The Galactic Cosmic Ray Electron Spectrum from 3 to 70 MeV Measured by Voyager 1 Beyond the Heliopause, What This Tells Us About the Propagation of Electrons and Nuclei In and Out of the Galaxy at Low Energies

TL;DR

Voyager 1's measurements of low-energy cosmic ray electrons reveal a steep spectrum and a rapid galactic outflow, providing insights into electron propagation and a potential dark energy component in intergalactic space.

Contribution

This study introduces a propagation model that explains the electron spectrum from 3 MeV to 30 GeV using a diffusion coefficient with a rigidity-dependent power law, linking low and high energy behaviors.

Findings

Voyager 1 detects electron intensities hundreds of times higher than Earth-based measurements.

A diffusion coefficient with P^0.45 above 0.5 GeV and P^-1.00 below 0.5 GeV fits the data.

Low-energy electrons form a dark energy component 100 times their rest energy.

Abstract

The cosmic ray electrons measured by Voyager 1 between 3-70 MeV beyond the heliopause have intensities several hundred times those measured at the Earth by PAMELA at nearly the same energies. This paper compares this new V1 data with data from the earth-orbiting PAMELA experiment up to energies greater than 10 GeV where solar modulation effects are negligible. In this energy regime we assume the main parameters governing electron propagation are diffusion and energy loss and we use a Monte Carlo program to describe this propagation in the galaxy. To reproduce the new Voyager electron spectrum, which is E-1.3, together with that measured by PAMELA which is E-3.20 above 10 GeV, we require a diffusion coefficient which is P 0.45 at energies above 0.5 GeV changing to a P-1.00 dependence at lower rigidities. The entire electron spectrum observed at both V1 and PAMELA from 3 MeV to 30 GeV can then be described by a simple source spectrum, dj/dP P-2.25, with a spectral exponent that is independent of rigidity. The change in exponent of the measured electron spectrum from -1.3 at low energies to 3.2 at the highest energies can be explained by galactic propagation effects related to the changing dependence of the diffusion coefficient below 0.5 GeV, and the increasing importance above 0.5 GV of energy loss from synchrotron and inverse Compton radiation, which are both E2, and which are responsible for most of the changing spectral exponent above 1.0 GV. As a result of the P-1.00 dependence of the diffusion coefficient below 0.5 GV that is required to fit the V1 electron spectrum, there is a rapid flow of these low energy electrons out of the galaxy. These electrons in local IG space are unobservable to us at any wave length and therefore form a dark energy component which is 100 times the electrons rest energy.