Inferring lunar wake potentials from electron phase space densities

TL;DR

This paper introduces a Hamiltonian inversion method to infer electric potential profiles from electron phase space densities in the lunar wake, addressing asymmetry and shocks, validated with simulations and lunar data.

Contribution

The paper develops a novel Hamiltonian inversion technique that accurately reconstructs electric potentials in complex plasma environments like the lunar wake.

Findings

Validated the method against particle-in-cell simulations at different wake stages.

Applied to ARTEMIS data, inferred potential drops of approximately 15 and 5 times the electron temperature.

Captured shock-related potential enhancements in the lunar wake.

Abstract

Inferring electric potentials from electron phase space density measurements in the lunar wake is complicated by two challenges: the asymmetry between the sunward and anti-sunward sides of the wake driven by the solar wind strahl, and the presence of ion acoustic shocks in the central wake. We develop the Hamiltonian inversion method, which infers the full spatial electric potential profile by exploiting the quasi-static Vlasov equilibrium condition $f = f(H)$, where $H$ is the electron Hamiltonian. The method addresses both challenges through a domain-decomposition strategy: on the two sides of the wake the potential is inferred independently by minimizing the misfit between the observed phase space density and a self-consistently reconstructed $f_\mathrm{interp}(\tilde{H})$, while in the central wake where flat-top trapped electron distributions are present the potential is inferred directly from the flat-top width. We validate the method against particle-in-cell simulation data at two evolutionary stages of the lunar wake: an early stage where strahl asymmetry is strong but no shocks have formed, and a later stage where ion acoustic shocks and flat-top distributions are present. We then apply the method to two ARTEMIS lunar wake crossings at the same evolutionary stages, inferring normalized potential drops of $e\Delta\varphi/T_e \sim 15$ and $\sim 5$ respectively and capturing shock-associated potential enhancements in the central wake. The method is broadly applicable to plasma environments where electrons are in quasi-static equilibrium with a field-aligned electric potential.