630 



BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



perturbation shall be said to have commenced. I consider we may be safe when we estimate the pos. 

 sible error at + 4 seconds. But the values of the slow-run magnetogram lie within this mar "in n t 

 Honolulu and Porto Rico, where, however, the readings are naturally not so trustworthy as those of the 

 quick-run magnetograms from Assouan. 



The curves from Sitka, Tucson, and Cheltenham show that the perturbations in those places have 

 had a somewhat different character from those at the three first-named stations, for it appears as though 

 a magnetic polar storm interferes. The curves for D and V show the same thing. 



The times we have been given from the Coast and Geodetic Survey for these stations are: 

 for Sitka 10** 21"', Tucson io'i 20"', and Cheltenham io' 2i, 9, and these refer to the "larger displace- 

 ment" in //. This occurs shortly after the first abrupt beginning, and the times are, as may be seen 

 with the exception of Tucson, slightly greater than the others. 



\ 



Honolulu 



furtoSico 



Assouan. 



slow Tim. 



Silku 



v 



Tucsu 



Cltcltcii- 

 lunn 



Ci-.M.T. 



1Aju: M 



Fig. 228. 



As regards Tucson, we notice that the first time-mark is considerably smaller than the later ones; 

 for this reason, I think, this value should perhaps be taken with some reservation. 



In Trondhjem, under the direction of Professor S^LAND, "quick-run" registerings were made simul- 

 taneously with our observations at Assouan, though not between 10 and midnight, Greenwich mean 

 time. As the above perturbation occurred just in this period, we unfortunately have only "slow-run" 

 registerings from this station. 



At this station the polar character of the storm is distinctly apparent, as might be expected from 

 so high a latitude. 



On the occasion of the magnetic storm we are here studying, the similar sudden changes occurred 

 around the terrestrial equator simultaneously, within the limits of error in the observations. 



When several observations of such magnetic storms around the equator obtained by quick-run 

 registerings, are available, as I hope may soon be the case, this important question of simultaneity v 

 be finally determined. 



It may be of interest in connection with this to call to mind that in 1900, quick-run registerings 

 were taken simultaneously in Potsdam and at my observatory at Haldde, near Bossekop. In my \ 

 "Expedition Norv6gienne de 1899 - 1900 pour 1'etude des aurores boreales" (Christiania, 1901), photo 



