Cosmic ray propagation in the galaxy and the heliosphere

Abstract

The local interstellar spectra (LIS’s) for cosmic rays (CRs) are still not fully determined over all energy ranges. Numerical modelling, such as with the GALPROP code, can be used to calculate LIS’s for a wide set of CR species. At low energies the LIS’s need to match the Voyager 1 (V1) observations made beyond the heliopause, while at higher energies very precise CR spectra are observed at the Earth by experiments such as PAMELA. To directly compare the observations at the Earth to the calculated LIS’s below at least 20 GeV, a comprehensive 3D modulation code is required to calculate the effect of solar modulation. This study uniquely aimed to implement the above numerical models and observations to produce LIS’s for electrons, positrons and protons simultaneously, while also considering CR Helium, Carbon, Boron and Oxygen. With the plain diffusion model of the GALPROP propagation code, LIS’s were calculated to match the two reported V1 electron spectra. Similarly the proton and Helium observations were matched, but Carbon and B/C observations necessitated the use of the GALPROP reacceleration model. With the reacceleration model the observed spectra could be matched, while also reproducing the B/C ratio much better than the plain diffusion model. The LIS’s were all tested against the corresponding observations at Earth by using the 3D modulation code. The positron LIS was investigated, but the LIS’s corresponding to the electron and proton results were not promising, neither for the GALPROP plain diffusion nor reacceleration models. To improve the positron LIS and the e+/e− ratio, while also taking into account electron and proton LIS’s, a single model was explored as positrons are secondary products related to nuclei, but propagate similarly to electrons. The initial tests did not improve the positron LIS as intended and as such a GALPROP reacceleration model that also includes convection, was tested by systematically adjusting the source and diffusion parameters. This resulted in the calculated electron LIS and the e+/e− ratio both matching the observations well. With this model LIS’s for electrons, positrons, protons, Helium, Carbon, Boron and Oxygen could all be calculated and shown to match the relevant observations well. A GALPROP plain diffusion model was sufficient when studying electrons, protons and Helium LIS’s separately, but including Carbon, the B/C ratio and positrons into the study, the constraints placed on the LIS’s by observations necessitate the use of a reacceleration model and ultimately the inclusion of convection. The inclusion of positrons proved the greatest challenge, indication that GALPROP in general is not yet optimally suited to calculate positron LIS’s, which may be the case for all secondary CR particles.