Elemental cosmic ray spectra reveal two populations of Galactic sources and an immediate transition to an extragalactic component after the knee

Abstract

The energy spectra for individual elements and/or for groups of elements in cosmic rays (CR) in the energy range between 100 $\times$ Z GeV and 10$^{3}$ $\times$ Z PeV (where Z is the charge number of the nucleus) have a number of features, including two steepenings ("knees") with the rigidity-dependent energies $E_{k1} \approx$ 15 $\times$ Z TeV and $E_{k2} \approx$ 3 $\times$ Z PeV and three hardenings ("ankles") at $E_{a1} \approx$ 500 $\times$ Z GeV; for protons $E_{a2-p} \approx$ 150 TeV and $E_{ a3-p} \approx$ 50 PeV. While the values of $E_{a1}$ for different nuclei are rigidity-dependent, the values of $E_{a2}$ (and probably of $E_{a3}$) are not: $E_{a2-He} \approx$ 1 PeV for Helium. The recent advances in precision measurements of the elemental CR spectra in the DAMPE and LHAASO experiments, and, to some extent, in IceTop and other experiments, make it possible, for the first time, to clarify the origin of the aforementioned spectral features. We show that the elemental CR spectra are reasonably well described with a sum of three components: 1) a low-energy Galactic component with a convex spectral shape reflecting the accelerated particle spectrum in the source; this component peters out after the TeV knee, 2) a high-energy Galactic component including the PeV knee, and 3) an extragalactic component. There is no need for any third, additional component of Galactic cosmic rays in the energy range between 10 PeV and 1 EeV.