Wind observations of extreme ion temperature anisotropies in the lunar wake

We describe Wind observations of two lunar wake encounters which occurred on 12-13 November 1996 and 18 July 2002. The observations were made at downstream distances of around 25 and 15 lunar radii (R-L), respectively. Both encounters occurred prior to the spacecraft entering the lunar shadow; one event took place within the magnetosheath. A characteristic feature of the lunar wake is the presence of two counterstreaming ion beams drawn in from either flank. We find that both ion components exhibit an extreme temperature anisotropy, often with T(perpendicular to)similar to10T(parallel to). The anisotropy is greatest in the central wake region. It appears that the anisotropy arises through the conservation of adiabatic invariants as solar wind plasma expands to fill in the cavity behind the Moon. Despite their large anisotropy, the proton distributions appear stable to the cyclotron instability. Correlated field and flow directional changes show that the wake geometry is dependent upon the prevailing magnetic field orientation.