SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 199 



with the abnormal records during the same time on the next day, 

 March 1. It innnediately appears that the disturbances blew the 

 ionosphere out into space — an effect especially pronounced during 

 the beginning of such disturbance. For this case the average density 

 of electrical charges during the magnetic storm was reduced to ap- 

 proximately one-third of the normal value observed on the previous 

 day. 



The nature of the relation between ionospheric and magnetic dis- 

 turbances is not yet fully evident. Ionospheric changes apparently 

 arise in part because of wave radiation and in part because of partial 

 (corpuscular) radiation from the sun. This part of the investigation 

 of geomagnetism has only begun. 



EARTH CURRENTS 



Electric currents produced by some natural agency circulate in the 

 earth at all times as was discovered after the introduction of the 

 telegraph 100 years ago. These earth currents are closely related 

 to changes in the earth's magnetism and bear some relationship to 

 polar lights and to certain conditions which affect radio transmission. 

 When polar lights appear, earth currents surge to and fro in an 

 unusual manner. These surgings often occur even when polar lights 

 are not sufficiently intense or extensive to be seen. They occur also 

 in equatorial regions where polar lights are never seen. 



They produce effects, resembling static in radio, which interfere 

 with the sending of messages. A study of these interferences as first 

 observed by Barlow on telegraph lines in England in 1847 shows 

 that they are clue to electric currents which come from the earth 

 and which to some extent are present at all times. 



Most earth-current storms which are observed in the middle lati- 

 tudes occur simultaneously everywhere on the earth, and hence 

 these must be due to an influence which acts directly upon a large 

 part of the earth. From a statistical study, a definite tendency is 

 found for an earth-current storm to recur every 27 days — once more 

 giving evidence of solar and terrestrial interrelationships. 



RECENT GEOMAGNETIC AND AURORAL DISTURBANCES 



The present cycle of solar and geomagnetic activity has been 

 noteworthy because of two of the greatest magnetic storms ever 

 recorded since the introduction of photographic recording in 1859. 

 These, as will be seen from table 1 of great magnetic storms during 

 1859 to 1941, indicate the current cycle as the most active in the 

 recorded annals of geomagnetism. 



