The comparative spectra of cosmic-ray protons and helium nuclei

Abstract

We have re-examined and extended the measurements of the primary cosmic ray proton and helium nuclei intensities in the range from a few MeV nuc−1 to ∼100 GeV nuc−1 using a considerable body of recently published data. The differential spectra obtained from this data are determined as a function of both energy and rigidity. The exponents of the energy spectra of both protons and helium nuclei are found to be different at the same energy/nucleon and to increase with increasing energy between 1 and 100 GeV nuc−1 reaching a value=−2.70 at higher energies and in addition, theP/He ratio changes from a value ≲5 at 1 GeV nuc−1 and below to a value ∼30 at 100 GeV nuc−1. On a rigidity representation the spectral exponent for each species is nearly identical and remains virtually constant above several GV at a value of −2.70, and in addition, theP/He ratio is also a constant ∼7 above ∼3 GeV. The changingP/He ratio and spectral exponent on an energy representation occur at energies well above those at which interplanetary modulation effects or interstellar ionization energy loss effects can significantly affect the spectra. In effect by comparing energy spectra and rigidity spectra in the intermediate energy range above the point where solar modulation effects and interstellar energy loss effects are important, but in the range where there are significant differences between energy and rigidity spectra, we deduce that the cosmic ray source spectra are effectively rigidity spectra. This fact has important implications regarding the mechanism of acceleration of this radiation and also with regard to the form of the assumed galactic spectrum at low energies. The relationship between the proton and helium spectra derived here and the heavier nuclei spectral differences recently reported in the literature is also examined. If rigidity spectra are adopted for protons and helium nuclei, then the source abundance ratio of these two components is determined to be ∼7:1. Some cosmological implications of this ratio are discussed.