Measurement of the Lunar Photoelectron Layer in the Geomagnetic Tail
The Charged Particle Lunar Environment Experiment (CPLEE), a part of the Apollo 14 ALSEP, is an ion-electron spectrometer capable of measuring ions and electrons with energies between 40 eV and 50 keV. The instrument, with apertures 26 cm above the surface, has detected a photoelectron gas layer above the sunlit lunar surface, with energies ranging up to 200 eV. Experimental data for periods while the Moon was in the Earth’s magnetotail for electrons with energies 40eV ≤5 E ≤ 200eV follow a power law spectrum j (E) = j 0(E/E 0)− μ with 3.5 ≤ μ ≤ 4. In the absence of photoelectrons with E > 200 eV, we assume that the surface potential is at least 200 V. The modulation 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 above the lunar surface were obtained. The solar photon spectrum I (hv), obtained from various experimental sources, and the photoelectron yield function of the surface materials, Y (hv), are two parameters 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)= =[Y 0(hv − W)]V/(W/2) for 6 eV ≲ hv ≲ 9 eV and Y (hv) = Y 0 for (hv) > 9 eV where W, the lunar surface work function, was set at 6 eV, we calculated a value of Y 0 = 0.1 electrons photon−1. The solution also showed that the photoelectron density falls by 5 orders of magnitude within 10 m of the surface, but the layer actually terminates several hundred meters above this height.
KeywordsSolar Wind Counting Rate Plasma Sheet Lunar Surface Electron Flux
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