ASTRONOMY AND ARTIFICIAL SATELLITES—GOLDBERG 293 
quencies is sometimes as high as that which would be radiated by 
a black body at a temperature in excess of 10” degrees. Many flares 
observed in recent years have also given rise to rather extraordinary 
increases in the intensity of cosmic rays from the sun while even 
rather modest flares can produce sharp bursts of gamma radiation, 
as have been observed by Winckler and his associates at the Uni- 
versity of Minnesota. Solar flares are clearly tremendously energetic 
phenomena and hence the explanation of their origin and physical 
nature is one of the fundamental problems of solar physics. 
1700 1730 1800 
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Ficure 3.—Comparison of Hae flare light curve (McMath-Hulbert Observatory) and 
concomitant 2800 Mc/s and 200 Mc/s solar radiation. The two radio-frequency rec- 
ords were recorded at N.R.C. (Ottawa) and Cornell University, respectively. 
Solar activity initiates various events on the earth, chiefly in the 
upper atmosphere, and hence its study has very important practical 
consequences for the human race. These events fall into two cate- 
gories: (1) sudden ionospheric disturbances (SID’s), which begin 
almost immediately upon the onset of a flare and must therefore be 
caused by radiation from the flare; and (2) geomagnetic storms and 
displays of northern hghts which start about 1 day later and hence 
must be triggered off by high-speed particles. 
Sudden ionospheric disturbances are caused by increases in the 
degree of ionization of the upper atmosphere in the height range 
50-100 kilometers, as a consequence of increased ultraviolet emission 
from the regions of solar flares. The SID’s are manifested in a 
variety of ways, the most dramatic of which is the sudden fadeout 
of shortwave radio communications on the earth, in the band 1.5-30 
megacycles. An example of such a fadeout in the 5-megacycle signal 
from station WWY, as received at Cornell University, is shown in 
figure 4, in relation to the Ha light curve of a flare. The lower fre- 
quencies are most strongly affected, although at much lower frequen- 
cies (about 50 ke.) the transmission is actually improved. The 
fadeouts may last for a few minutes or for as long as 8 hours, 
and hence can cause serious dislocation to an important phase of 
everyday life. The fadeouts can frequently but not always be pre- 
dicted in advance by observation of the sun in the Ha line of hydrogen. 
