relativistic, sizable departures from $\eta=1$ (the natural value from quasi-linear theory, see ) and/or large values of $V_{\rm A}$ (which allows for energy redistribution) may be needed to affect appreciably CRs whose $v\simeq c$. In a certain sense, $\eta$ is thus not a very valuable [*effective*]{} parameter for the problem at hand. Nonetheless, we keep the $\eta$ parameter in the discussion for historical reasons, since in combination with strong reacceleration it used to be an important ingredient in past studies of B/C data, notably at low energies [@SeoEtAl1994; @StrongEtAl1998; @JonesEtAl2001; @MaurinEtAl2001; @DonatoEtAl2004]. Sufficiently large negative values of $\eta<-\delta_{\rm l}$ (or, similarly, of $\delta_{\rm l}<-1$ if $\eta=1$) can also imply superluminal diffusion [@DunkelEtAl2007; @AloisioEtAl2009] in the non-relativistic regime. In this sense, we caution the reader never to extrapolate a-critically the functional forms obtained here too far from the rigidity range over which the fits have been obtained.

Let us now be more specific about reacceleration. It turns out that spatial diffusion can rather generically be linked to diffusion in momentum space (aka reacceleration) in most (but not all) cases [@BerezinskiiEtAl1990]. We include diffusion in momentum space through an additional diffusion coefficient $K_{pp}$—see . We follow the reacceleration model proposed in Refs. 