Physics
Scientific paper
May 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agusm...p22a03h&link_type=abstract
American Geophysical Union, Spring Meeting 2001, abstract #P22A-03
Physics
5109 Magnetic And Electrical Properties, 5421 Interactions With Particles And Fields, 5470 Surface Materials And Properties, 6250 Moon (1221), 7807 Charged Particle Motion And Acceleration
Scientific paper
The Electron Reflectometer (ER) on board Lunar Prospector (LP) was designed to measure the magnetic reflection of ambient electrons by lunar crustal magnetic fields. This magnetic reflection was expected to be adiabatic and constant as a function of energy. Instead, our measurements showed that electron reflection was often energy-dependent. Furthermore, on many occasions electron beams were observed traveling up magnetic field lines from the lunar surface. Both of these observations were quite puzzling until we realized that we were observing the effects of electric as well as magnetic fields. It has long been expected on theoretical grounds that the lunar surface, when in shadow, might charge up to large negative potentials. Estimates of the magnitudes of these potentials, however, varied widely and depended critically on rather uncertain assumptions of the ambient plasma parameters and the secondary emission characteristics of the lunar surface. Now, with LP data, we have the first direct measurements of lunar nightside electric potentials. Our data clearly shows that the lunar night side regularly charges to average values of 30-40 volts negative with respect to the LP spacecraft. We observe these potentials by measuring the distributions of reflected electrons, and also by observing beams of secondary electrons which are accelerated through the potentials. The results of these two methods are consistent, leaving little doubt that we are indeed observing electrostatic charging of the lunar night side. Simple current balance models of the electrostatic charging process, using measured ambient plasma temperatures and lunar secondary emission characteristics, give estimates which are of the same order of magnitude as the potentials we observe.
Acuña Mario Humberto
Binder Alan B.
Halekas Jasper S.
Hood Lon L.
Lin Robert P.
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