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Fully updated for the second edition, this book introduces the growing and dynamic field of particle astrophysics. It provides an overview of high-energy nuclei, photons and neutrinos, including their origins, their propagation in the cosmos, their detection on Earth and their relation to each other. Coverage is expanded to include new content on high energy physics, the propagation of protons and nuclei in cosmic background radiation, neutrino astronomy, high-energy and ultra-high-energy cosmic rays, sources and acceleration mechanisms, and atmospheric muons and neutrinos. Readers are able to master the fundamentals of particle astrophysics within the context of the most recent developments in the field. This book will benefit graduate students and established researchers alike, equipping them with the knowledge and tools needed to design and interpret their own experiments and, ultimately, to address a number of questions concerning the nature and origins of cosmic particles that have arisen in recent research.

In this chapter we summarize measurements of the spectrum and composition of cosmic rays with energies above 100 TeV and the implications for sources. We noted in Chapter 12 that the knee of the cosmic ray spectrum may coincide with the upper limit of shock acceleration by supernova remnants (SNR). We also noted that, whether the knee reflects the maximum energy of a class of accelerators or a rigidity-dependent change in propagation, the composition should change systematically from light to heavy in the knee region. Particles with energies greater than 3×10 18 eV are generally assumed to be from sources outside of the MilkyWay because they show no sign of the anisotropy that would be expected if they came from sources in the Galactic plane. The hardening of the spectrum at the ankle around this energy is often interpreted as the transition from Galactic to extragalactic cosmic rays [638]. If the knee reflects the upper limit of acceleration by SNR with a maximum energy for protons of ≈ 10 15 eV, then the major nuclear groups would follow the Peters cycle in rigidity culminating with E max (Fe) ≈ 3 × 10 16 eV. Interpretation of the ankle as the transition to extragalactic cosmic rays would then require a second kind of Galactic source capable of accelerating protons to ≈ 10 17 eV and iron to ≈ 3 × 10 18 eV. Hillas [639] suggests this possibility and identifies the contribution of unknown origin as Population B. On the other hand, the population of extragalactic cosmic rays may extend down to lower energy, A 10 17 eV, in which case an alternate explanation for the ankle is needed. Berezinsky et al. [308] proposed an extragalactic spectrum dominated by protons to explain the ankle as a pileup effect just below 3 × 10 18 eV, the energy above which losses due to e + e − pair production by protons in the CMB become more important than adiabatic expansion (see Figure 10.2). These conflicting possibilities require a detailed understanding of the spectrum and composition as a function of energy for their resolution.