Spatial origin of Galactic cosmic rays in diffusion models - I. Standard sources in the Galactic disk

The propagation of Galactic cosmic ray nuclei having energies between 100 MeV/nuc and several PeV/nuc is strongly believed to be of diffusive nature. The particles emitted by a source located in the disk do not pervade the whole Galaxy, but are rather confined to a smaller region whose spatial extension is related to the height of the diffusive halo, the Galactic wind and the spallation rate. Following the pioneering work of Jones (1978), this paper presents a general study on the spatial origin of cosmic rays, with a particular attention to the role of spallations and Galactic wind. This question is different, and to a certain extent disconnected, from that of the origin of cosmic rays. We find the regions of the disk from which a given fraction of cosmic rays detected in the solar neighborhood were emitted (f-surfaces). After a general study, we apply the results to a realistic source distribution, with the propagation parameters obtained in our previous systematic analysis of the observed secondary-to-primary ratios (Maurin et al. 2002a). The shape and size of these f-surfaces depend on the species as well as on the values of the propagation parameters. For some of the models preferred by our previous analysis (i.e. large diffusion slope delta), these f-surfaces are small and in some extreme cases only a fraction of a percent of the whole Galactic sources actually contribute to the solar neighborhood cosmic ray flux. Moreover, a very small number of sources may be responsible for more than 15% of the flux detected in the solar neighborhood. This may point towards the necessity to go beyond the approximations of both homogeneity and stationarity. Finally, the observed primary composition is dominated by sources within a few kpc.