First, the wave number k-dependence of the turbulent power
spectrum G(k) can be specified to derive analytical approximations
for Dμμ. The quasi-linear theory (QLT) derived by Jokipii
(1966) has been the standard theory, until it was realized that
QLT is not only inaccurate but, in fact, invalid for some scenarios.
For the example of isotropic turbulence, it has been known
from both qualitative arguments (Fisk et al. 1974; Bieber et al.
1988) and detailed calculations (Tautz et al. 2006) that QLT cannot
properly describe pitch-angle scattering, because it neglects
90◦ scattering and leads to infinitely large mean-free paths. This
problem was remedied by the application of the second-order
QLT (SOQLT, see Shalchi 2005; Tautz et al. 2008), which considers
deviations from the unperturbed spiral orbits that were assumed
in QLT.