Cosmic ray propagation for various galactic magnetic field models

Abstract

A survey has been made of the information about the galactic, magnetic field strength and configuration which has been obtained from a variety of astronomical measurements. These measurements comprise the. polarization of starlight, the background synchrotron radiation emitted by relativistic electrons, the Zeeman splitting observed in absorption lines produced by clouds of neutral hydrogen and the Faraday rotation of the plane of the electric, vector of linearly polarized radiation from extragalactic radio sources and. pulsars. Using these data four magnetic field models have been devised to represent the coherent galactic field. These are denoted models A, B, C, and D. Model A describes a field directed along the galactic spiral arms towards galactic longitude ℓ ~ 270º above the galactic plane and towards ℓ ~ 90º below the plane at the Sun. Model D also describes a longitudinal type of field but with no reversal above the galactic plane. The model C field has a helical configuration, while model B is a combination of model C near the Sun and model D at greater distances, A model has also been formulated for the irregular component of the magnetic field. The trajectories of particles with energy above 5. 10(^17) eV have been followed through the coherent galactic field and, using the assumption that the sources are uniformly distributed within the galactic disc or spiral arms predictions are made of the expected anisotropy of high energy cosmic rays of galactic origin reaching the Earth. Measurements of the arrival directions at the Earth of extensive air shower primaries obtained by the Volcano Ranch, Haverah Park and Pilliga Forest experiments are then compared with the arrival direction distribution predicted assuming various percentages of primary particles to be of galactic origin. Assuming that the metagalactic cosmic rays are isotropic upper limits are obtained for the percentage of cosmic rays that could be of galactic origin. Consideration of the irregular field component produces modifications in the values for these upper limits. In general, assuming that the galactic field can be represented by a model such as A or D, together with irregularities, and that all primary cosmic rays are protons, it would appear that most cosmic rays of energy ~6.10(^17) eV to 10(^19) eV must be of metagalactic origin.