The solar cycle variation in the solar wind and the modulation of cosmic rays

Evidence for a solar cycle variation in the interplanetary plasma is presented based on direct observations of the solar wind by the Pioneer and Vela spacecraft. These data indicate that between 1965 and 1971 there were more days associated with high speed streams during solar maximum than during solar minimum. These observations provide the first observational explanation for the solar modulation of cosmic rays. At solar maximum the probability is highest that cosmic rays entering the solar system will encounter high speed streams. As a result, there is more modulation of cosmic rays at solar maximum than at other times.