Modulation of low-energy galactic cosmic-ray hydrogen and helium

A study of the energy spectrum of the cosmicray hydrogen and helium nuclei as a function of the period in the solar cycle provides one of the better means of gaining insight into the mechanism that modulates the galactic cosmic radiation. An examination of these nuclei in the low-energy region, where the modulation effects are greatest, has the particular merit of permitting the separation of velocity and rigidity effects, since for a given rigidity the helium nuclei have a markedly different velocity from that of the protons. Considerable information related to this problem had been obtained before 1960 for the most recent period of increasing solar activity [Webber, 1962]. In order to continue the study of the problem of the variation of the galactic cosmic-ray proton and helium nuclei spectrums and extend it to lower energies, a series of balloon flights, beginning in 1961, has been made from Fort Churchill, Canada. It is the aim of this letter to present the results of the nuclear emulsion studies of the hydrogen and helium nuclei obtained in 1962 and 1963 and compare them to those obtained in 1961 [Fichtel et al., 1964].