Evidence for ion jets in the high‐speed solar wind

Abstract

Shapes of proton and alpha particle velocity distribution functions in the high‐speed solar wind are examined for the time period from 1973 to 1987. Whereas enhanced high‐energy extensions are a near universal feature of the proton distributions, no such enhancements are observed in alpha particle distributions. Such distributions are examined and documented in detail. An interpretation of this observation is presented in terms of a two‐component model of coronal heating and acceleration. We suggest that a modest amount of heating results from the damping of waves, whereas additional heating and the bulk of the acceleration is associated with the direct conversion of magnetic energy flux to plasma convection and enthalpy flux through the process of magnetic reconnection. This model predicts that the ambient flow of plasma from coronal holes is characterized by a low‐speed and a low‐mass flux. In particular, the proton flux is insufficient to drag alpha particles out of the solar gravitational potential well. This should lead to a considerable enhancement of the helium abundance close to the Sun. Plasma jets rich in helium and accelerated by reconnection close to the Sun buffet the ambient plasma from below. The slow‐moving ambient plasma is coupled to the faster‐moving jets by Coulomb friction close to the Sun, and then through microinstabilities far from the Sun. The net effect of this interaction is a high‐speed, double‐streaming proton population and a single component, but faster alpha particle flow at large heliocentric distances.