222 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



THE PERTURBATION OF THE 28th & 29th OCTOBER, 1902 



63. After the last polar elementary storm that occurred before midnight on the ayth October, the 

 conditions once more become comparatively calm, and continue so until about i8 h the following day, 

 when another perturbation of considerable power occurs. Sitka is the only place that forms an exception 

 to this, as there a perturbation of a rather considerable strength occurs about midnight, local time; but 

 its sphere of action is rather limited, as it is not noticed either at the Norwegian stations or at the other 

 stations in North America. 



The perturbation-conditions during this twenty-four hours closely resemble those of the preceding 

 day and night. On both days, the conditions at the Norwegian stations are characterised by two separate 

 storms; but on the 28th, these two storms are closer together, the first storm on that day being about 

 two hours and a half later than the first on the ayth, and the second on the 28th perhaps half an hour 

 earlier than that on the 27th. 



When we come to lower latitudes, we find the conditions during the time from I4 h to ao h rather 

 different on the two days. There is no trace on the 28th of the long storm that occurred on the 27th, 

 and was especially powerful at the equator; it is the intermediate storm that answers to the first storm 

 on the 28th. On the other hand, there is an astonishing resemblance between the conditions of the two 

 days in the last storms both at our Norwegian stations and in lower latitudes. We thus notice that the 

 deflection in H at Kaafjord are in the same direction on both days, and the Z>-curve has an undulating 

 form while the deflection in V is uniform in direction and very great. Farther south we find that the 

 //-curve on both days is of an undulating character; there are two intermediate more or less marked 

 maxima separated by a minimum. 



It appears from the curves that the distribution of strength in the northern hemisphere is about 

 the same on the two days. It is thus evident that on this occasion also there are two separate polar 



TABLE XXXV. 



