Transition from galactic to extragalactic cosmic rays

TL;DR

This paper reviews models of the transition from galactic to extragalactic cosmic rays, analyzing their predictions against observational data on spectra and composition changes around 10^17 to 10^19 eV.

Contribution

It compares three different transition models—dip, mixed composition, and ankle—and evaluates their consistency with recent observational data.

Findings

The ankle model suggests transition at ~10^19 eV with composition change to extragalactic protons.

The dip model indicates transition at the second knee (~4-8×10^17 eV) with a shift to extragalactic protons.

The mixed composition model proposes transition around 3×10^18 eV with a mixed nuclei composition.

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 $E_{\rm Fe}^{\rm max} \approx 1\times 10^{17}$ eV. At $E > E_{\rm Fe}^{\rm max}$ the spectrum is predicted to be very steep and thus the Standard Model favours the transition at energy not much higher than $E_{\rm Fe}^{\rm max}$. As observations are concerned there are two signatures of transition: change of energy spectra and elongation rate (depth of shower maximum in the atmosphere $X_{\rm max}$ 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 $E_a \approx 1\times 10^{19}$ 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 $(4 -8)\times 10^{17}$ eV and is characterised by change of mass composition from galactic iron to extragalactic protons. The mixed composition model describes transition at $E \sim 3\times 10^{18}$ 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.