Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using PICARD

TL;DR

This study explores how spiral-arm distributions of cosmic-ray sources affect cosmic-ray flux and composition across the Galaxy, revealing significant spatial variations and implications for modeling cosmic-ray propagation and anisotropies.

Contribution

It introduces a detailed 3D spiral-arm source model using PICARD, highlighting differences in primary and secondary cosmic-ray distributions compared to axisymmetric models.

Findings

Primary cosmic rays are more confined near sources.

Secondary cosmic rays are more homogeneously distributed.

Spatial variations in secondary/primary ratios are significant.

Abstract

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.