SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 191 



Magnetic activity provides one of the means of determining another 

 time change, namely, that of an 11-year cycle apparently agreeing 

 with the well-known cycle in sunspot frequency. Latest investigations 

 have shown, however, that solar activity as indicated in its reflected 

 effect in the magnetic field is not completely or always represented 

 by snnspottedness or other phenomena which astrophysicists observe 

 on the sun's surface ; magnetic observations apparently reveal distinct 

 solar influences of another kind and add in this way to our knowledge 

 of solar physics. 



Figure 8.— Geomagnetic activity and relative sunspot numbers. Upper, for 

 annual means, 1835-1U41; and lower, for monthly means, 1872-1941. 



As the earth revolves about the sun during the year there are 

 corresponding fluctuations in magnetic activity, the maximum or crest 

 occurring during the equinoctial months of March and September, 

 and the minimum or trough in the solstitial months of June and 

 December. This is revealed in the annual variation of magnetic ac- 

 tivity deduced from examination of many years' observations ob- 

 tained at several observatories. Graphs of sunspot areas for years 

 with many and years with few sunspots plotted against days before 

 magnetic storms give further evidence of relations between solar 

 activity and magnetic activity. 



