Galactic propagation of cosmic rays

Abstract

The widely used steady-state, rotational symmetric models (2D models) of cosmic ray (CR) propagation, assume smeared-out sources, which do not necessarily result in the same local CR

ux as the real local point sources. This suggests that the 2D models may not be adequate to describe the CR primary component originating from point-like CR sources. By means of 3D time-dependent calculations, it has been shown that the secondary CR component is not a ected by local point-like sources. When working with 2D models, concentrating on secondary, tertiary and higher CR nuclei may thus yield a better description of galactic CR propagation, as the ux of these nuclei does not depend on the local source history. Taking advantage of this fact and looking at CR primaries and secondaries separately, evidence of nearby CR point sources might be found with a 2D code. Conducting a parameter study, this should be seen in the di erent best t values for CR primaries and secondaries. The 2D version of the GALPROP code was adapted to a compute-cluster environment using the MPI framework and used to perform parameter studies comparing CR spectra with mainly primary and secondary CR data separately. The force eld approximation was implemented to account for heliospheric modulation. At Earth the approximation is valid, as only nuclei are studied and time-dependence is not considered, thus the disadvantages of the force eld are largely avoided. Using the GALPROP code to model CR propagation through the Galaxy, three of the parameters in the 2D plain di usion model were varied in the parameter study: the source spectral index ( ), the spectral index of the di usion coe cient ( ) and the magnitude of the di usion coe cient at particle rigidity 4GV (K0). The LIS produced by the models were compared to experimental CR data by means of a 2 test. For each set of data from di erent experiments, the LIS was inferred using the force eld approximation and the individual CR species were divided up into three groups according to the fraction of secondary and primary nuclei in each. The parameter values for the best t models were found to di er between these Primary, Mixed and Secondary CR component groups. The secondary CRs were found to be more easily t to data than the Primary component or the Mixed component group, implying that the 2D GALPROP model as used is indeed better suited for CR secondaries than for primaries. The results, together with the manner in which the 2D model handles CR sources, imply that there maybe local point sources of CRs that, so far, are not being taken into account. i