  Cosmic-ray scattering on magnetic turbulence leads to spatial diffusive
propagation; if the scattering medium is moving, this will inevitably also
cause changes in the momentum of the particles, so-called diffusive
reacceleration. This can be described as diffusion in momentum space. Diffusive
reacceleration has often been invoked to explain the peak observed in
secondary-to-primary ratios at a few GeV, in particular Boron-to-Carbon. This
avoids the necessity to postulate an ad-hoc break in the spatial diffusive
coefficient, and has become almost a standard in modelling cosmic-ray spectra.
However, at the levels invoked, the process implies a significant input of
energy from the interstellar medium into cosmic rays, so that in such models
interstellar space competes with the usual accelerators like supernova
remnants. The questions arise: is reacceleration really occurring at the high
level required to explain secondary-to-primary ratios? and are the energy
requirements physically plausible? We address this issue using both analytical
and numerical models of cosmic-ray propagation.
