Spectra of energized pick-up ions upstream of the two-dimensional heliospheric termination shock II. Acceleration by Alfvenic and by large-scale solar wind turbulences

Abstract

It is generally envisaged that pick-up ions originat- ingbyionizationprocessesfrominterstellarneutralatomsinthe region of supersonic solar windflow eventually act as seed pop- ulation for anomalous cosmic rays. It is, however, fairly unclear till now where and how the necessary energization from KeV- to 10MeV-energies takes place. Here we consider the continu- ous stochastic acceleration of pick-up ions at their convection to the outer heliospheric regions both by small-scale Alfvenic turbulence and by coherent nonlinear large-scale fluctuations of solar wind velocity and magnetic eld. For these latter fluc- tuations we develope a new turbulence concept by which we describe the average effect of corotating interaction regions in energizing pick-up ions. It is shown that large-scale turbulence isresponsiblefortheaccelerationofpick-upionsfrom10to100 KeV/nucleon while the preceded, primary pick-up ion acceler- ationfrom1to10KeV/nucleonisdonebysmall-scaleAlfvenic turbulence.OurresultsnicelycantULYSSESandVOYAGER data on energetic particles. We also conrm by our theory that a preferable acceleration of helium compared to hydrogen pick- ups occurs to 100KeV/nuc energies. Also the reflection rates at the termination shock are favourable for helium pick-ups, and thus we expect injection rates into the ACR regime favorable for ACR helium. In no case within the frame of our theoretical studies, the typical \anomalous cosmic ray energies of the or- der of 10 MeV/nucleon will be achieved before arrival of the pick-up ions at the shock. We can show, however, that the per- centage of pick-up ions undergoing reflection at the shock will be much increased by the pre-acceleration operating upstream of the shock. In ongoing reflection processes of rst and higher orders the ions permanently gain in energy till nally they are diffusively decoupled from the shock and can reappear in the inner solar system.