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Title: Spectrum and ionization rate of low-energy Galactic cosmic rays
Authors: Nath, Biman B.
Gupta, Nayantara
Biermann, P.L.
Keywords: ISM
cosmic rays
Issue Date: Sep-2012
Publisher: Royal Astronomical Society
Citation: Monthly Notices of the Royal Astronomical Society, 2012, Vol.425, pL86
Abstract: We consider the rate of ionization of diffuse and molecular clouds in the interstellar medium by Galactic cosmic rays (GCRs) in order to constrain its low-energy spectrum. We extrapolate the GCR spectrum obtained from PAMELA at high energies (≥200 GeV nucleon-1) and a recently derived GCR proton flux at 1-200 GeV from observations of gamma-rays from molecular clouds, and find that the observed average Galactic ionization rate can be reconciled with this GCR spectrum if there is a low-energy cut-off for protons at 10-100 MeV. We also identify the flattening below a few GeV as being due to (a) decrease of the diffusion coefficient and dominance of convective loss at low energy and (b) the expected break in energy spectrum for a constant spectral index in momentum. We show that the inferred CR proton spectrum of ? for Ekin≤ few GeV is consistent with a power-law spectrum in momentum p-2.45± 0.4, which we identify as the spectrum at source. Diffusion loss at higher energies then introduces a steepening by E-α with α˜ 1/3, making it consistent with high-energy measurements.
Description: Restricted Access. An open-access version is available at (one of the alternative locations)
ISSN: 0035-8711
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Copyright: 2012 Royal Astronomical Society
Appears in Collections:Research Papers (A&A)

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