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.

AMS-02 beryllium data and its implication for cosmic ray transport

“…The theoretical approach that we adopted to describe CR propagation is based on the diffusive halo model and is a modified version of the weighted slab technique already introduced in [9,10,41,42]. Within this approach, the CR sources are assumed to be located in a thin disc with half-width h H, where H is the half thickness of the Galactic halo.…”

“…The decay time of 10 Be becomes longer than the escape time from the Galactic halo for rigidity above 10-100 GV, depending on the size H of the halo, which is exactly the reason why the measurement of the flux of this isotope is sensitive to the parameter H.…”

Supernova Remnant-Cosmic Ray connection: a modern view

“…Such a model can well explain the observed break in the CR spectrum, but remains only a phenomenological description, lacking of a more deep physical explanation of the halo's structure. A more physically motivated scenario has been put forward by Blasi et al(2012), Aloisio & Blasi (2013) and Aloisio et al(2015), based on the idea that the magnetic turbulence could have two distinct components: one self-generated by the same CRs through the streaming instability and one resulting from the Kolmogorov cascade of turbulence injected at larger scale (presumably by SN explosions). In this scenario the diffusion is dominated at low energies by self-generated waves, while at larger energies, where the CR energy density becomes too low to generate a significant amount of waves, the Kolmogorov turbulence dominates and the spectral break in the CR spectrum should mark the energy where this transition occurs.…”

Anomalous Galactic Cosmic Rays in the Framework of AMS-02

“…Despite recent broad studies of galactic CR transport (e.g., Tomassetti 2012; Aloisio et al 2015), there are still difficulties to understand the spectral features of CRs, as well as their origins, acceleration mechanisms, and propagation. These long-standing problems are described in Schlickeiser (2002) and references therein.…”

“…Current theoretical models have explained these puzzling spectral features as source effect scenarios caused by different acceleration mechanisms (Biermann et al 2010) or different populations of CR sources (Yuan et al 2011). Moreover, there are other theoretical attempts to explain the observed upturn in the CR spectra based on the spatial change of CR diffusion properties of several sources in the Galaxy (see Tomassetti 2012;Evoli & Yan 2014;Aloisio et al 2015).…”