Cosmic-ray diffusion in a sectored magnetic field in the distant heliosheath

TL;DR

This paper investigates how a sectored magnetic field topology in the heliosheath allows cosmic rays to diffuse efficiently, explaining high cosmic-ray intensities near the heliopause through a combined drift and diffusive transport mechanism.

Contribution

It introduces a model of cosmic-ray diffusion in a sectored magnetic field, highlighting the role of a permeable modulation wall and stochastic sector variations in cosmic-ray transport.

Findings

Cosmic-ray transport is predominantly diffusive when sector width variations are considered.

The diffusion coefficients depend on sector structure and particle energy.

A test-particle model demonstrates efficient radial and latitudinal cosmic-ray diffusion.

Abstract

Very high intensities of galactic cosmic rays measured by Voyager 1 in the heliosheath appear to be incompatible with the presence of a modulation "wall" near the heliopause produced by a pile up of the heliospheric magnetic field. We propose that the modulation wall is a structure permeable to cosmic rays as a result of a sectored magnetic field topology compressed by plasma slowdown on approach to the heliopause and stretched to high latitudes by latitudinal flows in the heliosheath. The tightly folded warped current sheet permits efficient cosmic-ray transport in the radial direction via a drift-like mechanism. We show that when stochastic variations in the sector widths are taken into account, particle transport becomes predominantly diffusive both along and across the magnetic sectors. Using a test-particle model for cosmic rays in the heliosheath we investigate the dependence of the diffusion coefficients on the properties of the sector structure and on particle energy.