SPECTRA OF ENERGIZED PICK-UP IONS UPSTREAM OF THE HELIOSPHERIC TERMINATION SHOCK .1. THE ROLE OF ALFVENIC TURBULENCES

It is generally believed that keV pick-up ions in the outer heliospher e are processed by Fermi acceleration to MeV anomalous cosmic rays. He re we are studying the initial pick-up ion seed population by means of their energy distribution functions. In this paper we start out from the heliospheric pick-up ion production rates which are consistently o btained within the frame of the twin-shock interface model by Baranov & Malama (1993) and then consider the evolution of the pick-up ion vel ocity distribution function based on the relevant energy transport equ ation. This partial differential equation we first formally transcribe into the form necessary for so-called Ito-stochastic differential equ ations and then solve this set of equations by calculating statistical ly relevant numbers of stochastic particle trajectories applying Ito-l ike algorithms and taking account of Fermi acceleration processes due to resonant scattering at Alfvenic turbulences, As we can show when us ing this method energy spectra are obtained which clearly reveal their variation with solar distance and with the mass-per charge ratios of the different pick-up ion species that have been considered here. It i s also interesting to see the influence of different representations f or the energy diffusion coefficient which would result from different radial variations of relative fluctuation amplitudes of MHD turbulence s in the outer heliosphere. On the basis df the pick-up ion energy spe ctra obtained in this paper an energization to average energies of the order of 100 keV/nucleon can be achieved at 80 AU. Thus an additional acceleration mechanism has to be operating at the outer regions of th e heliosphere dose to the termination shock in order to produce starti ng from this level the energies of about 10 MeV/nucleon which are typi cal for the anomalous cosmic ray particles. We finally discuss in qual itative terms the differences in pick-up ion accelerations with solar latitude and the resulting conversion efficiencies of pick-up ions int o anomalous cosmic ray particles which also most likely are different at ecliptic and polar regions.