Plasma sheet at lunar distance: Characteristics and interactions with the lunar surface

The plasma sheet at lunar distance is investigated with the use of data from the charged particle lunar environment experiment (CPLEE), complemented with data from the Explorer 35/ARC magnetometer. It is shown that the presence of the lunar surface does not appreciably affect measurements of the plasma sheet characteristics by the lunar-based CPLEE instrument. In particular, the lunar surface generally does not shadow plasma sheet particles. This may be due to rapid random passage (>40 km/sec) of magnetotail field lines with respect to the lunar surface or to diffusion of plasma sheet electrons into the flux tubes in contact with the lunar surface. The plasma sheet is generally observed as a rapid increase in observed particle fluxes and a simultaneous decrease in field strength. Analysis of the CPLEE data shows that, for the plasma sheet at lunar distance, typical quiet time parameters are n = 0.10 ± 0.05 cm−3, kTe = 200 ± 50 ev, and kTi = 2.5 ± 0.75 kev. The typical ranges for the parameters are 0.05–0.20 cm−3 for n, 175–325 ev for kTe, and 1–5 kev for kTi. The typical total (plasma plus magnetic) energy density is in the range 150–350 ev/cm³. A statistical analysis of the CPLEE data shows that the plasma sheet in the midnight sector has a thickness of 5 RE ± 2 RE. Geomagnetic activity reduces the probability of encounters between the moon and the plasma sheet. This finding is consistent with the concept that geomagnetic activity increases the variance of the plasma sheet from its average location and/or decreases its thickness.