Charged Particle Lunar Environment Experiment (CPLEE)

The Charged Particle Lunar Environment Experiment (CPLEE), a part of the Apollo 14ALSEP, is an ion-electron spectrometer capable of measuring ions and electrons with energies between40 eV and 50 keY. The instrument, with apertures 26 em above the surface, has detected a photo-electron gas layer above the sunlit lunar surface, with energies ranging up to 200 eV. Experimentaldata for periods while the Moon was in the Earth's magnetotail for electrons with energies40 eV ~ E~ 200 eV follow a power law spectrumj (E)= jo(E/Eo)-'" with 3.5 ~ ,u ~ 4. In the absenceof photoelectrons with E > 200 eV, we assume that the surface potential is at least 200 V. Themodulation of this potential in the presence of intense plasma sheet fluxes has been observed.Numerical solutions for the variation of electron density and potential as functions of height abovethe lunar surface were obtained. The solar photon spectrum I (hv), obtained from various experi-mental sources, and the photoelectron yield function of the surface materials, Y (hv), are two para-meters of the solution. Energy spectra at the height of the measurements for various values of Y (hv)were computed until a fit to experimental data was obtained. Using a functional form Y (hv) ==[Yo(hv-W)]/(W/2) for 6eV;;;;hv;;;;9eV and Y(hv)=Yo for (hv)>9eV where W,thelunarsurface work function, was set at 6 eV, we calculated a value of Yo= 0.1 electrons photon-1 . Thesolution also showed that the photoelectron density falls by 5 orders of magnitude within 10 m ofthe surface, but the layer actually terminates several hundred meters above this height.