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Astron. Astrophys. 360, 381-390 (2000)

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

S.V. Chalov 1 and H.J. Fahr 2

1 Institute for Problems in Mechanics of the Russian Academy of Sciences, Prospect Vernadskogo 101, 117526 Moscow, Russia (chalov@ipmnet.ru)
2 Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany (hfahr@astro.uni-bonn.de)

Received 13 December 1999 / Accepted 8 May 2000


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 process 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 process of diffusive shock acceleration. Here we avoid this injection problem studying instead the single particle fate of pick-up ions arriving at the shock and undergoing multiple adiabatic reflections at the shock. We start out from preaccelerated pick-up ions arriving with a known isotropic distribution function at the shock and suffering reflections or transmissions through the shock depending on their actual velocity space coordinates. Upstream and downstream of the shock the ions in addition are subject to Fermi-2 acceleration processes described by means of a phase-space transport equation for a pitch-angle anisotropic distribution function. As we can show the spectral energy distribution resulting for the downstream pick-up ions consists of two parts, the low energy keV-part which is due to directly transmitted ions and the high energy MeV-part which is due to multiply reflected ions. We also show that the resulting spectrum is fairly sensitive to the location of the shock sector with respect to the upwind direction. This fact is best reflected in corresponding energetic neutral atom (ENA) fluxes reaching the Earth from different directions and thus serves as a unique diagnostic tool for the remote study of the 3-d properties of the termination shock.

Key words: acceleration of particles – shock waves – Sun: solar wind – inteplanetary medium

Send offprint requests to: H.J. Fahr

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© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000