SOLAR ACTIVITY — SPENCER JONES 239 



The great extension of the corona implies a rate of decrease of 

 density outward about 500 times slower than that in the sun's photo- 

 sphere. The only logical explanation of such a small density gradient 

 is that the coronal temperature is extremely high. It may be inferred 

 that it is at least one million degrees, as compared with the sun's effec- 

 tive temperature of about 6,000°. This conclusion may seem absurd 

 at first sight, but there is ample corroborative evidence. The corona 

 gives a continuous spectrum, on which are superposed a number of 

 bright lines ; none of these lines has ever been observed terrestrially, 

 nor, until the last few years, had they ever been detected in the spec- 

 trum of any other celestial body. Their origin was one of the major 

 unsolved problems of astronomy. In 1942 Edlen showed that the 

 principal lines were due to atoms of iron, nickel, and calcium from 

 which a large number of electrons — from 9 to 13 — had been stripped. 

 To produce such a high degree of ionization, a correspondingly large 

 amount of energy is needed, and this in turn requires a high tempera- 

 ture of the order of one million degrees. This high temperature pro- 

 vides an explanation, moreover, of why the Fraunhofer lines of the 

 solar spectrum are absent from the corona. The velocities of the elec- 

 trons in the corona are, because of its high temperature, so large that 

 the absorption lines are broadened through the Doppler effect to a 

 width of about 100 angstroms and are, therefore, completely washed 

 out. On the other hand, the velocities of the iron and nickel ions, 

 because of their greater mass, are only of the order of 20 km. a second ; 

 the width of the coronal emission lines should therefore be about one 

 angstrom, which agrees closely with observation. 



The radiation from the sun itself has maximum intensity at a wave- 

 length of about 5,000 A., whereas the radiation from the corona has 

 maximum intensity at about 30 A. and thus consists largely of X-rays. 

 In the visible region the radiation from the sun is about 10^ times that 

 from the corona, but at a wavelength of 600 A. the radiation from the 

 corona is about 10^ times that from the sun. It is the radiation of 

 about this wavelength that is responsible for the ionization in the 

 earth's atmosphere, so producing the ionosphere. Theoretical investi- 

 gations have led to the conclusion that the observed degree of ioniza- 

 tion in the E-layer of the ionosphere requires radiation whose intensity 

 is of the order of 10° that of a black body of a temperature 6,000° K. 

 This high intensity of radiation, which at first appeared puzzling, is 

 readily accounted for by the high temperature of the corona. The 

 radiation on radio wavelengths emitted by the normal quiet sun is also 

 explained by the high coronal temperature ; its source is in the corona 

 and not in the sun itself. 



There has been much speculation about the mechanism by which the 

 high temperature of the corona is maintained. One theory attributes 

 it to the infall of interstellar dust, the kinetic energy of the dust par- 



