Cosmic-ray diffusive reacceleration: a critical look

TL;DR

This paper critically examines the role and energy plausibility of diffusive reacceleration in cosmic-ray propagation models, questioning its necessity and physical realism in explaining observed cosmic-ray spectra.

Contribution

It provides a detailed analytical and numerical analysis to assess whether diffusive reacceleration is genuinely occurring at the levels used in models and if it is physically plausible.

Findings

Reacceleration may be overestimated in current models.

Energy input from interstellar medium could be physically implausible.

Reevaluation of cosmic-ray propagation models is necessary.

Abstract

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.