Plasma refilling of the lunar wake: plasma-vacuum interactions, electrostatic shocks, and electromagnetic instabilities

TL;DR

This study uses kinetic simulations to analyze the complex plasma-vacuum interactions, shocks, and instabilities involved in the lunar wake refilling process caused by solar wind interactions.

Contribution

It provides detailed kinetic insights into lunar wake refilling, confirming theoretical predictions and revealing electromagnetic instabilities and shock formation.

Findings

Confirmation of plasma-vacuum interaction theory during early refilling

Identification of electrostatic shocks from ion beam collisions

Detection of electromagnetic instabilities and wave growth

Abstract

A plasma void forms downstream of the Moon when the solar wind impacts the lunar surface. This void gradually refills as the solar wind passes by, forming the lunar wake. We investigate this refilling process using a fully kinetic particle-in-cell (PIC) simulation. The early stage of refilling follows plasma-vacuum interaction theory, characterized by exponential decay of plasma density into the wake, along with ion acceleration and cooling in the expansion direction. Our PIC simulation confirms these theoretical predictions. In the next stage of the refilling process, the counter-streaming supersonic ion beams collide, generating Debye-scale electrostatic shocks at the wake's center. These shocks decelerate and thermalize the ion beams while heating electrons into flat-top velocity distributions along magnetic field lines. Additionally, fast magnetosonic waves undergo convective growth via anomalous cyclotron resonance as they co-propagate with temperature-anisotropic ion beams toward the wake's center. Electromagnetic ion cyclotron waves may also be excited through normal cyclotron resonance, counter-propagating with these anisotropic ion beams. Our findings provide new insights into the kinetic aspects of lunar wake refilling and may enhance interpretation of spacecraft observations.