  Radiative transfer equilibrium models of nearby interstellar matter (ISM)
yield the boundary conditions of the heliosphere when constrained with
observations of ISM inside and outside of the heliosphere. Filtration factors
for interstellar neutrals crossing the heliosheath region, from charge exchange
with interstellar plasma, are given for H, He, N, O, Ar, and Ne. The best
models predict n(HI)~0.2 /cc, n(e)~0.1 /cc, however if the isotropic 2 kHz
emission observed by Voyager (Kurth & Gurnett 2003) is formed in the
surrounding ISM, an alternate model with lower electron densities is indicated.
Observations of nearby ISM, the radiative transfer models, and historical 10Be
records provide information on past variations in the galactic environment of
the Sun.
