Cosmic Ray Electrons and Protons ~1 MeV - A 40 Year Study of Their Intensities from the Earth to the Heliopause and Beyond Into Local Interstellar Space by the CRS Experiment on Voyager 1

TL;DR

This 40-year Voyager 1 study reveals how cosmic ray electrons and protons at ~1 MeV energies vary from Earth to interstellar space, highlighting the heliosheath as a source and the effects of solar modulation.

Contribution

It provides new insights into the radial dependence of low-energy cosmic rays and identifies the heliosheath as a significant source of sub-MeV electrons, with detailed observations beyond the heliopause.

Findings

Heliosheath is a source of sub-MeV electrons.

Solar modulation significantly affects MeV electrons.

Low-energy protons are part of the galactic cosmic ray spectrum.

Abstract

Studies on Voyager 1 using the CRS instrument have shown the presence of sub-MeV electrons in the interstellar medium beyond the heliopause. We believe that these electrons are the very low energy tail of the distribution of galactic GeV cosmic ray electrons produced in the galaxy. If so this observation places constraints on the origin and possible source distribution of these electrons in the galaxy. The intensities of these electrons as well as MeV protons and other higher energy electrons and nuclei have been followed outward from the Earth to beyond the heliopause during the 40 years of the Voyager mission. Among the other new features found in this study of the radial dependence of the electron intensity in the heliosphere are: 1. The heliosheath is a source of sub-MeV electrons as well as the already known anomalous cosmic rays of MeV and above, none of which appear to escape from the heliosphere because of an almost impenetrable heliopause at these lower energies; 2. Solar modulation effects are observed for these MeV electrons throughout the heliosphere. These modulation effects are particularly strong for electrons in the heliosheath and comprise over 90 percent of the observed intensity change of these electrons of 10-60 MeV between the Earth and the heliopause. Even for nuclei of 1 GV in rigidity, over 30 percent of the total intensity difference between the Earth and the LIM occurs in the heliosheath; 3. The 2 MeV protons studied here for the first time beyond the heliopause are also part of the low energy tail of the spectrum of galactic cosmic ray protons, similar to the tail noted above for sub MeV galactic cosmic ray electrons.