HR: 0800h
AN: SH51B-0277    [Abstracts]
TI: Large Negative Lunar Surface Potentials
AU: * Halekas, J S
EM: jazzman@ssl.berkeley.edu
AF: Space Sciences Laboratory, University of California, Berkeley, CA 94720 United States
AB: Previous observations by Lunar Prospector (LP) revealed the presence of monoenergetic field-aligned upward-going electron beams of moderate energies ($<$100 eV) coming from the lunar night side, both in the solar wind wake and in the magnetotail. Simultaneous observations of energy-dependent loss cones established that these beams likely consisted of secondary electrons accelerated from the lunar surface by an electrostatic potential. Due to the moderate potentials implied and the rather large Debye lengths (and therefore electric field scale heights) of $\sim$100-1000 m in these low-density regions, these potentials were not considered hazardous. A recent re-examination of LP data, however, has shown that, in more energetic plasma environments like the magnetotail plasmasheet, upward-going electron beams sometimes reach energies as high as 1-2 keV. These more energetic beams are commonly seen above the lunar night side and (though less frequently) sometimes even forward of the terminator, implying that plasmasheet currents are sometimes high enough to charge the surface negative despite photoemission. Higher plasma densities in the plasmasheet mean Debye lengths and therefore scale heights as small as $\sim$1-20 m, which along with the large potentials implied, could lead to electric fields as high as $\sim$100-1000 V/m. These fields are large enough to present a serious hazard to astronauts and equipment on the lunar surface. Though we are unfortunately limited by the low time and therefore spatial resolution of the LP measurements, I present here a first-cut characterization of the global distribution of large negative lunar surface potentials.
DE: 7815 Electrostatic structures
DE: 5421 Interactions with particles and fields
DE: 6250 Moon (1221)
DE: 2764 Plasma sheet
DE: 2780 Solar wind interactions with unmagnetized bodies
SC: SPA-Solar and Heliospheric Physics [SH]
MN: 2004 AGU Fall Meeting