SOLAR PROMINENCES IN MOTION — McMATH 127 



On September 23, 1938, a detached cloud which existed over a 

 very active area was photographed. During the time that the tele- 

 scope was directed at this limited portion of the sun's limb, a promi- 

 nence loop formed and disappeared at the point under examination. 

 Selected spectroheliograms from this record are reproduced in 

 plate 3. The plate covers a period of 2 hours and 20 minutes, and 

 in that short space of time a bright loop rose to a height of 60,500 

 km., broke at the crest, and fell back into the chromosphere. The 

 brilliance of the loop was remarkable. Up to the time when it began 

 to disintegrate, its intensity was even somewhat greater than that 

 of the adjacent chromosphere. In this case, apparently, the ma- 

 terial was caiTied upward and then returned completely to the active 

 area on the surface from which it had arisen. Except for the dis- 

 placement of the detached cloud, there remained scarcely a trace of 

 the phenomenon after its rapid subsidence. 



Another example of loop formation is found in the record of the 

 sunspot-type prominence of September 7, 1939. Although at certain 

 stages there was a similarity of form and intensity to that of Sep- 

 tember 23, 1938, the two evidence very different general characteris- 

 tics. Plate 4 illustrates several stages of the development of the loop 

 of September 7, 1939, from the time A, when there was the first 

 faint indication of a small stationary condensation at the right of 

 the main group of streamers, to the time F, when the loop had broken 

 up into an intricate pattern of loops and streamers. The phenome- 

 non began as a faint condensation about 42,000 km. above the limb, 

 which suddenly brightened and then extended branches in opposite 

 directions. The branches continued to grow rapidly, and both 

 curved downward, ultimately forming a complete arch. The time 

 occupied by the formation of the arch, as shown in E, was 8.27 

 minutes. The subsequent history of this loop is very different from 

 that of September 23, 1938, which disappeared into the chromo- 

 sphere. In contrast to this rapid dissolution, the loop of September 

 7, 1939, continued to develop into more and more small loops and in- 

 complete arches. At all times, prominence material was moving 

 downward from the crests of the loops along both branches to the 

 chromosphere. The appearance of the multiple arches 1 hour and 7 

 minutes after the loop had completely developed is shown in F. 

 This does not, however, represent the final stage, for the phonomenon 

 continued to become more intricate as it gradually lost its extreme 

 intensity. In plate 5 F, 2 hours and 2 minutes later than the last 

 stage shown in plate 4, the arches are shown as delicate, beaded 

 filaments. 



The prominence in which this arch development occurred is a 

 typical example of the sunspot-type classification which, together 



