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ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 6 



THE EFFECT OF TERRESTRIAL, SOLAR, AND GALACTIC MAGNETIC FIELDS 



The primary cosmic rays appear to come toward our earth uni- 

 formly from all directions, or nearly so. The earth is a huge magnet, 

 with a magnetic field of small intensity but large extent, so that the 

 paths of charged particles approaching the earth are bent. Considera- 

 tion of the influence of the magnetic field leads to the following con- 

 clusion : Particles of the same charge and momentum (relativistic mass 

 times velocity) are indistinguishable as regards the effect of the mag- 

 netic field upon them. 



While the student of cosmic rays usually speaks of particles in terms 

 of their momenta, the kinetic energy is a quantity more familiar to the 

 layman. It is, however, less simply related to the bending effect in 

 a magnetic field. Thus, for a given energy, heavy particles are bent 

 less in their paths than are light particles. 



Figure 4. — Paths of primary cosmic rays (in this case protons with IS billion electron volts 

 energy) as they are deflected by the earth's magnetic field. The paths are drawn in the 

 plane of the earth's magnetic equator, with the north pole upward from the paper. 



Any charged particle, such as a proton, has to have a certain mini- 

 mum energy before it can reach the earth's vicinity at all without 

 being bent back into space by the magnetic field. That minimum 

 energy amounts to 14 billion electron volts for a proton entering 

 vertically at the magnetic equator. It gets less and less as we proceed 

 toward the magnetic poles until, theoretically, at the poles themselves 

 particles with infinitesimal energy could reach the outer bounds of 

 the atmosphere. 



At any given geomagnetic latitude (we shall henceforth call this 

 simply latitude), the minimum energy for entry of positive particles 

 into the atmosphere varies as the direction of entry varies. It is 

 greatest for the east and least for the west, the value for the vertical 

 being intermediate in amount. 



