Cosmic ray anisotropies near the heliopause

Abstract

Context. The Voyager 1 spacecraft became the first man-made probe to cross the heliopause into the local interstellar medium and measure the galactic environment, including charged particle intensities, in situ. Aims. We qualitatively explain the observed anisotropies of galactic and anomalous cosmic rays in the interstellar medium. Methods. A pitch-angle-dependent numerical model was constructed and applied to the study of both heliospheric (anomalous cosmic rays and termination shock particles) and galactic cosmic rays near the heliopause region. Results. In accordance with the observations, the model is able to reproduce the observed anisotropic nature of both particle populations. In the interstellar medium, the heliospheric particle distribution shows a peak at pitch angles near 90°, while for galactic particles, their distribution shows a deficiency at these pitch-angle values. Conclusions. The observed anisotropies are related to the pitch-angle dependence of the perpendicular diffusion coefficient, and if this dependence is chosen appropriately, the anisotropies observed by Voyager 1 can be explained naturally.