126 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 



in B, a neutral density glass flat of 10 percent transmission -was intro- 

 duced to cover the image of the disk, and it enabled us to record 

 some of the surface details while exposing for the faint prominences. 

 The area of intense activity visible in B is covered at the limb by a 

 bright mound. This mound form frequently occurs over such regions 

 and has been named a "cap" prominence. 



Two subtypes, usually of very short duration, are illustrated in 

 plate 2, C and D which, in this case, occurred simultaneously over 

 the same sunspot area. These two spectroheliograms are separated 

 in time by only 6.42 minutes between midpoints of exposures. In 

 this short interval the small detached cloud, which is about 31,000 

 km. above the limb in 6', has been ejected to a point 79,000 km. 

 above the limb in Z>, and the bright triangular-shaped surge, directly 

 under the ejection in the first scene, has decreased in height from 

 23,600 to 15,800 km. A typical surge rises from an active area on 

 the chromosphere to its maximum extension and then, apparently 

 with a complete reversal of motion, returns to the surface along the 

 same trajectory. Rarely, at its highest point, a small cloud will be 

 ejected on outward while the main body of the surge subsides. 

 This latter phenomenon is classified as a secondary ejection. 



E and F^ plate 2, represent two stages in the development of a 

 quasi-eruptive prominence. The similarity of this prominence to 

 one strictly classified as active is evident in its general form and the 

 development and motion of its streamers. The eruptive characteris- 

 tics are illustrated in the later scene, F^ where the main body of the 

 prominence has been detached from the chromosphere. The motion- 

 picture record reveals that at this stage the velocities of the motions 

 increased, and that the whole prominence began to move along the 

 streamer trajectories. Ultimately, the prominence completely dis- 

 integrated, some material moving along streamers to the left, and 

 the major portion entering the chromosphere along the bright 

 streamers at the right. In this case the motion followed paths which 

 had slight curvatures and were, over considerable lengths, nearly 

 parallel to the limb. Of the complete record, the eruptive stage 

 occupies only a small portion and, because of its rapid changes, 

 requires many spectroheliograms to form a continuous story of the 

 behavior. 



Many records have depicted the last stages in the history of promi- 

 nences, but of the early stages very little is known. As yet, we 

 have no means of predicting when, or at what position on the sun, a 

 prominence is about to develop. Therefore, such records as we may 

 have of the beginnings of any type of prominence are the result of 

 particularly good fortune, in that the phenomenon took place in a 

 region under examination. 



