

400 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 19021903. 



At the time we believed that these more or less abnormal conditions must be due to polar precipi- 

 tation of some kind, concerning which we were then unable to express an opinion. Here, however, we 

 have distinct proofs of the existence of such polar systems during equatorial storms also. It was especi- 

 ally in America that this precipitation occurred then, and now we find the same thing occurring here. 

 The fact that it is in America that it occurs, is without doubt connected with the appearance of these 

 storms at more or less the same time of day; and the situation of the magnetic pole in those regions 

 is a circumstance of no little significance. 



The equatorial storm is represented on the first four charts. 



Chart I is drawn for a number of hours, to show the characteristic oscillation of the perturbing 

 forces, which we have previously always observed simultaneously with the commencement of the effect 

 of the equatorial storm. As the curves we have to go by are not continuous, but only readings for 

 every fifth minute, the variation cannot be followed as it might have been if we had had photograms. 

 It will be seen that the current-arrows at the stations in the south of Europe turn right round through 

 an angle of about 180, but it is not possible to determine whether the movement is clockwise or anti- 

 clockwise. Later on, when the movement is less pronounced, it can be followed. 



At the three southern stations the current-arrow moves in a direction contrary to that taken by the 

 hands of a clock from io h 25" until io )l 30, from which time until io h 35 m or io h 40 it reverses 

 its direction. 



At Gottingen, where the first two current-arrows are not in quite such opposite directions as at 

 the two other stations, it appears from the chart that the movement from io h 2o m to io h 25 has been 

 in the same direction as from io u 25 to io h 30, namely anti-clockwise, and that the principal pheno- 

 menon at the beginning of the equatorial perturbation would therefore be first a turn through 180 in a 

 direction contrary to that of the hands of a clock, and then a smaller, slower turn back, after which the 

 direction of the current-arrow remains constant as long as the effects of the equatorial system pre- 

 dominate. 



At Ssagastyr, on the other hand, the current-arrow moves through a smaller angle, clockwise, and 

 apparently more or less regularly, from io h 2o m to io h 30, and then remains more constant for tin- 

 remainder of the period represented on Chart I. 



At Uglaamie too, the movement of the current-arrow is similiar to that at Ssagastyr; but its direc- 

 tion, unlike that at most of the other stations, is northerly. 



At Fort Rae, the equatorial character of the perturbation is once more clearly apparent. The 

 direction of the current-arrow also undergoes a great change as the perturbation begins, exactly similar 

 to that which takes place at the southern European stations. 



At the other stations too, there are great deflections, at Godthaab, for instance, as much as 180. 

 At Cape Thordsen the movement is less, and anti-clockwise;, while south of that station it is generally 

 clockwise, at any rate after io h 25. 



While there are considerable perturbing forces from io h 30 to io h 40 at the stations round 

 Jan Mayen, those at Jan Mayen itself have now almost disappeared. There is evidently some connec- 

 tion between this circumstance and the fact that the current-arrows at Cape Thordsen and Godthaab 

 are now almost in opposite directions. At the first of these stations, the equatorial system appears to 

 exert a considerable influence, while in the region round Godthaab and Kingua Fjord, there seem to be 

 other influences at work, probably polar precipitation, which, as we have seen, subsequently developes 

 to a considerable strength in this very region. 



On the other charts which represent the conditions during the equatorial storm, the current-arrow 

 at the stations to the south of the auroral zone undergo, as a rule, little change in direction or size; 

 and the form of the field remains fairly constant. 



