Ion entry into the wake behind a nonmagnetized obstacle in the solar wind: Two‐dimensional particle‐in‐cell simulations

The ion entry into the wake behind an obstacle in the solar wind is studied using two‐dimensional, electromagnetic full‐particle simulations. A significant difference is found between the number of ions and electrons in the near wake, mainly due to the negative electric charge on the nightside surface of the obstacle. The ion acceleration toward the void is observed far from the position of the rarefaction wave expected in the self‐similar solution. The velocity profile of the ions in the wake approaches the self‐similar solution with finite ion temperature asymptotically until they reach the distance where the ions from both sides of the wake meet. The ions that entered from both sides are accelerated in the opposite directions. They raise the ion temperature and the pressure in the center of the wake, although each component remains cool when treated separately. The electron temperature appears nearly constant, except for the edge of the complete void of electrons. The large‐scale obstacle and a slow solar wind are favorable conditions for a detection of well‐accelerated ions near the nightside surface of the obstacle, because they have enough time to accelerate. The direction of the electric field in the wake seems consistent with the gradient of the electron pressure.