Neutral Solar Wind Generated by Lunar Exospheric Dust at the Terminator

TL;DR

This paper calculates the flux of neutral solar wind generated by lunar exospheric dust at the terminator, showing it varies with dust grain size and solar wind speed, and can be measured with current instruments.

Contribution

It provides the first detailed estimates of neutral solar wind flux from lunar dust grains of different sizes and speeds, offering a new method to study lunar exospheric dust.

Findings

Neutral solar wind flux is higher for smaller dust grains.

Flux ratios depend strongly on solar wind speed and dust grain size.

Measurements can inform on the distribution of small dust in the lunar exosphere.

Abstract

We calculate the flux of neutral solar wind observed on the lunar surface at the terminator due to solar wind protons penetrating exospheric dust grains with (1) radii greater than 0.1 microns and (2) radii greater than 0.01 microns. For grains with radii larger than 0.1 microns, the ratio of the neutral solar wind flux produced by exospheric dust to the incident ionized solar wind flux is estimated to be about 10^-4-10^-3 for solar wind speeds in excess of 800 km/s, but much lower (less than 10^-5) at average to slow solar wind speeds. However, when the smaller grain sizes are considered, this ratio is estimated to be greater than 10^-5 at all speeds, and at speeds in excess of 700 km/s reaches about 10^-3. These neutral solar wind fluxes are easily measurable with current low energy neutral atom instrumentation. Observations of neutral solar wind from the surface of the Moon would provide independent information on the distribution of very small dust grains in the lunar exosphere that would complement and constrain optical measurements at ultraviolet and visible wavelengths.