Observations of the helium focusing cone with pickup ions

¹ Department of Physics and Institute for Physical Science and Technology, University of Maryland, College Park, MD 20742, USA e-mail: gg10@umail.umd.edu
² Department of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA
³ Department of Physics and Space Science Center, University of New Hampshire, Durham, NH 03824, USA
⁴ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
⁵ Space Research Centre PAS, Bartycka 18A, 00-716 Warsaw, Poland
⁶ Institute for Space Research, Moscow, Russia
⁷ Institute of Astrophysics and Extraterrestrial Research, Bonn University, Auf dem Hügel 71, 53121 Bonn, Germany
⁸ Space Sciences Center, University of Southern California, Los Angeles, CA 90089, USA
⁹ National Astronomical Observatory of Japan, Hoshi-ga-oka 2-12, Misuzawa Iwate, Japan
¹⁰ Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

¹¹ Los Alamos National Laboratory, Los Alamos, NM 87545, USA
¹² Communications Research Laboratory, Koganei, Tokyo 184-8795, Japan

Received: 30 November 2003
Accepted: 30 January 2004

Abstract

The helium gravitational focusing cone has been observed using pickup He⁺, first during the solar minimum in 1984–1985 with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, , while observations of pickup He⁺⁺ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, cm^-3, in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He⁺ using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner (0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He⁺ is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He⁺ are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He⁺ in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He⁺ data without assuming loss rates that are significantly different from production rates.