A hydrodynamic model is developed for the structure of a cosmic-ray-mo
dified termination shock. The model is based on the two-fluid equation
s of diffusive shock acceleration (Drury and Volk, 1981). Both the ste
ady state structure of the shock and its interaction with outer helios
pheric disturbances are considered. Under the assumption that the sola
r wind is decelerated by diffusing interstellar cosmic rays, it is sho
wn that the natural state of the termination shock is a gradual decele
ration and compression, followed by a discontinuous jump to a downstre
am state which is dominated by the pressure contribution of the cosmic
rays. A representative model is calculated for the steady state which
incorporates both interstellar cosmic ray mediation and diffusively a
ccelerated anomalous ions through a proposed thermal leakage mechanism
. The interaction of large-scale disturbances with the equilibrium ter
mination shock model is shown to result in some unusual downstream str
ucture, including transmitted shocks and cosmic-ray-modified contact d
iscontinuities. The structure observed may be connected to the 2-kHz o
uter heliospheric radio emission (Caims et al., 1992a, b). The time-de
pendent simulations also demonstrate that interaction with solar wind
compressible turbulence (e.g., traveling interplanetary shocks, etc.)
could induce the termination shock to continually fluctuate between co
smic-ray-dominated and gas-dynamic states. This fluctuation may repres
ent a partial explanation of the galactic cosmic ray modulation effect
and illustrates that the Pioneer and Voyager satellites will encounte
r an evolving shock whose structure and dynamic properties are strongl
y influenced by the mediation of interstellar and anomalous cosmic ray
s.