On transiton from galactic to extragalactic cosmic rays

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Abstract
The transition from galactic to extragalactic cosmic rays is discussed. One of critical indications for transition is given by the Standard Model of Galactic cosmic rays, according to which the maximum energy of acceleration for iron nuclei is of order of Emax Fe � 1× 1017 eV. At E > Emax Fe the spectrum is predicted to be very steep and thus the Standard Model favours the transition at energy not much higher than Emax Fe . As observations are concerned there are two signatures of transition: change of energy spectra and elongation rate (depth of shower maximum in the atmosphere Xmax as function of energy). Three models of transition are discussed: dip-based model, mixed composition model and ankle model. In the latter model the transition occurs at the observed spectral feature, ankle, which starts at Ea � 1×1019 eV and is characterised by change of mass compostion from galactic iron to extragalactic protons. In the dip model the transition occures at the second knee observed at energy (4 8) × 1017 eV and is characterised by change of mass composition from galactic iron to extragalactic protons. The mixed composition model describes transition at E � 3 × 1018 eV with mass composition changing from galactic iron to extragactic mixed composition of different nuclei. These models are confronted with observational data on spectra and elongation rates from different experiments, including Auger.