On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium

TL;DR

This study investigates how galactic cosmic ray anisotropies vary with energy in the very local interstellar medium using Voyager 1 data, revealing a decrease in anisotropy amplitude with increasing energy and suggesting velocity-dependent scattering effects.

Contribution

It provides the first detailed analysis of the energy dependence of GCR anisotropies in the VLISM using Voyager 1 measurements, highlighting the role of velocity-dependent scattering.

Findings

GCR anisotropies decrease in amplitude with increasing energy during episodes.

Higher velocities are associated with stronger pitch-angle scattering.

The results support a velocity-dominated scattering mechanism for GCRs.

Abstract

We report on the energy dependence of galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP instrument includes a dual-ended solid state detector particle telescope mechanically scanning through 360 deg across eight equally-spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1 heliopause (HP) crossing in 2012, however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90{\deg} pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use V1 LECP count rate and pulse height analyzer (PHA) data from >=211 MeV channel together with lower energy LECP channels. Our analysis shows that while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch-angle scattering at the higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.