A Comparison of Galactic Cosmic Ray Proton and Helium Nuclei Spectra From ~10 MeV/nuc to 1 TeV/nuc Using New Voyager and Higher Energy Magnetic Spectrometer Measurements - Are There Differences In the Source Spectra of The Two Nuclei

TL;DR

This study compares cosmic ray proton and helium spectra from Voyager and other experiments, finding similar source spectra with some differences at high energies, and explores implications for their origins.

Contribution

It provides a detailed analysis of the source spectra of cosmic ray H and He nuclei across a wide energy range using new measurements and a Leaky Box model, highlighting differences at high energies.

Findings

Source spectra for H and He are approximately P-2.24 across the studied energy range.

Recent AMS-2 data suggest a steepening of the H spectrum above 8 GV.

The H/He source ratio is about 5.0 and varies with energy in the AMS-2 data.

Abstract

This paper determines the relative source spectra of cosmic ray H and He nuclei using a Leaky Box model for galactic propagation and the observed spectra of these nuclei from ~10 MeV/nuc to ~1 TeV/nuc. The observations consist of Voyager 1 measurements up to several hundred MeV/nuc in local interstellar space and measurements above ~10 GeV/nuc where solar modulation effects are small by experiments on BESS, PAMELA and AMS-2. Using BESS and PAMELA measurements which agree with each other, the observed spectra for H and He nuclei and the H/He ratio are well fit by source rigidity spectra for both nuclei which are ~P-2.24 over the entire range of rigidities corresponding to energies between 10 MeV/nuc and several hundred GeV/nuc. In this case, the H/He rigidity source ratio is 5.0 + 1. The recent and presumably more accurate measurements of these spectra above 10 GeV/nuc made by AMS-2 do not entirely agree with the earlier measurements, however. In particular the H spectrum is found to be steeper than that of He by about 0.10 in the spectral exponent. Using the same model for galactic propagation the AMS-2 data leads to source spectra of H and He which are ~P-2.24 up to a break rigidity ~6-8 GV. At higher rigidities the He source spectrum continues to be ~P-2.24 but the required source spectrum for H steepens to an index ~P-2.36 above ~8 GV and, as a result, the H/He source ratio decreases with increasing rigidity using the AMS-2 data.