Synopsis of the interstellar He parameters from combined neutral gas, pickup ion and UV scattering observations and related consequences

¹ Dept. of Physics and Space Science Center, University of New Hampshire, Durham, NH 03824, USA e-mail: Eberhard.Moebius@unh.edu
² Space Research Centre, Polish Academy of Sciences, Bratycka 18 A, 00-716 Warsaw, Poland
³ Institute for Problems in Mechanics, Russian Academy of Sciences, Prospect Vernadskogo 101-1, Moscow 117526, Russia
⁴ Institut für Astronomie und Extraterrestrische Forschung, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Dept. of Physics and IPST, University of Maryland, College Park, MD 20742, USA
⁶ Dept. of Atmospheric, Oceanic and Space Sciences, University of Michigan, Ann Arbor, MI 48109, USA
⁷ Department of Mechanics and Mathematics, Lomonosov Moscow State University, Moscow 119899, Russia
⁸ International Space Science Institute (ISSI), Hallerstr. 6, 3012 Bern, Switzerland
⁹ Service d'Aéronomie du CNRS, BP 3, 91371 Verrières-le-Buisson, France
¹⁰ Space Science Center, University of Southern California, Los Angeles, CA 90089, USA Now at: Naval Research Laboratory/Praxis, Inc., Space Science Division, Code 7660, 4555 Overlook Ave. SW, Washington DC 20375, USA
¹¹ National Astronomical Observatory of Japan, Hoshi-ga-oka 2-12, Mizusawa Iwate, Japan
¹² Department of Earth and Planetary Physics, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
¹³ Harvard Smithsonian Center for Astrophysics, 60 Garden St., MS-15, Cambridge, MA 02138, USA
¹⁴ Los Alamos National Laboratory, MS D466, Los Alamos, NM 87545, USA
¹⁵ Goddard Space Flight Center, Code 682.3, Greenbelt, MD 20771, USA
¹⁶ Space Sciences Laboratory, University of California Berkeley, Berkeley, CA 94720, USA
¹⁷ Max Planck Institut for Solar System Research, Max-Planck-Str. 2, 37191 Katlenburg-Lindau 3, Germany

Received: 9 December 2003
Accepted: 29 March 2004

Abstract

A coordinated effort to combine all three methods that are used to determine the physical parameters of interstellar gas in the heliosphere has been undertaken. In order to arrive at a consistent parameter set that agrees with the observations of neutral gas, pickup ions and UV backscattering we have combined data sets from coordinated observation campaigns over three years from 1998 through 2000. The key observations include pickup ions with ACE and Ulysses SWICS, neutral atoms with Ulysses GAS, as well as UV backscattering at the He focusing cone close to the Sun with SOHO UVCS and at 1 AU with EUVE. For the first time also the solar EUV irradiance that is responsible for photo ionization was monitored with SOHO CELIAS SEM, and the He I 58.4 nm line that illuminates He was observed simultaneously with SOHO SUMER. The solar wind conditions were monitored with SOHO, ACE, and WIND. Based on these data the modeling of the interstellar gas and its secondary products in the heliosphere has resulted in a consistent set of interstellar He parameters with much reduced uncertainties, which satisfy all observations, even extended to earlier data sets. It was also established that a substantial ionization in addition to photo ionization, most likely electron impact, is required, with increasing relative importance closer to the Sun. Furthermore, the total combined ionization rate varies significantly with solar latitude, requiring a fully three dimensional and time dependent treatment of the problem.