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

Abstract

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.

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