196 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



Although the ultimate source of energy is deep inside the sun, it is 

 the leakage of this energy through the surface that interests us tonight 

 in considering solar terrestrial effects. The surface has a temperature 

 of about 10,000° F. It radiates a remarkably steady flow of light, the 

 measurement of which has been a major part of the distinguished 

 career of Dr. Abbot of the Smithsonian. He found that, within 1 or 

 2 percent, the solar energy that can be observed from the ground is 

 constant. This is in marked contrast to the invisible ultraviolet radia- 

 tions which maintain and are absorbed in the ionosphere. 



If we look at the sun through a telescope at almost any time we 

 find its surface pocked with sunspots, great dark circular patches that 

 may be anything up to 100,000 miles in diameter. We know that they 

 are cooler than the surrounding regions and they are the seats of stu- 

 pendous magnetic fields, but their cause is a mystery. The most re- 

 markable thing about the sunspots is the cyclic variation in their 

 numbers. The sunspot cycle averages about II14 years. At maxi- 

 mum, many spots are always visible on the solar disk, but at sunspot 

 minimum no spots will be seen for days at a time. 



When we equip our telescope with various spectroscopic devices, we 

 can detect other solar features otherwise invisible. I have already 

 mentioned the flares. They are especially important to students of 

 solar terrestrial effects, because, of all solar activities, they are most 

 clearly connected with the most violent and troublesome of iono- 

 spheric disturbances. A flare is a very impressive sight. Somewhere 

 near a sunspot it suddenly appears as a small bright point of light on 

 the solar surface. In a matter of minutes it flashes up to its maximum 

 size and brightness, and then slowly fades away to invisibility in half 

 an hour or so. Its full size is about like that of the sunspots, and may 

 be anything from the smallest detectable up to 100,000 miles across. In 

 visible light a flare is a puny thing, which cannot be seen at all without 

 special optical aids that reject most of the light except that of the 

 flare. This appearance, however, grossly belies the true potency of 

 a flare, as the visible tip of an iceberg projecting above the sea belies the 

 great bulk below the surface. It is clear that the great bulk of flare 

 energy consists of ultraviolet quanta and corpuscles. This is evident 

 from the reaction of our ionosphere. When a flare appears a new 

 ionospheric screen very suddenly develops at a height of only about 

 40 miles, where the interference of air molecules becomes so great that 

 no reflection of radio waves is possible. We have a radio f adeout. An 

 estimate of the energy required shows that the ultraviolet output of a 

 large flare, which might have an area of one-thousandth of the visible 

 hemisphere of the sun, is comparable with the output of the whole nor- 

 mal sun. We conclude that each square inch of the flare would be 

 1,000 times as bright as a square inch of the normal solar surface, if 

 we could see it in ultraviolet light. 



