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Published August 2015 | Published
Book Section - Chapter Open

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


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.

Additional Information

Copyright owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-ShareAlike. Voyager data analysis is supported by NASA Grant NNN12AA012. GALPROP development is supported by NASA Grants NNX13AC47G and NNX10AE78G.

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