David L. Reasoner

ABSENCE OF THE PLASMA SHEET AT LUNAR DISTANCE DURING GEOMAGNETICALLY QUIET TIMES.

Abstract Plasma data from the Apollo XIV Charged Particle Lunar Environment Experiment (CPLEE) are presented to show that contrary to previously published analyses the plasma sheet does not extend to the lunar orbit with a thickness of 8 R H . Two electron spectral types are observed: (1) low energy photoelectrons with no statistically significant medium and high energy fluxes, and (2) double peaked medium and high energy electrons. The second type is observed either coincident with auroral substorms or at the center of the tail during quiet times. These spectra are one to several orders of magnitude less intense than plasma sheet spectra measured near 20 R E .

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.