For a three-month period in 1972 we have measured the low-energy galactic cosmic-ray proton and helium spectra using the University of Chicago IMP-S satellite experiment. Although the proton spectrum has a characteristic rising slope in the interval 30100 MeV, the helium spectrum shows a previously unobserved feature: an intensity essentially independent of kinetic energy below 80 MeV per nucleon. Indeed, below 35 MeV per nucleon, the helium flux exceeds the proton flux. It is pointed out that current models of solar modulation can account for the satellite observations 1965-1969, and the 1969-1971 period which included phase-lag effects during the recovery phase of current solar cycle modulation. However, we cannot extend these techniques with previously successful boundary conditions and diffusion coefficient changes to fit all the features of the 1972 May-July observations below 100 MeV per nucleon. Subject headings: cosmic rays - solar system
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García-Muñoz et al. (1973) studied this question.