STORMY WEATHER ON THE SUN — W. 0. ROBERTS 167 



This brilliant flash of light was first described in detail by a Cap- 

 tain Stannyan, who traveled to an eclipse in Berne, Switzerland, 

 around the year 1700. For some time astronomers believed it to 

 be a glow in the lunar atmosphere. Many years later, Sir Norman 

 Lockyer, at the suggestion of his friend, Dr. Sharpey, attached the 

 name "chromosphere" to this solar layer, because, in his words, "it is 

 the region in which all the various and beautiful color phenomena are 

 seen." The flash of red light results from the bright emission of 

 the spectral line Ha of hydrogen glowing intensely in the thin layer 

 of incandescent gases of the chromosphere. 



This chromosphere, transparent in some colors, opaque in others, is 

 one of the principal objects of modern solar research. In it are 

 tied up many of the problems of the definition and evaluation of 

 solar temperatures. Because of it and the need to understand its phys- 

 ical properties, large radio receivers of special design operating at 

 radar frequencies and designed to pick up the radio emanations 

 of the chromosphere, will, for example, travel to Khartoum in the 

 Anglo-Egyptian Sudan, for the eclipse of February 25, 1952, just as 

 they traveled to Attn Island in 1950, With these receivers, scientists 

 from the Naval Research Laboratory will make temperature determi- 

 nations for comparison with optical measurements that will be made 

 simultaneously at the eclipse. In the past there have been great 

 discrepancies between optical temperature determinations and those 

 made from radio observations. This shows that we have here a 

 major problem worthy of careful study. 



The chromospheric flash lasts a mere 2 or 3 seconds. After that, 

 the chromosphere gases remain obscured until just a second or two 

 before the end of totality. Then all the phenomena appear again in 

 reverse order. During the totality, which may last from a few seconds 

 to nearly 8 minutes, many other features of the sun's atmosphere can 

 be studied. The principal of these are the solar prominences and 

 corona. They differ greatly in appearance and physical composition. 



The prominences are huge clouds of solar atmospheric gases, extend- 

 ing high above the photosphere. They exhibit fascinating, jagged, 

 irregular shapes that change rapidly but that display regularities 

 that astrophysicists look upon today as revealing clues to their expla- 

 nation. They, like the chromosphere, shine with an intense rosy 

 color— characteristic of the Ha line of the spectrum of incandescent 

 hydrogen, one of their principal chemical constituents. Plate 1, figure 

 1, shows a typical solar prominence of large dimension at the south- 

 west limb of the sun on June 4, 1946. The prominences consist of 

 gases probably nearly identical in composition with the chromosphere, 

 even though the proportions of the various constituents probably 

 differ. 



981445—52 12 



