ea 



- 



i 



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



even a little light coming under the branches of the screen, as they do not lie close to the terrella bi 

 leave a millimetre here and there open between themselves and the terrella. These shadows and stripe 

 of light tell us the average straightness with which the rays descend towards the terrella. We sha 

 return to this, as the figures in fig. 221 are meant for such investigations. 



What we here first of all substantiate is that the precipitation on the night-side of the terrella i 

 the polar band has a tangential component eastwards. The magnetic effect of corresponding precipitatio 

 over the earth would thus be a positive current directed westwards, just as we have always found th 

 current-arrows directed in the -negative polar storms in the auroral zone. 



It is not only on the night-side of the terrella that we find precipitation on the west side of th 

 eight branches, but right round the connected luminous spiral, which we shall briefly call the auror; 

 zone. Even at the beginning of the spiral nearest the pole, where, in fig. 140, p-327, we saw a sudden ciirv 

 in the luminous band, we now see precipitation in two places on the west side of screen i, which point 

 towards the cathode; (see Nos. 2 and 8). But it is the precipitation on the night-side that is the st 

 and which comes out better, even when there is no precipitation on the day -side (see fig. 204); and 

 also has comparatively the greatest tangential component. This is thus in accordance with the fact tha 

 the negative polar storms are generally found on the night-side of the earth. 



It is also easy, however, to demonstrate in our photographs precipitation upon the screcn-branche 

 exactly analogous to the precipitation on the earth which occasions positive polar storms. With regan 

 to branches 3 and 2 especially (see, for instance, photograph 3), we also find on their east side a grea 

 precipitation of rays, which, close up to the terrella, has a strong tangential component westwards. 



The magnetic effect of corresponding precipitation over the earth would thus be a positive cum 

 directed eastwards along the auroral zone, just as we have always found the current-arrow directed 

 the positive polar storms. The time of day also suits these cases of precipitation exceedingly well, Co 

 the positive polar storms occur with a maximum in the afternoon, and, as is seen, branch 3 just answ 

 to a place on the terrella corresponding to 6 p. m. 



At the extreme end of branches 5 and 4 also, there is precipitation on the east side similar 

 that on 3 and 2, but not going down so close to the terrella. It occurs in much lower latitudes, \v 

 on branches 3 and 2 it has come quite up to the auroral zone. 



The photographs show plainly that the precipitation on the east side of branch 4 occurs in a m 

 more southerly latitude than that on the west side. On branch 3, too, the precipitation on the east 

 is farther south than that on the west side; but the two are considerably nearer to one another than 01 

 branch 4. On branch 2 they are still nearer to one another, looking as if they to some extent cover 

 one another. These conditions correspond in an astonishing degree with those on the earth duri 

 magnetic storms. We have frequently, indeed generally, seen that while there is a positive polar sto 

 in the southern part of the auroral zone, there is at the same time a negative polar storm in t 

 northern border of the zone. (See p. 445). These storms counteract one another in a horizontal direc 

 tion, and may sometimes neutralise one another's effect in the case of stations lying between the tw- 

 precipitations; but in a vertical direction the two storms art together. This has often been shown 

 discussing the observations from Jan Mayen, for instance. 



In the preceding pages, we have repeatedly put forward the opinion that this precipitation ofra; 

 with a tangential component westwards along the auroral zone, was due to rays of group B, that i: 

 say, rays that are first drawn down towards the terrella in its polar regions, and then deflected a 

 some of them thrown back. That certain rays have such a course is evident from the experiments 

 are described with photographs 13, 14 and 15, fig. 204. The distinct shadows of the conducting w 

 that are thrown upon the screen cannot be interpreted in any other way; and the experiments describe 

 in Art. 112 are also very conclusive. 





