The Range of Electron Plasma Temperature and Density at Lunar Orbit

The dayside lunar photoelectron sheath generates near-surface electrostatic fields that reflect a fraction of solar wind electrons and accelerate low-energy electrons. As the Moon passes from the solar wind through the magnetosheath and into the magnetotail plasma sheet, electron temperature ( T e ) and density ( n e ) can vary by more than 2 orders of magnitude, which leads to large variations of the lunar photoelectron sheath. In order to prepare for a human presence at lunar orbit, this paper quantifies the extremes of T e and n e observed by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS) extending over an entire solar cycle. A statistical analysis of ARTEMIS P1 data shows that in solar wind, T e is in the range 1.6–76 eV and n e is in the range 0.06–103 cm −3 . In the magnetotail (including magnetosheath, lobe, and plasma sheet), T e is in the range 6–1800 eV and n e is in the range 0.03–20 cm −3 (0.03 cm −3 is the instrument threshold, so the lowest density is likely smaller than this). These values were sustained for 1 minute such that the entire moon was immersed in plasma with a particular T e or n e . The solar wind T e and n e do not have a strong correlation with the solar cycle, but in the magnetotail near midnight, the range of T e and n e is significantly larger at solar minimum compared to solar maximum.