THE EDGE OF THE SUN — MENZEL 223 



mum positions to make the average flux equal to the rate of energy 

 generation deep in the interior. 



In the photospheric and subphotospheric layers, the solar atmos- 

 phere may be quite turbulent, though the convective flow is not par- 

 ticularly rapid. The rising hot gas alters the temperature distribution, 

 but kinetic energy of the convective matter does not appreciably 

 affect the atmosphere. At higher levels, however, marked changes 

 occur in the character of the convection. The speed of flow increases 

 as the bubbles expand into regions of lower density, until the gas 

 moves with a speed equal to or even greater than that of sound in 

 the medium. Intense shock waves may then occur. The gas becomes 

 hot as the kinetic energy of the moving clouds gradually dissipates 

 into random motions of individual atoms. This enhancement of con- 

 vection explains why the chromosphere, the layer immediately over- 

 lying the photosphere, is so jagged and rough, consisting of rapidly 

 changing jets and spicules. Motion pictures, taken by Richard Dunn 

 at Sacramento Peak Observatory, clearly demonstrate the violence of 

 activity in these outer layers, and Richard Thomas has studied the 

 heating effect of the jets. 



The chromosphere, which until recently could be studied only at 

 the time of total solar eclipse, presented the astronomer with many 

 interesting problems. Of particular significance was the appearance 

 of lines of ionized helium in the spectrum, radiation requiring ex- 

 tremely high excitation. At the same time the intensity of radiation 

 from neutral and ionized metals suggested a somewhat lower tem- 

 perature. The answer now becomes clear. We actually encounter a 

 wide range of excitation which varies according to the amount of 

 heating provided by the shock wave. The fact that the excitation 

 appears to increase upward is thus consistent with this new view. As- 

 tronomers were originally very much concerned to find that the upper 

 chromosphere had temperatures ranging up to 25,000° C. or more 

 when the effective surface temperature of the sun was only 6,000°. 

 Astronomers contributing to these recent advances include Athay, 

 Pecker, Thomas, Zirin, and Menzel. 



The concept of a dynamic solar atmosphere now extends to the 

 corona as well as the chromosphere. There are a few unsolved prob- 

 lems, however, chiefly the observed fact that motions of coronal gas 

 appear to be slower than those in neighboring prominences. Ap- 

 parently the corona derives some of its support from the sun's general 

 magnetic field. To derive such support, material must be shot out 

 from the sun at very high speeds. The observed coronal tempera- 

 tures, derived from widths of the coronal lines, are at least 1,000,000° 

 and in some regions as high as 5,000,000°. 



The idea recently suggested by S. Chapman, that the corona may 

 extend even beyond the earth, is particularly appealing after one 



