Introduction to Cosmic Rays

Abstract

Energetic particles, traditionally called Cosmic Rays, were discovered nearly a hundred years ago, and their origin is still uncertain. Their main constituents are the normal nuclei as in the standard cosmic abundances of matter, with some enhancements for the heavier elements; there are also electrons, positrons and antiprotons. Today we also have information on isotopic abundances, which show some anomalies, as compared with the interstellar medium. And there is antimatter, but no anti-nuclei. The known spectrum extends over energies from a few hundred MeV to 300 EeV (= 3×10²⁰ eV), and shows few clear spectral signatures: There is a small spectral break near 5 × 10¹⁵ eV, commonly referred to as the knee, where the spectrum turns down; there is another spectral break near 3×10¹⁸ eV, usually called the ankle, where the spectrum turns up again. Up to the ankle the cosmic rays are usually interpreted as originating from supernova explosions, i.e. those cosmic ray particles are thought to be Galactic in origin; however, the details are not clear. We do not know what the origin of the knee is, and what physical processes can give rise to particle energies in the energy range from the knee to the ankle. The particles beyond the ankle have to be extragalactic, it is usually assumed, because the Larmor radii in the Galactic magnetic field are too large; this argument could be overcome if those particles were very heavy nuclei as Fe, an idea which appears to be inconsistent, however, with the airshower data immediately above the energy of the ankle. Due to interaction with the cosmic microwave background (CMB), a relic of the Big Bang, there is a strong cut-off expected near 50 EeV (=5×10¹⁹ eV), which is, however, not seen; this expected cuto. is called the GZK-cuto. after its discoverers, Greisen, Zatsepin and Kuzmin. The spectral index α is near 2.7 below the knee, near 3.1 above the knee, and again near 2.7 above the ankle, where this refers to a differential spectrum of the form E ^-α in numbers. The high energy cosmic rays beyond the GZK-cuto. are the challenge to interpret. We will describe the various approaches to understand the origin and physics of cosmic rays.