Inhomogeneity in the Local ISM and its Relation to the Heliosphere
  This paper reviews past research and new studies underway of the local
interstellar environment and its changing influence on the heliosphere. The
size, shape, and physical properties of the heliosphere outside of the
heliopause are determined by the surrounding environment - now the outer region
of the Local Interstellar Cloud (LIC). Analysis of high-resolution HST spectra
led to a kinematic model with many interstellar clouds. This analysis
identified fifteen clouds located within about 10 pc of the Sun and their mean
temperatures, turbulence, and velocity vectors. With the increasing number of
sight lines now being analyzed, we find that temperatures and turbulent
velocities have spatial variations within the LIC and other nearby clouds much
larger than measurement uncertainties, and that these spatial variations appear
to be randomly distributed and can be fit by Gaussians. The inhomogeneous
length scale is less than 4,000 AU, a distance that the heliosphere will
traverse in less than 600 years. The temperatures and turbulent velocities do
not show significant trends with stellar distance or angle from the LIC center.
If/when the Sun enters an inter-cloud medium, the physical properties of the
future heliosphere will be very different from the present. For the heliosheath
and the very local interstellar medium (VLISM) just outside of the heliopause,
the total pressures are approximately equal to the gravitational pressure of
overlying material in the Galaxy. The internal pressure in the LIC is far below
that in the VLISM, but there is an uncertain ram pressure term produced by the
flow of the LIC with respect to its environment.
