There have been many new measurements of primary cosmic rays in the past 25 years over the whole energy range from around a GeV to above 100 EeV (1020 eV). Figure 2.1 is a global overview of the whole range of data from some of these experiments. A remarkable feature of the cosmic ray spectrum is the fact that it can be described by inverse power laws over large intervals of energy. Theoretical understanding of this fact (or lack of it) will be the subject of Chapters 9–12 on propagation, acceleration and sources. For now we simply note that the global spectrum can be divided into four regions. From 10 GeV to 1 PeV (1015 eV) the differential spectral index is α ≈ −2.7. From 10 PeV to 1 EeV (1018 eV) it is −3.1. Above 10 EeV the spectrum again flattens somewhat to α −2.6, and then it apparently cuts off around 1020 eV. Below 10 GeV the spectrum locally is modified by solar modulation from the interstellar index of α ≈ −2.7, as illustrated in Figure 1.5. The transition regions are known as the “knee” (∼ 3 PeV) and the “ankle” (∼ 3 EeV). The former is usually assumed to signal in some way the approaching end of the spectrum of galactic cosmic accelerators, while the ankle is sometimes associated with the emergence of particles of extragalactic origin. This picture is not final, and there are important hints of finer structure within the main regions that we will discuss.

Antiprotons and positrons are included on Figure 2.1 even though they are mostly (if not entirely) “secondary” in the sense that they are produced by collisions of “primary” cosmic ray nuclei during propagation in the interstellar medium. Positrons and antiprotons are therefore discussed in Chapter 11 following the introduction to cosmic ray propagation in Chapters 9 and 10. Although most electrons are “primary” in the sense of coming from cosmic ray acceleration sources, their spectra are significantly affected by propagation, so the discussion of electrons is also postponed until Chapter 11.

Fluxes of primary cosmic ray protons and nuclei are the starting point for all the topics of this book. On the one hand, the incident cosmic rays, by their interactions, generate the atmospheric hadrons, muons and neutrinos that reach the surface of the Earth.