Cosmic ray protons, nuclei and electrons are the only particles which have been detected from sources outside the Solar System. As observed at the top of the atmosphere, about 98% of the particles are protons and nuclei whilst about 2% are electrons. Of the protons and nuclei, about 87% are protons, 12% are helium nuclei and the remaining 1% are heavier nuclei.

Figure 15.1 provides a quick overview of the complete cosmic ray spectrum. A very wide range of energies is observed and the spectrum can be described by power-law distributions over many decades in energy. There are, however, important features in the spectrum, including the ‘knee’ at 1015 eV and the ‘ankle’ at 1018 eV. It is convenient to consider first cosmic rays with energies in the range 109−1015 eV and then those with higher energies. This division corresponds to the different techniques which are used to detect the cosmic rays, particle detectors in space observatories in the lower energy range and the cosmic ray air-shower technique at the higher energies. These are distinguished by the change of symbols in Fig. 15.1, which shows the results of a large number of experiments.

The energy spectra of cosmic ray protons and nuclei

The energy spectra of cosmic rays can be well represented by power-law energy distributions as illustrated in Fig. 1.16, which shows the differential energy spectra for protons, helium, carbon and iron nuclei as a function of the kinetic energy per nucleon of the particles (Simpson, 1983).