Propagation of Cosmic‐Ray Nucleons in the Galaxy

TLDR

The study presents a numerical method for computing the propagation of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. The method incorporates realistic interstellar gas and radiation field distributions, accounts for fragmentation, energy losses, and diffusive reacceleration, calibrates to observed B/C and 10Be/9Be ratios, sets an upper limit on convection velocity gradients, and provides full numerical solution details. The results show that diffusion–convection models poorly reproduce the B/C energy dependence, whereas reacceleration models fit it well, yielding halo heights >4 kpc for diffusion/convection and 4–12 kpc for reacceleration, and indicating a broader radial distribution of cosmic‑ray sources than current supernova remnant estimates.

Abstract

We describe a method for the numerical computation of the propagation of primary and secondary nucleons, primary electrons, and secondary positrons and electrons. Fragmentation and energy losses are computed using realistic distributions for the interstellar gas and radiation fields, and diffusive reacceleration is also incorporated. The models are adjusted to agree with the observed cosmic-ray B/C and 10Be/9Be ratios. Models with diffusion and convection do not account well for the observed energy dependence of B/C, while models with reacceleration reproduce this easily. The height of the halo propagation region is determined, using recent 10Be/9Be measurements, as >4 kpc for diffusion/convection models and 4-12 kpc for reacceleration models. For convection models we set an upper limit on the velocity gradient of dV/dz < 7 km/s/kpc. The radial distribution of cosmic-ray sources required is broader than current estimates of the SNR distribution for all halo sizes. Full details of the numerical method used to solve the cosmic-ray propagation equation are given.