Voyager 1 Observations of Galactic Cosmic Rays in the Local Interstellar Medium: Energy Density and Ionization Rates

Abstract

Voyager 1 (V1) has been in the local interstellar medium (LISM) since August, 2012. We present the galactic cosmic-ray (GCR) energy spectra of most elements from H through Ni, and also of electrons, for a period exceeding two years. The V1 energy spectra define the newly-revealed, low-energy part of the interstellar spectra of nuclei down to ~3 MeV nuc^(-1) and of electrons down to ~5 MeV. We use a leaky-box model and three GALPROP models of the local interstellar spectra of nuclei, constrained by the V1 observations at low energies and by observations at 1 AU from other missions at higher energies, to estimate the cosmic-ray energy density and the ionization rate of atomic H in the local interstellar medium by nuclei above 3 MeV nuc ^(-1). We use a model of the interstellar electron spectrum that approximately matches the V1 observations to derive the electron contributions to the cosmic-ray energy density and the ionization rate of atomic H above 3 MeV. We find that the total cosmic-ray energy density is in the range 0.82-0.97 eV cm^(-3), which includes a contribution of 0.023 eV cm^(-3) from electrons. We find the cosmic-ray ionization rate of atomic H to be in the range 1.45-1.58 X 10^-17) s^(-1), which is a factor of more than 10 below the cosmic-ray ionization rate in diffuse interstellar clouds based on astrochemistry methods. We discuss possible reasons for this difference.