Detailed structure and dynamics in particle-in-cell simulations of the lunar wake

The solar wind plasma from the Sun interacts with the Moon, generating a wa ke structure behind it, since the Moon is to a good approximation an insula tor, has no intrinsic magnetic field and a very thin atmosphere. The lunar wake in simplified geometry has been simulated via a 1 1/2 D electromagneti c particle-in-cell code, with high resolution in order to resolve the full phase space dynamics of both electrons and ions. The simulation begins imme diately downstream of the moon, before the solar wind has infilled the wake region, then evolves in the solar wind rest frame. An ambipolar electric f ield and a potential well are generated by the electrons, which subsequentl y create a counter-streaming beam distribution, causing a two-stream instab ility which confines the electrons. This also creates a number of electron phase space holes. Ion beams are accelerated into the wake by the ambipolar electric field, generating a two-stream distribution with phase space mixi ng that is strongly influenced by the potentials created by the electron tw o-stream instability. The simulations compare favorably with WIND observati ons. (C) 2001 American Institute of Physics.