Microstructure of the heliospheric termination shock: Full particle electrodynamic simulations

A particle-in-cell code is used to examine kinetic properties of pickup ions at the heliospheric termination shock. The code treats the pickup ions self-consistently as a third component. The simulations are one-dimensional in spatial variations. We use a relative pickup ion density of 30% and two different values of the magnetic field-shock normal angle: Theta(Bn) = 90 degrees and Theta(Bn) = 87 degrees. The oblique shock is chosen in order to allow for wave vectors parallel to the magnetic field due to instabilities in the foot. In addition, a run is presented with a 60% relative pickup ion density to investigate a pickup ion-dominated shock. Upstream of the ramp is an extended foot due to reflected pickup ions. In this foot the magnetic field continuously increases, and the bulk speed of the pickup ions as well as the bulk speed of the solar wind ions decrease and reach at the magnetic field ramp the downstream value. The positive bulk velocity of the pickup ions in the extended foot perpendicular to the magnetic field and to the shock normal causes an electric field in the shock normal direction. This leads to a large increase of the shock potential well upstream of the magnetic field ramp. The maximum value of the potential is similar to 0.35 the shock ram energy and is by a factor 5 larger than expected for a weak shock without pickup ions. Pickup ion reflection at the shock is almost 100%; part of the pickup ions are essentially specularly reflected by the magnetic field force term of the Lorentz force in the overshoot and part of the pickup ions are reflected in the extended foot due to a combination of the magnetic force term and the cross-shock potential. In the 30% pickup ion case, about 90% of the total thermal energy gain in the shock is gained by pickup ions, a little under 10% by the solar wind ions. The thermal energy gain by pickup ions increases as the pickup ion relative density increases. The pickup ion temperature increases continuously from the upstream edge of the extended foot to the shock ramp and then stays constant through the overshoot and downstream.