There's a relatively hard boundary at the heliopause.
This is not correct. The particles are not in orbit about the Sun, they're coming from the Sun--they're the solar wind. The heliopause is where the solar wind particles are stopped by the pressure of the surrounding interstellar medium. When Voyager passed that point (the heliopause), the number of particles hitting it dropped drastically.
The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance. The crossing of the heliopause should be signaled by a sharp drop in the temperature of solar wind-charged particles,[30] a change in the direction of the magnetic field, and an increase in the number of galactic cosmic rays.[34]