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KR. BIRKELAND. M-.N. Kl. 



Luminous Phenomena Over the Auroral Zone. 



4. The rays will precipitate towards the earth in the so called 

 auroral zone, two almost circular bands round the points where the mag- 

 netic axis intersects the earth's surface, one not very far from the North 

 Pole (actually in North West Greenland) and a point correspondingly 

 distant from the South Pole. A most precise idea of the wa}' the rays 

 fall in round the polar region of the earth is given by my terrella ex- 

 periments illustrated on pp. 327 and 598 in »Aurora Polaris«. 



Greenland is within the auroral zone, and Japan and Kamtschatka 

 far from it, in fact Japan is near the magnetic equator. 



The conditions which must exist if the bulk of corpuscle rays shall 

 penetrate deepl}^ into our atmosphere are easy to determine. Today we 

 may say that most of the rays during great magnetic storms arrive at and 

 turn at a height of about 500 km. above the earth's surface. This is at 

 any rate the result at which I arrived when the precipitation of rays came 

 down between my two stations Kaafjord and Axeløen in 1902 — 1903. 

 See »A. P.« p. 308. The energy of such corpuscular precipitation is, as 

 we shall see, puzzlingly enormous. 



But even if most of the rays at the present time pass at a height of 

 five hundred km., there will be some rays in the lower parts of the pre- 

 cipitation which come so near to the earth that the atmosphere will be 

 luminous. Some rays, but very few, may come straight down and almost 

 parallel to the magnetic lines of force and give brilliant north-light 

 curtains and are so absorbed totally in the atmosphere. 



But there are two other classes of luminous phenomena to which I will 

 draw special attention. Mr. Krogness, who has now several years' expe- 

 rience as Director of the Haldde Observatory, has got the impression that 

 always during great magnetic storms a tranquil auroral arc is seen crossing 

 the sky high up in the atmosphere just under the path where on an 

 average the bulk of cosmic ra}' current passes nearest to the earth along 

 the auroral zone eastwards or westwards. * 



I think this is well established, but it must be more closely examined. 

 The other luminous phenomenon alluded to above, are the remarkable lumi- 

 nous nights which have aroused reflection in so many observers in the 

 polar region. I have observed it myself many times. The evening and 

 night sky till after mignight, even in the middle of the winter, can be 

 wonderfully bright day after day. Probably the ordinary radiation of heat 

 from the earth's surface will be less at night under such a bright sky, 

 than under a usually clear sky. There can be no doubt that this lumi- 



