ASTRONOMY AND ARTIFICIAL SATELLITES—GOLDBERG 29] 
ionized magnesium at 625 A. Now the continuous spectrum with its 
dark lines is radiated by the visible surface of the sun, the so-called 
photosphere, where the temperature is in the neighborhood of 6,000°. 
As predicted by the well-known laws of black-body radiation, gases 
at such a relatively low temperature give off very little ultraviolet 
radiation, and hence the continuous spectrum ceases to be conspicu- 
ous at 1,600 A. However, the layers above the photosphere are very 
much hotter. Ata height of about 50,000 kilometers above the photo- 
sphere, in the region of the lower corona, the temperature has already 
risen to about a million degrees. Between the photosphere and the 
corona lies the chromosphere, the transition region in which the 
temperature rises precipitously from 6,000° to a million degrees. 
Through the chromosphere are propagated the hydromagnetic waves 
that originate in the layers below and must supply the energy to 
maintain the corona at its very high temperature. The mechanism 
by which the energy is transferred is not well understood and forms 
one of the most pressing problems of solar physics. The solution of 
this problem will come from the study of the spectrum of the chromo- 
sphere, particularly the ultraviolet spectrum. The significance of the 
spectrum shown in the plate is that the bright emission lines (with 
the exception of the magnesium line at 625 A., which probably is 
radiated by the hot corona) do, in fact, originate in the chromosphere. 
Furthermore, since the temperature is increasing very steeply with 
height in the chromosphere, lines of high excitation will come from 
the higher levels. Thus, by exploration of the ultraviolet emission 
spectrum of the sun the physical state of the chromosphere can be 
probed through its entire thickness. 
Friedman and his collaborators at the Naval Research Laboratory 
have also, by means of photon counters, recorded the emission of 
X-rays in the band from about 5 to 150 A. Most of the emission in 
this region of the spectrum comes from the highly stripped atoms 
that prevail at the million-degree temperatures found in the corona. 
Because the X-ray emission from the sun is expected to be highly 
variable and extremely sensitive to solar activity (see below) its 
systematic observation from satellites will be of the greatest 
importance. 
One of the most important activities of the International Geo- 
physical Year has been the continuous monitoring of the sun from 
a network of stations all around the earth. The purpose of this 
patrol] has been to obtain a complete record of solar activity, or tran- 
sient disturbances in the solar atmosphere, because of the effect of 
such solar activity upon the earth. The most common of these tran- 
sient disturbances are the sunspots, which occur in the photosphere 
and may therefore be seen visually in the telescope or photographed 
