SOLAR ACTIVITY — SPENCER JONES 231 



culi. In the Ha light of hydrogen, the fiocculi are finer and more 

 threadlike in structure and in the vicinity of sunspots show evidence 

 of circulatory or vortex motions, which are in the opposite direction 

 in the two hemispheres. 



In the highest-level photographs, long dark markings, called -fila- 

 ments, appear. The filaments are much longer-lived than sunspots 

 and are among the most stable of the solar markings. Many of them 

 are associated with sunspots, from which they spread out in a north- 

 south direction. Because the rotation of the sun is most rapid at the 

 Equator and is slower the higher the latitude, a filament progressively 

 changes its orientation in the course of its lifetime. When a filament 

 comes to the edge of the disk it is seen to project beyond the limb, 

 appearing as a prominence at the limb. The prominences, and neces- 

 sarily also the filaments, consist of flames of incandescent gas at tem- 

 peratures of from 10,000° to 20,000°, standing up above the surface of 

 the sun, and often extending to heights of many thousands of miles. 

 The prominences absorb light from the solar surface and the atoms of 

 the gaseous matter re-emit it at discrete wavelengths in all directions ; 

 they therefore appear dark when seen in projection on the disk. 



The frequency and latitudes of occurrence of the prominences vary 

 through the cycle of solar activity. There are two principal prom- 

 inence zones in each hemisphere. The more important of these coin- 

 cides with the sunspot zones ; prominences commence to appear in lati- 

 tudes of about 30° N. and S., within a year or two after the epoch of 

 minimum activity and, becoming more frequent as the spots become 

 more frequent, follow the drift of sunspots toward the equator 

 with a lag of several degrees in latitude ; like the spots, they die out 

 around minimum activity. 



A second prominence zone commences in latitudes of about 45° N. 

 and S. at about the time of sunspot minimum and moves toward the 

 poles, reaching latitudes of 75° N. and S. shortly after sunspot max- 

 imum. In this zone the prominences occur most frequently some 2 

 years before maximum sunspot activity. 



The prominences assume a great variety of forms. They may per- 

 sist in a comparatively stable form for many months and then sud- 

 denly develop into an eruptive type, when the whole prominence may 

 be rapidly dissipated away into space with a speed of several hundred 

 miles a second. 



In the vicinity of a large spot, particularly when it is in the stage 

 of active development, a limited region of the sun's surface may become 

 intensely bright. The area of the region affected may be as great as a 

 few thousand million square miles. Such a solar eruption or -flare (pi. 

 1, fig. 2), as the phenomenon is usually called, normally lasts for about 



370930—56 16 



