Third, rather than entirely specifying the turbulent magnetic
fields, one can perform direct numerical simulations to compute
solutions to the magnetohydrodynamic equations, while the testparticle
trajectories are still integrated as in the previous method.
Such computations (see, e.g., Beresnyak et al. 2011; Spanier &
Wisniewski 2011; Wisniewski et al. 2012) do not require assumptions
regarding the turbulence spectrum that is seen by the
energetic particles. They are, however, limited regarding the extent
of the inertial range of the turbulence spectrum, owing to
computational constraints. In the present study we, therefore, restrict
ourselves to the first and second approaches.