Galactic propagation of cosmic ray nuclei in a model with an increasing diffusion coefficient at low rigidities: A comparison of the new interstellar spectra with Voyager data in the outer heliosphere

Abstract

Using both a Monte Carlo Diffusion Model and a Leaky Box Model for propagation in the galaxy, we have determined the interstellar spectra of H, He, and heavier nuclei with emphasis on energies below ∼1–2 GeV nucleon−1. This calculation uses diffusion coefficients that are increasing at rigidities below ∼2–3 GV and which are based on those used to fit the interstellar electron spectrum to the galactic nonthermal radio synchrotron spectrum by Webber and Higbie (2008). The resulting interstellar spectra of the cosmic ray nuclei are thus reduced at low energies in a charge‐dependent way from those derived using a diffusion coefficient = constant as was the case in some earlier calculations. The resulting interstellar intensities at ∼200 MeV nucleon−1 near the peak in the differential spectrum have been reduced by factors of 2.00 and 1.46 for H and He nuclei, respectively. These new interstellar intensities are then compared with recent Voyager measurements beyond the heliospheric termination shock. Solar modulation effects, corresponding to values of ϕ between 60 and 80 MV in simple force field models, are still evident in the 2008.5 Voyager 1 H and He spectra at 107 AU. In 2009, when the 11‐year solar modulation effects are expected to be a minimum in the outer heliosphere, a comparison of future Voyager measurements at ∼110 AU and beyond and these new interstellar spectra along with the corresponding electron spectrum measured by Voyager 1 will provide an important test of both modulation and interstellar propagation models.