Three-dimensional Hybrid Simulations of the Evolution of the Martian Plasma Environment

We have studied the plasma environment of Mars by means of a three-dimensional hybrid code. In this model, the solar wind protons as well as the planetary heavy ions are represented by macroparticles. The electrons, on the other hand, form a massless, charge-neutralizing fluid. The use of a hybrid model is necessary because the pick-up gyroradius of the oxygen ions significantly exceeds the characteristic size of the planetary obstacle. The ionosphere is modeled by a Chapman layer, i.e. the local production rate depends on both the solar zenith angle and the altitude above the surface. The code also includes a multi-species representation of the ionosphere and exosphere of Mars. The simulation results regarding the shape and position of the plasma boundaries (bow shock and magnetic pile-up boundary) have shown to be in good quantitative agreement with measurements conducted by the ASPERA instuments on Phobos-2 and Mars Express. The direction and strength of the convective solar wind electric field have a decisive influence on the orientation of the ionospheric tail and the formation of the pick-up region. We will present 2d and 3d views of the heavy ion flow around Mars as well as of the magnetic field topology. The model also allows an analysis of the anicient Martian plasma environment. We compare two scenarios: The present situation and the case 3.5 billion years ago. This comparative study illustrates that the structure of the interaction region and the ionospheric loss rates are not only highly sensitive to changes in the solar UV intensity, but an important role must be ascribed to the solar wind parameters (magnetic field, density and velocity) as well.