Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1

TL;DR

This study reports the detection of protons in the near-lunar wake by Chandrayaan-1, revealing higher-than-expected velocities and challenging existing analytical models of plasma expansion.

Contribution

It provides new observational data on lunar wake protons and highlights discrepancies with current analytical models, advancing understanding of lunar plasma interactions.

Findings

Detected protons in the lunar wake with higher energies than solar wind protons.

Observed proton velocities exceeded model predictions by 2-3 times.

Simple models cannot fully explain ion dynamics near the Moon.

Abstract

Significant proton fluxes were detected in the near wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a $140^{\circ}$ solar zenith angle, i.e., ~50$^{\circ}$ behind the terminator at a height of 100 km. The protons come from just above the local horizon, and move along the magnetic field in the solar wind reference frame. We compared the observed proton flux with the predictions from analytical models of an electrostatic plasma expansion into a vacuum. The observed velocity was higher than the velocity predicted by analytical models by a factor of 2 to 3. The simple analytical models cannot explain the observed ion dynamics along the magnetic field in the vicinity of the Moon.