3.1. General remarks
The usual definition of pitch-angle scattering can be found in the
so-called Taylor-Green-Kubo (TGK) formalism (Taylor 1922;
Green 1951; Kubo 1957; Shalchi 2011) as
Dμμ(μ) =
∞
0
dt ˙μ(t) ˙μ(0)
(2a)
=
1
2
d
dt
(Δμ(t))2
, (2b)
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R. C. Tautz et al.: Pitch-angle scattering in magnetostatic turbulence. I.
where the second line employs the definition Δμ(t) = μ(t)−μ(0).
It can be shown (e.g., Shalchi 2011) that both versions agree
with each other, if t is high enough that the expression becomes
asymptotically time-independent.