Characteristics of the lunar photoelectron layer in the geomagnetic tail

Abstract

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.

Lunar surface electric potential changes associated with traversals through the Earth's foreshock

“…but no flux with enellies above 200 eV. Reasoner and Burke (1972) took this result to indicate that the sunlit surface was positively charged, and perhaps with a potential of 200 V.…”

A survey of the plasma electron environment of Jupiter: A view from Voyager

“…In Figure 1 we have plotted the empirical "return current relation" derived from our cruise analysis LSittler and Scudder (1981) exp+ectx,tiona at 1 AU ( see Reasoner and Burke (1972) With the return current relation developed during cruise, the analysis of the electron data taken within Jupiter's bow shock is completely independent of the ion measurements. Our procedure is to assume a fi, st guess of the floating potential, 0SC(1) , on the order of the most probable energy of the data under consideration; under this assumption the return current to the spacecraft implied by the measurements of electrons with observed energies E > et SC (1) is determined.…”

The possibility of studying the lunar ionosphere with the SELENE radio science experiment

“…Charged particles that are present near the lunar surface are mostly the result of the interaction of the solar ultraviolet radiation and the solar wind with the lunar atmosphere and regolith (Stern, 1999). Very near the dayside surface a photoelectron sheath with a density of the order of 10 4 cm −3…”

“…Above this layer is an ionosphere, whose density is generally thought to be on the order of 1 cm −3 in the range from the surface to 100 km altitude (Stern, 1999). This ionosphere is probably produced by the photo-ionization of the tenuous neutral atmosphere (exosphere), which is composed mostly of Ar, Ne and He with a density of ∼10 5 cm −3 on the dayside (Hodges et al, 1974).…”

“…and a height of a few hundred meters exists (Reasoner and Burke, 1972;Walbridge, 1973). Above this layer is an ionosphere, whose density is generally thought to be on the order of 1 cm −3 in the range from the surface to 100 km altitude (Stern, 1999).…”