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

Abstract

The energy spectra of cosmic-ray low-energy antiprotons (p's) and protons (p's) have been measured by BESS in 1999 and 2000, during a period covering the solar field reversal. Based on these measurements, a sudden increase of thep/p flux ratio following the solar field reversal was observed as predicted by a drift model of the solar modulation.PACS numbers: 98.70. Sa, 96.40.Kk, 95.85.Ry The real underlying physics of the sun is the 22 year solar magnetic cycle with recurrent positive and negative phases. The magnetic field polarity flips when the solar activity is maximum and the global magnetic field profile reverses in the heliosphere. The most recent field reversal should happen in the beginning of 2000. The solar modulation of cosmic rays is caused by expanding solar wind, which spreads out locally irregular magnetic field and therefore modifies energy spectra of the cosmic rays entering the heliosphere. The positive and negative particles drift in opposite directions, during their propagation in the large scale heliospheric magnetic field. The charge-sign dependence is, therefore, a natural consequence [1] in the solar modulation, and it explains alternate appearances of "flat" and "peaked" periods in neutron monitor data around solar minima. In spite of an emerging understanding that the drift became unimportant for several years around the solar maximum [2], recent works [3][4][5] indicated that the drift produces a strong differentiation between the positive and negative particles even during the high solar activity. This view is supported by measurements of temporal variation in cosmic-ray ratios, such as electrons to helium nuclei (He) [6] and electrons to protons (p's) [7], where the largest variation is associated with the solar field reversal. Antiprotons (p's) and their ratio to p's may be novel probes to study the solar modulation becausep's differ from p's only in the charge sign while electrons would behave differently from He and p's due to their lighter mass [4].In the last solar minimum period, the BESS experiment revealed that the cosmic-rayp spectrum has a distinct peak around 2 GeV [8], which is a characteristic feature of secondaryp's produced by cosmic-ray interactions with interstellar (IS) gas. It has become evident thatp's are predominantly secondary in origin, because several recent calculations of the secondary spectrum basically agree with observations in their absolute values and spectral shapes [4,5,9,10].We report here new measurements of cosmic-rayp and p fluxes and their ratios in the energy range from 0.18 to 4.2 GeV collected in two BESS balloon flights carried out in 1999 and 2000, when the solar activity was maximum. Based on the solar magnetic field data [11], the Sun's polarity reversed between these two flights [12]. With our full set of data [8,[13][14][15], we observed the temporal variation of thep flux andp/p ratio covering the solar minimum, the maximum, and the field reversal.The BESS spectrometer was designed [16,17] and developed [18-21] as a high-resolution ...

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