HR: 1340h
AN: SH14A-1697    [Abstracts]
TI: The Boundary Conditions of the Heliosphere: Photoionization Models Constrained by In Situ and Interstellar Data
AU: Frisch, P C
EM: frisch@oddjob.uchicago.edu
AF: University of Chicago, Dept. Astronomy and Astrophysics 5640 S. Ellis Ave., Chicago, IL 60637, United States
AU: * Slavin, J D
EM: jslavin@cfa.harvard.edu
AF: Harvard-Smithsonian Center for Astrophysics, 60 Garden St. MS 83, Cambridge, MA 02138, United States
AB: The Sun is immersed in a small, warm, and low density interstellar cloud, known as the Local Interstellar Cloud (LIC). The LIC thickness, less than about 100,000 AU, and low density, n~0.25 cm^-3 at the Sun, yield a LIC that is transparent to helium-ionizing radiation while having significant optical depth to hydrogen-ionizing radiation. As a result, the heliosphere boundary conditions depend on the radiative transfer properties of the surrounding LIC. At the heliosphere, elements with first ionization potential of 13.6-25 eV, e.g. H, He, N, O, Ne, and Ar, are partially ionized with fractional ionizations of about 0.2 to 0.7. We present results on the boundary conditions of the heliosphere that are based on models that include the radiation field gradient, and the thermal balance and photoionization of the surrounding interstellar material. The model constraints are data on interstellar material inside of the heliosphere, such as pickup ions, He, N, O, Ar, Ne, and anomalous cosmic rays, as well as astronomical observations of interstellar gas towards the star eta CMa. The interstellar radiation field is made up of stellar FUV and EUV, as well as diffuse soft X-ray emission from the hot gas of the Local Bubble and the thermally conductive boundary between the LIC and the Local Bubble. We find that we can satisfy the observational constraints, including the temperature and density of neutral He in the heliosphere, for a range of model parameters that affect the radiation field. Despite the wide range of possible input parameters, the output values for quantities important for shaping the heliosphere are confined to a fairly small range: n(H°)=0.19 -- 0.20 cm^-3, n(H⁺)=0.04 -- 0.07 cm^-3. The best models indicate that the ISM creating the heliosphere boundary conditions is low density and partially ionized, n(H°)=0.19 cm^-3 and n(H⁺)=0.05 cm^-3. About 38% of the helium is ionized, and about 22% of the hydrogen is ionized. At the heliosphere edge, the neutral hydrogen density is always be higher than ten times the neutral helium density, or n(H°)>0.151 cm^-3, because of the radiation field hardness. Our results favor a reference abundance pattern for the LIC that is comparable to the protosolar abundances. The depletion pattern of refractory elements onto interstellar dust grains is characteric of warm low density interstellar material. The LIC gas-to-dust mass ratio derived from astronomical data depends strongly on the assumed reference abundance set, and ranges from 140 to 490. In contrast, in situ observations of interstellar dust in the heliosphere yield a value 116 to 124.
DE: 2124 Heliopause and solar wind termination
DE: 2126 Heliosphere/interstellar medium interactions
DE: 2129 Interplanetary dust
DE: 2144 Interstellar gas
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2007 Fall Meeting