Proton entry into the near-lunar plasma wake for magnetic field aligned flow

TL;DR

This study reports the first direct observation of protons entering the near-lunar plasma wake during aligned magnetic and solar wind flow, revealing new insights into lunar-solar wind interactions.

Contribution

It provides the first observational evidence of proton entry into the lunar wake during aligned flow conditions, challenging conventional fluid models.

Findings

Protons are observed in the lunar wake during aligned IMF and solar wind flow.

Protons originate from the solar wind and have large gyro-radii.

Entry of protons may involve wake boundary electric fields.

Abstract

We report the first observation of protons in the near (100--200 km from surface) and deeper (near anti-subsolar point) lunar plasma wake when the interplanetary magnetic field (IMF) and solar wind velocity ($v_{sw}$) are parallel (aligned flow, angle between IMF and ($v_{sw} \le 10^\circ$). More than 98% of the observations during aligned flow condition showed the presence of protons in the wake. These observations are obtained by the SWIM sensor of the SARA experiment on Chandrayaan-1. The observation cannot be explained by the conventional fluid models for aligned flow. Back-tracing of the observed protons suggests that their source is the solar wind. The larger gyro-radii of the wake protons compared to that of solar wind suggest that they were part of the tail of the solar wind velocity distribution function. Such protons could enter the wake due to their large gyro-radii even when the flow is aligned to IMF. However, the wake boundary electric field may also play a role in the entry of the protons in to the wake.