Successive Measurements of Cosmic-Ray Antiproton Spectrum in a Positive Phase of the Solar Cycle

TL;DR

The study addresses the origin of low-energy cosmic-ray antiprotons and the role of solar modulation. It uses four successive BESS flights (1993, 1995, 1997, 1998) to measure the antiproton spectrum from 0.18 to 4.20 GeV with a high-precision spectrometer, applying event selection, background subtraction, and atmospheric corrections, and comparing to secondary production models under solar modulation. The key findings are that the 1998 data agree with secondary production models and that the antiproton-to-proton ratio is nearly consistent across positive-polarity periods, suggesting charge-dependent modulation with no compelling evidence for a soft primary component; future measurements during polarity reversal are planned to test this. These results constrain solar modulation and propagation models, inform searches for exotic sources of antiprotons, and guide future experimental strategies in cosmic-ray physics.

Abstract

The energy spectrum of cosmic-ray antiprotons has been measured by BESS successively in 1993, 1995, 1997 and 1998. In total, 848 antiprotons were clearly identified in energy range 0.18 to 4.20 GeV. From these successive measurements of the antiproton spectrum at various solar activity, we discuss about the effect of the solar modulation and the origin of cosmic-ray antiprotons. Measured antiproton ratios were nearly identical during this period, and were consistent with a prediction taking the charge dependent solar modulation into account.

Figures (5)

• Figure 1: Cross-sectional view of the BESS 1998 detector with one of the $\bar{p}$ events.
• Figure 2: The identification of $\bar{p}$ events. The solid curves define the $\beta^{-1}$--$R$ region and the $\bar{p}$ mass band used for the spectrum measurement.
• Figure 3: The BESS 1998 antiproton spectrum at the top of the atmosphere, together with the previous data. The thick solid curve represents the expected spectrum for secondary $\bar{p}$FRB at the 1998 flight ($\phi_{98}$=610MV) and well reproduces the BESS 1998 spectrum in the whole energy region. The expected spectrum at the 1997 flight ($\phi_{97}$=500MV) was also calculated in the same way as the 1998, represented by the thick dashed curve. Although the curve agrees with the BESS 1997 spectrum in the peak region, the agreement is less consistent in the low energy region. Also shown are other calculations BERGBIEB for secondary $\bar{p}$ at solar minimum (corresponding to the solar activity level at the BESS 1997 flight).
• Figure 4: Comparison of the BESS 1993, 1995, 1997 and 1998 $\bar{p}/p$ ratios with previous data CAPRICE, and the calculation BIEB taking the charge dependence of the solar modulation into account. The solid and dashed curves represent the expected $\bar{p}/p$ ratio at the solar minimum and at the solar maximum in the positive Sun's polarity. When the polarity switches, the ratio is expected to increase as represented by the dotted curve.
• Figure 5: The ratios of the $\bar{p}/p$ ratio normalized by the BESS 1997 data are distributed around the unity within the statistical fluctuation. The BESS 1993, 1995, 1997 and 1998 data were measured in the positive polarity phase. Therefore, it is shown that the $\bar{p}/p$ ratio is nearly identical during the positive polarity phase.