The Different Shapes of the LIS Energy Spectra of Cosmic Ray He and C Nuclei Below ~1 GeV/nuc and The Cosmic Ray He/C Nuclei Ratio vs. Energy -V1 Measurements and LBM Propagation Predictions

TL;DR

This study investigates cosmic ray He and C nuclei spectra below 1 GeV/nuc, revealing that a modified propagation model with nucleus-specific truncation parameters better explains the observed low-energy spectral features and the He/C ratio increase.

Contribution

The paper introduces a truncated Leaky Box Model with nucleus-specific parameters to accurately reproduce low-energy cosmic ray spectra and the He/C ratio, highlighting the influence of source distribution and energy loss effects.

Findings

A simple LBM cannot reproduce low-energy spectra and He/C ratio increase.

A truncated LBM with nucleus-specific parameters matches observations.

Predicted spectra agree with AMS-2 measurements within 5%.

Abstract

This paper examines the cosmic ray He and C nuclei spectra below ~1 GeV/nuc, as well as the very rapid increase in the He/C ratio below ~100 MeV/nuc, measured by Voyager 1 beyond the heliopause. Using a simple Leaky Box Model (LBM) for galactic propagation we have not been able to simultaneously reproduce the individual He and C nuclei spectra and the large increase in He/C ratio that is observed at low energies. However, using a truncated LBM with different truncation parameters for each nucleus that are related to their rate of energy loss by ionization which is ~Z2/A, these different features can be matched. This suggests that we are observing the effects of the source distribution of cosmic rays in the galaxy on the low energy spectra of cosmic ray nuclei and that there may be a paucity of nearby sources. In this propagation model we start very specific source spectra for He and C which are ~dj/dP = P-2.24, the same for each nucleus and also for all rigidities. These source spectra become spectra with spectral indices ~-2.69 at high rigidities for both charges as a result of a rigidity dependence of the diffusion coefficient governing the propagation which is taken to be ~P-0.45. This exponent is determined directly from the B/C ratio measured by AMS-2. These propagated P-2.69 spectra, when extended to high energies, predict He and C intensities and a He/C ratio that are within +3-5% of the intensities and ratio recently measured by AMS-2 in the energy range from 10 to 1000 GeV/nuc.