Are anomalous cosmic rays the main contribution to the low-energy galactic cosmic ray spectrum?

Abstract

While the high-energy part of the Galactic cosmic ray spectrum is well observed, its nature at energies below about 1 GeV nucleon-1 is still not known well. Recent in situ measurements made with the Voyager 1 spacecraft in the heliosheath between the solar wind termination shock and the heliopause have added further constraints on the local interstellar spectrum of Galactic cosmic rays at low energies. We show here that they also suggest how the low-energy proton part is formed locally in the heliosphere and globally in the Galaxy. The measured flux of anomalous cosmic rays in the heliosheath is unexpectedly high compared to expectations before Voyager 1 reached the shock, which might be a temporal effect or due to an additional acceleration beyond the termination shock. Combining this finding with recent model results for astrospheres immersed in different interstellar environments shows that the astrospheric anomalous cosmic ray fluxes of solar-type stars can be a hundred times higher than thought earlier and, consequently, their total contribution to the lower end of the interstellar spectrum can be significant