The variability of the proton cosmic ray flux on the Sun’s way around the galactic center

Abstract

The discovery of direct evidences for the acceleration of high energetic particles at the shell supernova remnant RXJ1713.7-3946 underlined the need to calculate the cosmic ray (CR) distribution in the Galaxy on a spatial grid fine enough to resolve the changes in the CR density due to these kind of objects. It was shown before by Büsching et al. [Büsching, I., Kopp, A., Pohl, M., Schlickeiser, R., Perrot, C., Grenier, I. Cosmic-ray propagation properties for an origin in supernova remnants. ApJ 619, 314–326, 2005] that the discrete nature (both in space and time) of super novae (SN) as sources of Galactic CR leads to CR spectra changing in space and time, resulting in a range of possible CR spectra at a given location in the Galaxy. As the most frequent SN types Ib and II are found within spiral arms, one can expect a significant difference of the range of possible spectra in and outside spiral arms. We investigate the variability of the local interstellar CR proton spectrum during the motion of the Sun in and out of spiral arms in its journey around the Galactic center. Using the code described by Büsching et al. [Büsching, I., Kopp, A., Pohl, M., Schlickeiser, R., Perrot, C., Grenier, I. Cosmic-ray propagation properties for an origin in supernova remnants. ApJ 619, 314–326, 2005], the proton CR density in the Galaxy is calculated with high spatial and temporal resolution (75 pc in galactocentric radius r and azimuth φ at the position of the Sun and 20 pc in z (perpendicular to the galactic plane), with a time step of 1 kyr), assuming stochastically distributed point sources with a probability distribution that resembles the spiral structure of our Galaxy. We find the averaged CR flux outside the spiral arm to be about 50% of that inside a spiral arm. We further find spatial and temporal variations of the CR flux inside spiral arms.