Measurements of Cosmic-Ray Low-Energy Antiproton and Proton Spectra in a Transient Period of Solar Field Reversal

The energy spectra of cosmic-ray low-energy antiprotons ( $\overline{p}$'s) and protons ( $p$'s) have been measured by BESS in 1999 and 2000, during a period covering reversal at the solar magnetic field. Based on these measurements, a sudden increase of the $\overline{p}/p$ flux ratio following the solar magnetic field reversal was observed, and it generally agrees with a drift model of the solar modulation.