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 similar to 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 similar to 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 x 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.