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 e†ects on the low-energy antiproton/proton ratio at 1 AU. We Ðnd that the antiproton/proton ratio at energies above D3 GeV is a useful diagnostic of cosmic-ray transport in the Galaxy. However, at energies below D1 GeV the expected ratio is much more uncertain because of di†erences 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 Ðnd 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. Subject headings : cosmic rays È interplanetary medium