Abstract
The observed spectra of Cosmic Rays (CRs) depend on two basic processes: the propagation in the interstellar medium of our Galaxy, described in this chapter, and the acceleration in the astrophysical sources. Upon leaving the source regions, high-energy charged particles diffuse in the random galactic magnetic field that accounts for their high isotropy and relatively long confinement time. The galactic diffusion model explains the observations on energy spectra, composition, and anisotropy of CRs. It also provides a basis for the interpretation of radio, X-ray, and γ-ray measurements, since a continuous radiation with a non-thermal spectrum is produced during propagation by the energetic electrons, protons, and nuclei. As discussed in this chapter, relevant information on CR propagation arise from the measurements of the abundances of some particular nuclei: the so-called light elements Li, Be, and B. Light elements are mainly of secondary origin, i.e., produced as the result of interactions of heavier primary nuclei with interstellar matter. We use the observed ratio between light and medium elements to assess an analytic description of the CR propagation and a first-order estimate of their escape time from our Galaxy. Electrons, as the lightest stable-charged particles, are subject to additional energy loss mechanisms with respect to protons and nuclei. The presence of magnetic fields induces synchrotron emission, which produces intense electromagnetic radiation in the proximity of the electron accelerators. In addition, a diffuse emission is produced during electron propagation in the galactic disk. Therefore, severe limits on the electron energy spectrum and on the distance of CR electron sources can be derived.
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Notes
- 1.
“Spallation” refers to inelastic nuclear reactions that occur when energetic particles interact with an atomic nucleus. Cosmic ray physicists usually refer to reactions induced by cosmic rays as “fragmentation”. For our practical purposes, the two words are synonymous.
- 2.
In the literature, this amplitude is usually denoted with δ (lower case). We adopt the upper case Δ to avoid confusion with the diffusion parameter defined above and with the declination, also used in this chapter.
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Spurio, M. (2018). Diffusion of Cosmic Rays in the Galaxy. In: Probes of Multimessenger Astrophysics. Astronomy and Astrophysics Library. Springer, Cham. https://doi.org/10.1007/978-3-319-96854-4_5
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