We study the impact of possible spiral-arm distributions of Galactic
cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our
Galaxy. We investigate model cosmic-ray spectra at the nominal position of the
sun and at different positions within the Galaxy. The modelling is performed
using the recently introduced numerical cosmic ray propagation code
\textsc{Picard}. Assuming non-axisymmetric cosmic ray source distributions
yields new insights on the behaviour of primary versus secondary nuclei.
We find that primary cosmic rays are more strongly confined to the vicinity
of the sources, while the distribution of secondary cosmic rays is much more
homogeneous compared to the primaries. This leads to stronger spatial variation
in secondary to primary ratios when compared to axisymmetric source
distribution models. A good fit to the cosmic-ray data at Earth can be
accomplished in different spiral-arm models, although leading to decisively
different spatial distributions of the cosmic-ray flux. This results in very
different cosmic ray anisotropies, where even a good fit to the data becomes
possible. Consequently, we advocate directions to seek best fit propagation
parameters that take into account the higher complexity introduced by the
spiral-arm structure on the cosmic-ray distribution. We specifically
investigate whether the flux at Earth is representative for a large fraction of
the Galaxy. The variance among possible spiral-arm models allows us to quantify
the spatial variation of the cosmic-ray flux within the Galaxy in presence of
non-axisymmetric source distributions.Comment: 38 pages, 16 figures, accepted for publication in Astroparticle
Physic
