Earth Planets Space, Vol. 64 (No. 2), pp. 73-82, 2012
J. S. Halekas1,2, A. Poppe1,2, G. T. Delory1,2, W. M. Farrell4,2, and M. Horányi2,3
1Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720, USA
2NASA's Lunar Science Institute, NASA Ames Research Center, Moffett Field, CA 94035, USA
3Laboratory for Atmospheric and Space Physics, and Department of Physics, University of Colorado, Boulder, CO 80309, USA
4NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
(Received January 18, 2011; Revised March 10, 2011; Accepted March 22, 2011; Online published March 8, 2012)
Electron distributions measured by Lunar Prospector above the dayside lunar surface in the solar wind often have an energy dependent loss cone, inconsistent with adiabatic magnetic reflection. Energy dependent reflection suggests the presence of downward parallel electric fields below the spacecraft, possibly indicating the presence of a standing electrostatic structure. Many electron distributions contain apparent low energy (<100 eV) upward-going conics (58% of the time) and beams (12% of the time), primarily in regions with non-zero crustal magnetic fields, implying the presence of parallel electric fields and/or wave-particle interactions below the spacecraft. Some, but not all, of the observed energy dependence comes from the energy gained during reflection from a moving obstacle; correctly characterizing electron reflection requires the use of the proper reference frame. Non-adiabatic reflection may also play a role, but cannot fully explain observations. In cases with upward-going beams, we observe partial isotropization of incoming solar wind electrons, possibly indicating streaming and/or whistler instabilities. The Moon may therefore influence solar wind plasma well upstream from its surface. Magnetic anomaly interactions and/or non-monotonic near surface potentials provide the most likely candidates to produce the observed precursor effects, which may help ensure quasi-neutrality upstream from the Moon.
Key words: Moon, solar wind, precursor effects, wave-particle interactions.