GALACTIC COSMIC RAYS IN THE LOCAL INTERSTELLAR MEDIUM: OBSERVATIONS AND MODEL RESULTS.

Since 2012 August has been observing the local interstellar energy spectra of Galactic cosmic-ray nuclei down to 3 MeV nuc and electrons down to 2.7 MeV. The H and He spectra have the same energy dependence between 3 and 346 MeV nuc, with a broad maximum in the 10-50 MeV nuc 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, consistent with models having no local interstellar gradient. The energy spectrum of electrons ( + ) with 2.7-74 MeV is consistent with 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 and electrons with >3 MeV is 0.83-1.02 eV cm and the ionization rate of atomic H is in the range of 1.51-1.64 × 10 s. 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.