Properties of Quiet Magnetotail Plasma Sheet at Lunar Distances

The Earth's magnetotail at lunar distances (R ≈ 60 RE) serves as a unique laboratory to study plasma dynamics in a weak, highly fluctuating magnetic field with a strong magnetic field gradient. In particular, studies of the of quiet‐time plasma in the lunar‐distant magnetotail can inform us about plasma entry to the magnetosphere and sources of magnetospheric plasma populations. We use the data collected by the two lunar‐orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft during its 2013–2019 magnetotail traversals to infer the average thermodynamic and spectral properties of plasma populations in the quiet‐time (i.e., low geomagnetic activity and absence of fast plasma flows) magnetotail at lunar distances. We found that plasma temperature and density in the quiet‐time magnetotail at R ≈ 60 RE are organized by the magnetic field. Three distinct regions with plasma β ≫ 1, β ∼ 1, and β ≪ 1, the central plasma sheet (CPS), outer plasma sheet (OPS), and lobes are sampled. We found that temperatures and energy spectra of ion populations in CPS, OPS, and lobes regimes are different: the hotter CPS temperatures scale with the kinetic energy of solar wind protons; cold OPS/lobe ions are, likely, of ionospheric origin. The ion and electron particle spectra in CPS, OPS, and lobes are nonthermal and reasonably well fitted by the Kappa function, with κ exponent varying between 2.5 and 3.5. Plain Language Summary Earth's only natural satellite, the Moon orbits around the Earth at a distance of about 239,000 miles (385,000 km). The two lunar‐orbiting Acceleration, Reconnection, Turbulence and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft orbit around the Moon in stable equatorial, high‐eccentricity orbits, of 100‐km × 19,000‐km altitude. The Moon and the ARTEMIS satellites spend about 25 days in the solar wind and 3–4 days inside the Earth's magnetosphere in the magnetotail. Using the comprehensive instrumentation of the ARTEMIS satellites, we study properties of the plasma and the magnetic field in the Earth's magnetotail at lunar distances. Our statistical results are valuable for the NASA Lunar Gateway HERMES mission preparation. Key Points We examine the properties of plasma populations in the magnetotail at R ≈ 60 RE under quiet conditions Non‐Maxwellian ion and electron populations dominate at βi ≤ 1 and βi > 1 regimes Average ion and electron equatorial temperatures