292 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
in white light. The most spectacular disturbances, however, occur 
higher up in the chromosphere and are normally visible only in the 
monochromatic radiations emitted by atoms of ionized calcium and 
hydrogen. These disturbances comprise what is referred to as “solar 
activity,” or “solar phenomena.” Solar phenomena are exceedingly 
diversified and complex, and it is difficult even to give an organ- 
ized account of them. The more striking among them include (a) 
the sunspots; (0) the so-called plages, which are relatively stable 
formations that usually, but not always, occur near sunspots and 
appear perhaps 50 to 100 percent brighter than their surroundings; 
(c) the solar flares, the most catastrophic of all events on the sun, 
which always break out in plage regions; (d) the prominences, great 
clouds of gas which jut out beyond the limb of the sun and are 
frequently in violent turbulent motion; and (e) the dark flocculi 
and filaments, which are prominences seen in projection against the 
solar disk. Plate 3 is a photograph of the sun in the monochromatic 
red line of hydrogen, Ha, made with a narrow-band filter, and there- 
fore called a filtroheliogram. It shows several types of solar activity, 
including plages, filaments, and three flares. 
Except for some types of prominences, solar activity is generally 
confined to sunspot regions and indeed the level of activity roughly 
parallels the sunspot cycle. The lifetimes of solar phenomena are 
highly variable. Some types of activity, e.g., the surge prominences 
ejected at the limb, and the high-speed dark flocculi that accom- 
pany flares, are exceedingly ephemeral, and last but a few minutes. 
Great flares may persist for several hours, some types of prominences 
for weeks, and plages and sunspots for months. 
The flare phenomenon is probably the most spectacular of all solar 
events, because of its complexity, its abrupt commencement, its rela- 
tionship to other solar phenomena, and its often immediate and dra- 
matic impact upon the earth. It is characterized by a sudden 
increase in the radiation intensity from relatively small areas up to 
two or three-tenths of a percent of the total area of the solar disk. 
The enhanced radiation is almost always in the form of bright lines, 
and only rarely does the intensity of the continuous spectrum increase. 
After the initial brightening, the excess radiation intensity dies out 
slowly in times of from one-half to 3 or 4 hours. 
The flare is not an isolated phenomenon on the sun in the sense 
that it often interacts with and is accompanied by other phenomena. 
Thus, dark filaments in the vicinity of flares previously quiescent 
may suddenly become active. High-speed gas clouds are sometimes 
ejected from flares at several hundred kilometers per second. Bright 
flares are almost always accompanied by great bursts of radio noise, 
particularly in the low-frequency band 20-600 megacycles (see fig. 
3). The absolute intensity of these radio bursts at the lowest fre- 
