  Voyager 1 has explored the solar wind-interstellar medium interaction region
between the Terminal Shock and the Heliopause, following the intensity
distribution of the shock accelerated anomalous component of cosmic rays in the
MeV energy range. The sudden disappearance of this component at 121.7 AU from
the sun is discussed in terms of three models for the transition into the
interstellar plasma flow. Particles trapped flowing parallel to the boundary
may penetrate up to one Larmour radius beyond. If the boundary is stationary,
Voyager 1 directly samples this distance. The boundary could flap, depending on
Heliosheath pressure changes and Voyager 1 then samples the extent of this
motion. Finally, a turbulent boundary layer is considered in which the MeV
particle distribution falls off with distance, thus measuring diffusion within
the layer.
