Interstellar Spectra of Primary Cosmic Ray Nuclei from H through Fe Measured at Voyager and a Comparison with Higher Energy Measurements, An Interpretation of the Spectra from 10 MeVnuc to Over 100 GeVnuc Using a Leaky Box Model

TL;DR

This study combines Voyager and other spacecraft data to analyze cosmic ray nuclei spectra from 10 MeV/nuc to over 100 GeV/nuc, comparing observations with a Leaky Box Model and exploring low-energy spectral behaviors.

Contribution

It provides a comprehensive interstellar spectra analysis of primary cosmic ray nuclei across a wide energy range using Voyager data and tests the Leaky Box Model's predictions.

Findings

Spectra above 50-100 MeV/nuc follow a P^-2.28 source spectrum and P^-2.78 at higher energies.

Heavier nuclei spectra fall more rapidly at low energies than lighter ones, indicating additional processes.

H and He spectra suggest a local component possibly from heliospheric sources.

Abstract

Utilizing new Voyager measurements at lower energies and higher energy spacecraft measurements near the Earth, the interstellar spectra of primary cosmic ray nuclei from H to Fe have now been determined from ~10 MeV/nuc to > 100 GeV/nuc. These measurements are compared with the predictions from a Leaky Box propagation model. It is found that above ~50-100 MeV/nuc the spectra of all the nuclei, H, He, C, O, Ne, Mg, Si and Fe are well connected between 100 MeV/nuc and 10 GeV/nuc and above by simple source rigidity spectra proportional to P-2.28, with the exponent independent of rigidity and using a rigidity dependent diffusion coefficient ~P0.50 above ~1.0 GV. This leads to intensities and spectra ~P-2.78 at ~100 GeV/nuc and above, which are consistent with new AMS-2 and PAMELA measurements of H, He and C to within + 10%. Below 50-100 MeV the spectra of these primary charges fall into two groups. The spectra of primary nuclei with Z > 6 fall more rapidly at low energies than those of H and He and more rapidly than would be expected from a pure diffusive LBM, where energy loss by ionization is important at low energies. Increasing the fraction of ionized hydrogen in the interstellar medium from 15% to 30% does not satisfactorily explain the systematics of the intensity decreases of the heavier primary nuclei at low energies. Other processes beyond those assumed in a simple leaky box model, such as a lack of cosmic ray sources near the Sun are being investigated as the source of this more rapid intensity decrease of these nuclei. The spectra of H and He are very similar to each other but both contain more low energy particles than would be expected from a correspondence with the heavier nuclei spectra. We believe that this may indicate the presence of a local (heliospheric) component of these nuclei in addition to the galactic component.