  The transition between the supersonic solar wind and the subsonic
heliosheath, the termination shock (TS), was observed by Voyager 2 (V2) on 2007
August 31-September 1 at a distance of 84 AU from the Sun. The data reveal
multiple crossings of a complex, quasi-perpendicular supercritical shock. These
experimental data are the starting point for a more sophisticated analysis that
includes computer modeling of a shock in the presence of pickup ions (PUIs).
here, we present two-dimensional (2-D) particle-in-cell (PIC) simulations of
the TS including PUIs self-consistently. We also report the ion velocity
distribution across the TS using the Faraday cup data from V2. A relatively
complete plasma and magnetic field data set from V2 gives us the opportunity to
do a full comparison between the experimental data and PIC simulation results.
Our results show that: (1) The nonstationarity of the shock front is mainly
caused by the ripples along the shock front and these ripples from even if the
percentage of PUIs is high. (2) PUIs play a key role in the energy dissipation
of the TS, and most of the incident ion dynamic energy is transferred to the
thermal energy of PUIs instead of solar wind ions (SWIs). (3) The simulated
composite heliosheath ion velocity distribution function is a superposition of
a cold core formed by transmitted SWIs, the shoulders contributed by the hot
reflected SWIs and directly transmitted PUIs, and the wings of the distribution
dominated by the very hot reflected PUIs. (4) The V2 Faraday cups observed the
cool core of the distribution, so they saw only a tip of the iceberg. For the
evolution of the cool core distribution function across the TS, the computed
results agree reasonably well with the V2experimental results.
