  We combine different measurements of the polytropic index of the proton
plasma in the heliosphere: i) near-adiabatic index in the inner heliosphere
~1AU, ii) subadiabatic indices in the outer heliosphere ~20-40AU, and iii)
near-zero indices in the inner heliosheath. These observations are unified by a
single theoretical model of the polytropic index throughout its radial extent
in the heliosphere; the corresponding fitting reveals the decreasing trend of
the polytropic index with increasing heliocentric distance R. We anticipate
that with increasing R, (i) the Debye length and mean-free-path decreases; (ii)
the Landau damping is less effective, transferring thus less wave energy to
particles; and (iii) the collisionality degree increases, indicating that the
proton plasma in the inner heliosheath might be collisionless.
