. Popular science monthly. andpulsates, sometimes lighting up the northern sky with the most brilliantdisplay of red and green discharges. Birkeland and Stormer have worked out a theory which accountsin a very satisfactory way for these phenomena. They suppose that elec-trified particles, shot out from the sun with great velocity, are drawnin toward the earths magnetic poles along the lines of force. Strikingthe rarified gases of the upper atmosphere, they illuminate them, justas the electric discharge lights up a vacuum tube. There is reason tobelieve that the highest part of the earths atmos
. Popular science monthly. andpulsates, sometimes lighting up the northern sky with the most brilliantdisplay of red and green discharges. Birkeland and Stormer have worked out a theory which accountsin a very satisfactory way for these phenomena. They suppose that elec-trified particles, shot out from the sun with great velocity, are drawnin toward the earths magnetic poles along the lines of force. Strikingthe rarified gases of the upper atmosphere, they illuminate them, justas the electric discharge lights up a vacuum tube. There is reason tobelieve that the highest part of the earths atmosphere consists of rari-fied hydrogen, while nitrogen predominates at a lower level. Some ofthe electrons from the sun are absorbed in the hydrogen, above aheight of 60 miles. Others reach the lower-lying nitrogen, and descendto levels from 30 to 40 miles above the earths surface. Certain still 2 Figs. 4, 6 and 7 represent the same region of the sun, photographed atsuccessively higher levels. THE EARTH AND SUN AS MAGNETS in. Fig. 5. The Aueora. more penetrating rays sometimes reach an altitude of 25 miles, thelowest hitherto found for the aurora. The passage through the at-mosphere of the electrons which cause the aurora also gives rise to theirregular disturbances of the magnetic needle observed during magneticstorms. The outflow of electrons from the sun never ceases, if we may reasonfrom the fact that the night sky is at all times feebly illuminated bythe characteristic light of the aurora. But when sun-spots are numer-ous, the discharge of electrons is most violent, thus explaining the fre-quency of brilliant auroras and intense magnetic storms during sun-spot maxima. It should be remarked that the discharge of electronsdoes not necessarily occur from the spots themselves, but rather fromthe eruptive regions surrounding them. Our acquaintance with vacuum tube discharges dates from an earlyperiod, but accurate knowledge of these phenomena may be said tobegin with the work of
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