L. A. Fisk University of Michigan, Ann Arbor, Michigan, USA
The distribution functions of superthermal particles in the solar wind – interstellar pickup ions and inner source pickup ions – show pronounced tails at speeds in excess of the solar wind flow speed, particularly in the slow solar wind. The presence of these tails does not appear to be correlated with the passage of shock waves; rather they appear to be more ubiquitous. A likely mechanism for the formation of the tails is statistical acceleration. Various mechanisms for statistical acceleration will be reviewed and compared with observations. The role of statistical acceleration in the creation of a particle population that can readily be injected into shock acceleration mechanisms will also be discussed.