Characteristics of the lunar photoelectron layer in the geomagnetic tail

The charged particle lunar environment experiment (CPLEE), a part of the Apollo 14 lunar surface package, 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. No detectable flux above 200 ev has been observed. Experimental data for periods while the moon was in the earth's magnetotail for electrons with energies 40 ev ≤ E ≤ 200 ev follow a power-law spectrum j(E) = j0(E/E>0)−μ with 3.5 ≤ μ ≤ 4. In the absence of photoelectrons with E > 200, we assume that the surface potential is at least 200 volts. The modulation of this potential in the presence of intense plasma-sheet fluxes has been observed. Also, a detailed history of the February 10, 1971, total lunar eclipse, to determine the source distribution of high-energy solar photons, is presented. A classical penumbral-umbral behavior indicates that at the time of the eclipse the emission of higher-energy photons was uniform over the solar disc. 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(hν), obtained from various experimental sources, and the photoelectron yield function of the surface materials, Y(hν), are two parameters of the solution. Energy spectra at the height of the measurements for various values of Y(hν) were computed until a fit to experimental data was obtained. Using a functional form Y(hν) = [Y0(hν − W) / (W / 2)] for 6 ev ≲ hν ≲, 9 ev and Y(hν) = Y0 for hν >9 ev, where the lunar-surface work function W was set at 6 ev, we calculated a value of Y0 = 0.1 electrons/photon. The solution also showed that the photoelectron density falls by 5 orders of magnitude within 10 meters of the surface, but the layer actually terminates several hundred meters above this height. A hydrostatic model of the photoelectron layer has also been developed. It is shown that the numerically calculated pressure, density, and potential can be approximated by solving the hydrostatic equations with an equation of state P/n1/2 = constant out to 200 cm from the surface. Beyond this height, the equation of state shifts toward the isothermal case, P/n = constant.