Interaction of the solar wind with weak obstacles: Hybrid simulations for weakly active comets and for Mars

Obstacles in the solar wind are weak when their extension is small or comparable with respect to the characteristic ion gyroradii. Then the interaction gets kinetic features and hybrid models are an adequate description. A hybrid code operating on a curvilinear 3D grid is discussed and it is applied to the solar wind interaction with weak obstacles. As examples small comets and planet Mars are studied. The cometary case is adapted to 67P/Churyumov-Gerasimenko (CG) which is the target of the Rosetta mission. The hybrid simulations follow the evolution of the plasma environment between 3.5AU and 1.75AU heliocentric distance. Beyond 3.5AU CG is extremely weak and the cometary ions behave mainly as test particles. For decreasing heliocentric distances the cometary ion production goes up and thus the re-action of the cometary plasma to the solar wind becomes significant. Plasma structures like cometopause, bow shock and precursors of a magnetic cavity appear. The cometopause separates cometary plasma and solar wind plasma in a pronounced way. There is a typical asymmetry in the interaction region including the tail. The largely constant distance of Mars generates much less evolution in the plasma structures. However, partly the structures at Mars are well comparable to those of CG. The cometopause is replaced by the ion composition boundary which separates Martian plasma and solar wind plasma. The bow shock and the asymmetry in the interaction region are similar to the cometary case