Measurement of the cosmic ray proton and helium fluxes from 200 MeV nucl−1 to 100 GeV nucl−1 during the 1987 solar minimum

Abstract

The galactic cosmic ray proton and helium spectra have been measured with a balloon-borne superconducting magnetic spectrometer over the energy range from 200 MeV nucl⁻¹ to 100 GeV nucl⁻¹ during the 1987 solar minimum between the 21st and 22nd solar cycle. The absolute fluxes were approximately equal to the highest fluxes observed at the previous solar minimum in 1977. Above 10 GV the observed spectra are represented by a power law in rigidity with spectral indices of 2.74± 0.02 for protons and 2.68 ± 0.03 for helium. The data are consistent with rigidity power law interstellar spectra modulated with the solar modulation parameter φ = 500 MV.