232 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, igO2 1903. 



THE FIELD OF FORCE. 



66. (i) Charts I to VIII represent the conditions during the time between g h and I2 h 30. 



During this comparatively long time, the form of the field in the eastern hemisphere remains almost 

 constant. It may be briefly charaterised in the following manner: 



At the equator there are powerful perturbing forces directed southwards. In Central and Southern 

 Europe, the force is only about half as great as at Dehra Dun and Batavia, and throughout is south- 

 west in direction. At Kaafjord and Matotchkin Schar, the current-arrow is directed all the time east- 

 wards along the auroral zone, a circumstance that seems to have some connection with the fact that 

 during this time these stations are situated on the day-side. At Axeleen the force is almost in the 

 opposite direction. The current-arrow is at first directed southwards, but in the course of the above- 

 mentioned period turns clockwise until at I2 h 30 its direction is WSW. 



In medium and northern latitudes in the western hemisphere the conditions are more variable, 

 whereas at Honolulu there is a powerful perturbation that remains almost constant all the time. The 

 conditions there are very similar to those at Dehra Dun; the current-arrow at both places is directed 

 westwards, but is a little smaller at Honolulu. 



The conditions in North America are very interesting, and require a fuller description. 



At Sitka, as already mentioned, the perturbation is extremely violent; and the curve presents the 

 same very serrated appearance that is so characteristic of the powerful storms about the auroral zone. 

 On looking at the charts, we see that the perturbing force remains more or less constant in direction. 

 The current-arrow is directed principally westwards, sometimes a little WSW. The strength too, varies 

 but not much on the whole. 



During the polar elementary storms that occur about midnight, and have their centre in the regions 

 round the Norwegian stations, we have always found that there is only little difference between the condi- 

 tions at Sitka and those at Toronto and Baldwin; but on this occasion there is a very great difference 

 between them, and even considerable difference between Toronto and Baldwin. In the case of the last- 

 named two stations, moreover, there is great variableness from time to time, which makes these perturba- 

 tions very distinct from those in the eastern hemisphere with their more constant conditions. This cir- 

 cumstance is to be explained by the fact that the perturbation in the north of North America is due 

 in a great measure to the occurrence of more or less independent storms that are confined to those 

 regions. 



In order to obtain a clear idea of the field that is produced by these storms in the north of 

 North America, we should examine it at those times when the force is greatest, as we may then most 

 safely disregard the other forces that are acting through other systems. Let us look then at Charts IV 

 to VIII. We see that the arrow at Sitka remains almost constant. The arrows at Toronto and Baldwin 

 show that there is an area of convergence there, with very great convergence, of the perturbing force. 

 We cannot help noticing that this field exhibits the same properties that characterised the field in the 

 previously-discussed polar elementary storms with their centre at the Norwegian stations. At Sitka there 

 is a comparatively powerful perturbation with constant direction of the perturbing force, corresponding 

 to the conditions at the Norwegian stations; and in both cases the current-arrow is directed towards the 

 west. The area of convergence in North America on this day corresponds with the area of conver- 

 gence in the European district under the above mentioned elementary storms. 



The correspondence appears still greater when we notice that the centre of these storms has about 

 the same position in relation to the sun as the previously-mentioned polar elementary storms at the 

 Norwegian stations, the storm-centre in these cases being in the district that has midnight at the time of 

 the storm, or often on the morning side. In the case of the perturbation here described we also find 

 the same. The chart for 9 h 3o m forms an exception to this. In the first place it must be remarked 





