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Astron. Astrophys. 335, 746-756 (1998)

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Phase space diffusion and anisotropic pick-up ion distributions in the solar wind: an injection study

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 (e-mail: chalov@ipmnet.ru)
2 Institut für Astrophysik und Extraterrestrische Forschung der Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany (e-mail: unf308@ibm.rhrz.uni-bonn.de)

Received 9 December 1997 / Accepted 27 March 1998

Abstract

Pick-up ions are produced all over the interplanetary space by ionization of neutral interstellar atoms. Over the last decade it was generally expected that after pick-up these ions quickly are isotropized in the velocity space comoving with the solar wind by strong pitch-angle scattering, though not assimilating to the thermodynamic state of solar wind ions. Recent studies of pick-up ion data obtained with plasma analyzers on AMPTE and ULYSSES have, however, revealed that during extended time periods substantially anisotropic distributions prevail. In this paper we want to improve the understanding of the evolutionary behaviour of pick-up ions in interplanetary phase space by an pick-up ion injection study taking into account all relevant diffusion terms describing pitch angle scattering, adiabatic cooling, drifts and energy diffusion. For particles injected at 1 AU the resulting distribution function stays substantially anisotropic up to distances of the order of 6 AU, unless increased isotropic turbulence levels and non-dissipative wave spectra are considered. The average bulk velocity of pick-up ions within this distance range is smaller than the solar wind velocity by factors of between 0.6 and 1.0. Pick-ups are shown to substantially become spread out from that solar wind parcel into which they were originally injected. Derivations of interstellar gas parameters using pick-up ion flux data can only be dared with additional care in the interpretation. As a consequence the location of the LISM helium cone axis and the LISM helium temperature are faked in the associated He+ pick-up ion flux pattern.

Key words: turbulence – acceleration of particles

Send offprint requests to: H.J. Fahr

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© European Southern Observatory (ESO) 1998

Online publication: June 18, 1998
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