X-rays From the Sun^ 



By Herbert Friedman 

 VS. Naval Research Laboratory 



Only a few decades ago solar X-ray emission was unknown. The 

 sun was viewed as a glowing sphere of hot gas radiating at a tem- 

 perature of 6,000° K. and incapable of producing any significant flux 

 of X-rays. Today, with information gained from experiments carried 

 in rockets and satellites, we know that solar X-rays shape some of the 

 major features of the ionosphere. Sporadic, explosive outbursts of 

 X-rays are synchronized with solar flares and linked directly with 

 radio fadeouts. More modest eruptions are associated with active 

 prominences and coronal condensations. The stormy character of 

 solar X-ray emission far exceeds that of any portion of the ultra- 

 violet or visible spectrum, and is matched only by the violent outbursts 

 observed at radio frequencies. 



At the time of a solar eclipse, a corona of faintly luminescent gas 

 is visible above the disk. This thin white halo, with a slightly greenish 

 cast, reaches millions of miles into space. The source of solar X-ray 

 emission lies within the corona very near its base where the tempera- 

 ture is of the order of a million degrees Kelvin. How the corona 

 reaches this remarkably high temperature when the visible surface 

 of the sun is only 6,000° K. is explainable in terms of the dissipation of 

 shock-wave energy. Immediately below the surface of the sun, energy 

 is transported outward by the violent convection of hydrogen gas. 

 Starting as sound waves generated by turbulence within the hydrogen 

 convection zone, they propagate outward, increasing in amplitude as 

 the density decreases until shock waves develop. Energy is thus 

 transferred from the interior to the corona. Because the corona is 

 so thin, it radiates poorly and only a small fraction of the sun's energy 

 need be dissipated in the corona to achieve very high temperatures. 



In the 25th annual James Arthur lecture on the sun,^ Dr. Leo Gold- 

 berg described the earth's atmosphere as a barrier to astronomical 



^ The 27th annual James Arthur lecture on the sun, given under the auspices of the 

 Smithsonian Institution on Feb. 2, 1961. 



* See Annual Report of the Smithsonian Institution for 1939, p. 285. 



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