Nonlinear cosmic ray Galactic transport in the light of AMS-02 and Voyager data

Abstract

Context. Features in the spectra of primary cosmic rays (CRs) provide invaluable information on the propagation of these particles in the Galaxy. In the rigidity region around a few hundred GV, these features have been measured in the proton and helium spectra by the PAMELA experiment and later confirmed with a higher significance by AMS-02. We investigate the implications of these data sets for the scenario in which CRs propagate under the action of self-generated waves. Aims. We show that the recent data on the spectrum of protons and helium nuclei as collected with AMS-02 and Voyager are in very good agreement with the predictions of a model in which the transport of Galactic CRs is regulated by self-generated waves. We also study the implications of the scenario for the boron-to-carbon ratio: although a good overall agreement is found, at high energy we find marginal support for a (quasi) energy independent contribution from the grammage, which we argue may come from the sources themselves. Methods. The transport equation for both primary and secondary nuclei is solved together with an equation for the evolution of the self-generated waves and a background of pre-existing waves. The solution for this system of nonlinear equations is found with an iterative method elaborated by the same authors in a previous work on this topic. Results. A break in the spectra of all nuclei is found at a rigidity of a few hundred GV, as a result of a transition from self-generated waves to pre-existing waves with a Kolmogorov power spectrum. Neither the slope of the diffusion coefficient, nor its normalization are free parameters. Moreover, at rigidities below a few GV, CRs are predicted to be advected with the self-generated waves at the local Alfvén speed. This effect, predicted in our previous work, provides an excellent fit to the Voyager data on the proton and helium spectra at low energies, providing additional support to the model.