Spectra of energized pick-up ions upstream. of the heliospheric termination shock I. The role of Alfvenic turbulences.
Astronomy and Astrophysics, v.304, p.609
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It is generally believed that keV pick-up ions in the outer heliosphere are processed by Fermi acceleration to MeV anomalous cosmic rays. Here 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 obtained within the frame of the twin-shock interface model by Baranov & Malama (1993) and then consider the evolution of the pick-up ion velocity distribution function based on the relevant energy transport equation. This partial differential equation we first formally transcribe into the form necessary for so-called Ito-stochastic differential equations and then solve this set of equations by calculating statistically relevant numbers of stochastic particle trajectories applying Ito-like algorithms and taking account of Fermi acceleration processes due to resonant scattering at Alfvenic turbulences. As we can show when using 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 is also interesting to see the influence of different representations for the energy diffusion coefficient which would result from different radial variations of relative fluctuation amplitudes of MHD turbulences in the outer heliosphere. On the basis of the pick-up ion energy spectra obtained in this paper an energization to average energies of the order of 100keV/nucleon can be achieved at 80AU. Thus an additional acceleration mechanism has to be operating at the outer regions of the heliosphere close to the termination shock in order to produce starting from this level the energies of about 10MeV/nucleon which are typical for the anomalous cosmic ray particles. We finally discuss in qualitative terms the differences in pick-up ion accelerations with solar latitude and the resulting conversion efficiencies of pick-up ions into anomalous cosmic ray particles which also most likely are different at ecliptic and polar regions.