ON THE TRANSPORT OF PICK-UP IONS IN THE HELIOSPHERE

A study of the transport of pick-up ions throughout the heliosphere is presented. First, the basic features of the pick-up ion velocity dist ributions and their evolution under the influence of either the second -order Fermi process or transit-time damping as mechanisms for momentu m diffusion are studied. Second, the distributions are computed for va rious regions of the heliosphere and the two types of momentum diffusi on are compared with respect to their efficiency to form tails of the distributions towards higher energies. Third, it is assumed that both processes are operating simultaneously. For all cases, implications fo r the acceleration efficiency of anomalous cosmic rays across the sola r wind termination shock surface are discussed. The distributions are obtained as solutions of a transport equation including, in addition t o momentum diffusion, terms to describe the convection and adiabatic c ooling of pick-up ions in the expanding solar wind plasma as well as, in difference to earlier work, a source function based on a realistic model of the distribution of neutral particles within the heliosphere.