Fokker--Planck equations for charged-particle transport in random fields

The Fokker-Planck equations for charged-particle dynamics are rederived, extending somewhat the elegant discussion of Hasselmann and Wibberenz. It is shown that the usual results are obtained and the conclusions in many cases are correct over a very broad range in energy. In particular, the rate for pitchangle scattering (and hence K i ) may be accurately given down to energies much lower than previously thought. Recent claims that these Fokker-Planck equations are in general incorrect are thus shown to be in error.