230 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



THE PERTURBATIONS OF THE 31st OCTOBER & 1st NOVEMBER, 1902. 



(PL VII). 



65. After the last storm on the 28th October, quiet conditions once more prevail; but at about i8 h 

 on the following day, the storm bursts out again, and continues until midnight, and it seems, that the 

 two polar perturbations, that occured rather destinctly on the 28th now come so near one another, that 

 they form a single one (cf. PI. VI). 



On the next day again, this is repeated. At Axeleen in particular, there are powerful perturba- 

 tions, but they commence at about i6 h . In southern latitudes, this twenty-four hours is fairly quiet; but 

 during the morning of the 3ist, a storm begins, which lasts uninterruptedly for nearly twenty-four hours. 

 It appears at the poles with tremendous violence, although perhaps its strength is even more unusual at 

 the equatorial stations. Considering its long duration and its universal distribution, we may say that it 

 is the greatest storm that has been observed by us. 



A circumstance which adds still more to the interest of this storm is that it occurs at the new 

 moon, and what is more, there was even an eclipse of the sun during the perturbation. This eclipse 

 began at 5 h 58.5 m on the 3ist October, and ended at io h 2.3. It was only partial, and the greatest 

 phase (0.699) occurred at 8 h o.4 m , in longitude 100 56' East, and latitude 70 53' North. The eclipse 

 cannot in itself be considered as affecting this perturbation in any essential degree. Whatever direct 

 effect there may possibly be of the eclipse itself this must at any rate be very small as compared with 

 the total amount of the perturbation, as no special change is observable in the curves, coinciding with 

 the time of the eclipse. We know that powerful storms often occur at the same time as an eclipse, 

 without being directly due to it; but it has been stated "that an observable magnetic variation makes 

 itself felt during the time of a solar eclipse, and that this variation is analogous in its nature to the 

 solar diurnal variation, differing from it only in degree." ( a ) In this case it is difficult for us to study 

 this direct influence, as we have no material from the places at which the eclipse was greatest. 



If the moon can be supposed to exert any influence on the perturbation, it must be owing to 

 the fact that it is a new moon. We will not here, however, enter more particularly into these questions 

 but only describe the perturbation, and find out its actual distribution and course. 



It exhibits great variableness round the Norwegian stations. The curves have a very serrated 

 appearance, resulting from great vibration in the field of perturbation. Notwithstanding this, however, 

 the conditions of the perturbation as a whole, run a fairly simple course, which may be characterised 

 as follows. 



During the time that the perturbation lasts, namely from about g h on the 3ist October to 3 h 

 on the ist November, most of the curves for the magnetic elements form a single undulation with 

 crest and sinus. This wave differs, however, in phase at the three stations. At Kaafjord the deflec- 

 tions changes sign in all three elements between i8 h and i8 h 30. At Matotchkin Schar it changes 

 in H at about i6 h , in D at i6 h 45", and in V at I9 h 15"", thus taking place on the whole earlier than 

 at the former station. At Axeleen, the undulating form is very marked in the declination, the change 

 not taking place until about 22 b . The smaller variations must be regarded as ripples upon this princi- 

 pal undulation. Two of these shorter variations in particular are considerable and worthy of notice. 

 One of them appears at about I4 h , the other at about midnight, with maximum about 23 h 45. At 

 Axeleen, where the main undulation was somewhat less marked in H, these two intermediate storms are 

 very prominent. 



(') L. A. Bauer: Terrestial Magnetism Vol. 7, p. 192. 



W. van Bemmelen: Contribution to the Knowledge of the Influence of Solar Eclipses on Terrestrial Magnetism. 

 C. Nordmann, Bulletin Astronomique, Mars 1907. 



