no THE POPULAR SCIENCE MONTHLY 



we come to magnetic storms and auroras, which are almost certainly of 

 solar origin. 



Here is a photograph of the sun, as it appears in the telescope 

 (Fig. 4).^ Scattered over its surface are sun-spots, which increase and 

 decrease in number in a period of about 11.3 years. It is well known 

 that a curve, showing the number of spots on the sun, is closely similar 

 to a curve representing the variations of intensity of the earth's mag- 

 netism. The time of maximum sun-spots corresponds with that of 

 reduced intensity of the earth's magnetism, and the parallelism of the 

 two curves is too close to be the result of accident. We may therefore 

 conclude that there is some connection between the spotted area of the 

 sun and the magnetic field of the earth. 



We shall consider a little later the nature of sun-spots, but for the 

 present we may regard them simply as solar storms. When spots are 

 numerous the entire sun is disturbed, and eruptive phenomena, far 

 transcending our most violent volcanic outbursts, are frequently visible. 

 In the atmosphere of the sun, gaseous prominences rise to great heights. 

 This one, reaching an elevation of 85,000 miles, is of the quiescent type, 

 which changes gradually in form and is abundantly found at all phases 

 of the sun's activity. But such eruptions as the one of March 25, 1895, 

 photographed with the spcctroheliograph of the Kenwood Observatory, 

 are clearly of an explosive nature. As these photographs show, it shot 

 upward through a distance of 1-46,000 miles in 24 minutes, after which 

 it faded away. 



When great and rapidly changing spots, usually accompanied by 

 eruptive prominences, are observed on the sun, brilliant displays of the 

 aurora (Fig. 5) and violent magnetic storms are often reported. The 

 magnetic needle, wliich would record a smooth straight line on the 

 photographic film if it were at rest, trembles and vibrates, drawing a 

 broken and irregular curve. Simultaneously, the aurora flashes and 

 pulsates, sometimes lighting up the northern sky with the most brilliant 

 display of red and green discharges. 



Birkeland and Stormer have worked out a theory which accounts 

 in a very satisfactory way for these phenomena. They suppose that elec- 

 trified particles, shot out from the sun with great velocity, are drawn 

 in toward the earth's magnetic poles along the lines of force. Striking 

 the rarified gases of the upper atmosphere, they illuminate them, just 

 as the electric discharge lights up a vacuum tube. There is reason to 

 believe that the highest part of the earth's atmosphere consists of rari- 

 fied hydrogen, while nitrogen predominates at a lower level. Some of 

 the electrons from the sun are absorbed in the hydrogen, above a 

 height of 60 miles. Others reach the lower-lying nitrogen, and descend 

 to levels from 30 to 40 miles above the earth's surface. Certain still 



*Figs. 4, 6 and 7 represent the same region of tbe sun, photographed at 

 successively liiijluM- levels. 



