NASA Goddard Space Flight Center, Laboratory for Extraterrestrial Physics, Greenbelt, Maryland 20771

We report on WIND magnetic field observations at ~ 6.8 R_L downstream of the moon on 27th December 1994. The moon was in the solar wind during the encounter. IMP-8 observations are used to determine baseline IMF conditions, and therefore determine those features in the WIND data which are related to its proximity to the moon. Previous Explorer 35 observations suggest that the lunar wake is not detectable beyond a downstream distance of ~ 4 R_L. However, despite the distance of WIND from the moon, we observe a slight decrease in field intensity just prior to the spacecraft entering the optical shadow, a slight field strength enhancement whilst in shadow, and perhaps a weak depression once the spacecraft re-emerges into sunlight. These signatures closely resemble, but are weaker than, the previous observations. We conclude that a lunar wake did extend to these distances at the time of this encounter. We also note a rotation in field direction some distance outside of the wake signature which may be attributed to the crossing of the lunar mach cone boundary. We discuss the observations in terms of simple models of the solar wind interaction with an insulating body.

Geophys. Res. Lett., 23, No. 10, 1263-1266, May 1996