Pitch‐angle diffusion of electrons in the magnetosphere

This paper reviews the experimental and theoretical knowledge concerning pitch‐angle diffusion of trapped electrons in the earth's radiation zones. Experimental information bearing on pitch‐angle diffusion has been obtained from data on electron flux decay following artificial injection by a nuclear explosion or following natural injection during a period of magnetospheric activity. Useful information has also been obtained by studying the trapped‐electron pitch‐angle distributions and the precipitation of electrons into the atmosphere at low altitudes. The theory of pitch‐angle diffusion based upon the Fokker‐Planck equation is discussed, and some simple cases of this theory are worked out. Several different pitch‐angle scattering mechanisms that may be important for trapped electrons, such as cyclotron resonance, bounce resonance, and atmospheric Coulomb scattering, are also discussed.