168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



The giant prominence clouds are supported in delicate equilibrium 

 against the powerful forces of gravitation, more than 25 times as 

 potent as terrestrial graviation. What holds the prominences up has 

 long been a matter of major astronomical concern. Now it seems that 

 we can say, thanks to the recent work of Donald H. Menzel, of the 

 Harvard College Observatory, that they are supported in large part 

 by the magnetic fields of the sun. We can also say that the thousands 

 of tons of gaseous hydrogen, helium, and other gases in the promi- 

 nences are constrained to move generally parallel to the lines of force 

 of the magnetic fields of the solar atmosphere. Menzel's work indi- 

 cates that prominences probably become luminescent as they are af- 

 fected by powerful compressive forces generated in the presence of 

 streams of prominence gases. The reason for the very complex shape 

 and appearance of the prominences is to be found, according to the 

 new theory, in the complex structure of the magnetic fields surround- 

 ing sunspots. Initial deformities in the magnetic fields may be fur- 

 ther aggravated by the gravitational and other forces that also af- 

 fect prominences, once they come into being. Menzel's new theories 

 have resulted in part from his extensive studies of prominence motion 

 pictures, most of them photographed at our High Altitude Observa- 

 tory station at Climax, Colo. 



The corona, on the other hand, surrounds the eclipsed sun like a 

 giant halo; it radiates a soft pearly- white light. Today we know 

 that the corona is no simple phenomenon, but that it consists of sev- 

 eral components. Recent work has carried us far tow^ard under- 

 standing the different constituents of the corona, while, on the other 

 hand, spectroscopic studies have told much in detail about the ele- 

 ments from which originate the bright-line spectrum that makes up 

 a significant part of the corona's light. 



It was a mere 11 years ago that the distinguished Swedish astron- 

 omer Bengt Edlen first succeeded in establishing that the long-known 

 lines of the coronal spectrum arise from gaseous iron, nickel, and 

 calcium, at fantastically high temperatures. The story of the iden- 

 tification of these lines of the spectrum, long believed due to some 

 mysterious nonterrestrial element called "coronium," is a fascinating 

 one, but too long for telling here. 



The coronal gases described above form one of the three principal 

 components of the corona, but they do not explain its soft pearly- 

 white light. That, in turn, is not simple either. Researches of the 

 postwar years have succeeded in identifying it as consisting of at 

 least two components, the so-called "electron corona" and the "dust 

 corona." One of these is not solar, in the strictest sense. The former 

 is made of streams of electrons in the sun's atmosphere which are so 

 numerous as to scatter the brilliant sunlight that passes through them. 



