International School of Cosmic Ray Astrophysics
14th Course: "Neutrinos and Explosive Events in the Universe"
A NATO Advanced Study Institute
2-13 July 2004
Ettore Majorana Centre
Erice, Sicily, Italy
Topic: The Knee in the Energy Spectrum of Cosmic Rays
Lecturer: Joerg R. Hoerandel
The all-particle energy spectrum of cosmic rays follows a power law over manyorders of magnitudes in energy. Around 4 PeV the measurements exhibit a kink in the spectrum, resulting in a change of the spectral index of the power law. This feature is usually called the knee. It's origin has not been clarified and is generally believed to be a cornerstone in the understanding of the origin of high-energy cosmic rays. In this energy region the cosmic ray flux is too low to be measured directly above the atmosphere. Instead, extensive air showers are investigated, generated by high-energy interactions of cosmic ray particles in the atmosphere. The present status of air shower measurements is reviewed, data on the energy spectra and mass composition of cosmic rays are compiled. The data are set in context to predictions of contemporary models to describe the acceleration and propagation of cosmic rays as well as the origin of the knee.