Abstract
The question at which energy the transition from Galactic to extragalactic cosmic rays takes place has been a long-standing conundrum in cosmic ray physics. In the past, the transition energy has been usually associated with one of the evident features of the cosmic ray spectrum: The second knee around E≃5 ×1017 eV or the ankle at E≃3 ×1018 eV. I review anisotropy and composition data and show that they require that the transition from Galacticto extragalactic CRs has to happen early, at few ×1017 eV. As a result, a successful model for the transition should explain the ankle as a feature of the extragalactic CR spectrum. I review briefly such models and their implications.
Highlights
The question at which energy the transition from Galactic to extragalactic cosmic rays (CRs) takes place is one of the major unresolved issues of cosmic ray physics
Its answer is fundamental to our understanding of Galactic CR sources and the requirements on their acceleration mechanisms as well as the nuclear composition and the injection spectrum of extragalactic sources
The main disadvantage of this suggestion is the enormous pressure it puts on acceleration models for Galactic CR sources
Summary
The question at which energy the transition from Galactic to extragalactic cosmic rays (CRs) takes place is one of the major unresolved issues of cosmic ray physics. The two most promising choices for the transition energy were to associate it with one of the evident features of the cosmic ray spectrum: The ankle at E ≃ 3 × 1018 eV or the second knee around E ≃ 5 × 1017 eV The former case offered a simple explanation for the sharpness of the ankle as the cross-over between the end of Galactic flux and the start of the extragalactic component. Knowledge about the high-energy end of the Galactic CR spectrum and its nuclear composition is crucial in addressing the question of the transition to extragalactic CRs. A problem of particular importance is the question, if the second knee corresponds to the proton knee shifted by Z = 26 in energy. The question how strong the Galactic fluxes are suppressed at energies above ZEk are important to determine theoretically the extragalactic fluxes in the transition region and below
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