Kinetic Electron and Ion Instability of the Lunar Wake Simulated at Physical Mass Ratio

TL;DR

This study uses advanced simulations to reveal how kinetic electron effects cause early disruption of ion beams in the lunar wake, highlighting phenomena that previous models overlooked.

Contribution

It introduces the first 1D PIC simulations with a physical ion-electron mass ratio to analyze lunar wake stability, emphasizing the role of non-linear electron hole growth.

Findings

Kinetic electron effects disrupt ion beams closer to the moon than hybrid models predict.

Electron holes from a velocity distribution dimple grow large enough to destabilize the wake.

Non-linear electron dynamics are crucial for understanding lunar wake stability and signatures.

Abstract

The solar wind wake behind the moon is studied with 1D electrostatic particle-in-cell (PIC) simulations using a physical ion to electron mass ratio (unlike prior investigations); the simulations also apply more generally to supersonic flow of dense magnetized plasma past non-magnetic objects. A hybrid electrostatic Boltzmann electron treatment is first used to investigate the ion stability in the absence of kinetic electron effects, showing that the ions are two-stream unstable for downstream wake distances (in lunar radii) greater than about three times the solar wind Mach number. Simulations with PIC electrons are then used to show that kinetic electron effects can lead to disruption of the ion beams at least three times closer to the moon than in the hybrid simulations. This disruption occurs as the result of a novel wake phenomenon: the non-linear growth of electron holes spawned from a narrow dimple in the electron velocity distribution. Most of the holes arising from the dimple are small and quickly leave the wake, approximately following the unperturbed electron phase-space trajectories, but some holes originating near the center of the wake remain and grow large enough to trigger disruption of the ion beams. Non-linear kinetic-electron effects are therefore essential to a comprehensive understanding of the 1D electrostatic stability of such wakes, and possible observational signatures in ARTEMIS data from the lunar wake are discussed.