Properties of Quiet Magnetotail Plasma Sheet at Lunar Distances

Abstract

AbstractThe 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.