On the relative magnitude of streaming velocities of alpha particles and protons at 1 AU

The occurrence at a heliocentric distance of 1 AU of alpha particle streaming velocities larger than proton streaming velocities, v_α/v_p>1 (Ogilvie, 1975) is investigated on the basis of the theory suggesting the existence in the solar wind of an accelerating force acting preferentially on the alpha particles. Accurate solution of the three-fluid model equations for the quiet solar wind indicates that a necessary and sufficient condition for (v_α/v_p)_{1 AU}>1 is the presence of a relatively weak accelerating force acting in a limited region in the vicinity of 1 AU. If the force is effective only at small heliocentric distances, the alpha particle streaming velocity excess vanishes at distances less than 1 AU, because of the (equalization) action of the dynamical friction force.