192 



AKNTJAIi REPORT SMITHSONIAN INSTITUTION, 1942 



As lias been stated, the recurrence of sunspots with the sun's rota- 

 tion varies for different solar latitudes from 27 to 28 days. There- 

 fore, if solar activity has an effect on the earth's magnetic field, we 

 may expect some like recurrence in geomagnetic activity. 



The simplest method of establishing the existence of a recurrence 

 tendency and of determining any average of its interval is the super- 

 posed epoch method due to Chree. This proof depends only on mag- 

 netic data without any reference to the sun. The average character 

 figures for epochs of 5 days per month that have the highest character 



FiauEE 9. — Superposed-epocb method to show solar period in geomagnetism. 



(After Chree.) 



figure of magnetic activity and for each of the 5 preceding and 30 or 

 more days following are compiled. When plotted, these averages 

 show first marked peaks corresponding to the selected epochs followed 

 by repetitions of the peaks after 27 days, thus indicating the tendency 

 for magnetic disturbance to recur in about 27, 54, and 81 days. The 

 same method applied to selected epochs of the 5 quietest days per 



