174 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1951 



And finally, just to mention a few of the major new fields of work 

 with the Climax coronagraph, we are paying great attention to the 

 regions from which a still-unidentified line of the sun's corona comes. 

 This unidentified line is the yellow coronal line at 5694 A. This is 

 the only major line of the coronal spectrum that is still not certainly 

 identified. Edlen has stated that it may be calcium in a very high 

 state of ionization. If Edlen's identification is correct, it demands 

 the highest temperature of all the lines of the sun's corona, up in the 

 millions of degrees Kelvin. But the remarkable thing that our spec- 

 trograms show, and that is also confirmed in the observations of Wald' 

 meier with the coronagraph in Switzerland, is that the yellow coronal 

 line is excited in close association with certain very intensely active 

 and rapidly moving prominences. These prominences associated 

 with the yellow coronal line are invariably of special form known as 

 the "sunspot" type. It seems highly likely that the excitation of 

 the yellow coronal line is in some way related to the formation of the 

 explosive solar flares that produce instantaneous and drastic influences 

 on long-distance radio communications over the entire sunlit hemi- 

 sphere of the earth whenever they are very large. At the present time 

 we have a program of work under way that should lead us to definite 

 conclusions about this strange relationship between the sun's corena 

 and the activity of its prominences. Just where this work will finally 

 take us we do not yet know. 



For the astronomer, the basic science of the physical conditions of 

 the solar atmosphere is fully as interesting as the applications. And 

 so today a host of people work daily at the study of the temperatures, 

 pressures, gas densities, and other physical conditions of the sun's 

 chromosphere, corona, and prominences. From their research will 

 come ever-expanding knowledge. No matter how improbable it may 

 seem today, this knowledge will rapidly find practical application 

 in some new activity of importance to man. If only we can stabilize 

 sufficiently the political affairs of our planet, we have a long and 

 profitable time ahead of us for the exploration of the nearest star, 

 our sun. 



