176 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



the region where an old group has been, sometimes not even waiting 

 for the old group to disappear. The group of January 24, 1926, may 

 have been a return of the group of December 29, 1925 ; certainly both 

 were in the same region of the sun. A small round spot closely follow- 

 ing the large group of January 1926 was probably identical with the 

 largest spot of the December 1925 group. The large spots of the 

 January group, however, behaved like new spots which had developed 

 near the waning members of the December group. The two groups 

 were therefore probably not actually identical. The groups of March 

 and April 1947 may not be identical but their relationship seems closer 

 than that between the groups of December 1925 and that of January 

 1926. 



Large sunspots or, more precisely, the solar activity associated with 

 them, definitely affect the earth. The obvious direct terrestrial effects 

 are confined to the ionosphere, that high region of the atmosphere in 

 which electric currents can flow and from which radio waves are re- 

 flected. The most spectacular effects are brilliant auroras (northern 

 and southern lights). Closely associated with auroras are changes in 

 the electric currents in the ionosphere, which produce marked fluctua- 

 tions in the earth's magnetic field (magnetic storms). These disturb- 

 ances can be so violent that long-distance telegraph lines and cables 

 are affected, making communications difficult or impossible. Other 

 terrestrial effects are produced simultaneously with very intense solar 

 "flares," phenomena which nearly always appear in or near large spot 

 groups. Intense flares produce minor changes in the earth's magnetic 

 field and also cause high-frequency radio waves to be absorbed so that 

 long-distance short-wave communication is impossible on the daytime 

 part of the earth while a flare is in progress. These effects are in all 

 probability due to an increase in ultraviolet radiation from the sun at 

 the time of the flare. Although the change in solar radiation due to 

 the presence of a large sunspot must affect the earth's lower atmosj^here, 

 and therefore the weather, buch effects are very small and are difficult 

 to measure and interpret. 



