Effects of solar modulation on the low-energy cosmic-ray antiproton/proton ratio

We examine the transport of cosmic-ray protons and anitprotons from local interstellar space through the interplanetary medium to Earth and discuss the resulting effects on the low-energy antiproton/proton ratio at 1 AU. We find that the antiproton/proton ratio at energies above similar to 3 GeV is a useful diagnostic of cosmic-ray transport in the Galaxy. However, at energies below similar to 1 GeV the expected ratio is much more uncertain because of differences in the energy spectra and the resulting relative modulation of protons and antiprotons over the solar cycle, as well as uncertainties in the interstellar spectra. Using calculated interstellar spectra as references, we find that the antiproton/proton ratio at low energies varies by as much as an order of magnitude over the solar cycle. As a result, we recommend that attention be given instead to interpretation of the measured cosmic-ray antiproton energy spectrum rather than to the antiproton/proton ratio.