STEADY-STATE AND DYNAMICAL STRUCTURE OF A COSMIC-RAY-MODIFIED TERMINATION SHOCK

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.