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Astron. Astrophys. 320, 659-671 (1997)

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Spectra of energized pick-up ions upstream of the two-dimensional heliospheric termination shock

II. Acceleration by Alfvénic and by large-scale solar wind turbulences

Sergei V. Chalov 1, Hans J. Fahr 2 and Vlad Izmodenov 3

1 Institute for Problems in Mechanics of the Russian Academy of Sciences, Prospect Vernadskogo 101, 117-526 Moscow, Russia
2 Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
3 Moscow State University, Moscow, Russia

Received 4 June 1996 / Accepted 16 August 1996


It is generally envisaged that pick-up ions originating by ionization processes from interstellar neutral atoms in the region of supersonic solar wind flow eventually act as seed population for anomalous cosmic rays. It is, however, fairly unclear till now where and how the necessary energization from KeV- to 10 MeV-energies takes place. Here we consider the continuous stochastic acceleration of pick-up ions at their convection to the outer heliospheric regions both by small-scale Alfvénic turbulence and by coherent nonlinear large-scale fluctuations of solar wind velocity and magnetic field. For these latter fluctuations 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 is responsible for the acceleration of pick-up ions from 10 to 100 KeV/nucleon while the preceded, primary pick-up ion acceleration from 1 to 10 KeV/nucleon is done by small-scale Alfvénic turbulence. Our results nicely can fit ULYSSES and VOYAGER data on energetic particles. We also confirm by our theory that a preferable acceleration of helium compared to hydrogen pick-ups occurs to 100 KeV/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 order of 10 MeV/nucleon will be achieved before arrival of the pick-up ions at the shock. We can show, however, that the percentage 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 first and higher orders the ions permanently gain in energy till finally they are diffusively decoupled from the shock and can reappear in the inner solar system.

Key words: Heliosphere – interstellar neutral atoms – pick-up ions – energy diffusion – particle acceleration

Send offprint requests to: H.J. Fahr

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998