The general features of the cosmic-ray spectrum have been known for a long time. Although the basic approaches to understanding cosmic-ray propagation and acceleration have also been well understood for many years, there are several questions of great interest that motivate the current intense experimental activity in the field. If the energy-dependence of the secondary to primary ratio of gala...

The isotropic component of the γ-ray flux between 100MeV and 100EeV (10 eV) contains important information about the origin of highest energy cosmic rays and the extragalactic magnetic field (EGMF). Using numerical simulations of extragalactic cosmic and γ-ray propagation we show that an average fraction of ≃ 10% γ-rays in the total cosmic ray flux around 10EeV would imply both a non-accelerati...

The problem of cosmic ray propagation is briefly discussed.

We have studied the impact of cosmic-ray acceleration in SNR on the spectra of cosmic-ray nuclei in the Galaxy using a series expansion of the propagation equation, which allows us to use analytical solutions for part of the problem and an efficient numerical treatment of the remaining equations and thus accurately describes the cosmic-ray propagation on small scales around their sources in thr...

Observations of cosmic-ray helium energy spectra provide important constraints on cosmic ray origin and propagation. However, helium intensities measured at Earth are affected by solar modulation, especially below several GeV/nucleon. Observations of helium intensities over a solar cycle are important for understanding how solar modulation affects galactic cosmic ray intensities and for separat...

As ultra-high energy cosmic ray protons propagate through the universe, they undergo photo-meson interactions with the cosmic microwave background, generating the ‘cosmogenic’ neutrino flux. If a substantial fraction of the cosmic ray primaries are heavy nuclei rather than protons, however, they would preferentially lose energy through photo-disintegration, so the corresponding neutrino flux ma...

The cosmic-ray propagation code GALPROP has recently been enhanced in the areas of Galactic magnetic field and synchrotron radiation. A full 3D treatment is now implemented. We describe these enhancements and show applications to the interpretation of non-thermal radio surveys and gamma rays, to probe the Galactic cosmic-ray electron spectrum and magnetic fields.