Wind observations of extreme ion temperature anisotopies in the lunar wake

D. Clack, J. C. Kasper, A. J. Lazarus, J. T. Steinberg, and W. M. Farrell

Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139

Abstract:

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.

A characteristic feature of the lunar wake is the presence of two counter-streaming ion beams drawn in from either flank. We find that both ion components exhibit an extreme temperature anisotropy, often with T_^ ~ 10T_||. 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. We also find evidence that the plasma void in the central wake is being replenished by electrostatic processes. Correlated field and flow directional changes suggest that the wake geometry is dependent upon the prevailing magnetic field orientation, and might display an elliptical cross-section drawn out along the field direction.

submitted, Geophys Res. Lett., 2003