SUN AND THE EARTH'S MAGNETIC FIELD — FLEMING 177 



about 4° from its pole and thus Jnuch nearer than is the case for the 

 earth whose magnetic axis makes an angle of 11. °5 with the axis of 

 rotation. From variation in the inclination of the magnetic axis 

 of the sun as the sun rotates, it appears that the magnetic pole rotates 

 about the geographical pole once in about 31i/^ days. The sun's mag- 

 netic field is about 50 gausses or 100 times greater at its surface than 

 the maximum value of the magnetic field of the earth and its intensity 

 decreases rapidly with height. 



Exceedingly brilliant clouds occasionally burst forth suddenly on 

 the sun and reach maximum brightness in a few minutes and then 

 slowly subside. These spectacular bright eruptions usually last from 

 10 to 30 minutes, depending upon their brightness, although some of 

 the most brilliant have remained as long as a few hours. They 

 usually occur in the neighborhood of magnetically complex — usually 

 also abnormally active — sunspots, which change rapidly in form and 

 size. They flare up into the solar atmosphere far above the level of 

 the sunspots like sheets or tongues of flame extending outward; 

 often, brilliant fountainlike prominences are observed over such active 

 spot groups. The upper region of the solar atmosphere where these 

 clouds appear is called the chromosphere, and in it are also the many 

 other fainter and more stable prominences. Although the latter are 

 much fainter, they usually extend higher above the solar surface than 

 the very bright clouds that appear so suddenly. Because of their 

 brightness and almost explosive nature, these active prominences are 

 called chromospheric eruptions to distinguish them from less intense 

 eruptions. Spectroscopic analysis of the light from the cliromospheric 

 eruptions shows that they are composed mainly of hydrogen and 

 helium. 



It has been found that chromospheric eruptions produce terrestrial 

 effects throughout the daylight hemisphere on the earth. The visible 

 radiations of these bright eruptions cannot account for the observed 

 results on the earth, indicating that an immense increase in the in- 

 visible or ultraviolet light must accompany them. These eruptions, 

 with their direct almost simultaneous effects on the earth furnish the 

 first and only evidence which has so far been obtained of positive 

 terrestrial changes produced by specific solar phenomena and have 

 opened the way to the solution of many problems of influences of 

 solar activity on the earth's magnetism. 



THE EARTH'S MAGNETIC FIELD 



The complex nature of geomagnetism — the general magnetic field 

 of the earth — and of its varied phenomena is still a riddle. Despite 

 several centuries of speculation and research, there is as yet no ade- 

 quate explanation of how the earth became magnetic or why it 

 remains so. Associated with this problem is the "perpetual vari- 



