230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



and at its stage of maximum development covered an area exceeding 

 6,000 million square miles, or about 130 times the equatorial cross 

 section of the earth. 



A typical spot consists of an umbra or dark center, surrounded by 

 a penumbra in the form of a more or less complete ring, which is 

 darker than the surrounding solar surface, though not so dark as the 

 umbra. This darkness is only apparent, being the effect of contrast 

 against the brighter surrounding disk, and is the consequence of the 

 temperature of the spots being lower than that of the normal solar 

 surface. 



Around every sunspot group there is an irregular bright patch. 

 In integrated light these patches are best seen near the sun's limb, 

 where the surface brightness, because of absorption of light in the 

 sun's outer atmosphere, is less than at the center of the disk. These 

 bright patches are regions of higher temperature and are called 

 faculae; they do not occur exclusively aromid sunspots, often appear- 

 ing in regions devoid of spots. The total area of the faculae increases 

 and decreases in close correlation with the increase and decrease of the 

 sunspot areas and numbers. 



Further information about the sun can be obtained by photograph- 

 ing it in light of one particular wavelength, either by the use of the 

 spectroheliograph or by the use of a monochromatic filter of the 

 type designed by Lyot, which transmits a narrow band about 1 ang- 

 strom in width. Spectroheliograms are usually obtained in either 

 the Ha line of hydrogen or the K line of ionized calcium ; the Lyot 

 filter transmits a narrow band at Ha. 



With the spectroheliograph, different levels in the sun's atmosphere 

 may be photographed by narrowing the slit so that the light from the 

 center of the line, or from one of its wings, is used. The principle 

 can be illustrated as follows : when we look vertically downward into 

 perfectly clear water we are able to see to the bottom ; if the water is 

 slightly turbid we can see to a lesser depth ; if it is veiy turbid, we can 

 see only to a small depth. In the center of an absorption line, the 

 absorption is a maximum; a photograph taken in the light from the 

 center of the line therefore gives a representation of a high level in 

 the atmosphere; one taken in the light a little away from the center 

 gives a representation of a lower level ; one taken in the light from the 

 wing gives a representation of a still lower level. 



Photographs at the lowest level closely resemble those in integrated 

 light (pi. 2) . As the level is raised the faculae become more prominent 

 and at the highest level the spots are usually completely hidden by 

 them. In the K line of ionized calcium a coarse granulation structure 

 appears, giving a mottled appearance, somewhat like the surface of a 

 Seville orange. These clouds, fairly round in shape, are called -floG- 



