3. Pitch-angle scattering
Perhaps the most important transport process of high-energy particles
is represented by pitch-angle scattering, i.e., by stochastic
variations in μ = cos ∠(u, B0) = v/v with range [−1, 1], where
B0 = B0ˆez is the mean magnetic field and u is the particle velocity.
This process is related to diffusion along the mean magnetic
field, which is described by the parallel diffusion coefficient, κ,
or the parallel mean free path, λ = (3/v)κ, which are also related
to the cosmic ray anisotropy (Schlickeiser 1989; Shalchi
et al. 2009).