The process can be analyzed by means of analytic calculations
and numerical Monte-Carlo simulations, which are methods
based on the kinetic Vlasov theory and the integration of
the equation of motion for a large number of test particles,respectively. Whereas the use of a Fokker-Planck approach to
determining the pitch-angle scattering and, based thereupon, the
parallel mean-free path, has been established decades ago, it has
been difficult to reproduce the process using test-particle simulations.
The reason is not only that, owing to the required pitchangle
resolution of the Fokker-Planck coefficient, a large number
of particles are required, but that, additionally, the underlying
algorithm is not entirely clear. While there are previous simulations
(Qin& Shalchi 2009) that confirm quasi-linear results, here
we have shown that the time dependence of the Fokker-Planck
coefficient cannot be neglected.