Galactic Cosmic Rays in the Local Interstellar Medium: Voyager 1 Observations and Model Results

Since 2012 August Voyager 1 has been observing the local interstellar energy spectra of Galactic cosmic-ray nuclei down to 3 MeV nuc^-1 and electrons down to 2.7 MeV. The H and He spectra have the same energy dependence between 3 and 346 MeV nuc^-1, with a broad maximum in the 10-50 MeV nuc^-1 range and a H/He ratio of 12.2 ± 0.9. The peak H intensity is ∼15 times that observed at 1 AU, and the observed local interstellar gradient of 3-346 MeV H is -0.009 ± 0.055% AU^-1, consistent with models having no local interstellar gradient. The energy spectrum of electrons (e ^- + e ⁺) with 2.7-74 MeV is consistent with E ^-1.30±0.05 and exceeds the H intensity at energies below ∼50 MeV. Propagation model fits to the observed spectra indicate that the energy density of cosmic-ray nuclei with >3 MeV nuc^-1 and electrons with >3 MeV is 0.83-1.02 eV cm^-3 and the ionization rate of atomic H is in the range of 1.51-1.64 × 10^-17 s^-1. This rate is a factor >10 lower than the ionization rate in diffuse interstellar clouds, suggesting significant spatial inhomogeneity in low-energy cosmic rays or the presence of a suprathermal tail on the energy spectrum at much lower energies. The propagation model fits also provide improved estimates of the elemental abundances in the source of Galactic cosmic rays.