THE STORY OF COSMIC RAYS — SWANN 259 



rial in the apparatus itself, places a lower limit on the energy of the 

 rays that can be observed.) 



If the intensity of the magnetic field of the sun at its magnetic pole 

 is known, we can calculate its value at the earth's orbit, and we can 

 calculate the energy below which no rays are to be found striking the 

 earth's atmosphere. We can then compute the corresponding latitude 

 on the earth at which further increase of latitude would yield no addi- 

 tional rays, as these would have energies lower than those permitted 

 by the sun to be present. 



The magnitude of the sun's magnetic field has been debated for a 

 long time, and particularly within recent years. Optical measure- 

 ments of the Zeeman effect led, about 40 years ago, to the conclusion 

 that the sun had a field of about 50 gauss at its pole. If the sun acted 

 like an ordinary magnet, the corresponding field in the earth's vicinity 

 would prevent protons reaching us if they had less energy than 3 

 billion electron volts, which is the energy for entry through the earth's 

 magnetic field at the latitude of 50° geomagnetic. Consequently, on 

 this basis, we should expect that even at the outer limits of the atmos- 

 phere there would be no increase of intensity with latitude from 50° to 

 the pole. 



However, recent experiments by M. A. Pomerantz, under the aus- 

 pices of the Bartol Research Foundation, the Office of Naval Research, 

 and the National Geographic Society, have shown that over the range 

 of latitude from 52° to 69° there is an increase of 46 percent in the 

 vertical primary cosmic radiation intensity. Also, this radiation is 

 composed of rays of such small energies that they could not possibly 

 have come to us from outer space through the sun's magnetic field if 

 it had more than 6 percent of the strength originally assumed from 

 the Zeeman effect. 



This argument should be accepted with reservation. There is in- 

 creasing evidence that some primary rays may come to us from the sun 

 itself, and such rays might reach us in spite of the sun's magnetic field 

 because their short journey to us would not permit enough bending 

 in their paths to keep them away from us. At the present time, the 

 whole question of the magnitude of the sun's magnetic field and its 

 bearing upon the primary cosmic rays calls for further elucidation. 



Astronomers have come to doubt the existence of a solar magnetic 

 field as high as 25 or 50 gauss at the poles of the sun. Indeed, G. 

 Thiessen, who was originally one of the strongest supporters of the 

 earlier value for the sun's field, concluded in 1949 that a strict analysis 

 of the original data, while not denying the existence of the larger field, 

 does not support such a field with any certainty. Moreover, his recent 

 careful observations by improved methods, and those of others, using 

 the new solar magnetograph, have led to a solar value of only about 

 one gauss, and in the opposite direction to that formerly found. 



