Astron. Astrophys. 363, L21-L24 (2000)

Letter to the Editor

Keplerian injection velocities reflected in helium pick-up ion spectra

S.V. Chalov ¹ and H.J. Fahr <sup>2

1</sup> Institute for Problems in Mechanics of the Russian Academy of Sciences, Prospect Vernadskogo 101-1, 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 25 September 2000 / Accepted 27 October 2000

Abstract

It is usually assumed that pick-up ions at the event of their production, seen in the solar rest frame, have peculiar velocities which are negligible with respect to solar wind velocities. This assumption, however, is no more valid for pick-up ion injections at small solar distances. As we are showing here, the He⁺ - pick-up ion spectra to be expected at distances inside 1 AU clearly reflect the fact that ion injection took place with non-negligible velocities due to the Keplerian motion of parent He atoms entering from the interstellar medium, as was already recognized in SOHO CELIAS measurements by Möbius et al. (1999). We demonstrate that these injection signatures are less pronounced under conditions of high-speed compared to those of low-speed solar winds and are variable with the off-upwind angle. Careful study of these injection features can help to identify the actual He ionization rate.

Key words: acceleration of particles Sun: solar wind inteplanetary medium ISM: atoms, ions

Send offprint requests to: S.V. Chalov

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000

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