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.