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

Cummings, A. C. and Stone, E. C. and Heikkila, B. C. and Lal, N. and Webber, W.R. and Jóhannesson, G. and Moskalenko, I. V. and Orlando, E. and Porter, T. A. (2016) Galactic Cosmic Rays in the Local Interstellar Medium: Voyager 1 Observations and Model Results. Astrophysical Journal, 831 (1). Art. No. 18. ISSN 0004-637X. PMCID PMC8507217. doi:10.3847/0004-637X/831/1/18.

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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.

Item Type:Article
Related URLs:
URLURL TypeDescription CentralArticle
Cummings, A. C.0000-0002-3840-7696
Stone, E. C.0000-0002-2010-5462
Webber, W.R.0000-0001-9238-9145
Jóhannesson, G.0000-0003-1458-7036
Orlando, E.0000-0001-6406-9910
Additional Information:© 2016. The American Astronomical Society. Received 2016 March 29; revised 2016 April 21; accepted 2016 April 22; published 2016 October 21. We appreciate discussions with Mark Wiedenbeck and George Gloeckler. Voyager data analysis is supported by NASA Grant NNN12AA01C. GALPROP development is supported by NASA Grants NNX13AC47G, NNX10AE78G, NNX16AF27G, and NNX15AU79G.
Group:Space Radiation Laboratory
Funding AgencyGrant Number
Subject Keywords:cosmic rays; ISM: abundances; ISM: clouds
Issue or Number:1
PubMed Central ID:PMC8507217
Record Number:CaltechAUTHORS:20161021-094336650
Persistent URL:
Official Citation:A. C. Cummings et al 2016 ApJ 831 18
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:71334
Deposited By: George Porter
Deposited On:21 Oct 2016 17:04
Last Modified:09 Feb 2022 00:10

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