schar lasting from i6 h 40 to about midnight. 



The almost simultaneous appearance of the polar storm and the positive equatorial perturbation has 

 been already mentioned as of frequent occurrence. The explanation of the positive equatorial perturba- 

 tion given in Art. 31, also at once suggests the connection. Fig. 38 b shows the descent upon the screen 

 of those rays that would turn back before reaching the terrella. It was these rays which we assumed 

 to be the cause of the positive equatorial perturbation. The figure also distinctly shows, however, that 

 this descent of rays upon the screen occurs simultaneously, and is connected, with the descent in the 

 polar regions on the terrella. 



The field of force for the perturbation in question is shown in Table XXIII and in the two charts 

 following. 



TABLE XXIII. 

 The Perturbing Forces on the 291)1 & 3Oth October, 1902. 



Birkeland. The Norwegian Aurora Polaris Expedition, 1902 1903. 



