Energy spectrum and elemental composition of high energy cosmic rays

In spite of lapse of time of nearly 90 years since the discovery of cosmic rays, the basic questions: "Where are cosmic rays coming from? How are they accelerated to extremely high energies and propagate through the interstellar and intergalactic space?" are largely not clarified. Contemporary theoretical models describe the acceleration of nuclei in the Cosmos by strong shocks, either of galactic or extragalactic origin, which are effectively produced in supernova remnants, supersonic stellar winds, active galactic nuclei and other phenomena. All the models and conjectures towards an explanation of the energy spectrum, in particular of the conspicuous discontinuity ("knee") observed in the energy region of about 3.10(15) eV, do not only predict the shape of the spectrum, they imply also specific variations of the elemental composition of the primary cosmic rays. The lecture discusses the experimental approaches investigating the shape of the primary spectrum and the elemental composition of cosmic rays.