148 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



uniform magnetic properties of a magnetized steel sphere are not at 

 all surprising. The phenomenon of the secular variation, or the 

 rotation of the magnetic poles about the geographical poles, is one 

 of the peculiarities toward the solution of which both theory and 

 experiment should be directed. 



Passing over other remarkable phenomena of terrestrial mag- 

 netism, we come to magnetic storms and auroras, which are almost 

 certainly of solar origin. 



Here is a photograph of part of the sun, as it appears in the tele- 

 scope (fig. 3) .^ 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 magnetism. 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 abun- 

 dantly found at all phases of the sun's activity. But such eruptions 

 as the one of March 25, 1895, photographed with the spectrohelio- 

 graph of the Kenwood Observatory, are clearly of an explosive na- 

 ture. As these photographs show, it shot upward through a distance 

 of 146,000 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. 6) and violent magnetic storms are often reported. 

 The magnetic needle, which would record a smooth straight line on 

 the photographic film if it were at rest, trembles and vibrates, draw- 

 ing a broken and irregular curve. Simultaneously, the aurora flashes 

 and pulsates, sometimes lighting up the northern slvy with the most 

 brilliant display of red and green discharges. 



Birkeland and Stormer have worked out a theorj^ which accounts 

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

 electrified particlas, 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 



1 Figs. ^, 4, and 5 represent the same region of the sun, photographed at successively 

 higher levels. 



