Differential energy spectra and intensity variation of 1-20 MeV/nucleon protons and helium nuclei in interplanetary space (1964-66)

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Abstract
Measurements made in interplanetary space with a ΔE-range telescope on the OGO I and OGO III satellites and on the Pioneer 7 space probe showed that protons and helium nuclei in the energy range I–20 MeV/nucleon are present during "quiet times" in late 1964, in May–October–November 1965, and in August 1966. The intensities for both proton and helium nuclei are found to decrease with increasing energies. The spectra join those of the particles above 20 MeV/nucleon, which are known to be of galactic origin. For both protons and helium nuclei, the fluxes were higher in the period of minimum solar modulation in 1965 than in 1964. While in 1966 the helium flux decreased to its 1964 level, the proton flux showed a further increase over this period. It is believed that most of the particles observed during the period of minimum solar modulation were of galactic origin. As solar activity increases again, it seems that to the galactic low-energy fluxes, particularly for protons, there is added a steady contribution of solar origin.