Fitting the Low Energy Spectra of Cosmic Ray Primary Nuclei from C to Fe as Measured on Voyager 1

Abstract

The intensities of the low energy part of the spectra of primary cosmic ray nuclei including C, O, Ne, Mg, Si and Fe measured by Voyager 1 beyond the heliopause are deficient relative to the spectra measured at energies above ~100 MeV/nuc as calculated using a standard Leaky Box Model with the path length a function of rigidity. Modifications to the normal exponential distribution of path lengths at a single rigidity as is used in a simple LBM will provide a good fit to this new Voyager 1 data at low energies. These modifications, sometimes called a truncation, lead to a deficiency of short path lengths relative to an exponential distribution. This deficiency in the intensities can be described by a truncation parameter = 0.12. This modification can be produced in several ways including a non-uniform local distribution of cosmic ray sources. A uniform source distribution in the galactic plane that is deficient in sources within 0.2-0.4 Kpc of the Sun is indicated by the data. Further studies of these low energy spectra in more detail will improve these estimates and help define other features of the local galactic distribution of these cosmic rays.