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

1. Introduction

Recently Möbius et al. (1999) have presented observations of He⁺ pick-up ions obtained with SOHO CELIAS CTOF in 1996 for the time period when the Earth was on the upwind side of the interstellar medium flow. As shown in this paper the high-velocity cut-off of the pick-up ion spectra in the solar rest frame is at higher values compared to the standard value In contrast analogous He⁺ data obtained with AMPTE SULEICA in 1985 on the downwind side show a shift of the cut-off to lower velocities. It has also been demonstrated that the normalized value of the shift is anticorrelated with the solar wind velocity. Möbius et al. (1999) consider these shifts as due to a manifestation of the interstellar neutral helium velocity at the injection point in the inner heliosphere.

When modelling the pick-up ion transport in the heliosphere the authors usually assume that initial velocities of freshly created ions, since considered to be small compared to solar wind velocities, in the solar wind frame simply are equal to the negative local solar wind velocity. This assumption is valid as long as the solar wind velocity is considerably larger than the peculiar velocities of the parent neutral He atoms. In the inner heliosphere, however, the He atoms suffer considerable acceleration by the solar gravity. The effect of acceleration due to the absence of radiation pressure is more pronounced for neutral helium as compared to neutral hydrogen atoms. The effect of the proper motions of parent He atoms on velocities of freshly created He⁺ pick-up ions in the resulting spectra thereby is different on the upwind and downwind hemispheres as has been shown by Möbius et al. (1999).

In the present paper we study pick-up helium velocity distributions in the ecliptic plane on the basis of solutions of the Fokker-Planck type transport equation for anisotropic distribution functions taking into account the actual velocities of He⁺ pick-up ions at the moment of their injection and also all relevant physical phase-space transport processes occurring after injection, like adiabatic cooling and focusing, pitch-angle scattering, and energy diffusion.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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