Models of the Knee in the Energy Spectrum of Cosmic Rays
  The origin of the knee in the energy spectrum of cosmic rays is an outstanding problem in astroparticle physics. Numerous mechanisms have been proposed to explain the structure in the all-particle spectrum. In the article basic ideas of several models are summarized, including diffusive acceleration of cosmic rays in shock fronts, acceleration via cannonballs, leakage from the Galaxy, interactions with background particles in the interstellar medium, as well as new high-energy interactions in the atmosphere. The calculated energy spectra and mean logarithmic masses are compiled and compared to results from direct and indirect measurements. 
Joerg R. Hoerandel