The boundary conditions of the heliosphere: photoionization models constrained by interstellar and in situ data

¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MS 83, Cambridge, MA 02138, USA e-mail: jslavin@cfa.harvard.edu
² University of Chicago, Department of Astronomy and Astrophysics, 5640 S. Ellis Ave., Chicago, IL 60637, USA e-mail: frisch@oddjob.uchicago.edu

Received: 15 June 2007
Accepted: 7 May 2008

Abstract

Context. The boundary conditions of the heliosphere are set by the ionization, density, and composition of inflowing interstellar matter.

Aims. Our aim is to constrain the properties of the Local Interstellar Cloud (LIC) at the heliosphere, which requires radiative transfer ionization models.

Methods. We modeled the background interstellar radiation field using observed stellar FUV and EUV emission and the diffuse soft X-ray background. We also modeled the emission from the boundary between the LIC and the hot Local Bubble plasma, assuming that the cloud is evaporating because of thermal conduction. We created a grid of models covering a plausible range of LIC and Local Bubble properties, and used the modeled radiation field as input to radiative transfer/thermal equilibrium calculations using the Cloudy code. Data from in situ observations of He⁰, pickup ions and anomalous cosmic rays in the heliosphere, as well as from absorption line measurements towards ϵ CMa were used to constrain the input parameters.

Results. A restricted range of assumed LIC H i column densities and Local Bubble plasma temperatures produced models that match all the observational constraints. The relative weakness of the constraints on N(H i) and T_h contrast with the narrow limits predicted for the H⁰ and electron density in the LIC at the Sun, n(H⁰) = 0.19 - 0.20 cm^-3and n_e = 0.07 ± 0.01 cm^-3. Derived abundances are mostly typical of low-density gas, with sub-solar Mg, Si, and Fe, possibly subsolar O and N, and S about solar; however, C is supersolar.

Conclusions. The interstellar gas at the Sun is warm, low-density, and partially ionized, with n(H) ≈ 0.23 - 0.27 cm^-3, T ≈ 6300 K, X(H⁺) ~ 0.2, and X(He⁺) ~ 0.4. These results appear to be robust since acceptable models are found for substantially different input radiation fields. Our results favor low values for the reference solar abundances for the LIC composition.