258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1956 



there is more of the atmosphere above, and so on. We might thus ex- 

 pect that the knee of the latitude curve — the place where the hori- 

 zontal portion starts — would occur at a latitude which was greater the 

 greater the altitude. 



Now the simplicity of the foregoing picture is disturbed by the fact 

 that the knee does not seem to vary with latitude as the altitude is 

 varied, but seems to occur around 50° latitude for all altitudes. This 

 led to the view that the origin of the knee was not to be explained by 

 the absorption of low-energy rays by the atmosphere, but by something 

 outside the atmosphere, something that creates, for the energy spec- 

 trum, a lower limit of energy which is nevertheless sufficiently great to 

 permit penetration of the whole atmosphere. Under such conditions, 

 the intensity-versus-latitude curve would show, at all altitudes, a knee 

 corresponding to this energy. 



To solve this paradox, it was suggested that the knee of the latitude 

 curve owed its origin to the sun's magnetic field. The bending of the 

 paths of the rays near the earth, that is, within a few earth radii, is 

 caused mainly by the earth's magnetic field, which is here considerably 

 stronger than the field of the sun. At greater distances from the 

 earth, however, the sun's magnetic field could predominate. 



Consider a sphere containing the earth's orbit and centered at the 

 sun. Then, neglecting the influence of the earth's magnetic field, we 

 can ask what energy a cosmic ray of assigned type must have to enter 

 that sphere at all in the vicinity of the earth's orbit, which orbit lies 

 roughly in the plane of the sun's magnetic equator. No rays of energy 

 less than this amount could reach the earth's orbit at all. If this energy 

 is enough to penetrate the atmosphere, we would expect, as found, 

 that the cosmic rays falling upon the earth would increase in intensity 

 with increase of latitude from the equator only to the point at which 

 all the rays permitted access to the earth's orbit by the sun had been 

 received by the earth. Increase of latitude beyond this point would 

 yield no further rays because there would be no more rays. The knee 

 of the latitude curve would occur at a definite latitude which would be 

 the same for all altitudes. 



The foregoing considerations become complicated by what happens 

 to the primary cosmic rays as they enter our atmosphere. However, 

 a fairly clean-cut story appears if observations are made so high in 

 the atmosphere that down to that depth nothing in particular has 

 happened to the primary rays. At such altitudes, and for the case 

 where the primary radiation contains rays of all degrees of smallness 

 in energy, we should expect the intensity to show a continual increase 

 with latitude right up to the poles. (However, the matter is not quite 

 as simple as here stated because even the small amount of atmosphere 

 above the apparatus at high altitudes, and indeed the absorbing mate- 



