SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 197 



dently the last is an important factor in the geomagnetic diurnal 

 variation. 



Of the abnormal ties between sun and earth, the radio fade-out is 

 the one outstanding example of a direct relationship, namely, the 

 observed association of the bright chromospheric eruption on the 

 sun's surface, the simultaneous disappearance of radio echoes, and 

 characteristic pulse in the earth's magnetism. McNish has shown 

 that the unique geomagnetic change associated with the fade-out is 

 an augmentation of the normal diurnal variation at all places where 

 it is observed. Because the atmospheric region below 100 kilometers 

 appears almost solely affected during the fade-out, it seems probable 

 that not only the unique geomagnetic pulse but also the whole diurnal 

 variation, of which the pulse is but an augmentation, arises from 

 electric current flow at about these levels. 



Thanks to the extensive network of stations distributed over the 

 earth keeping constant watch for these chromospheric eruptions, a 

 similar and older network of magnetic observatories, and the chan- 

 nels of radio communication, data on these associated phenomena 

 accumulate rapidly. Dellinger's compilation of such data soon 

 showed that chromospheric eruptions always accompany fade-outs 

 and the concomitant magnetic changes. The first actual simul- 

 taneous observations of a fade-out, a bright eruption in the solar 

 chromosphere, and a unique baylike kind of geomagnetic pulse were 

 obtained on August 28, 1937, at the Huancayo Magnetic Observatory 

 in Peru. Characteristically the echoes disappeared rather suddenly 

 at a level between 70 and 90 kilometers. This is probably because of 

 absorption produced by the intense ionization of this region by the 

 ultraviolet light emanating from the solar eruption. 



Another powerful influence upon terrestrial conditions is the result 

 of bombardment of the earth by minute particles which are shot out 

 in streams from the sun. This is as though streams of small parti- 

 cles were sprayed toward the earth as from a moving nozzle. The 

 development of such streams is generally associated with large or 

 active sunspot groups. These streams travel relatively slowly and 

 normally require about 1 to 4 days to cover the distance between 

 sun and earth. When they reach the earth's upper atmosphere, 

 their effect is felt and observed as magnetic and ionospheric storms 

 as well as auroral displays. 



The magnetic storm of March 1, 1941, associated with the passage 

 of a large active sunspot group across the sun's central meridian about 

 2 days before, gave a typical example of the behavior of the iono- 

 sphere caused by corpuscular radiation during a magnetic disturb- 

 ance. The ionospheric effect was produced in this case as a result 

 of the earth's bombardment by these solar particles. We may com- 

 pare the normal records on the undisturbed day, February 28, 1941, 



