IX. CYCLES 

 CYCLE ORIGINS 



It is now generally recognized that certain small climatic variations 

 are caused by changes in the sun. The study of tree-growth in this 

 volume, and especially its correlation with solar cycles described in 

 this chapter, provide the motive for seeking in the sun the real origin 

 of larger climatic cycles and in the trees a detailed history of the 

 effects of such cycles on organic life. 



SOLAR THEORY* 



Nature of sunspots — The work at the Mount Wilson Solar Obser- 

 vatory and elsewhere shows that two-thirds of the sunspot groups are 

 dual, with a leader and follower in the direction of daily rotation. 

 These are connected below the apparent surface of the sun and form 

 the two exposed ends of a partial vortex-ring. The brilliant work of 

 Hale has shown that during the recent sunspot cycles the leaders in 

 the north and south hemispheres have exhibited opposite magnetic 

 polarity and that during the two minima under observation, 1913 and 

 1923, the polarity reversed between the two hemispheres. This 

 suggests a double sunspot cycle as the fundamental period. Hale 

 (1926 to 1927) finds evidence that this polarity results from direction 

 of rotation in the lower parts of the spot. Lighter gases in the upper 

 and thinner layers of the solar atmosphere are sucked downward into 

 the spot. Their direction of rotation resembles usually the rotation of 

 storms on the earth and so is independent of sunspot minimum. 



Periodicity theories — No recent advance has been made in explain- 

 ing the periodicity of sunspots. The weight of evidence favors internal 

 causes; for example, the polarity phenomenon and the " butterfly" 

 diagram (by Maunder; it refers to the continued decrease in mean 

 latitude of sunspots, as each cycle begins, reaches maximum, and ends) 

 both point to internal causes. The possible extension of solar cycles 

 back into geologic ages is more agreeable with an internal cause than 

 with a meteoric hypothesis, using a swarm subject to perturbations 

 and possible dissipation. On the other hand, there is a possibility that 

 several cycles will need explanation, and it is hard to think of several 

 mechanical pulsations in the sun going on at the same time. Mechani- 

 cal disturbance between a dense core and a lighter shell have been the 

 foundation of some thought on this subject. Snyder and others have 

 been at work on a theory involving atomic energy. This might be 

 called chemical pulsation. 



♦Continuing a related topic in Vol. I, p. 84. 



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