240 ANTSfXTAL REPORT SMITHSONIAN ESrSTITirTION, 1956 



tides being distributed as heat throughout the coronal matter. An- 

 other view is that it is produced by magnetic processes, associated with 

 magnetic fields on the sun and the emission of high-speed particles, the 

 corona possibly being heated by electric currents flowing within it. A 

 third suggestion is that sound waves are generated by the turbulent 

 motion of gases in the sun's atmosphere and that as these waves move 

 outward into more tenuous regions they develop into shock waves, 

 which produce the high temperature of the corona. These alternative 

 views are all very tentative ; no completely satisfactory explanation of 

 the high coronal temperature has yet been given. 



The increase in effective temperature from centimeter to meter wave- 

 lengths that is required to account for the observed intensities of radia- 

 tion on radio wavelengths from the quiet sun, can be explained on the 

 assumption that the centimeter waves originate in the level just above 

 the visible surface of the sun, while the meter waves originate at a 

 much greater height, where the temperature is much higher. If these 

 latter waves were generated near the surface of the sun they would be 

 absorbed and would be unable to escape into space. There is in fact 

 for each level above the surface an appropriate critical frequency of 

 escape, which progressively increases as we penetrate deeper and deeper 

 through the corona toward the visible surface of the sun. 



The increase in the lag between the visible flash and the radio out- 

 burst accompanying a flare, as the frequency decreases and the wave- 

 length increases, is an indication, as already mentioned, of some effect 

 that travels outward from the sun. It is probable that a stream of 

 particles is shot out with high velocity when the flare occurs and that 

 as the stream passes outward through the corona the highly ionized 

 gas is set into oscillation and radio waves are emitted of wavelengths 

 which are appropriate to the electron density at the particular height 

 reached by the stream. We should expect the radiations of very high 

 frequency to be excited first, then, as the stream moves outward, radia- 

 tions of progi'essively diminishing frequency to be excited. This is in 

 agreement with what is actually observed. 



There are many problems presented by the sun which are as yet 

 unsolved. The corona probably holds the key to many of the hidden 

 mysteries. Further observations, supplemented by theoretical investi- 

 gations, are required before a complete and satisfactory explanation 

 can be provided. Intensive observations of the sun will be made at 

 many observatories during the International Geophysical Year, when 

 the sun will be at or near its maximum sunspot activity ; during the 

 same period there will be worldwide observations of ionospheric, geo- 

 magnetic, auroral, and other phenomena. The coordination of the 

 solar and terrestrial observations will lead, it is hoped, to a more com- 

 plete understanding of phenomena occurring on the sun and of the 

 manner in which the associated terrestrial effects are produced. 



