Imaging the heliosheath using HSTOF energetic neutral atoms and Voyager 1 ion data

¹ Space Research Center, Polish Academy of Sciences, Bartycka 18A, PL 00716 Warsaw, Poland e-mail: ace@cbk.waw.pl
² Max-Planck-Institut für Sonnensystemforschung, Max-Planck-Strasse 2, 37191 Katlenburg-Lindau, Germany e-mail: hilchenbach@mps.mpg.de
³ Physics Department, University of Arizona, Tucson, AZ 85721, USA e-mail: hsieh@physics.arizona.edu
⁴ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA e-mail: kota@.lpl.arizona.edu

Received: 11 February 2008
Accepted: 1 May 2008

Abstract

Context. Voyager 1 and 2 LECP instruments measure the distributions of the heliosheath ions of energies ≥40 keV. This threshold energy is an order of magnitude higher than the maximum energy (6 keV) of the energetic neutral atoms (ENA) to be measured by the forthcoming IBEX mission. On the other hand, the energy range of SOHO/CELIAS/HSTOF ENA measurements is 58-88 keV for H and 28-58 keV/n for He atoms, which overlaps with the energy range of Voyagers LECP. This offers a unique opportunity to combine HSTOF ENA measurements at 1 AU with LECP ion measurements in the heliosheath and obtain information about the large-scale structure of the heliosphere.

Aims. We use energetic neutral atoms data at 1 AU and a Voyager 1 spectrum in the heliosheath to estimate the average column density of neutral hydrogen in selected sectors of the forward heliosheath.

Methods. We reanalyzed the energetic neutral hydrogen and helium data from HSTOF to identify the contribution to the neutral atoms flux originating in directions close to the apex of the Sun's motion relative to the local interstellar medium (LISM). We combine the data from HSTOF with the parent ion spectrum in the heliosheath measured by Voyager 1 to derive the background neutral hydrogen column density N_H in the heliosheath and estimate the thickness L of the heliosheath within ±45° from the apex direction and in two 90° wide flank sectors.

Results. In the forward sector of the heliosheath N_H = (0.63±0.19) 10¹⁴ cm^-2, corresponding to the thickness L = (42±12 AU)/n_H/(0.1 cm^-3) where n_H = average H density in the heliosheath. This is within the range of values predicted by theoretical models, but suggests that the heliosheath is thinner than expected. The hydrogen column densities in the flank sectors are not symmetric relative to the apex, but the difference is within the statistical uncertainty. The H/He ratio measured by HSTOF is lower than the value following from Voyager 1 heliosheath spectra.