Effects of Magnetic Boundary Layer on Galactic Cosmic Rays during the Heliopause Crossing for Voyager 2

Abstract Voyager 2 crossed the heliopause in 2018 and observed a small-amplitude modulation of galactic cosmic rays beyond the heliopause, within a boundary layer approximately 0.57 au thick. Prior to this, it detected a similar boundary layer for both the cosmic rays and the magnetic field, with a scale of about 0.82 au inside the heliopause. In this study, we investigate the modulation of cosmic rays near the heliopause using numerical simulations that account for magnetic boundary layers on both sides of the heliopause. Our results indicate that the boundary layer beyond the heliopause leads to a significant reduction in the parallel diffusion coefficient for the cosmic rays, compared to values further into interstellar space, thereby causing modulation of cosmic rays beyond the heliopause. Inside the heliopause, the boundary layer reduces the perpendicular diffusion coefficient and potentially increases the parallel diffusion coefficient, relative to regions deep inside the heliosphere. This results in a steady increase in intensities of cosmic rays until a sudden jump at the heliopause. Our findings confirm that the presence of the boundary layer near the heliopause is crucial for explaining the observations of cosmic rays made by Voyager 2.