# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1905.11990