Introduction to the galactic cosmic radiation

Although the astrophysical sites for the cosmic radiations and their acceleration mechanisms are still not firmly established, this radiation has turned out to be a vital contributor to our understanding of high energy phenomena in our galaxy. For example, the cosmic rays constitute approximately one-third of the energy density of the interstellar medium and, on a galactic scale, they form a relativistic gas whose pressure is important to take into account in the dynamics of galactic magnetic fields. The electronic component is a source of bremsstrahlung and synchrotron radiation which reveals the configurations and intensities of large scale galactic and stellar magnetic fields. The cosmic ray nuclei are the only direct and measurable sample of matter from outside the solar system. It is a unique sample since it includes all of the elements from Hydrogen up to the actinides. The isotopic abundances and elemental abundances of these nuclei convey not only the 'imprint' of their nucleosynthesis origins at astrophysical sites in the galaxy, but also, through their radioactive species, reveal the time required for their acceleration and the history of their propagation in interstellar magnetic fields. (orig.)