Wind Observations of Extreme Ion Temperature Anisotropies Within the Lunar Wake

We describe Wind observations of a lunar wake encounter which occurred on July 18, 2002. The observations were made at a distance of 15 lunar radii (R_L) downstream, and the data suggest that the moon was immersed in magnetosheath plasma at this time. The large azimuthal flows associated with the sheath region mean that the wake crossing was completed some 90 minutes before the spacecraft entered the lunar shadow. The most extensive observations of spacecraft encounters with the lunar wake reported previously occurred at ~ 2 R_L (Explorer 35) and ~ 6 R_L (Wind), thus, this is the first time such an event at this far distance has been reported on. A characteristic feature of the lunar wake is the presence of two ion beams moving with different velocities. Improvements in the analysis procedure for Wind/SWE mean that for the first time we are able to discern the thermal anisotropy of these ion components. We find that both ion components exhibit an extreme temperature anisotropy with T_bot > 4T_ł. We present an examination of the possible causes and implications of this extreme anisotropy.