Volume 558, October 2013
|Number of page(s)||5|
|Published online||25 October 2013|
Pitch-angle scattering in magnetostatic turbulence
II. Analytical considerations and pitch-angle isotropization
Zentrum für Astronomie und Astrophysik, Technische Universität
Received: 25 June 2013
Accepted: 21 August 2013
Aims. The process of pitch-angle isotropization is important for many applications ranging from diffusive shock acceleration to large-scale cosmic-ray transport. Here, the basic analytical description is revisited on the basis of recent simulation results.
Methods. Both an analytical and a numerical investigation were undertaken of the Fokker-Planck equation for pitch-angle scattering. Additional test-particle simulations obtained with the help of a Monte-Carlo code were used to verify the conclusions.
Results. It is shown that the usual definition of the pitch-angle Fokker-Planck coefficient via the mean-square displacement is flawed. The reason can be traced back to the assumption of homogeneity in time which does not hold for pitch-angle scattering.
Conclusions. Calculating the mean free path via the Fokker-Planck coefficient has often proven to give an accurate description. For numerical purposes, accordingly, it is the definition that has to be exchanged in favor of the pitch-angle correlation function.
Key words: plasmas / magnetic fields / turbulence / solar wind / cosmic rays
© ESO, 2013
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