Transport of cosmic rays across the heliopause

Abstract:

The heliopause (HP) is a boundary that separates the flow with embedded magnetic field of solar origin in the inner heliosheath from that of the interstellar origin in the outer heliosheath. According to the theory of ideal MHD, it should be a tangential discontinuity, but magnetic reconnection or instability can make it more complicated. Voyager 1 crossed the HP in August 2012 at a radial distance of 122 AU from the Sun. The behaviors of Galactic cosmic rays (GCR) and anomalous cosmic rays (ACR) at the HP crossing are very complex. The intensity of GCR experiences step-like increases to reach a nearly steady interstellar level in the outer heliosheath. Its angular distribution changes from isotropic inside the HP to bidirectional anisotropy that appear on and off for several periods of time in the outer heliosheath. The ACR intensity experiences several episodes of decreases near the HP before it eventually disappears. The anisotropy of ACR in the partial depression regions is pancake-like, indicating there is some temporary trapping of particles of near-90$^\circ$ pitch angles. The information has provided us clues for understanding the properties of particle transport in the turbulence of the interstellar magnetic field. In this paper, we review results of model calculations of GCR and ACR transport across the HP. With the observations and modeling results, we can now establish constraints on the properties of particle scattering, diffusion, and interstellar magnetic field turbulence level.