234 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1957 



doubt very glad of its steadiness. But in its outer layers there occur 

 phenomena of very great violence — a kind of meteorology where the 

 scale and the speeds are enormous compared with those on the earth 

 and where the forces responsible are evidently of quite a different 

 nature. One particular phase of this atmospheric violence is called 

 a flare. It is not known what gives the phenomenon its great sudden- 

 ness and its terrific power, but it is certain that electric forces play 

 a very large part. Speeded-up movies of motions in the sun's atmos- 

 phere are taken at the high-altitude observations at Climax, Colo., 

 and at Sacramento Peak and in those one can see the powerful guid- 

 ing effects that are undoubtedly magnetic. Basically, though not in 

 detail, it is understood how magnetic fields would guide the motions of 

 gases and it is known independently from optical observations that 

 very strong magnetic fields occur in the vicinity of sunspots which are 

 also localities of the greatest visible disturbances. 



Flares are seen by the great increase of the light in some parts of 

 the spectrum. In a matter of a few minutes some region in the sun's 

 atmosphere lights up in bright emission-line light, and in the case 

 of the most intense flares a big and interesting chain of events is 

 then initiated. It is a more or less fortuitous circumstance that we do 

 not often take notice of the great flares without the use of instru- 

 mentation. Almost all the great effects of a flare are in forms that 

 either do not penetrate through the atmosphere of the earth, or that 

 we cannot perceive by our own sense organs. For this reason it is not 

 easily appreciated just how terrific an event a great flare is. The first 

 effects that arrive at the earth are usually intense radio noise that 

 can readily be received on modern radioastronomical equipment, and 

 the ultraviolet light that does not penetrate the atmosphere but 

 results in characteristic effects in the upper layers where it is absorbed. 

 Sudden interruption of all long-distance radio communication may 

 set in on the entire day side of the earth. Also the sudden change in 

 the electrical properties of the upper atmosphere gives rise to slight 

 but immediate disturbances in the earth's magnetic field. A day or 

 two later a great magnetic disturbance may set in, being no doubt 

 due to some ejected gas having then reached the earth from the sun. 



All these effects have been known for a long time, and many parts 

 of the phenomena have been explained, but the basic effect that hap- 

 pens so suddenly and with such violence on the sun is still not under- 

 stood, although of course there are a number of theories. 



As a result of the work of Forbush and Ehmert, it has become 

 known that another type of event is related to flares. The rate of 

 bombardment of the earth by cosmic rays shows occasionally a sharp 

 increase clearly related to the very greatest of the solar outbursts. 

 Since 1942 only five such events have been detected. But strangely 



