Galactic Cosmic-Ray Anisotropies: Voyager 1 in the Local Interstellar Medium

TL;DR

This paper reports on Voyager 1 observations of cosmic ray anisotropies in the local interstellar medium, revealing a broad, shallow depletion region likely caused by magnetic trapping and cooling effects near the heliopause.

Contribution

It provides the first detailed characterization of cosmic ray anisotropies and depletion regions beyond the heliopause using Voyager 1 data.

Findings

Up to 3.8% reduction in cosmic ray intensity observed.

Identification of a broad, shallow depletion region centered at 90° pitch angle.

Evidence suggests magnetic trapping and cooling influence the anisotropy.

Abstract

Since crossing the heliopause on August 25, 2012, Voyager 1 observed reductions in galactic cosmic ray count rates caused by a time-varying depletion of particles with pitch angles near 90-deg, while intensities of particles with other pitch angles remain unchanged. Between late 2012 and mid-2017, three large-scale events occurred, lasting from ~100 to ~630 days. Omnidirectional and directional high-energy data from Voyager 1's Cosmic Ray Subsystem are used to report cosmic ray intensity variations. Omnidirectional (greater than ~20 MeV) proton-dominated measurements show up to a 3.8% intensity reduction. Bi-directional (greater than ~70 MeV) proton-dominated measurements taken from various spacecraft orientations provide insight about the depletion region's spatial properties. We characterize the anisotropy as a "notch" in an otherwise uniform pitch-angle distribution of varying depth and width centered about 90 degrees in pitch angle space. The notch averages 22-deg wide and 15% deep - signifying a depletion region that is broad and shallow. There are indications that the anisotropy is formed by a combination of magnetic trapping and cooling downstream of solar-induced transient disturbances in a region that is also likely influenced by the highly compressed fields near the heliopause.