Magnetosheath plasma response to a slow compression

Effect of a slow compression on the magnetosheath plasma is studied using two-dimensional hybrid expanding box model. In this model the plasma is under an externally driven continuous compression and responds to this compression self-consistently. The evolution in the simulated plasma has two distinct phases: during the first one, when the system is stable with respect to the Alfven ion cyclotron (AIC) and mirror instabilities, the ions behave double adiabatically. This behavior leads to strong ion temperature anisotropies and eventually the system crosses the treshold of the AIC instability. After a short transition the second phase starts: the plasma follows the marginal stability condition of the AIC instability. During this phase the temperature anisotropy decreases whereas the ion parallel beta increases. In higher beta plasma the mirror instability appears but the mirror waves does not alter the overall system behavior. We investigate a role of the compression speed and of the presence of alpha particles. We discuss the simulation results in the magnetosheath context.