Pick-up ion acceleration at the termination shock and the post-shock pick-up ion energy distribution

It is discussed since quite some time in the literature that the solar wind termination shock may act as an efficient particle accelerator though the underlying physics of injecting particles into this shock acceleration proc ess was not well understood up to now. Most of the earlier work required an ad-hoc prescription of the rate by which particles are injected into the p rocess of diffusive shock acceleration. Here we avoid this injection proble m studying instead the single particle fate of pickup ions arriving at the shock and undergoing multiple adiabatic reflections at the shock. We start out from preaccelerated pickup ions arriving with a known isotropic distrib ution function at the shock and suffering reflections or transmissions thro ugh the shock depending on their actual velocity space coordinates. Upstrea m and downstream of the shock the ions in addition are subject to Fermi-2 a cceleration processes described by means of a phase-space transport equatio n for a pitch-angle anisotropic distribution function. As we can show the s pectral energy distribution resulting for the downstream pick-up ions consi sts of two parts, the low energy keV-part which is due to directly transmit ted ions and the high energy MeV-part which is due to multiply reflected io ns. We also show that the resulting spectrum is fairly sensitive to the loc ation of the shock sector with respect to the upwind direction. This fact i s best reflected in corresponding energetic neutral atom (ENA) fluxes reach ing the Earth from different directions and thus serves as a unique diagnos tic tool for the remote study of the 3-d properties of the termination shoc k.