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. 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.

Imprints of cosmic rays in multifrequency observations of the interstellar emission

“…(i) PDDE: We adopt the hadronic best-fit CRs injection spectra and propagation parameters from the very recent work by Cummings et al (2016). This corresponds to their plain diffusion model.…”

“…Additional details on the fitting technique for the hadronic and isotopes are described in the appendix of Cummings et al (2016). A GALPROP plain diffusion model (and a diffusivereacceleration model presented below as DRE model) with standard propagation parameters shows good agreement with Voyager 1 measurements of CR species from H to Ni in the energy range 10 -500 MeV/nucleon (Cummings et al 2016). The reason of such an agreement may be the absence of a recent source of low-energy CR hadrons in the solar system neighborhood (Cummings et al 2016).…”

“…Very recent Fermi electron measurements (Abdollahi et al 2017) are found in agreement with AMS-02 data. Further recent CR measurements by Cummings et al (2016) are performed with Voyager 1 (Stone et al 1977). Launched in 1977, Voyager 1 has reached interstellar space, providing measurements of CRs beyond the influence of the solar modulation.…”

Propagation of cosmic rays in the AMS-02 era

“…However, as shown in Ref. [83], the very low en- ergy ( 50 MeV/n) B/C spectrum measured by Voyager-1 is difficult to be modelled in various models. Further tuning of the modelling and/or better understanding about the measurements may be necessary.…”

Inside out: unveiling local interstellar spectra of cosmic ray species

“…These advances are built on solid results of earlier missions, such as ACE/CRIS, SuperTIGER, ATIC, BESS, CAPRICE, CREAM, HEAO-3, HEAT, ISOMAX, TIGER, TRACER, Ulysses, and many others. Launched in 1977 Voyager 1, 2 spacecrafts are providing unique data on the elemental spectra and composition at the interstellar reaches of the Solar system [36,37]. Other high-expectations missions are recently launched (CALET and DAMPE) or are awaiting for launch (ISS-CREAM).…”