COSMIC RAYS FROM THE SUN — GOLD 235 



enough, although the effect is very rare, when it occurs it does not 

 seem to be of a marginal nature. All five events are easily detectable 

 and substantially smaller events would have been observed had they 

 occurred. The rarity of the phenomenon is thus not to be thought 

 of as due to very few only having reached a detectable level. 



These events have made it clear that a particle accelerator can 

 occur in the atmosphere of the sun and hence presumably also in a 

 great number of other stars. So one might think that here is the clue 

 to the entire process. Cosmic rays are perhaps all made in the at- 

 mospheres of stars. After all, there are many stars on which we 

 might well suppose that far more violent effects are taking place than 

 on the sun. Could they not supply the entire stream? Some people 

 think so, but there are serious difficulties in this. The sun's cosmic 

 rays when they occur are all among the lowest energy particles 

 that can reach us on the earth's surface. In the general flux there 

 is a much greater proportion of high-energy particles. And, after 

 all, the difference between the low- and the high-energy particles 

 is really great. Their impacts are about as different as being hit by 

 a fly or a truck. There are no stars where we could really suppose 

 the high-energy particles to be accelerated. Presumably then, there 

 must be mechanisms operating on a larger scale than the stars. The 

 solar process is the one that we really know something about now, 

 and we can watch. We hope that it will show us a basic mechanism, 

 and there is, of course, the hope that a similar mechanism will in 

 different circumstances be found in other places. Accordingly one 

 is hard at work trying to understand ways in which these magneti- 

 cally controlled hurricanes and typhoons on the sun's surface can 

 produce the accelerated particles. Perhaps I should mention one 

 interesting hint that we have. 



In Russia, and here too I presume, people have made experiments 

 with very strong electric sparks in the hope of reaching temperatures 

 at which the great energy-generating process of nuclear fusion will 

 set in. One curious and quite unexpected byproduct of these sparks 

 has been the generation of fast particles accelerated to much higher 

 energy than could be accounted for by the voltage that had been 

 applied altogether. So there, in front of our eyes, nature is per- 

 forming such a trick as using in some way the violence of a spark 

 to accelerate a very tiny fraction of the gas molecules present to 

 enormous speeds. It may well be that the same trick is done also on 

 the sun and perhaps in larger regions still. Experimental research 

 and the observations of the solar phenomena may together give the 

 answer. 



The best information that we have about the details of the solar 

 cosmic-ray production comes from the great event of February 22-23, 



