HEOS 1 helium observations in the solar wind

Results of alpha-particle observations performed by the European satellite HEOS-1, in the period from December 9, 1968 to April 13, 1969 and from September 6, 1969 to April 15, 1970, are presented. The average bulk velocities of protons (Vp) and alpha-particles (V-alpha) appear to be equal: however, due to an instrumental bias, the possibility of V-alpha being lower than Vp cannot be ruled out. Comparison with observations of Vela 3 and Explorer 34 satellites give evidence of a dependence of helium abundance on the solar cycle. The problem of the stability of differences between the bulk velocities of protons and alpha-particles is investigated. The behaviour of alpha-particles through interplanetary shock waves is illustrated in connection with magnetic field measurements.